Macropores and micropores are the spaces found between particles of soil, much like the holes seen in a sponge. Like a sponge, those holes can hang onto water, or they can be filled with air. They also provide habitat for important soil microorganisms. 

Macropores and micropores are critical to the movement of air, water, and roots through the soil. While both micropores and macropores can hold air and water, their sizes play different roles in plant and soil health.

Being larger, and normally found between soil aggregates, macropores allow water to drain away through gravity and are often filled with air and soil microorganisms. Because of this, macropores determine a soil’s aeration and porosity. Insufficient macropores can mean compacted soil and drainage problems.

Micropores, often found between and within soil aggregates, are so small that surface tension holds water in place. Instead of draining away, water moves through micropores only when suction is created by thirsty roots. This determines a soil’s water holding capacity.

If you want to get really picky, there are also mesopores, ultramicropores, and cryptopores. Cryptopores are so small (<0.1 μm), most organisms cannot enter, preventing decomposition, and water is held too tightly for plants to use. Ultramicropores are 0.1-30 μm and tend to be populated by microorganisms. And mesospores are larger than macropores at 30 μm–75 μm. Mesospores are filled with easily accessible water at field capacity, providing plants with plenty of water. [Field capacity is the amount of water found in soil after the excess has drained away.] Most of us, however, don’t need to go into that much detail, so we will stick with macropores and micropores.

​Too many, or not enough?

Sandy soil can have so many macropores and micropores that water and nutrients simply leach away. Heavy clay soil, at the opposite end of the spectrum, has more micropores, so water and nutrients are held tightly. Loamy soil, in the middle, provides a healthy balance of micropores and macropores within the soil structure.

One common mistake people make, when trying to improve the structure of clay soil is to incorporate sand. It sounds right, but it’s not. The tiny clay particles fill the spaces between sand particles, creating an even denser soil, frequently referred to as concrete.

To improve soil structure, the best methods are to regularly incorporate organic matter and to apply 3 to 4 inches of coarse wood chips as mulch to unplanted areas. The wood chips will, over 2 or 3 years, break down, adding organic material that helps create a range of aggregate sizes, with plenty of macropores and micropores.

Thank you, Moshe and Robyn! I owe you both a pack of seeds!

Evapotranspiration describes the way water moves from land up into the atmosphere through soil evaporation and transpiration by plants.

As the sun heats the Earth’s surface, the water stored in soil evaporates, rising up into the atmosphere. Sunlight also increases the surface temperatures of lakes and oceans, causing some of that water to evaporate.

Transpiration refers to the way water is lost as plants open pores, called stomata, found on the underside of leaves, and on stems and flowers. These cells open to allow for the gas exchange necessary for photosynthesis. A large, well-watered tree can lose 100 gallons of water each day in hot weather through evapotranspiration!

Hot weather increases a plant’s need for water. At the same time, higher temperatures increase evaporation of the water found in soil. The combined affect is the reason why you need to water your plants more often and more deeply in summer. 

​Now you know.

Alluvial soil is a deposit of clay, silt, sand, and gravel left by flowing streams in a river valley or delta, typically producing fertile soil.

This is some of the most fertile soil on earth. In fact, over 12,000 years ago, our agricultural beginnings nearly all started around creeks and rivers. (Carrying water is hard work). 

Rain, snow, and flowing water erode rocks, carrying minerals downhill, where they collect in lowlands with regular supplies of water. Nutrient rich alluvial soil plus readily available water and you have it - a garden!

Vermiculture refers to the care and feeding of worms. Vermicomposting refers to raising worms to generate valuable worm castings by feeding them compostable materials.

Worms are amazing at breaking plant and animal materials into the best compost available while improving soil structure. While worms prefer temperatures between 60 and 80°F, they can tolerate 40 to 90°F. Anything hotter or colder than that and the worms may be harmed and feeding is slowed.

Raised in artificial beds, worms provide high quality fertilizer by eating the equivalent of their bodyweight in yard and kitchen waste each day. Properly maintained worm beds do not have a smell. According to the EPA, 20-30% of the material currently found in landfills could be used more productively as compost and worm food.

You can put worms to work for you, even in an apartment, using these simple steps:

1. Get a container. You can buy readymade worm bins or you can make your own. Those big covered plastic bins (2’ x 3’) work well, as do 5-gallon buckets. You can also use a wooden box, just make sure it is not made with pressure treated lumber, which contains toxins. You can find many different design ideas online or at your local library. Your worm bin will need drainage holes, or a slightly raised floor and a spigot. The liquid that comes out of your worm bin holds many valuable plant nutrients, but it may need to be diluted to avoid burning plant roots. If a plastic container is used, be sure there are ventilation holes.
2. Add bedding. To a worm, bedding can be shredded newspaper, printer paper, or even cardboard, just avoid using waxed or colored material. Moisten the shredded material, but not to the point of being soggy. Put the bedding in the container a couple of days before adding worms, as it may get very warm, at first. Moisture levels must be maintained between 70 to 80% for worm health.
3. Add worms. Red wigglers (Eisenia fetida) and redworms (Lumbricus rubellus) are the best worms for home composting and you can get them online or at your local bait shop. The earthworms you find in the garden are not well suited to vermiculture - they need the wide open spaces of your garden. If your worm bin will be exposed to higher temperatures, you may want to use Indian blues (Perionyx excavatus). You will want to start with one pound of worms. A family of four will need two pounds. Just make sure you do not introduce invasive Asian jumping worms into the mix!
4. Feed your worms. Worms can be fed every day, twice a week, or once a week. Simply place the food under an inch or two of bedding. You can feed your worms moldy bread, tea bags, coffee grounds and paper filters, eggshells, paper towels (free of cleaning chemicals), pasta, and any other kitchen or yard waste. Do not feed meat, bones, oil, or dairy products to your worms.
5. Maintenance. Every couple of months, you will need to care for your worm bin and harvest the worm castings. To do this, dump everything out on a tarp. Leave the pile alone for a few minutes and go wash the tub, dry it, and add fresh bedding. Then, take the old bedding and add it to your outdoor compost pile or use it as a mulch or top dressing in the garden. When you come back to the tarp, your worms will have clumped together, trying to avoid the light. Collect the top of the pile, push everything into a smaller mound, and wait for the worms to hide again. Repeat as needed until the majority of your nutrient rich worm castings have been harvested.
6. Remove the largest worms and share them with friends, use them for fish bait, or feed them to your chickens. Return the smaller, younger worms (and any seed casings), back to the worm bin and start over. Everything left on the tarp is worth its weight in gold to your garden and house plants.

