Agronomy

The quality of agricultural soils can be easily degraded by various cultural influences, but especially: (1) nutrient removal; (2) loss of organic matter; (3) acidification; and (4) erosion that is caused when soils are plowed and crops are harvested. Soil degradation is one of the most important problems associated with agricultural activities, because of the obvious implications for the longer-term sustainability of productivity and harvests. Understanding the causes of soil degradation and devising ways of preventing or mitigating this problem are among the most important objectives of agronomy.

Nutrient losses from agricultural soils are caused by a number of influences:

1. Whenever crop biomass is removed from the land during harvesting, nutrients such as nitrogen, phosphorus, potassium, calcium, and others are also extracted. These removals occur in the form of nutrients contained in the biomass. Depending on the crop, nutrient removals during annual harvesting are not necessarily large, but over a longer period of time the cumulative losses become significant, and the soil becomes impoverished.
2. Severe disturbance of the integrity of the soil surface, for example, by plowing, makes the soil quite susceptible to erosion by water and wind. Associated with the physical losses of soil are losses of the nutrients that it contains.
3. Nutrient losses are also encouraged when there are decreases in the concentrations of organic matter in the soil, a phenomenon that is also associated with tillage of the land. Organic matter is important because it helps to bind nutrients in relatively immobile and insoluble forms, thereby helping to ensure their continuous supply, and preventing the leaching of nutrients from the site.

When nutrient losses from soil have been severe, it may be possible to compensate for the losses of fertility by adding nutrients. Fertilization is an important activity in modern agriculture. However, fertilization is expensive, and it causes important environmental impacts. Therefore, many agronomists are engaged in research designed to reduce the dependence of modern agriculture on intensive fertilization, and on increasing the efficiency of nutrient uptake by crops.

Losses of soil organic matter are another important problem that agronomists must address. Soil organic matter is important because of its great influence on the tilth, An irrigated area along a river near Yazd, Iran. Note the unirrigated desert in the background. JLM Visuals. Reproduced with permission. or physical structure of soil, which is closely associated with the concentration of humified organic matter. Tilth is very influential on the water- and nutrient-holding capacities of the soil, and is highly beneficial to the growth of crops. The emerging field of organic agriculture is largely involved with managing and optimizing the concentration of organic matter in soils. Commonly used techniques in organic agriculture include the use of green manures and composts to maintain the concentration of organic matter in soil, as well as the use of carefully designed crop rotations and mixed-cropping systems.

Soil acidification is another important agricultural problem. Acidification is caused by the removal of calcium and magnesium during cropping, through erosion, leaching, and actions of certain nitrogen-containing fertilizers, such as ammonium nitrate and urea.

Acidification may also be partially caused by atmospheric pollution, especially in regions where the air is contaminated by sulfur dioxide, and where acidic precipitation is important. Acidification is routinely countered in agriculture by mixing limestone or lime into the soil.

Erosion is another common agricultural problem that is largely caused by disturbance of the soil surface through plowing. Erosion represents a mass wasting of the soil resource, with great implications for fertility and other important aspects of land capability. Erosion also causes secondary impacts in the aquatic ecosystems that typically receive the large wastage of eroded materials. Erosion is particularly severe on lands with significant slopes and coarse soils, especially if these occur in a region with abundant precipitation. Erosion can be substantially prevented by plowing along contour lines rather than down-slope, and by maximizing the amount of time during which the land has a well-established plant cover. The latter can be accomplished through the use of no-tillage agricultural systems, and wherever possible, by cultivating perennial crops on sites that are vulnerable to erosion.