Cation exchange capacity (CEC) is a measure of soil fertility.

The chemistry and science behind how and why a soil’s cation exchange capacity works the way is does is fascinating (and a little too complex for this venue). To put it simply, cation exchange capacity (CEC) is a measurement of how many positively charged minerals can be held by the surface of a soil particle. To learn how this affects the plants in your garden, we will need to touch up on some basic chemistry. Don’t panic - you can do this!

Basic chemistry
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Everything is made up of atoms and molecules that are either positively charged (cations), negatively charged (anions), or neutral. Organic materials and clay tend to be negatively charged. This means these anions attract and hold cations, such as potassium, calcium, and iron, which are important plant nutrients. Many anions, such as phosphorus, sulfur, and boron are held in the water that is found in the spaces between soil particles. Soils with high CEC ratings generally hold onto more water, as well as nutrients. Below, you can see the electrical charge for each plant nutrient.

Soil science

Plant food exists as atoms and molecules of minerals. Minerals make up 45 to 49% of a soil sample. These mineral particles come in a range of sizes, with sand being the largest and clay being the smallest. Here in San Jose, California, we have clay. Chemically, sand is relatively unreactive and neutral. The spaces between particles (macropores and micropores) tend to be large. These spaces, and the lack of an electrical charge, make it more likely that plant nutrients will be leached out, or washed away. This is why sand has a low CEC rating. Clay, on the other hand, is made up of many negatively charged secondary minerals that love to attract and hold cations. Also, the smaller particle size of clay means that it has 100,000 times more surface area than sand, in the same size sample, so there are plenty of places for attachments to occur.

Soil test results

When I had my soil tested, it came back with a CEC rating of 20.6, but what does that mean? A CEC rating of 20.6 is considered relatively high. One way to look at a CEC rating is to think of it as a power strip - just how many cords can be plugged in? CEC is measured using mEq/100g. We won’t get into it, but it basically means how many parts of something there are in a certain volume of soil. Different soil types have very different CEC ratings:

•     Sand                       2 to 6 mEq/100g
•     Sandy loam          3 to 8
•     Loam                      7 to 15
•     Silt loam               10 to 18
•     Clay/clay loam    15 to 30

My soil’s CEC rating of 20.6 means it can hold onto a significant amount of plant nutrients. Another, related figure found on a soil test is called base saturation. Base saturation is the percentage of available connections being used. You will normally see separate figures of base saturation for calcium, magnesium, and potassium. [You can think of them as different sized electrical plugs.] Below, you can see my base saturation results with “values found” (left) and “optimal ranges” (right).

CEC and pH

Soil pH also plays a role in a soil’s ability to hold onto plant nutrients. This is because pH is a function of rogue hydrogen cations (H+) floating around in the soil. Soils with a higher, more alkaline pH, tend to have a higher CEC rating. Of course, too much of a good thing turns out to be a bad thing. If the soil becomes too alkaline, nothing can grow in it! This is true in other ways, as well. Too much ammonium (NH4+) in the soil can interfere with the uptake of potassium (K+), calcium (Ca2+), and magnesium (Mg2+). This is why soil tests are so important. Armed with the information they provide, you can look at fertilizer labels with a more informed idea of what your soil actually needs. Acidifying our local clay is one way to make more nutrients available to your plants. The opposite is true in areas with acidic soil. There is a happy medium, but soils with a higher CEC rating are more difficult to alter, when it comes to pH.

The carbon to nitrogen ratio (C:N) describes relative proportions of carbon and nitrogen in a substance. That substance can be soil, compost, or plants.

Carbon is used as energy and a building material, much the way carbohydrates and sugars are used by us. Nitrogen is needed by to generate proteins, amino acids, and enzymes, making it the steak and salad portion of a garden diet. The energy contained in carbon can only be used if there is enough good health provided by the nitrogen.

With the proper C:N ratio, plants can thrive, soil can support plant growth, and compost can decompose quickly. Improper C:N ratios can interfere with the soil microorganisms that make everything else in the garden possible.

Soil microorganisms

Soil microorganisms are responsible for the decomposition of dead plant and animal matter. They also break down minerals and chemicals found in the soil, turning them into plant food. Soil microorganisms prefer a C:N ratio of 24:1. This means they perform best in an environment that is 24 parts carbon and 1 part nitrogen. Since some carbons break down more slowly than others, the Golden Rule of C:N ratios is 30:1.

Microorganisms have a C:N ratio of 8:1. When they consume carbon and nitrogen, 16 parts of the carbon is burned off as energy, while 8 parts are used for maintenance. If higher levels of carbon are available, such as right after applying straw as a mulch, the microorganisms will pull nitrogen from the soil to maintain the balance they need. This means there will be less nitrogen in the soil for your plants. This tying up of nitrogen is called immobilization because the nitrogen is unavailable until the microorganisms die and decompose. The latter half of this cycle is called mineralization, because minerals are returned to the soil. Materials added to the soil with a C:N ratio of less than 24:1 means there will be an abundance of nitrogen left over for your plants. 

What’s your soil’s C:N ratio?

Complex lab tests are used to calculate C:N ratios for commercial agriculture. Since nitrogen doesn’t stick around for very long, you don’t need to go to the trouble or expense for an actual test. Instead, you can manipulate the C:N ratio with cover crops, crop rotation, mulch, and composting, If you have the correct C:N ratio, your plants will be able to eat and your compost will break down quickly, If the C:N ratio is out of whack, things won't be running as smoothly.

Cover crops and C:N ratios

Many cover crops, such as fava beans and other legumes, are grown to both protect the soil from erosion and compaction, and to add nutrients to the soil. You need to find a balance point between allowing the cover crop to grow, when to cut it, and what to replace it with as it decomposes in place. Cover crops are an excellent way to protect and feed the soil between regular crops. Depending on the plants used as a cover crop, you can ensure that your soil microorganisms and your plants have the nutrients they need.

Secondary plant nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). 

The nutrients plants use the most are called primary nutrients. Nitrogen, potassium, and phosphorus are primary nutrients. On the other hand, only tiny amounts of boron, copper, iron, chloride, manganese, molybdenum, and zinc are needed. These micronutrients used to be called trace elements. In the middle are the secondary nutrients. Secondary nutrients rarely need to be supplemented, but they are very important to plant health.

Most soils already contain high enough levels of these secondary nutrients, but you don’t know for sure without a lab-based soil test. The effects of not enough or too much of any one nutrient can create a domino effect that is difficult to diagnose. Simply adding more fertilizer can often makes problems worse, rather than better.

Why are these secondary nutrients important and what are some signs of toxicity or deficiency? Let’s find out!

Calcium

Plants use calcium to build strong cell walls, to move materials across cell membranes, to grow primary root systems, and to maintain the cation-anion balance. [Cations and anions are electrically charged atoms of minerals that plants use for food.] Optimal levels of calcium range from 1000 to 1500 parts per million (ppm).

Calcium is relatively immobile inside a plant. It takes a lot of water to move a calcium molecule around inside a plant. That’s why blossom end rot is more of an irrigation problem than a calcium deficiency problem. Calcium deficiencies, whether caused by real lack or insufficient irrigation, are rare in nature. When they do occur, they can cause bitter pit in apples, cavity spot in carrots, and leaf tip burn in several different plants.

Too much calcium is also rare, but it can interfere with the absorption of magnesium and potassium, causing deficiencies in those nutrients. Bottom line with calcium: irrigate adequately, regularly and consistently.

Magnesium

Magnesium is essential for plant health. Ideal levels of magnesium range from 50 to 120 ppm. Magnesium stabilizes cell membranes, making plants better able to withstand drought and sunburn. Magnesium is found in enzymes that plants use to metabolize carbohydrates. Most important, magnesium is contained in the chlorophyll molecules that convert the sun’s energy into food. This process, the Calvin Cycle, is what makes photosynthesis possible. 

