Soil Depletion

Soil depletion is the loss of soil fertility through a decline in organic matter content, major mineral nutrients, and trace elements. Lowered levels of nutrients lead to poor crops and deficiencies in the diet of grazing animals and, thus, eventually in the human food chain. It is a major handicap in developing sustainable agriculture. The loss of nutrients is the result of both natural and anthropogenic processes. Soils more than 200,000 yrs. (years) old on rocks poor in base elements are particularly prone to nutrient and trace element depletion. In the southeastern United States, in forest soils up to 2 million yrs. old on base-poor igneous and metamorphic rocks, calcium (Ca) in [Page 2583]the rooting zone has been leached out of the soil profile. This means that the potential for replenishment of soil-exchangeable Ca through mineral weathering is low. The natural nutrient cycle maintains sufficient soil fertility for tree growth, but once that cycle is disrupted, through timber harvesting, base elements are rapidly depleted. Exploitative agriculture from 1800 to 1920 in these forests depleted much of the original pre-European Ca in the soil. Mid-20th-century acid deposition (sulfate [SO4] loading) accelerated soil Ca losses. Such effects are widespread in Europe and the United States. Soil depletion also occurs on younger postglacial soils, for example, in Maine, but less rapidly than in the southeastern United States, because the levels of acid deposition and rates of Ca accumulation in trees are lower. The conifer trees of Maine grow more slowly and require less Ca than the southeastern U.S. hardwoods.

Soil depletion is of concern to most low-latitude countries. All continental African countries, except Libya, experience annual nutrient deficits. Soils in densely populated parts of the semiarid, arid, and Sudano-Sahelian areas lose 60 to 100 kg ha-1 yr.-1 (kilograms per hectare per year) of nitrogen, phosphorus, and potassium (NPK). These shallow, highly weathered soils are subjected to intensive cultivation but low levels of fertilizer application. In subhumid and humid African regions, rates of nutrient depletion range from 30–60 kg ha-1 yr.-1 NPK in the humid forests and wetlands in Southern Central Africa to more than 60 kg ha-1 yr.-1 in the East African highlands.

Globally, in 2000, soil nutrient deficits averaged (in kg ha-1 yr.-1) 18.7 for N, 5.1 for P, and 38.8 for K, covering 59%, 85%, and 90% of harvested area in the year 2000, respectively, and annual total nutrient deficit was 5.5 Tg (teragrams, 1 Tg = 1,012 g) for N, 2.3 Tg for P, and 12.2 Tg for K, coupled with a total potential global production loss of 1,136 Tg/yr. Soil depletion is producing fertility problems throughout the world.