Are commercial agricultural practices impoverishing our soils? In particular, is the practice of replacing soil minerals with commercial fertilizers impairing the nutritional content of plants?
Rich Soil, Poor Soil
Soil is the Earth's thin covering of unconsolidated, largely mineral material. It is ubiquitous and, to most people, uninteresting. Yet, like air and water, soil is one of the ultimate resources necessary for human survival. It has been called the excited skin of the Earth, for it is in the upper few meters of the planet that we grow our crops, build our structures, dispose of our waste, and bury our dead. Concern over the health of our soil is well-founded: Our supply is large but limited; some soils are of low quality; and all soils unlike air and water are relatively immovable.
Although it is actually the suitability of soil for a particular purpose that determines its quality, most agriculturalists define high-quality soils as those that: (a) do not have in their upper meter layers that limit roots; (b) contain a moderate amount of clay; (c) abound in both humus (organic matter) and "weatherable" (decomposable) minerals; and (d) have a near neutral pH (pH is a measure of acidity and alkalinity). Less than 10 percent of all the soil in California meets this definition.
Mineral weathering the natural process of mineral decomposition that supplies nutrients to plants is too slow to supply modern high-yielding crop varieties with the nutrients they require for normal growth. Nevertheless, with some horrible exceptions, food production has kept pace with the rapid growth of the human population in the last hundred years. Reasons for this include:
* the substitution of machine power in short, petroleum for human and animal power;
* the development of new plant varieties;
* the development of new methods for controlling pests, weeds, and plant diseases;
* the use of artificial fertilizers and soil conditioners (called "amendments") to correct soil nutrient deficiencies and chemical imbalances; and
* the great increase in the number of irrigated acres.
Overall, the twentieth-century increase in food production which now supports a population of nearly six billion has been accomplished without significant conversion of uncultivated land to farmland. Indeed, in the United States land that is relatively unsuitable for agriculture goes unfarmed. Further increases in food production will require either continued increases in yields per acre or conversion of unfarmed land to farmland. The trouble is, in the case of most of the unfarmed lands conversion would entail increasing the risk of environmental problems due to wind or water erosion. For example, the increase in the rate of erosion from cultivating soil on a ten-percent slope would be over six times the increase from cultivating identical soil on a two-percent slope.
Moreover, our ideas about conversion have changed. For example, we used to consider all swamps and other wetlands ripe for draining; now we reserve them for wildlife.
Increasing food production to meet future world needs will require increasing intensity of land use. However, some critics worry that increased use of artificial fertilizers and soil conditioners will result in soil-nutrient bankruptcy.
Any element without which a particular plant cannot complete its life cycle (the series of stages from seed to seed) is termed an essential nutrient for that plant. Seventeen elements are essential nutrients for all plants: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), and chlorine (Cl). Plants obtain all essential nutrients except C, H, and O (which are not considered mineral nutrients) from the soil. Plants contain other elements, but it has not been determined whether these other elements are essential to all plants.
The relative concentration of a nutrient determines whether it is classified as a macronutrient or as a micronutrient. Sodium, cobalt, vanadium, and silicon are essential micronutrients for some plants. Selenium, an essential micronutrient for humans that is toxic at high intakes, is not considered an essential nutrient for plants and not used as a fertilizer. In any case, humans need to ingest only very small amounts of this element. In the United States selenium deficiency appears nonexistent in free-living people.
Soil nutrient losses are a natural consequence of plant growth, which is indispensable to human life. Plants stabilize soils and oxygenate the atmosphere. Humans derive essential nutrients both from plants and from animals that derive these nutrients from plants. If plants did not "rob" the soil of these nutrients, anyone unable to obtain mineral supplements would die.
Consider the simple cycle for potassium (K), a macronutrient whose primary source is soil minerals (see Figure 1). The arrows in the diagram illustrate the paths that K may take as it moves from one form to another. Some crops take up much more potassium than is present in the harvested portion of the crop; the potassium in the unharvested portion is recyclable.
Figure 1: The potassium cycle. Adapted from Singer and Munns, Soils: An Introduction (3rd ed.) Prentice-Hall, 1996.
The potassium in unfertilized soils originally came from certain primary mineral compounds (mica, for example) that form when hot solutions solidify below or on the Earth's surface. Although such compounds constitute the single largest potassium reservoir in soils, they give up the mineral only very slowly. Secondary sources of potassium include clay minerals, potassium ions (K+) on clay surfaces (exchangeable K), potassium incorporated in clay (fixed K), and potassium in solution. Fixed K is unavailable to plants. Exchangeable K is held loosely by clay minerals and can go into the soil solution, where it becomes available to plants and microorganisms. As plants die and decay, they give up some of their cellular potassium to the soil solution, and K again becomes available to plants and microorganisms. Microorganisms also use some of the potassium that remains in the organic matter as it breaks down to dark-colored humus.
If the potassium and other essential elements in soil merely shifted continually in form, fertilization would not be necessary. However, because of erosion and leaching, soil fertility declines even in natural ecosystems.
In tropical rain forests, where weathering is rapid and mineral losses due to leaching are high, most of the nutrients are in plant materials. Thus, removal of vegetation entails removal of much of the nutrient supply. However, in cooler, drier environments, where a lower proportion of the nutrient supply is in plant materials, the harm from removal of vegetation is more gradual.
No matter what the particular ecosystem, we must replace plant nutrients to sustain our current rates of harvest. How we replace them depends on cost, on the accessibility of nutrient sources, and on philosophy. We can replace soil K, for example, with minerals derived from salt deposits and seawater (the source of commercial K fertilizers); from recycled livestock manure; and from wood ashes. The replacement of choice is the one that provides the essential nutrient to the plant at a rate conducive to the health of the plant. Some critics of the use of commercial K fertilizers recommend substituting rock powders such as pulverized granite. But rock powders do not give up potassium rapidly enough to maintain healthy plant growth unless the rate of application is uneconomically high.
Farmers dislike wasting money no less than nonfarmers do. Most farmers add fertilizer to soil in amounts determined by observations of plant responses and tests of soil and plant tissues. Although overfertilization with nitrogen has led to pollution of groundwater subterranean water that supplies wells and springs in some parts of the United States, it is not clear whether organic methods, which either do not utilize artificial fertilizers or use them only sparingly, manage nitrogen better than nonorganic methods.
Soil nutrient losses enable plant growth. Judicious use of artificial fertilizers and soil conditioners counters soil nutrient depletion and, combined with soil conservation, is necessary to meet world food needs.
Dr. Singer is a Professor of Soil Science, and Dr. Pettygrove a Cooperative Extension Specialist, in the Department of Land, Air and Water Resources, University of California, Davis.
(From Priorities, Vol. 9, No. 4)