Too much magnesium in the soil makes it difficult for plants to absorb calcium and other nutrients, which can lead to blossom end rot, bronzing, and many other problems. This is a common problem in areas with alkaline soil. The opposite is true in areas with acidic soil. 

Insufficient magnesium symptoms look like potassium toxicity symptoms. Older leaves, at the bottom of the plant, start turning brown, between and alongside the leaf veins, working upward through the plant. Magnesium deficiencies in stone fruits often start out as slightly brown areas along leaf edges (margins) that expand inward, causing cracking, necrosis, and leaf loss. Magnesium deficiency in California is extremely rare.

Sulfur

Plants use a surprising amount of sulfur. This secondary nutrient is used in making chlorophyll and certain proteins and enzymes. Sulfur is also part of the arrangement between legumes and rhizobia bacteria that allow them to make use of atmospheric nitrogen.

Sulfur deficiency is seen first in new growth. Leaves are pale and growth is spindly. If sulfur levels become toxic, leaves will be smaller than normal and have scorched edges. Sulfur is commonly used as an organic fungicide and to acidify the soil.

Do not use horticultural oil within 2-4 weeks of applying sulfur. Sulfur and horticultural oil are phytotoxic (poisonous to plants) when combined. Also, it is better to use fixed copper, rather than sulfur, on apricot trees.

Your plants may not need as much of these secondary nutrients, but they are just as important to plant health. Get a lab-based soil test to find out what is in your soil.

Micronutrients are elements that plants only need in small (micro) doses. These used to be called trace elements, but the American Society of Agronomy and the Soil Science Society of America are urging us to use the term micronutrient instead. Whatever you call them, your plants need them to grow and produce.

Plant nutrition
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Like us, all plants need water, oxygen, and carbon to live and thrive. They also need the Big Three: nitrogen, phosphorus, and potassium. Beyond that, to function properly, plants need secondary nutrients and micronutrients to stay healthy. All together, these are called essential nutrients. But, before we learn about the various players in the world of plant nutrition, you need to understand that nutrient uptake is a delicate, intricate dance that occurs between root hairs, soil minerals, moisture, soil pH, and some mind-blowing processes, all of which are occurring at the molecular level!

What’s important to keep in mind is that too much of a good thing can be a bad thing. Essential nutrients can reach toxic levels, causing just as much damage as not having enough. To make matters even more confusing, the symptoms of one nutrient’s toxicity make look exactly like a different nutrient’s deficiency. And none of this occurs in isolation. A deficiency in one nutrient can domino into the deficiency of several other nutrients. Also, some plants can  block the entry of some nutrients, preventing toxicity, while others cannot. If that weren’t confusing enough, some nutrient deficiencies and toxicities can look exactly like herbicide overspray damage, pest feeding, or a disease. Fear not! We will sort this out as best we can. Read on!

​Plant nutrients

If you don’t know what your plants need, you can’t give it to them. A basic understanding of plant nutrition can help you help your plants. For a quick refresher of the three macronutrients:
    Nitrogen (N) - used in photosynthesis for rapid leaf growth
    Phosphorus (P) - used in photosynthesis for flower, fruit and root growth
    Potassium (K) - used to fight disease and improve fruit quality

Secondary nutrients also play vital roles in plant health:
    Calcium (Ca) - used for cell wall structure and to move other nutrients around
    Magnesium (Mg) - essential for photosynthesis and it activates plant enzymes
    Sulfur (S) - used to create amino acids for root and seed production

For plants, secondary nutrients and micronutrients work much the way vitamins do for us: you probably won’t die without them, but your teeth may fall out and you certainly won’t be at your best.

Nutrient mobility

As you learn about plant nutrients, you need to know that some of them are free to move around within a plant and others are not. Highly mobile nutrients, such as nitrogen and potassium, go where they are needed. This means that deficiencies are seen in older growth, as the nutrients are pulled away to provide for new growth. The opposite is true for nutrients that do not readily move around. Once they are absorbed, they tend to stay where they are. This means that deficiencies are usually seen in young leaves and new buds.

Below, you will find a brief description of each plant micronutrient. This will help you to understand how these nutrients are used, and what the plants look like if they are missing a nutrient (or have too much).

Aluminum (Al)

Aluminum acidifies the soil by removing hydroxide ions out of water, which leaves acidic hydrogen ions behind. Aluminum is not exactly a plant nutrient, though it is believed to act as a fungicide for some root rots. The reason for its inclusion is that it can be absorbed by plants to the point of toxicity. Aluminum toxicity causes slowed root growth. At the same time, aluminum is frequently applied to tea crops because of the way it prevents copper, manganese, and phosphorus toxicity.

Boron (B)

Boron is used to make cell walls. It also helps plants use and regulate other nutrients by facilitating the production of sugar and carbohydrates. Boron helps plants reproduce. This means plants use boron to flower, fruit, and go to seed. It is also used in pollen generation and cell division. Boron is only available to plants when the soil pH is between 5.0 and 7.5. [My last soil test came back with a pH of 7.7, which is not uncommon in the Bay Area.] Boron is generally created by decomposing organic matter that is deposited on the soil surface. Unfortunately for many of our summer plants, boron cannot be absorbed once the soil is dry. Boron is an immobile nutrient that works in tandem with calcium, so a deficiency of one can lead to the unavailability of the other. Boron deficiency appears as stunted, crinkled, or otherwise distorted fruit or buds (meristem tissue), dark rings on leaf petioles (those tiny stems that attach leaves to twigs), roots that are shorter, thicker, and highly branched, or upper leaves that turn reddish yellow. Too much boron in the soil makes leaves look scorched, with browned areas on leaf tips and edges.

Chlorine (Cl)

Chlorine aids plant metabolism during photosynthesis. It is necessary for osmosis and fluid balance within plants. It is a mobile micronutrient. Too much chlorine in the soil, common in areas with hard water, can interfere with a plant’s ability to absorb nitrogen. [San Jose tap water ranges in pH from 7.0 to 8.7.]

Cobalt (Co)

Cobalt is not needed by all plants. It is only used by legumes in nitrogen fixing.

Copper (Cu)

Copper is used to make reproductive enzymes - that means flowers, fruits, and seeds. Copper also helps plant roots eat and breathe, and it metabolizes proteins. Since copper is not mobile inside the plant, deficiencies are usually seen near the top or in new growth, rather than established leaves and stems. Copper deficiencies can cause leaf rolling and curling.

Iron (Fe)

Iron is essential in the production of chlorophyll and moving electrons around within the plant. Iron is also used in enzyme functions that help your plants absorb many other nutrients. This means your soil, like mine, can contain plenty of everything else, but insufficient iron makes it difficult for plants to get at all that food. Iron deficiencies usually appear as chlorosis (yellowing) and necrosis (dying) between young leaf veins, especially at the top of the plant. Iron is not readily mobile. If your soil is alkaline or deficient in copper, it can make an iron deficiency even worse.

Manganese (Mn)

Manganese is used to make chlorophyll and activates plant enzymes that breakdown carbohydrates and nitrogen into usable bits. It is also used as an antioxidant. Manganese is not mobile, so symptoms of deficiency are usually seen near the top of the plant. Yellowing between the veins of young leaves, with tan flecks, while the areas next to veins stays dark green, is the first symptom of a manganese deficiency. Deficiencies are more likely in alkaline soil. Too much calcium can cause a manganese deficiency, which can, in some cases, be counteracted by adding more nitrogen.

Molybdenum (Mo)

Molybdenum helps plants use nitrogen by working with certain enzymes. If molybdenum is deficient, or if any of those enzymes become sluggish, overall plant growth will slow significantly. Molybdenum is partially mobile.

