The fertility of soil refers to its ability to function in the growing of crop plants. Soil fertility is frequently defined narrowly as the ability of the soil to provide nutrients that are essential for plant growth and the soil having a pH in the range needed for the specific crops grown (most plants do well in the pH range of 6 to 7). If soils are very acidic, below pH 5.5, many crop plants are harmed by the lack of nutrients as well as the presence of soluble aluminum, which can be toxic to plants. On the other hand, some crops such as cassava do well in the acidic and low-nutrient soils of the tropics. Most of the eighteen essential elements for plants come from the soil. Of the nutrients supplied by the soil, nitrogen, phosphorus, and potassium are the ones needed in the largest amounts and are frequently applied in commercial fertilizers. Other nutrients, such as calcium, sulfur, and magnesium, are needed in lesser amounts while the micronutrients, such as iron, manganese, copper, and zinc, are needed in very small amounts and are not as commonly deficient.
A broader view of soil fertility has recently developed that is sometimes referred to as soil health or soil quality. In this more comprehensive view, equal consideration is given to the physical and biological properties of soil that are important to plants, along with the chemical properties. For example, a soil may have excellent nutrient availability but be too wet or compact for crops to grow properly or might have high populations of plant disease organisms or parasitic nematodes. The chemical fertility of the soil is usually determined by laboratory analysis of a composite sample taken from a field. The tests normally included in routine soil analysis are for pH, available nutrients such as phosphorus, potassium, calcium, and magnesium, and lime requirements. Other tests commonly done are salinity and percent saturation with sodium (both in arid regions) and amount of organic matter. In determining the broader health of the soil, the degree of compaction, and root health are usually evaluated in the field, and some laboratories determine water-holding capacity and indicators of biological activity.
Topsoil, usually the top six inches or more, is the most fertile part of the soil. It contains higher amounts of organic matter and nutrients, has more biological activity, retains water more easily, and is usually better aerated than subsoil. Plentiful organic matter is the key to healthy, fertile soils because it has profound and positive effects on almost all soil properties—chemical, biological, and physical. It helps soils store water, resist compaction, make nutrients more available to plants, maintain a thriving and diverse populations of organisms, and so on.
When forests or grasslands are converted to agricultural use, these virgin soils are usually productive for a number of years, even without addition of fertilizers or amendments. The soils usually have good physical, chemical, and biological properties. This is the basis for tropical slash-and-burn agriculture, a twenty- to forty-year cycle in which the trees in a forest plot are first felled and burned. For a few short years, the nutrients stored in the soils and the positive effect of the ash on soils allow reasonable crop productivity. Much of the fertility of these soils is associated with organic matter or elements in the ash that are leached readily into the soil. Thus, in the traditional slash-and-burn system where no fertilizers or organic amendments are applied, after two or three years these soils are allowed to revert to forest.
DEGRADATION OF SOIL FERTILITY
Soil degradation is a worldwide phenomenon that has occurred in many places since the development of agriculture. Soils can be degraded in a number of ways, such as nutrient depletion, organic matter depletion, development of saline or sodic (excess sodium) soils, removal of topsoil by accelerated erosion, and compaction.
In ancient times, crop production decreased in parts of Mesopotamia (in the Tigris and Euphrates valleys), as salts accumulated from irrigation water were not washed out regularly. The hillside agriculture of Greece, Turkey, and the Middle East was first practiced without terraces. Removal of the original forests and repeated plowing and working of these soils left their exposed surfaces smooth or with aggregates that were easily broken under the impact of rainfall. Rainfall, not able to penetrate the soil at the rate it fell, caused runoff and soil erosion. As Homer put it in the Iliad, "Many a hillside do the torrents furrow deeply, and down to the dark sea they rush headlong from the mountains with a mighty roar, and the tilled fields of men are wasted" (cited in Hillel 1992, p. 103). The large area of denuded hillsides in the region, with large areas of exposed bedrock or boulders, testify to the magnitude of this problem, which many feel severely stressed these societies to the point of either decline or aggressive adventures to find extra sources of food in other countries.
On highly leached and geologically old landforms such as occur in much of Africa, the loss of soil’s ability to provide adequate nutrition to crop plants can happen very rapidly—within two or three years following cutting down the trees and burning them. (In places where the cycle of crops to forest and back to crops is greatly reduced, soils are much decreased in their fertility.) On the other hand, on the deep fertile grassland soils that developed in humid temperate regions, it can take decades of cropping before added nutrients are needed to maintain plant growth. These soils, naturally high in organic matter and able to supply crops with large amounts of nitrogen for years, usually also have large supplies of other plant nutrients. In his classic book Slavery and Capitalism (1961), Eric Williams makes the case that in the southeastern United States and the Caribbean it was the institution of slavery that allowed the large-scale exploitation of the land for cash crops (cotton and sugarcane). If free laborers were brought for the purpose of working large estates, as was the case in Australia, they would have abandoned the estates and started their own small farms. These smaller farms, Williams argued, would have not have been so hard on the soil because they would have grown more varied crops and used crop rotation. While this argument makes sense, the soils of the Caribbean and southeastern United States are easily depleted of their nutrients and, when on sloping land, prone to erosion. Thus, small farms applying the practices of the time might also have led to soil degradation, but perhaps not as rapidly as under the institution of slavery.
