To understand how best to raise healthy, nutritious crops, one should know a bit about how plants grow. Although plants are much different from animals or ourselves, they still are living organisms, and they have some very similar needs for growth. Just as our cells do, the cells of plants need oxygen, water, amino acids, enzymes, vitamins, minerals, and so on. We get many of them in the food we eat.
The major difference between plants and animals is that nearly all plants produce their own food, through the process of photosynthesis (simple plants such as fungi and most bacteria, along with a few parasitic higher plants, do not make their own food). Photosynthesis is a complex step-by-step process by which plants take water and carbon dioxide (from the air), and using the energy in light, combine them to make carbohydrates called glucose and fructose (simple sugars). A by-product of photosynthesis is oxygen, which animals and humans need. Later, the glucose and fructose are usually changed into other types of substances, sometimes with the addition of other raw materials (nitrogen, sulfur, calcium, phosphorus, magnesium, etc.). These new products may include complex carbohydrates (sucrose, starch, cellulose), fats and oils, amino acids and proteins, nucleic acids (DNA, RNA), plant hormones, flavors, and substances that can ward off pests or diseases.
The water that plants use comes from precipitation that seeps into the soil and is absorbed by the plant’s roots. Roots mainly absorb water through a mass of tiny root hairs (narrow extensions of some of the root’s epidermal cells). Another source of water is from the beneficial fungi called mycorrhizae which inhabit the roots of most plants. Mycorrhizae extend farther out into the soil than root hairs, so they provide more water than the roots alone, and they can save the plant from wilting or death under drought conditions.
Normally, water can enter roots from the soil freely, but if there is little actual water between soil particles (drought conditions) or if the salt content of the soil is high, roots cannot absorb enough water and the plant will wilt (wilting can also occur on hot, sunny or windy days). Too much salt in the soil can occur naturally in arid regions, but similar conditions can occur if the farmer uses too much synthetic salt fertilizers [see the link on Fertilizers]. Not all soil salt is the same as table salt (sodium chloride), but several kinds of chemical salts may be involved, such as potassium chloride, ammonium nitrate, potassium nitrate and urea.
The water absorbed by the roots enters part of the plant’s “plumbing system,” called the xylem, tiny tubular cells found in the “veins” of roots, stems and leaves. Normally, the xylem tubes will easily carry the water, along with nutrients that are in it, to all parts of the plant; however, sometimes the plant’s “plumbing” gets clogged, just as your house’s plumbing can, and the plant’s functions and growth are slowed or stopped. Plugged “plumbing” can occur when a plant is under some type of stress, including low-oxygen or waterlogged soil conditions, pest or disease attack, high soil salts, or unbalanced nutrients.
The other part of the plant’s “plumbing” is the phloem, cells found near the xylem that mainly transport the food made in the leaves to other parts of the plant (perhaps into a growing fruit, or into the large root of a carrot). The phloem cells can also become plugged or die when the plant is under stress.
Besides the water and carbon dioxide that plants use to make food, they also need relatively smaller amounts of several nutrients, mostly absorbed from the soil along with water. These nutrients are needed in various ways in order for the plant to make a wide variety of substances. They can be subdivided into two groups, the major nutrients (or macronutrients), needed in larger amounts, and the minor nutrients (micronutrients, or trace elements), needed in very small amounts. Both types are equally necessary for the plant to grow normally and produce nutritious food.
The major nutrients are:
- Nitrogen. Although the air is 78% nitrogen gas, plants cannot use it in that form. Instead, plants mainly need two forms of “combined” nitrogen, nitrate (nitrogen plus oxygen) and ammonium (nitrogen plus hydrogen).
- Nitrogen is used by plants to make amino acids and proteins, enzymes, chlorophyll and nucleic acids, among others.
- When organic matter decomposes, the nitrogen it contains is at first in the ammonium form, but soil bacteria quickly convert it into the nitrate form. The ammonium form of nitrogen is held on soil particles, while nitrate is dissolved in soil water and thus easily leached away.
- Since plants need nitrogen especially during the rapid growth of their stems and leaves, it is important to have enough in the soil all along. This will happen if the soil contains a supply of compost (humus) or decomposing organic matter (but not too much raw organic matter at a time). Other sources of nitrogen are from lightning and rainfall (only a little), from certain bacteria (nitrogen-fixing bacteria) that are able to take it from the nitrogen in the air (some of these bacteria live on the roots of legumes, while others live freely in the soil), and from human-applied fertilizers [see the link on Fertilizers]. Other soil bacteria can cause loss of soil nitrogen by converting it into nitrogen gases (called denitrification).
- Most soils contain plenty of phosphorus, but most of it is “locked up” in soil mineral particles, especially in very acid conditions; however, an abundance of humus and soil organisms can make a lot of it available to plants (the special root-fungi called mycorrhizae are especially valuable). Unfortunately, soluble forms of phosphorus in commercial fertilizers are quickly converted into insoluble, “locked-up” forms in the soil, so having an adequate supply of humus and soil organisms is vital in promoting a sustainable system of agriculture.
The micronutrients, or trace elements, include iron, manganese, zinc, copper, boron, molybdenum and chlorine. They are just as important as the major nutrients (above), but are needed in very small amounts by plants (humans and animals need them, too). In plants, the trace elements mainly function as parts of enzymes and other substances that are used in most cellular functions.
Soil micronutrients come primarily from the weathering of minerals. Since they are needed in tiny amounts, soils may have enough available trace elements; however, sometimes one or two may be deficient.
For plants to grow at an optimal rate, to resist pests and diseases, and to produce nutritious food for humans and animals, it is important that they absorb a proper balance of soil nutrients. A deficiency (or excess) of one or more nutrients can greatly upset a plant’s metabolic activities, leading to poor growth, pest or disease attack, and unhealthful food.
Different crops require a somewhat different balance of nutrients; for example, corn needs plenty of nitrogen and zinc, while alfalfa does best with plenty of calcium and boron. The familiar tomato disease, blossom end rot, can be due to deficient calcium or excess magnesium or potassium.
Those three nutrients – calcium, magnesium and potassium – are related in a balance, since an excess of one will result in a relative deficiency of the others (the total of all three in any particular soil is fairly constant). You can have your soil tested for its cation exchange capacity (CEC), or base exchange capacity, a measure of how much of the above nutrients it can hold (which depends mostly on its clay and humus content). The soil can also be tested for the percentage or proportion of those three nutrients that it actually contains. A good target balance is 75 – 85% calcium, 12 – 19% magnesium and 1 – 5% potassium. Too high potassium or magnesium means too little calcium, possibly resulting in a poor, sick crop.
The two other nutrients that are often deficient are nitrogen and phosphorus. Nitrogen deficiency is often seen when the lower leaves of the plant turn yellow. Phosphorus deficiency may result in purplish, reddish or brownish leaves. A nitrogen excess may result from over-fertilization, with leaves appearing dark green and with excessive stem growth.
Any serious nutrient imbalance can be corrected by adding supplementary organic matter and fertilizers, as well as adjusting soil pH, since the availability of most nutrients to plants decreases at very acid or alkaline pHs [see the links on Fertilizers and Improving soil].