Nickel

Nickel is used to activate certain enzymes. Insufficient nickel causes a condition called mouse ear, in which stems are shorter than normal and leaves are smaller and more rounded. Insufficient nickel allows urea to collect within plants, causing lesions.

Selenium

Selenium isn’t exactly an essential nutrient, but plants grown on selenium-depleted soils end up being less nutritious for us. Selenium is believed to stimulate plant growth and to counteract stress, pests, and disease. [In the human body, selenium makes antioxidant enzymes that prevent cell damage.]

Silicon

Silicon is found everywhere, so we generally do not think of it as an essential nutrient. That being said, silicon is used by plants to build cell walls and to improve plant health and productivity. Silicon is believed to help plants counteract drought and frost damage.

Sodium

Plants, like us, can suffer or even die from too much sodium, though, for them, it’s not from heart failure. Sodium can replace other critical nutrients within a plant, potassium and nitrogen, in particular. At the same time, low levels of sodium are needed by plants to stimulate growth, maintain a water balance, and improve fruit flavor.

Vanadium

Vanadium is not used by all plants. When it is used, plants are using it as a substitute for molybdenum.

Zinc (Zn)

Zinc helps plants breakdown carbohydrates, and it regulates sugar consumption. Zinc is also used to activate enzymes. It is not mobile. Zinc deficiencies show up as yellowing between young leaf veins and overall bleaching that does not reach leaf edges or midribs. This bleaching can also take the form of narrow yellow or white stripes between the veins of upper leaves. Zinc deficient leaves may also roll or curl, and leaves may be smaller than normal. Zinc deficiencies are common in areas with alkaline soil and/or insufficient organic matter. (So keep composting!)

Mulder’s Chart

Back in 1953, a man named Mulder created a chart that shows how he believed different nutrients interact. The relationships shown in the graph are either antagonistic (red lines) or enhancing (green lines). It takes a little getting used to, and we will discuss it further, but give it a ponder. To get you started, take a look at the lines that point to potash (potassium). You can see green lines connecting it to iron and manganese. That means the presence of potassium, iron and manganese facilitates the uptake of all three, simply because of their chemical makeup.

At the same time, red lines can be seen connecting potassium to boron, calcium, magnesium, nitrogen, and phosphorus. This means that potassium competes with these elements on the root hairs. It also alters soil chemistry, making it so alkaline that iron and boron cannot be absorbed. Of course, there are dozens of other variables at play, so this is an oversimplified explanation of what's happening in your soil, but it might get you to take a closer look at your soil test results!

​What are your plants trying to tell you about the soil they live in?

Phosphorus is an essential plant nutrient, second only to nitrogen in plant health. In fact, phosphorus is found within every living cell on earth. It is part of our DNA and every cell wall.

Phosphorus helps plants use and store energy. It is also used make oils, sugars, and starches, within each plant. Most important to the home gardener, phosphorus supports flower and root growth. Much the way nitrogen supports vegetative growth, and potassium supports crop quality and size, phosphorus is the reproduction nutrient, the in-between stage between growing and fruiting. It supports root, flower, seed, and fruit growth. Phosphorus is also part of photosynthesis.

Sources of phosphorus

Despite being so important, phosphorus is rarely found in a form plants can use. This is because phosphorus is highly reactive. When elemental phosphorus is exposed to oxygen, it actually glows! [This is where we get the word ‘phosphorescence’!] Mostly, phosphorus exists as an acid- or salt-version of its former self, called phosphates. Scientists recently discovered that phosphorus is created when a star goes supernova. Here on Earth, organic sources of phosphorus include animal manure, urine, guano, compost. blood meal, and bone meal. Most of the phosphorus found in bags of fertilizer is mined in China, Russia, and the midwest, with 50% of the world’s supply found within the borders of Arab nations. [Perhaps phosphorus will become the new petrol...] Phosphorus is commonly applied around seeds at planting time in a process called banding.

Worldwide, the demand for phosphorus is growing twice the rate as the human population, mostly for agricultural use as fertilizers and pesticides. Phosphates are also used as nerve agents and in detergents. [I prefer soap nuts.] Experts predict a phosphorus shortage by 2040, and a complete end of mineable phosphorus in 345 years. Other experts claim this will happen much sooner. Yet another reason for not applying fertilizers your soil does not need.

So, before we run out, how do you know if your plants have enough (or too much) phosphorus?

Nutrient mobility

Phosphorus is a mobile nutrient, which means that it can be moved around, within a plant, after it has been absorbed. Other nutrients, such as iron and calcium, are described as ‘immobile’ because they generally stay where they were first dropped off by the vascular system. The reason this matters is that it helps in identifying deficiencies and toxicities. Mobile nutrient deficiencies are normally seen in older leaves first. This is because mobile nutrients are pulled out of older leaves to provide support for new leaves. With immobile nutrients, the opposite is true. Older leaves have already gotten their nutrients and hold them in place. The new leaves do not have access and so exhibit deficiency symptoms.

Phosphorus deficiency and toxicity

Phosphorus deficiency is practically unheard of in California home gardens. The optimal range, in parts per million, is 4 to 14. My soil test results reported a value of 84.3 - nearly 10 times what my plants need! The problem wasn’t phosphorus deficiency, but accessibility. Without enough iron in the soil, my plants could not access all that phosphorus (and several other important nutrients). In addition to being rare, phosphorus deficiency can be difficult to identify. In the early stages of growth, a deficiency may appear as nothing more than sluggish growth or mild stunting.

​Since phosphorus is an important part of genetic information transfer, deficiencies ultimately result in smaller and fewer leaves, and fruit set failure. This deficiency also causes a procedural imbalance between photosynthesis (carbohydrate production) and carbohydrate storage. This imbalance leads to too many carbs in the leaves, which makes them darker and more purple or red than normal, especially on the underside, with a shiny almost metallic appearance on the top surface. These symptoms cannot be relied upon as a diagnostic tool, because the same symptoms may indicate several other conditions. Soil testing and plant material testing are the only way to know for sure.

Too much phosphorus can interfere with a plant’s ability to absorb copper and zinc, but this condition is extremely rare in garden environments. It can be seen in containerized plants, or those being grown hydroponically. Zinc and copper deficiencies appear as chlorosis, twig dieback, and bronzing.

Testing for phosphorus

Soil tests are invaluable in learning about what is in your soil. The reason for using a local reputable lab lies mostly in the tests for phosphorus. There are two tests generally used when calculating phosphorus levels: the Bray P1 test and the Olsen sodium bicarbonate test. Their effectiveness lies in soil pH. The land west of the Rockies tends to have alkaline soil, which is better suited to the Olsen test. More acidic midwest and eastern seaboard soils give a more accurate reading when the Bray test is used. If you send your soil samples to the other side of the country, your results may be less accurate (and less useful).

So, before you add any more phosphorus to your soil, take the time to find out if it is actually needed.

Finally, did you know that the rough surface used to strike a match is made with glue, ground up glass, and phosphorus? Now you know.

Banding is a way to help your seeds get a better start on life.

As much as making music is a great way to expand your mind and improve your math skills, banding in the garden has nothing to do with tempo or harmonics. Banding refers to the practice of incorporating fertilizer in the soil close to your seeds at planting time. Just picture, in your mind’s eye, a band of seeds planted in the ground, with a band of fertilizer right next to them. As new roots grow, they are sure to find a good meal to help them get big and strong.