Between the two extremes of highly leached tropical and subtropical soils and deep grassland soils of the temperate regions are many of the soils of Europe and the eastern United States. These are the regions where modern agriculture first developed and the more intensive nature of cropping required large quantities of external nutrients to maintain plant growth year after year. Modern agriculture developed in a symbiotic relationship with industrial capitalism, and through the break in nutrient cycling capitalism has added new dimensions to the issue of soil degradation. Part of the reason for the need for large quantities of external nutrients is that, with the development of cities and capitalist farmers who sold most or all of what they produced, large amounts of nutrients were being removed from farmland and transported to cities in the form of food products. As Karl Marx wrote in volume 1 of Capital:
Capitalist production … disturbs the metabolic interaction between man and the earth, i.e. it prevents the return to the soil of its constituent elements consumed by man in the form of food and clothing; hence it hinders the operation of the eternal natural condition for the fertility of the soil____ All progress in capitalist agriculture is a progress in the art, not only of robbing the worker, but of robbing the soil; all progress in increasing the fertility of the soil for a given time is a progress towards ruining the more long-lasting sources of that fertility.. Capitalist production, therefore, only develops the techniques and degree of combination of the social process of production by simultaneously undermining the original sources of all wealth—the soil and the worker (1867, pp. 637-638).
USE OF AMENDMENTS TO ENHANCE SOIL FERTILITY
Although various animal manures and green manures were used for thousands of years to enhance the fertility of soils (and still are), in the nineteenth century the application of modern science to agriculture resulted in the knowledge that depletion of a few nutrients—nitrogen, phosphorus, and potassium—were likely to be limiting plant growth on many soils that had been farmed for years. The depleted, "worn-out" soils of Europe and the eastern United States created a strong demand for external nutrient sources. Bones, a source of phosphorus, were imported into Europe, and farmers even raided the graves of the Napoleonic battlefields to obtain them. In the 1840s phosphate became the first industrially produced fertilizer. But industrial production of nitrogen fertilizer would not happen until the after World War I (1914-1918), when the Haber-Bosch process for fixing nitrogen would begin to make this essential element more available. (And it was not until after World War II [1939-1945], when the munitions plants turned to making ammonium nitrate fertilizers, that the low price of nitrogen fertilizer greatly stimulated its use.) In the mid-1800s, in the absence of a ready supply of synthetically produced nitrogen, the use of guano from Peru, high in both nitrogen and phosphorus, came under British control. This caused the United States, in competition with European countries, to search abroad for sources of nutrients to help maintain soil fertility, referred to by the phrase "guano imperialism." During this period, from 1856 (after passage of the Guano Island Act) to the early 1900s, the United States seized close to 100 island sources of guano.
A second major change in the cycling of nutrients took place after World War II. In this period, low-cost nitrogen fertilizers allowed farmers to forgo planting legume forage crops that had been part of an integrated crop farming system that included complex rotations and raising animals as well as crops. Farmers, no longer needing legumes or animal manures to supply their grain crops with nitrogen, began to specialize in one or two crops such as wheat and corn (and later soybeans). Animals, especially poultry, beef cows, and hogs, started to be raised on large farms that frequently imported feeds, made from mainly corn and soybeans, from other regions. The growth of these large-scale industrial animal facilities was brought about by the location of meat-processing facilities by highly integrated corporations. Many farmers are now raising chickens and hogs under contract, with little control over the actual production process. This separation of a large number of animals from the land that grows their feed has resulted in the necessity for crop farmers to use large quantities of fertilizers (to make up for the nutrients exported in their crops), while at the same time nutrients accumulate on the animal farms and result in significant pollution of groundwater and surface waters.
ECOLOGICAL MANAGEMENT OF SOIL FERTILITY
Although commercial fertilizers still have their place in agriculture, the high cost of nitrogen fertilizer in terms of energy use (and, therefore, price) and the ecological harm caused by phosphate mining mean that in the future more emphasis needs to be placed on more efficient nutrient cycling, the reintegration of animals back into cropping systems, and the return to legumes and manures as major sources of nitrogen and other nutrients for nonleguminous crops. Most soils can be greatly improved by implementation of ecologically sound practices for building healthy soils. One of the best approaches to improving soil health is to use various means to build up soil organic matter levels—such as adding manures and composts or other local sources of organic materials, using cover crops routinely, rotating row crops with hay-type forages, and reducing tillage intensity.
