Nutrient deficiencies (and toxicities) interfere with plant health. And it’s not a simple matter of being present. Soil can be chock full of nutrients. And plants may still be deficient.
The 20 or so minerals used by plants as food exist in soil as ions. Ions are atoms and molecules that have either a positive or negative charge. These cations and anions, respectively, attach themselves to water molecules and are pulled into the plant by root hairs.
Negatively charged clay and organic matter can attract and hold positively charged nutrients, such as calcium and potassium. Clay, in particular, holds onto more water and nutrients than other soil textures. Sometimes, it holds on so tightly that plants cannot access the food they need. Positively charged sand and water hold on to negatively charged particles, such as phosphorus and sulfur.
The ability of a plant to pull nutrients in also depends on soil texture, structure, and pH.
Nutrient deficiencies and soil composition
We describe soil by its texture (size) and structure (arrangement). Soil texture can be large (sand), medium (loam), or incredibly small (clay). Soil texture determines how well it drains. It also impacts which nutrients are easy for plants to absorb.
Soil structure describes how minerals clump together with microbes, earthworms, and organic matter. Spaces, called macropores and micropores, occur within and between these particles. Compacted soil makes it difficult for plant roots to get to their food. Sandy soil often allows water and nutrients to drain away before plants can get to them.
Soil pH ranges from 0 to 14, with lower numbers indicating acidity and higher numbers indicating alkalinity. More nutrients are available, and there is more microbial activity when soil pH is between 6.0 and 7.0. Most plants can survive in soil pH from 5.2 to 7.8, but each species has a narrower range that allows them to thrive. As plants absorb these anions and cations, the soil pH changes slightly. Too much or too little of certain minerals in the soil may interfere with nutrient availability. Mulder’s Chart illustrates this concept nicely.
Without the necessary nutrients, plants cannot thrive. But too much of a good thing can be a bad thing too. If there is a lot of a specific nutrient in your soil, plants may take what they need, or they may eat themselves to death. It depends on the nutrient and the plant species.
In my old yard, the soil was compacted clay with a severe iron deficiency. Since plants use iron to help them absorb several other nutrients, it didn’t matter that the previous owner kept adding a balanced fertilizer. As soon as the plants used any iron present, there was still too much of everything else.
So, how do you know if there’s a deficiency, and what can you do about it?
Lab-based soil tests are the only way to know if your soil is deficient. Over-the-counter tests look great, but they cannot provide accurate information. Luckily, laboratory soil tests are not expensive. They cost about the same as a large bag of fertilizer and can save you more than that by preventing overfeeding and poor plant health.
All too often, people see what they think is a nutrient deficiency, so they apply a 10-10-10 fertilizer. At first, everything looks better. But it never lasts. That is because the balance of nutrients in the soil is what matters.
Symptoms of deficiency
Nutrient deficiencies are often present in specific parts of a plant. Some nutrients are more mobile than others. Plants can move mobile nutrients, such as nitrogen, to wherever they need them. Older leaves start looking chlorotic when plants move nutrients to support new growth. The opposite is true for immobile nutrients, such as calcium. These nutrients require significant amounts of water to move around within a plant, so symptoms first occur in fruit and new growth. Let’s take a closer look.
Magnesium, nitrogen, phosphorus, and potassium deficiencies are visible on older leaves or near the bottom of the plant. We see molybdenum and sulfur deficiencies in the whole plant or around the middle. New growth and the tops of plants exhibit boron, calcium, copper, iron, manganese, and zinc deficiencies.
Deficiency symptoms can overlap, making the diagnosis a bit tricky. This table may help.
What can you do to prevent or correct deficiencies?
First, get your soil tested. You can’t know what’s going on without it. Once you have your test results in hand, only add what’s needed. If toxicities are present, you can reduce those numbers over time by harvesting as much as possible. Plant and harvest heavy feeders, such as cereals, melons, or squash, to help reduce excess nutrients.
In some cases, such as potassium deficiencies, the damage to an individual leaf is irreversible. But you can still improve conditions in general, helping the plant recover. The following good practices will help keep your plants healthy and well-fed:
You can grow a surprising amount of food in your own yard. Ask me how!
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