It makes obvious sense, but you do have to be a little careful. Banding falls into the “too much of a good thing can be a bad thing” category. According to Montana State University, benefits of proper banding include:

• higher overall crop yield
• less weeds, and plants better able to compete with weeds
• higher fertilizer efficiency

How to band seeds at planting time
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Unlike top dressing, which means leaving aged compost or fertilizer on top of the soil, banding requires a little more effort. For the home gardener, we don’t need to get too particular about the depth. Professional, large-scale farmers have this down to an art and science, but we can safely apply our banded fertilizer 3 or 4 inches deep, 1 to 3 inches on either side of the row of seeds being planted. These “starter fertilizers” make valuable nutrients available to early roots, helping the seedling to reach its full potential. Before you jump on the fertilizer band wagon [Sorry, I couldn’t resist], you need to find out what you are working with and which fertilizers are best for your plants.

​I’ll say it again: soil test!

You can’t know what to add if you don’t already know what you have. Your soil may have an abundance of phosphorus. Adding more could be detrimental to your plants, and it’s a waste of money. Adding unnecessary fertilizer also puts the environment at risk, you know, global health and all that. Find a reputable, local soil test company and use them. The results are really fascinating and useful. [Over the counter soil test kits are not reliable or useful. Yet.] If your soil already has plenty of everything, banding is unnecessary. If your soil is lacking any of the Big Three plant nutrients, banding can help your seeds overcome this handicap.

Choosing the right fertilizer for banding
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All fertilizers are required to provide information about the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K). Think about this for a moment. A 10-pound bag of 10-20-10 fertilizer contains 1 pound nitrogen, 2 pounds phosphorus, 1 pound potassium, and 6 pounds of filler. After you have gotten the results from your soil test, you can select the best fertilizer for your crop.

According to Pennsylvania State Extension, nitrogen and phosphorus are the “key nutrient components in a starter fertilizer.” If all your plants need is nitrogen, blood meal is an excellent choice. Be cautious with fertilizers that contain urea (46-0-0) or diammonium phosphate (10-34-0), as these substances can burn or even kill young plants.

So, find out what’s in your soil. If something is lacking, put it where seeds are sure to find it with banding!

Green manure probably isn’t what you think. Not a manure at all, green manure refers to certain fast growing cover crops.

Green manure crops are grown to be cut down while they are still green or just after flowering. Traditionally, green manures were plowed into the soil, but it has been discovered that this damages networks of root fungi (mycorrhizae) that help plants absorb nutrients. Like animal manures, green manures provide many benefits to the soil.

Nitrogen banking

Some green manure crops are grown to add nitrogen to the soil. These plants include alfalfa, fava beans, cowpeas, sweet clover, Egyptian (or berseem) clover, crimson clover, lana (or woollypod) vetch, and hairy vetch. These plants are all members of the legume family. Legumes have a working relationship with certain soil bacteria (Rhizobia) that allow them to ‘fix’ atmospheric nitrogen and convert it into a form usable by other plants. Growing these plants as a green manure can increase the amount of nitrogen available to the next crop by 40 to 60%. That’s like dumping 40 to 200 pounds of nitrogen on an acre of land!

Other nutrients

In the same way as nitrogen banking, other nutrients are returned to the soil as green manure crops are broken down by soil microorganisms. These important nutrients include calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S).

Acidifiers

As green manures break down on (or in) the soil, they tend to lower soil pH. This is because acids are formed in the decomposition process. In San Jose, California, where we tend to have heavy, alkaline clay, this acidification can benefit many acid-soil loving crops, such as blueberries, raspberries, potatoes, and parsley.

Soil structure

As soil microbes, worms, and other critters go to work on a cut green manure crop, fungi and slime add their efforts at getting a portion of the banquet. As they all feed (and poop), the amount of organic matter, or biomass, increases and the soil is aerated, increasing the number of macropores and micropores that carry food, water and air. This also improves water infiltration and retention, and makes it easier for tender, young roots to reach the water and nutrients they need to thrive. Medic, berseem clover, and woollypod vetch are good choices for improving soil structure. Deep rooted green manure crops, such as mustard, drought-tolerant alfalfa (Medicago sativa), and alsike clover (Trifolium hybridum), can also help break up compacted soil and pull nutrients closer to the surface for your garden plants to enjoy in the next growing season.

Attract pollinators and predators

If a green manure crop is allowed to flower before being cut, those flowers can attract and feed a wide range of pollinating insects. Phacelia (Phacelia tanacetifolia) is particularly effective. Green manure crops can also provide habitat and protection for many beneficial predator insects. This can lead to a reduced need for insecticides and bigger harvests.

Weed suppression

Since green manure crops tend to be fast growing, they often block common weeds from getting the sunlight, water, and nutrients they need to thrive and reach seed-producing status. Buckwheat (Fagopyrum esculentum), fenugreek (Trigonella foenum-graecum), sorghum, and sweet clover are especially good at blocking weeds.

Erosion control

Cover crops and green manures can be used to reduce erosion. Plant roots stabilize slopes and protect the top layer of soil from sun and wind damage. White clover, barley, rye, and ryegrass are especially good as erosion control. These crops can also help prevent runoff and urban drool.

Pest management

Sorghum, crimson clover (Trifolium incarnatum), and rye provide habitat and food for many beneficial insects. If nematodes are problem in your landscape, you can grow a green manure of white mustard (Sinapis alba) and radish (Raphanus sativus). Nematodes hatch and are attracted to the roots of these plants. After burrowing into the plants’ roots, the nematodes are unable to reproduce. Populations of beet cyst nematodes and Columbian root knot nematodes can be reduced by as much as 70 to 99% using this method. If you have citrus trees, planting bell beans, woollypod vetch, Austrian winter pea, or New Zealand white clover will attract a predator mite (Euseius tularensis) that attacks citrus thrips.

Problems with green manures

Like everything else in life, there are downsides to green manures. First, these crops must be cut before they start reproducing. Legumes stop adding nitrogen to the soil once they start their own reproductive cycle. If any green manure crops are allowed to go to seed, they may overtake an area. Also, all that green, shady moisture can attract slugs and snails. Another potential problem is the bacteria that cause clubroot in members of the brassica family (cabbages, broccoli, turnips, mustards, and cauliflower) may also be encouraged by the presence of green manures. A slightly trickier aspect of growing green manures is the carbon to nitrogen ratio.

Carbon-nitrogen (C:N) ratio

A healthy compost pile* will have a C:N ratio of 20:1 to 35:1. This means it contains a mass that is 20 to 35 parts of carbon to one part nitrogen. [Ratios greater than 35:1 will slow the composting process.] Farmers use a 24:1 ratio for simplicity sake. Healthy soil has a similar ratio. So do the microorganisms in soil that break down all that organic matter. These microbes maintain that ratio with the food they eat. If there is too much carbon available, say, if you mulch an area with straw, soil microbes will devour all that straw and then eat all the available nitrogen, leaving nothing for your plants! But don’t panic. When the microbes die, they return all that nitrogen to the soil, it just takes some time.

*Generally speaking, carbon is in the brown stuff, while nitrogen is in the green stuff. That isn’t exactly accurate, but it will help you to understand how to maintain a healthy compost pile. Since plants are made up of both, aim to provide your compost pile with equal parts green and brown.
​
Carbon content

Non-legume plants have higher carbon contents than the legumes. Also, carbon content changes as a plant ages. You can avoid this problem by planting a mix of green manure crops at certain times of the year and ensuring that they are mowed or cut before they start producing seeds of their own. [For you science nerds, the average adult human body has a C:N ratio of 54:1.] Here is a list of C:N ratios for many green manure crops (animal manure tends to be 20:1):

Ammonium sulfate is a good source of iron for your plants, even though it doesn’t contain any. How can that be? Read on!

Ammonium sulfate (AS) is the oldest form of manufactured nitrogen fertilizer. Chemically, ammonium sulfate [(NH4)2SO4] is a salt that contains 21% nitrogen and 24% sulfur.

What’s in the fertilizer bag?

