12.1.2: The Nutrition of Plants

Much of what we’ve discussed of human nutrition may also be applied to the nutrient needs of plants. Like us, they are organized groups of cells. Like us, their cells must also be supplied with the raw materials of life, and the materials which they require are determined by the DNA in their cells. Their chemical makeup is, like ours, ordained by heredity.

Figure 20-1: Microbes are our greatest food hazard.

If we think about it for a moment, we realize that the life chemistry of plants must be somewhat similar to our own. For we get our fuel (e.g., fat) and building blocks (e.g., amino acids) from plants, either by consuming them directly, or by eating animals which have fed on them.

Of course, there are also striking differences. One is that plants don’t consume other life forms as we do to get their nutrients. Instead, they take in nutrients in very simple form, and use chlorophyll to trap solar energy. Another difference is that, unlike us, plants can’t move around to seek food. So they depend on natural phenomena—or on us—to bring them food.

From the beginning of our dependence on agriculture, we’ve tended to provide plants with a kind of room service. This service was unnecessary when we simply gathered plants by foraging over the land, seeking out those which were edible. But once we began, some 9,000 years ago, growing plants in batches for a more dependable and convenient food source, “man-made” growing methods became essential to the survival of the crops and us.

At first glance, it may seem that there’s a striking difference between the ways in which we and the plants take in food. But there’s a basic similarity. Like us, the plants can absorb and take into their cells only the simpler structural units of their food.

Of course, our food first has to be processed through our digestive tract, so that our digestive enzymes can break it down into forms that we can absorb and use. But even here, there’s some similarity. The roots of a plant may be seen as similar to our intestinal walls. If soil nutrients are in too complex forms, the enzymes of soil bacteria do the work of breaking them apart into forms that plants can absorb and use. This is what happens when we make compost. By mixing complex organic matter (gardening and food scraps, manure, etc.) into the soil, the bacteria work to “digest” it into plant food.

The Need for Nitrogen

What sort of nutrients must plants take in? Like us, plants mostly need carbon, hydrogen, oxygen, and nitrogen. Plant cells have no problem getting their carbon, hydrogen, and oxygen from water and the carbon dioxide in the air. But they must get their nitrogen from the soil. (We get ours from the protein we eat.)

The plant can’t absorb protein or amino acids. It takes up its nitrogen in a simpler form from the soil, combined with oxygen as a nitrate (NO₃). In nature, the nitrogen comes mainly from the protein in decaying plant materials, such as leaves and stems. But it must wait for the soil bacteria to turn the plants’ decaying proteins into nitrates, and for rainwater to carry these chemicals to its roots for absorption (see Fig. 20-2). Then, the plant uses the nitrates to make its own amino acids and proteins.

By trial and error, early farmers found that manure and decomposing life materials would keep soils fertile. These materials provide nitrogen. We learned that we must keep the soil supplied with nitrogen, or plants won’t grow—without nitrogen they can’t form the proteins for new cells. Once we understood this chemistry, we realized that simpler, more concentrated sources of nitrogen supplied these needs faster and more efficiently— resulting in a larger crop in a shorter time.

Our modern “chemical” fertilizers supply nitrates rapidly in a simple, immediately usable (inorganic) form. “Organic” farming, with its compost piles of decaying manure and plants, doesn’t directly supply the nitrate. Its nitrogen is tied up in elaborate carbon-skeleton (organic) molecules. The ironic reality of organic farming is that plants can’t use nitrogen in organic form.

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Industrial production of fertilizer from nitrogen gas uses a lot of energy, whether for the strong electrical currents passed through contained air, or the compression of nitrogen, hydrogen, and oxygen gases at extremely high temperatures. Energy sources for this process (and required technology) are out of reach for many developing countries.

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Plants can’t use nitrogen in gaseous form either. Ironically, three-fourths of the air we breathe is the nitrogen that both we and plants need so urgently. But in this gaseous state (N₂), it’s a chemical loner, reluctant to join with any other element and so is useless to us or plants.

But nitrogen gas can be made (“fixed”) into usable form (ammonia, NH₃) by nitrogen-fixing bacteria, which invade the roots of legumes (e.g., soybeans, peas), and provide them with nitrogen usable to make protein. (Recall that legumes are rich plant sources of protein.) The conversion of nitrogen gas to ammonia requires a lot of energy (ATP), made by photosynthesis in the legumes. About one-fifth of the ATP made by photosynthesis in the pea plant, for example, is used by the nitrogen-fixing bacteria residing in its roots.

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Legumes and the nitrogen-fixing bacteria have a symbiotic relationship: The bacteria provide the legumes with usable nitrogen; the legumes provide the bacteria with the ATP energy to do so.

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How did nitrogen get into the soil in usable form before life began on earth? Scientists believe that continual lightning storms struck through the primeval atmosphere. These storms are thought to have supplied the energy for the nitrogen gas to combine with hydrogen gas (forming ammonia, NH₃) and with oxygen gas (forming nitrates, NO₃).