You can raise worms under your kitchen sink, in the garage, outside, or pretty much any place you want, as long as the temperatures are reasonable. The nice thing about raising worms is, even if you totally fail the first time around, you still have valuable compost for your plants!

The Bad Guys

Some microorganisms are disease pathogens that can damage or kill plants. There are bacteria in the soil that can cause fireblight, cankers, and soft rot. Fungi may bring powdery mildew, eutypa dieback or rust to your garden. Viruses can cause spotted tomato wilt, cucumber mosaic, and many other diseases.

The Good Guys

Legumes, such as peas and beans, have evolved a symbiotic relationship with Rhizobium soil bacteria. These bacteria live in and around the roots, converting atmospheric nitrogen into a form usable by plants. This is just one part of the Nitrogen Cycle.

In similar fashion, many other soil microorganisms make nutrients available to plants. Plants trade the carbon they create through photosynthesis for mineral nutrients mined from the soil by microbes. These networks of beneficial microorganisms can extend a surprising distance from the plant. In the case of giant redwoods, the network of microorganisms that feed the tree can be over seven miles long, in all directions!

Scientists have learned that some soil microorganisms trigger defensive plant behavior, while others produce toxins that kill disease pathogens. Large populations of beneficial microbes also use up resources, making life more difficult for disease pathogens.

Networks of soil fungi allow plants to pick up on alarm signals emitted by neighboring plants, giving them a head start on producing disease-fighting enzymes. These soil fungi provide an information and material network that few could have imagined hundred years ago.

Researchers have found that plants cultivate relationships with beneficial microorganisms, depending on current needs, and that those alliances can change as conditions change. There are soil microorganisms that promote photosynthesis, improve drought tolerance and respiration efficiency, and sensitivity to high salt levels, just to name a few. Biodiversity of soil microbes is one of the best ways to maintain plant health. And those benefits are carried into future generations. Even if a beneficial microbe’s population declines, the pathways that were forged during its active phase are retained, creating the plant equivalent of a vaccine. This information is passed on to future generations, creating natural immunities. Unfortunately, as far as researchers can tell at this time, those benefits only last through the second generation. The third generation has to start the process from scratch.

Therapeutic microorganisms

One soil microorganism in particular, Mycobacterium vaccae, has recently been found capable of uplifting your mood! This particular microorganism is absorbed through tiny cuts and is inhaled on dust particles, as we garden. Once inside, these microorganisms cause a chemical reaction similar to the effects of prozac. Most gardeners claim that gardening is their therapy. Ends up, they were right!

Microorganisms, like other living things, can be poisoned with herbicides, pesticides, and fungicides. The truth is, we don't really understand all of the interactions between these tiny life forms. Throwing a chemical monkey wrench into what looks like a delicate balance is probably not in our best interest. A lab-based soil test every few years can tell you a lot about what is going on in your soil.

The best way to keep a healthy balance of microorganisms in your soil is to keep the soil healthy. Very often, chemicals cause too much change too quickly. Mulching with coarse wood chips and regularly adding aged compost to your soil are the best ways to keep your soil healthy.With those numbers, each handful of soil can contain more microorganisms than there are human beings on Earth. Try wrapping your brain around that, the next time you’re outside pulling weeds!

Most of these microorganisms have not yet been identified, or even named, but scientists at the Earth Microbiome Project, and elsewhere, are working on that. What we do know, in the world of soil microorganisms, is that there are beneficial microbes, and there are microbes that cause us grief.

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!

Soil tends to form layers on top of bedrock. These layers of soil are called horizons. Just above the bedrock is a layer of unbroken rock. There are generally no plant roots here. This is called the regolith. 

There are four basic soil horizons:

• O horizon - very thin, dark top layer made up of decomposed organic material
• A horizon - topsoil where most plant roots are found, which is made up of decomposed organic material and minerals
• B horizon - lighter subsoil, with more minerals and less organic material
• C horizon - regolith layer of unbroken rocks

All of this normally sits on top of a hard layer of solid bedrock. The U.S. Department of Agriculture has an amazing interactive map that can help you learn more about your local soil.

​Artificially and naturally occurring layers of impenetrable hardpan can thwart even the best gardeners.

The best way to understand your soil is with a lab-based soil test.