Most people know that plants use nitrogen to grow. If you buy a 10-pound bag of 5-5-5 fertilizer, that means you are getting 5% of each of the primary nutrients - nitrogen (N),  phosphorus (P), and potassium (K). This works out to 1/2 pound of each nutrient and 8-1/2 pounds of filler. If you buy a 10-pound bag of ammonium sulfate, you get 2.10 pounds of nitrogen, 2.40 pounds of sulfur, and 5.5 pounds of filler. Now, don't think that those fillers simply take up space, though sometimes that’s exactly what they do. Mostly, these fillers are sand or granulated limestone. Whether or not those are good for your soil depends on your unique situation. Personally, I prefer less filler and more substance. Also, in many regions, gardens often don’t need anything besides nitrogen for plant growth. You can't know without an inexpensive lab-based soil test.

The soil at my old house had nearly 10 times the optimal amount of phosphorus, twice as much potassium and calcium, and 8 times more magnesium than my plants needed, the last time I had it tested. Adding more would be a complete waste of money. My problem was iron. I had less than one-third of the optimal amount. And my plants couldn’t even get to what little there was because of soil pH.

Ammonium sulfate and soil pH

In areas with alkaline soil, sulfur acts as an extremely mild acidifier. If you want to grow acid-loving plants, such as blueberries, artichokes, or potatoes, lowering the soil pH can seriously improve your harvest and the overall health of your plants. Ammonium sulfate has a pH value of 5.5 and the sulfur it contains will provide a tiny bit of help. The real pH reduction occurs when soil microbes convert the ammonium into nitrate, in a process called nitrification. If your soil has a pH of less than 6.0, you should not use ammonium sulfate. Soil that is too alkaline (or too acidic) make it difficult for plants to absorb nutrients and thrive.

Ammonium sulfate and food safety

Unlike many fertilizers, which can be dangerous, ammonium sulfate is a food additive. The U.S. Food and Drug Administration lists ammonium sulfate as “generally recognized as safe”. [I still wouldn't eat it out of the fertilizer bag!] It is commonly added to flours and breads to regulate acidity. It is also added to many vaccines to improve their effectiveness. Ammonium sulfate is hygroscopic, which means it absorbs moisture from the air, so be sure to keep the bag tightly closed.

Applying ammonium sulfate

As with any soil treatment, read the label and follow the directions. Seriously. When applying ammonium sulfate to your lawn or garden, be sure to water or work it into the soil right away. If it sits on top of everything, much of the ammonia (nitrogen) will be lost to the atmosphere.

Ammonium sulfate and iron

So, how can ammonium sulfate provide your plants with iron if it doesn’t contain any? Here’s the rub: soil pH dictates the absorbability of many nutrients. Slightly acidic soil makes it easier for plants to absorb the available iron. Of course, if your soil is low on iron, all the ammonium sulfate in the world won’t help. Get your soil tested so that you KNOW what you are working with. And if you live in an area with alkaline soil, ammonium sulfate can be an excellent way to add nitrogen and reduce soil pH for healthier plants.

Calcium inside plants

We all know that calcium makes for strong bones and teeth. It also helps plants stay healthy. Calcium is critical to plant growth and development. Plants use calcium to build strong cell walls, move materials across cell membranes, grow primary root systems, and maintain the cation-anion balance. [Cations and anions are electrically charged atoms of minerals that plants use for food.]

Researchers learned, in 2016, that the movement of calcium molecules in plant root cells triggers proteins that tell the plant that nitrogen-fixing bacteria are nearby. This causes the plant to start building nodules on the roots that will serve as homes to those helpful bacteria. Bottom line, as calcium walks in the door, the welcome mat for nitrogen-fixing bacteria gets unrolled, setting the stage for healthier and more productive plants. [This stuff amazes me.]

Calcium deficiency

​Calcium deficiency is often caused by irregular irrigation. Unlike more mobile nutrients, such as nitrogen, calcium does not move around within a plant easily. Once it stops traveling through the xylem, it pretty much stays where it is. This is why calcium deficiency is rarely seen in older plant tissue. Normally, calcium is moved through a plant by evapotranspiration, which uses a lot of water. Calcium deficiency can also occur when there is too much nitrogen in the soil, causing plants to grow faster than they can move the available calcium. When plants do not have enough calcium, you may see stunted growth, leaf curling, dead terminal buds and root tips, and leaves with brown spots along the edges that spread toward the center. These damaged areas make it easier for pests and diseases to strike. Some crop-specific symptoms of calcium deficiency include:

• apple - bitter pit
• cabbages - tip burn and heart rot
• carrot - cavity spot
• celery - black heart and stunted central leaves
• strawberries, caneberries, lettuce, spinach, artichoke, cauliflower - leaf tip burn
• peppers and tomatoes - blossom end rot

Drought and minerals

Minerals, such as calcium, are affected by drought in ways that might surprise you. Reduced water supplies often mean we get our tap (irrigation) water from reservoirs that are scraping the bottom of the proverbial barrel. That water already has high salt and mineral contents. The chemical reactions that occur between those salts and plant nutrients can make life difficult for everyone involved.

​California pistachio growers have found that, by adding more calcium to the soil, they can reduce the amount of salt absorbed by plants. This is not something you should attempt in your garden because what you just read is an oversimplification of a complex condition. I only use it to point out the amazing balancing act going on all the time to get you the foods you love. Another factor that involves drought and calcium is drip irrigation emitters. They tend to get clogged by calcium the same way your coffee maker and iron do. If your region has hard (high mineral content) water, you may want to invest in a filter.

Sources of calcium

Before adding calcium to your soil, it is important to find out what it already contains. Most soils west of the Rocky Mountains contain abundant calcium. The optimal range is 1000-1500 ppm. A soil test, conducted by a reputable lab, is the only way to know for sure. Over-the-counter soil tests are not reliable or accurate enough. If you are growing anywhere there used to be an ocean, there’s probably plenty of calcium already present. If you live east of the Rockies, it’s a different story. Agricultural lime and calcium chloride sprays can replenish depleted soils. Sorry, but egg shells do not add calcium to your soil.

Calcium uptake problems

Let’s assume that your soil has plenty of calcium in it and that you are watering regularly and properly. Other problems can interfere with a plant’s ability to absorb this important nutrient. Excessive potassium (K) is one. Too much magnesium (Mg), sodium (Na), iron (Fe), or ammonium (NH4+) can also slow the uptake of calcium. Soil alkalinity or acidity (pH) also plays a role.

The molecular balancing act that occurs between minerals within your soil and plants is mind-boggling, to say the least. Suffice to say, your average gardener (or gardening blogger) only groks the tip of this iceberg. This is not something to guess about. Get your soil tested. Your plants will thank you.

​Fertilizer isn’t just NPK

Most gardeners are very familiar with the NPK found on most bags of fertilizer. These letters represent the three primary macronutrients needed by plants: nitrogen, phosphorus, and potassium, respectively. Of course, what you mostly get when you buy fertilizer is filler. A 20-pound bag of 10-10-10 fertilizer actually contains 10%, or 2-pounds, of each element. The remaining 14-pounds is just filler. Plants also use calcium (Ca), magnesium (Mg), and sulfur (S) as secondary macronutrients. Plants also need these micronutrients: copper (Cu), manganese (Mn), zinc (Zn), boron (B), and iron (Fe).