Today, this is the process used by industry to make our fertilizers. In one method, electrical charges are passed through air in closed containers, trapping the nitrogen gas as ammonia and/or nitrates. These ultimately are added to the soil, often in water, just as primeval rains washed nitrogen compounds out of the atmosphere and into the earth’s crust.

Figure 20-2: Nutritional Sustenance of Plants Plants take in water and carbon dioxide and, using solar energy, combine the two chemicals into sugar and starch, releasing oxygen. Birds use the sugar and starch for energy by using oxygen to oxidize them. The birds’ waste products in this reaction are carbon dioxide and water, which plants use to make sugar and starch, continuing the cycle.

The Need for Minerals

Of course, plants need more than nitrogen to grow. They need minerals. These needs are also somewhat parallel to those of humans. The minerals come from the rock and soil of the earth. The plants need as wide a variety as we do—where else would we get ours? Dominant among their needs are phosphorus, potassium, sulfur, and calcium.

Phosphorus is usually incorporated in ammonium phosphates (ammonium, phosphorus, oxygen) or calcium phosphates. Potassium is acquired almost entirely from mined potassium chloride ores. During the last century we began to seek out phosphorus and potassium deposits, grind and modify them into useful form, and return them to our fields to feed the crops.

Calcium sulfate (calcium, sulfur, oxygen) is another “chemical” fertilizer. And, of course, trace minerals such as iron, which we seek for our own nutrition, are part of the hereditary plant chemistry—how else could we depend on getting iron from eating beans and greens?

Does Agriculture Deplete Soil Minerals?

Certainly. That’s why we use fertilizers to supply minerals as well as nitrogen. The plants, responding to their necessities, need minerals just as much as we do. These minerals are called for by the plant’s metabolism. If the minerals are missing, the plants can’t grow.

What if the minerals aren’t entirely missing, but are merely in short supply? Then only as many plants as can be nourished by the available minerals will grow. Or, like undernourished humans, their growth and development will be stunted. As surely as human development is thwarted by inadequate nutrition, so is the development of plants. The farmer who exploits the soil learns the hard way from a crop with very poor yield.

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Plants don’t need an outside source of vitamins, as we do. Vitamins are used in their metabolism, much as they are used in ours, but plants make their own—and ours too. It’s useless to feed plants vitamins.

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So plants which exist at all must be complete. They must contain a full complement of substances called for in their genes. Otherwise, they wouldn’t exist. All of the elements known to be essential in plant and human nutrition are easily identified in soils by modern analytic methods. Our soils and foods are regularly tested, for good reason.

For one, farmers have an acute economic interest in their soil. It’s their most costly and basic asset. A flawed soil, untreated, will produce a poor crop or none at all. The fertilizers which the farmers add are expensive. They can’t afford to guess—so they have their soil analyzed.

In addition, the soils are important economic assets to the society, especially in those states that produce most of our food. Governments of agricultural states such as California and Florida have a vested interest in keeping watch over their soils.

No evidence has come to light that our crops are grown on depleted soil, making them nutritionally inferior. Nor has there been any nutritive differences found between plants fertilized “organically” and plants fertilized “chemically.”

Are There Unobservable Deficiencies?

A few trace minerals can be low. These are the elements which are essential for animal life, but have little use in plant chemistry. So the plant content of these elements varies, because they are taken up incidentally. These elements are only a few—chiefly iodine, selenium, cobalt, and zinc. Fluoride might be considered another. There are easy safeguards against shortages of iodine and fluoride—iodine through the iodizing of salt, and fluoride through fluoridation of water.

The human needs for some of these trace elements were identified largely through inadequacies in the diets of pasture-raised livestock, mainly sheep and cattle. Nutritionally, our main sources of these elements are animal foods. (Recall, for example, that cobalt is a critical constituent of vitamin B₁₂, a vitamin not found in plant foods. Since cobalt isn’t needed by the plant, it’s only there incidentally when it’s in the soil.) Where these elements are lacking in the soil, the deficiency signs are apparent in the animals that graze off the land, and in general are easily supplemented, by adding them to the soil or to animal feed.

Note that the shortages of such trace minerals aren’t due to farming practices, but to regional soil differences. Plants grown in certain areas will, for example, contain little iodine or selenium. If such plants are used for compost, they will obviously not supply any deficiencies of the soil in which they grew.

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Many of the world’s poor only have locally grown produce available. If their soil happens to be low in selenium, the crops grown in them—and the local animals and people who eat them exclusively—also will be low in selenium.

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Note, also, that plants can incidentally take up undesirable elements as well. Plants grown in lead-contaminated soil, for example, can contain lead, a human toxin. Plants grown in soil contaminated by radioactive fallout (e.g., as from the Chernobyl nuclear accident) can contain radioactive iodine, which can damage the thyroid gland. (Iodine—radioactive or not—concentrates in this gland.) Plants can, in fact, be used in this way for environmental clean up, e.g., to remove toxic metals from polluted ground water.¹