​Nutrient movement within plants

Some plant nutrients are highly mobile. This means they can be moved around, within a plant, to where they are needed. Nitrogen is so mobile that you can identify nitrogen deficiencies in the soil by looking at the leaves. Older leaves will begin to turn yellow as nitrogen is pulled away to make fresh, bright green, new leaves. Iron, on the other hand, is relatively fixed once it enters a plant. Older leaves will stay green, but new leaves will be pale or look bleached.​​

​Iron deficiency symptoms

Plants grown in iron deficient soil often turn yellow and they cannot thrive because they are starving. This is because iron is needed to produce chlorophyll and in plant respiration. [Note: plant respiration is not the same thing as mammalian breathing - plant respiration refers to breaking down stored food reserves to release usable energy into the plant.] Yellowing (chlorosis) due to iron deficiencies normally begins in the areas between leaf veins, which stay green. Young leaves may look bleached. Symptoms are more pronounced in acid-loving plants, such as blueberries, raspberries, and camellias. Over time, leaf size will be reduced, and dead (necrotic) patches will appear along leaf edges (margins) and between veins. Leaves will also die and drop prematurely. Shoots and canes can also die back.

Iron and soil pH

Often, you will see the word ‘chelated’ on a soil amendment that includes iron. This is because free iron molecules can become unavailable to plants when pH levels are not between 5.0 and 6.5, or when phosphate concentrations are out of balance. This balancing act has to do with the way micronutrients interact with each other in the soil. If they bind to one another, plants can’t get to them. This is particularly troublesome in our highly alkaline, compacted clay soil.

Other causes of iron being unavailable to plants include soil that has been waterlogged (due to flooding or a leaking sprinkler), or too many other nutrients. High levels of copper, manganese, phosphorus, calcium, or zinc will bind to the iron, making everything unavailable to plants. This is a case where adding fertilizer just makes things worse, rather than better.

Case in point

I have a beautiful yard and I love to garden. We moved here in 2012. There were several fungal disease problems present, as a result of leaky and poorly placed sprinklers. The soil is heavy clay and compacted. Borers, scale insects, and aphids were problematic. We stopped using the sprinklers and switched to soaker hoses. Then we started adding compost and mulch to the landscape. And I sent a soil sample to a lab for testing. The results were educational, to say the least.

My soil had an excess of every nutrient, except iron. My soil’s iron levels were far below the recommended level, making every other nutrient unavailable to my plants. As a result, the plants were not healthy enough to fight off pests and diseases as well as they would have been otherwise. Without that soil test, I might have added even more fertilizer, making things even worse for my plants. I can’t stress this enough: get your soil tested by a reputable, local lab. The information is invaluable.

Counteracting iron deficiencies

If your soil is low on iron, you can jump-start your plants’ health with a foliar spray of iron. Spraying leaves with chelated iron or ferrous sulfate allows plants to absorb the mineral directly through leaf tissue. Once the iron is inside the plant, other nutrients can be used. This treatment will need to be repeated as new leaves emerge. Also, it should not be done during hot weather, and it might stain your patio. Longer term solutions include:

• selecting plants that will thrive in the soil (and microclimate) you already have
• acidifying the soil with iron sulfate, elemental sulfur, or ammonium sulfate (which also contains nitrogen) - do not use aluminum sulfate, which can be toxic to root systems
• reducing soil compaction with aeration and mulch
• trunk injections of iron by a licensed professional
• fertilizing the soil with chelated iron

These treatments should only be done AFTER a laboratory soil test has indicated that iron levels are deficient. Over-the-counter soil tests are not effective enough to justify any adjustments.

What is nitrogen?

Nitrogen is an element, like hydrogen or oxygen. The Earth’s atmosphere is 78% nitrogen, but it is in a form that plants cannot use. Nitrogen is the first number you see on a bag of fertilizer. It is the “N” of NPK. Since pure nitrogen boils away at -320 degrees Fahrenheit, you won’t be buying a bag of pure nitrogen at your local garden center. [If you’ve ever had a dermatologist “freeze” off a wart or precancerous area, they are often using nitrogen.]

How plants use nitrogen

Nitrogen is a fundamental building block for chlorophyll and plant enzymes and proteins, including a plant’s DNA. Without nitrogen, photosynthesis cannot occur. Some crops use more nitrogen than others. Cucurbits, such as melons and squash, are relatively light feeders. Heavy feeders include sage, artichoke, potatoes, onions, lemongrass, and corn. If you are growing plants in containers or straw bales, plants should be monitored closely for signs of insufficient nitrogen.

Not enough nitrogen

Stunting and chlorosis are the two most common signs of insufficient nitrogen. Nitrogen is highly mobile within the soil and in plants. Nitrogen deficiencies are frequently seen as a pale area down the middle of each leaf, with older leaves affected first. This happens because the plant pulls nitrogen from older leaves to feed newer leaves. Nitrogen deficiencies in peach and nectarine tend to show as red areas on leaves (where photosynthesis is no longer occurring properly). Heavy clay also reduces nitrogen levels in the soil.

​Too much nitrogen

Too much nitrogen can be just as bad as not enough. Excessive nitrogen is seen as darker than normal leaves and more vegetative growth than fruit or flowers. Too much nitrogen can burn plants, and it can cause erratic or reduced budbreak. Too much nitrogen can also stimulate new  growth that may be vulnerable to cold weather, thrips, leaf spot, Verticillium wilt, aphids, and scale. This is why the timing the use of fertilizer is so important.

Types of nitrogen

The Nitrogen Cycle refers to the conversion of atmospheric nitrogen into chemically reactive forms that attach themselves to other elements, creating ammonia or nitrate based fertilizers. Crops that prefer more acidic soil, such as blueberries and potatoes, seem to prefer ammoniacal nitrogen based fertilizers over nitrate based fertilizers. As plants absorb nitrates, they increase the soil pH, making it more alkaline. When plants take up ammonium, the soil becomes more acidic.

Nitrogen - a fleeting plant nutrient

Nitrogen is quickly used up by nearby plants. It also deteriorates rapidly and is leached out of soil by rain. This deterioration is largely a function of moisture and temperature. As temperatures rise, there tends to be less organic matter in soil. As moisture increases, so does organic matter. This is why it is so important in hot, dry weather to regularly add compost to our gardens and landscapes.

Nitrogen sources

​Native Americans used the Three Sisters Method of growing corn, beans, and squash together. Beans, being a legume, are host to bacteria that convert atmospheric nitrogen into forms usable by plants. Planting them all together provided the corn and squash with extra nitrogen early in their growing season. Some tribes added dead fish or eels when planting, which provided even more nitrogen. Fish emulsion is a mild source of nitrogen. According to study by the Washington State University Extension Office, coffee grounds contain 10% nitrogen after brewing. Blood meal, cottonseed meal, alfalfa meal, and feather meal are all good sources for nitrogen. Urea and urine both provide high levels of nitrogen.

Slime molds are the red-headed stepchildren of the garden world. Not a plant, not an animal, recent research has created more questions than answers about this garden visitor.

This species will appear in your garden. Slime molds do not hurt your crops. Instead, they make nutrients more readily available.

We’ve all heard about fish emulsion, but what’s really in it and how does it actually work in the garden?

Some of the down sides of using fish emulsion are cost and smell. Let’s face it, dead fish have a pretty short half-life. The smell may also attract some unwanted wildlife, such as skunks and flies. Some fish emulsion manufacturers claim that their product is “non-odorous”. I can’t imagine how that happens, but maybe.

Now, most commercially available fertilizers range in significantly higher numbers than fish emulsion, when it comes to nutrients. If you recall, the NPK numbers found on fertilizer packaging refers to the percentage by weight of nitrogen, phosphorous, and potassium, respectively. Commonly, the nitrogen and potassium numbers will be in the double digits, while phosphorus is usually a single digit number. For fish emulsion, the numbers are much lower. The NPK ratio of fish emulsion ranges from 4-1-1 to 5-2-2. This is neither good nor bad, it simply shows that the nutritional value of fish emulsion is lower than many other sources of plant food.

The science behind fish emulsion studies has come to some specific conclusions:

• While some gardeners claim that soaking seeds in fish emulsion will increase germination rates, scientific research has shown that this is not true.
• Transplanted tomatoes and peppers drenched with 1/2 ounce of fish emulsion per gallon of water grew bigger than all of the other plants in one study.
• Studies on the use of foliar (leaf) sprays of fish emulsion showed no significant difference between sprayed and unsprayed plants.
• In a study using fish emulsion, combined with humic acid, on a soybean crop, yields were not impacted by the fish emulsion.

So, the bottom line on fish emulsion: it is an effective way to feed delicate seedlings and transplants, providing easy to use nutrients, but it doesn’t do much of anything for larger, more mature plants. 

In my book, I categorize fish emulsion as a plant baby food.

Plants use a surprising amount of sulfur. This secondary nutrient is used in making chlorophyll and certain proteins and enzymes. Sulfur is also part of the arrangement between legumes and rhizobia bacteria that allow them to make use of atmospheric nitrogen.

Plants tend to pull equal amounts of sulfur and phosphorus from the soil. Imbalances can cause problems.

Chemical balance in the garden

Most plants prefer a relatively neutral to slightly acidic pH. Some plants, such as blueberry, prefer more acidic soil. Sprinkling sulfur throughout the garden and then watering it in creates sulfuric acid. This is not the acid that will dissolve your car’s paint, but it will help make many nutrients available to plant roots. Before treating soil with sulfur to adjust the pH, it is important to get a soil test from a reputable lab. Too much of a good thing can be a bad thing. If sulfur levels become too low, some plants, such as clovers, will disappear completely. Sulfur deficiency is seen first in new growth. Leaves are pale and growth is spindly. If sulfur levels become toxic, leaves will be smaller than normal and have scorched edges.

Sulfur as fungicide

This bright yellow mineral has antifungal properties. Dusting plants with sulfur can prevent or counteract many fungal diseases, such as powdery mildew, brown spot, crown rot, and others. Fungi generally avoid acidic environments, which is what makes sulfur so effective as an organic fungicide. Sulfur in the soil also helps reduce salt levels.

WARNING: 

Do not use horticultural oil within 2-4 weeks of applying sulfur. Sulfur and horticultural oil  are phytotoxic (poisonous to plants) when combined. Also, do not use sulfur on apricot trees. Use fixed copper, instead.

Before feeding plants or amending soil with bone meal, it is very important to have your soil tested by a reputable local lab. Unfortunately, over-the-counter soil tests are too unreliable to be worthwhile. Also, amending your soil with bone meal may attract raccoons or dogs, who will dig up your plants in search of a hidden treat that they will never find.

Invest in a good soil test to see if your garden can benefit from bone meal.

Fertilizer is one of those things that falls under the, “Too much of a good thing is a bad thing” category. All too often, at the first sign of unhealthy plants, people grab a bag of fertilizer before checking for inhospitable soil conditions, unhealthy roots, irrigation problems, nutrient toxicities, and pest or disease infestation. ​

​N, P & K

Most gardeners are familiar with the three numbers displayed on bags and boxes of fertilizer, but we’ll do a quick review, just to be sure. Those three numbers represent the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K). Think about this for a moment. A 10-pound bag of 10-20-10 fertilizer contains 1 pound nitrogen, 2 pounds phosphorus, 1 pound potassium, and 6 pounds of filler. Yes, filler. If all your plants need is nitrogen, blood meal may be a better choice.

Plant nutrients

Plants use 16 chemical elements as food. Those elements include oxygen, carbon, and hydrogen, along with 13 mineral nutrients. Those mineral nutrients are broken down this way:
   Macronutrients
        Nitrogen (N) - leaf growth
        Phosphorus (P) - root, fruit and flower growth
        Potassium (K) - stem and cell growth
    Secondary Macronutrients
        Calcium (Ca)
        Magnesium (Mg)
        Sulphur (S)    
    Micronutrients
        Copper (Cu)
        Iron (Fe)
        Manganese (Mn)
        Molybdenum (Mo)
        Zinc (Zn)
        Boron (B)

Soil tests

Before applying fertilizer, invest in a good soil test. It’s worth it. And it’s a fascinating snapshot of what is really going on in the garden. Now, I don’t mean one of those over-the-counter test tube kits. Those are a waste of money, in my humble opinion. When searching for a soil lab, it is best to pick one near you. The east and west coasts have very different soils (ours is alkaline; theirs is acidic, for one thing). This means that different types of tests are used to analyze soil samples. 

I learned some surprising facts about my soil when I sent in a sample. Most important, I learned that my soil already has a ton of everything, except iron. Without iron, the plants weren’t able to absorb the abundance of available nutrients. Adding fertilizer would have been a complete waste of time and money. Instead, because I had the knowledge, I was able to apply a foliar (leaf) spray of iron and my garden plants had access to everything they needed! So, get your soil tested before adding anything.

Types of fertilizer

If you are like me and prefer a more natural approach, use compost instead of fertilizer. Since I raise chickens, composting is even more effective. Chicken poop is high in nitrogen, and practically anyone can raise hens or build compost. Yard and kitchen scraps that would normally end up in landfills can be transformed into nutrient rich compost that that also improves soil structure.

If you decide fertilizer really is necessary: READ THE BAG. Seriously. Federal law requires that specific instructions and useful information are printed on the container, and for good reason. Follow directions carefully and wash your hands when you’re done. Technically, there is no chemical difference between nitrogen from compost and nitrogen formulated in a lab. Nitrogen is nitrogen. The difference lies in everything else. What are the fillers? What else is in the compost that plants need? Honestly, there’s a lot we don’t yet understand about how living things interact. I prefer to err on the natural side, just in case.

​For a hysterical read about the effects of too much fertilizer, check out Don Mitchell’s Moving/Living/Growing Up Country series.

What is in your wood ash?

Before adding wood ash to the garden, it is important to know what it was before it was burned. Ashes from BBQ grills, plywood, pressure treated, stained or painted wood, cardboard and even paper bags should never be used in the garden. These materials can contain toxic chemicals that you certainly don’t want in your soil, especially if you are growing food plants.

Wood ashes vary in their nutrients, depending on the type of wood that was burned. Soft woods, such as pine, contain only one-fifth of the nutrients held in hardwood. Ashes from good quality hardwoods contain a lot of potassium, or potash. This is the “K” in the standard N-P-K found on fertilizer packaging. On average, wood ash contains 0-1-3 using this scale. Wood ash generally contains the following micronutrients as a percentage of weight:

• calcium 15%
• potassium 2.6%
• aluminum 1.6%
• magnesium 1%
• iron 0.8%
• phosphorus 0.5%
• manganese 0.4%

Potassium supports root growth and cell structure. Stronger roots and cells make plants more resistant to pests, diseases, and environmental stresses. Wood ashes can also improve soil structure, but there’s a price.

Wood ashes and pH

Wood ashes are very alkaline. Using the pH scale, substances are measured on a range from 0 to 14, with 7 being neutral. Lower numbers are more acidic and higher numbers are more alkaline. Wood ash can have a pH of 9-13! Our San Jose soil tends to already be alkaline, so adding wood ash might end up being a bad idea. The only way to really know what you are working with is through a soil test from a reputable lab. Over-the-counter soil test kits are not currently reliable or accurate enough to be useful.

If your soil could benefit from applying wood ash, the best time of year to do this is in the fall and winter. Save cold wood ashes until then and apply over a period of time. Smaller plants and seedlings can be dramatically impacted, for better or worse, with sudden changes in pH.

Other garden uses for wood ash

If your soil does not need its pH altered, there are several other uses for wood ash in the garden: 

• Wood ash can be added to compost piles to help maintain the neutral pH preferred by microorganisms that break down organic material. Simply sprinkle wood ash on each layer of compost as it is added.
• Plants can also be protected against slugs and snails with wood ash. As long as the wood ash stays dry, it will desiccate any mollusks before they get a chance to feed. Of course, irrigation and rain will wash the wood ash into the soil, altering its pH and raising salt levels, so it must be used judiciously.
• Adding a ring of wood ash around basil plants may help keep cutworms away.
• If you raise chickens, wood ashes make an excellent dust bath that helps get rid of lice and other irritants.

If you’re feeling really adventurous, you can use those wood ashes to make soap or shine up Granny’s silver. Just be sure to wear protective clothing when working with wood ashes. Wood ash particulates can irritate your lungs and wet wood ash (lye) can dissolve your fingernails, so be careful!

Put on your science caps, dear readers! Today we are learning about the nitrogen cycle!
​
In its most basic terms, the nitrogen cycle is a process by which bacteria convert atmospheric nitrogen into a form available to plants. Plants use the nitrogen to grow. The plants are then eaten and turned into organic waste. This waste enters the soil where it is again taken up by plants. And so it goes!

Nitrogen (N2) makes up 78% of our atmosphere and it is a necessary component of every living thing. Nitrogen, in the form of nitrates (N03), nitrites (NO2), and ammonium (NH4) help create amino acids and nucleic acids, which turn into proteins and DNA.

The nitrogen cycle consists of:

• fixation - bacteria convert atmospheric nitrogen (N2) into ammonium (NH4)
• nitrification - bacteria convert ammonium (NH4) into nitrates (N03)
• assimilation - plants absorb the nitrates (N03)
• ammonification - decaying matter is converted back into ammonium (NH4)
• denitrification - bacteria convert fixed nitrogen back into atmospheric nitrogen (N2)

Atmospheric nitrogen is generally unavailable to plants, but they really need it. For nitrogen to be absorbed by plants it must be “fixed”. When lightning strikes the Earth, nitrogen is fixed, but the majority of fixation occurs via microorganisms called diazotrophs.

Diazotroph bacteria carry an enzyme that converts atmospheric nitrogen into ammonia, which is then converted into other organic compounds by the bacteria in a process called nitrification. Other microorganisms, such as mycorrhizae, carry similar enzymes that can also fix nitrogen into the soil. In exchange for their efforts, plants provide these bacteria with carbohydrates and sugars. The most effective nitrogen fixing bacteria are from the Rhizobium family. These nitrogen fixing bacteria are found in the root nodules of legumes, such as beans and peas. If your soil is low on nitrogen, it is a good idea to plant members of the legume family and let their microscopic nitrogen fixing factories get to work! 

Industrially, a method called the Haber-Bosch process uses heat and pressure to convert atmospheric nitrogen (N2) into 30% of the ammonium (NH4) used in agriculture. Since the agricultural revolution started 10,000 years ago, humans have been responsible for doubling the amount of nitrogen available to plants. The level of ammonia (NH3) has tripled and nitrous oxide (N20) has begun to break down our protective ozone layer. Rather than applying chemical fertilizers, there are other ways to improve the nitrogen cycle in your garden.

Adding nitrogen naturally

An excellent way to add valuable nitrogen to your soil is to raise or encourage worms in the garden. When worms poop, or die, the nitrogen released is available to plants through ammonification. Worms have the added advantage of improving soil structure, aeration and percolation. Without good soil structure, heavy rains can saturate the soil, creating a bog. In a bog environment, other bacteria step in and convert fixed nitrogen back into the unavailable atmospheric variety in a process called denitrification. Blood meal is an excellent source of nitrogen.

Applying mulch and compost can also improve soil health and soil structure. Nature has evolved effective, sustainable mechanisms for all living things. We can best continue by following those examples.

No, this is not a ghoulish new smoothie flavor, but it is an excellent source of natural nitrogen. Nitrogen is the primary nutrient needed by plants and it is highly volatile, which means it disappears quickly.

Rather than inundating your plants with chemicals and nutrients they don’t need, blood meal is an excellent way to keep your plants well fed. Blood meal is collected at slaughter houses and dried. It can be added to container plants, spread on lawns, or added to established plants. You can find blood meal at your local nursery or big box store.

Be sure to water thoroughly after applying blood meal and you will probably be astounded at the results. Within just a couple of days, your plants will be stronger, healthier, greener, and more productive.

As an added benefit, blood meal repels such pests as raccoons and deer.

Soil pH can make or break your plants' ability to absorb nutrients and thrive.

What is pH?

Everything is existence is either acidic, alkaline, or somewhere in the middle. The pH scale is a simplified version of an algorithmic equation that measures the number of hydrogen ions in a specific quantity of a material in solution. The scale ranges from 0 to 14, with lower numbers indicating acidity and higher numbers indicating alkalinity. In the middle; 7.0 indicates neutral.

Testing soil pH

Soil pH can be tested with an over-the-counter product found at all garden supply stores. Testing the soil will tell you if your soil is neutral (7.0), alkaline (greater than 7.0) or acidic (less than 7.0). While over-the-counter pH tests are accurate enough, other soil tests available from retail outlets are not. To get your soil tested, and I urge you to do so, use a local, reputable soil test lab.

Nutrient availability and soil pH
​
Plants grow best when they have access to all of the nutrients they use to grow and reproduce. At certain pH levels, some nutrients become unavailable. At the same time, soil microbes, which help plants absorb nutrients, are also restricted by certain pH extremes. Also, some plants, such as blueberries, prefer more acidic soil. Using the chart below, you can see that more nutrients are available, and there is greater microbe activity, when soil pH is between 6.0 and 6.5. Most plants can survive in soil pH from 5.2 to 7.8, but the narrower range allows plants to thrive.

Altering soil pH

Soil pH is, for the most part, a function of your local bedrock material. This isn’t going to change any time soon. What you can do is integrate certain practices in to your normal gardening routine that will temporarily alter soil pH.

East of the Rocky Mountains, soil tends to be more acidic; west of the Rockies, soil is more alkaline. Traditionally, acidic soil is treated with lime, to bring is closer to a neutral pH. If your soil pH is too high, you can acidify your soil with sulfur. Some people claim that adding peat moss or pine needles to the soil can increase its acidity, but research has not shown this to be true. Unfortunately, altering pH takes time and repetition to see any results.

Also, it is more difficult to alter the pH of clay soils. Once you begin treating your soil, it is important to continue monitoring pH levels.

In the long run, a soil pH of 6.0 to 7.0 will help your plants become healthier and more productive.

Gardens may look peaceful and calm, but there’s really a lot going on, especially at the level of atoms and molecules. Don’t let this freak you out or chase you away. It’s actually pretty amazing.

If you’ve ever taken a chemistry class, you know that atoms and molecules can be stable or unstable. Unstable atoms and molecules have the wrong number of electrons spinning around. When an atom or molecule is unstable, it is called an ion. 

So what in the world does this have to do with gardening?

Simple. Soil, minerals, and plants are all made up of atoms and molecules, just like us humans. Nutrients in solution, such as liquid fertilizer, or rain or irrigation water passing through compost, have a tendency to stick to the surrounding solids. This is called adsorption.

Don’t let the words confuse you. While adsorption looks a lot like absorption, they behave very differently. Imagine yourself at a party. As you enjoy a sip of your drink (absorption), you spill some on your shoe (adsorption).

Generally speaking, soil is negatively charged. This means soil is using adsorption to grab electrons from nearby atoms and molecules of minerals. Adsorption is a good thing because it gets the nutrients closer to where the plants need them. This is especially relevant when adding amendments or fertilizer to poor soil.

​In many cases, it is soil microbes, called mycorrhizae, that actually move nutrients from the surrounding soil and into the roots themselves. As you can see, soil health is not as simple as it may appear.

Put simply, it doesn't help to add it if your plants can't get to it!