|CERES No. 122 (FAO Ceres, 1988, 50 p.)|
"Africa's soils are no less problematic than her climate," writes Paul Harrison in The Greening of Africa. "Only 19 per cent of the soils of the continent have no inherent fertility limitations.... The heavy rainfall leaches out the soluble nutrients. High temperatures break down organic matter more rapidly and inhibit the work of bacteria that fix nitrogen from the air. As a result, Africa's soils are among the least fertile in the world. They are often low on nitrogen, which builds leaves, and on phosphorus, essential for root growth. The predominance of coarse particles and the lack of organic matter make for soils that are poor at holding water or nutrients."
So much for what erosion leaves behind. But what exactly does it wash, or blow, away? How much nitrogen, phosphorus, and organic matter is contained in the soil lost to erosion and what are they worth in dollars and cents?
In 1985 the Soil Conservation Programme of FAO's Land and Water Development Division commissioned a pilot study to put a price tag on soil loss from erosion. The study concentrated on Zimbabwe because that country possessed a large and unique body of data on soil erosion. The data had been collected in the late 1950s and early 1960s at the Henderson Research Station 20 kilometres outside Harare, then Salisbury, during a series of experiments on soil loss, runoff, and nutrient losses. Researchers Michael Stocking and Henry Elwell undertook the "long and painstaking task" of classifying, documenting, and analyzing these records. But 30 files from a separate subprogramme, damaged by rain and eaten by insects, escaped notice until 1984, when, finally, they "were seen to contain a storm-by-storm record of nitrogen, phosphorus and organic carbon concentrates in the sediment samples collected from the Henderson research plots".
The project of analyzing data from more than 2 000 storm soil loss events on four soil types and numerous crops, treatments, and slopes began at the Institute of Agricultural Engineering's Soil Conservation Research Section at Hatcliffe, outside Harare, and the University of East Anglia, Norwich, UK.
There are two trends discernible in soil erosion research today, says Stocking. The first is concern with the on-site effects of erosion rather than with the soil loss itself; the second is the growing need to measure the problem of its seriousness accurately and objectively. Techniques for quantification, he says, offer "an almost bewildering variety of types of measurement and techniques" of which expressing the cost of erosion in monetary terms is "the most useful and relevant" as the information can be used directly in decision-making about land use, forms of soil conservation, and investments in agricultural development.
Stocking concludes that "the appropriate quantification of the impact of erosion should be the highest priority on the agenda of erosion research in the next decade". Among the study's findings: - Enrichment ratios (how much more concentrated a nutrient is in eroded soil than it is in the soil that remains) average about 2.5. That is, soil lost to erosion is 2.5 times richer in nutrients than the soil from which it came. Thus even a very small loss of soil can have dramatic effects on yields.
- The more soil is lost, the more nitrogen, phosphorus, and organic carbon are lost - from all types of soils, slopes, and crops. If erosion rates can be estimated, nutrient losses can be predicted. - Typical rates of erosion range from 3 to 75 metric tons per hectare, depending on type of land use and farming. - The yields of severely eroded soils are often reduced by half. - Investments in fertilizers and chemicals are literally washed away by erosion. - Every year Zimbabwe loses 1.6 million metric tons of nitrogen, 0.24 metric tons of phosphorus, and 15.6 metric tons of organic carbon, which is the equivalent of US$1.5 billion of N and P in fertilizer. - Commercial farmers lose much of the fertilizer they apply. The natural fertility of soil where little or no fertilizer is used is declining. - The soil profile will grow shallower and shallower to the point that within 35-50 years, if present rates of erosion continue, no viable production will be possible from soils now under subsistence farming. - Technology (chemicals, improved seeds, agricultural engineering) has concealed much erosion-induced decline in productivity.
"Soil erosion and soil productivity are inextricably linked.... There is abundant evidence of the erosion productivity relationship: in yields; in nutrient level in eroded soils; in the way the fertility-enhancing crops and good cover crops allow less erosion; in trends in world farming productivity," Stocking writes. "Because erosion selectively removes the finer and more fertile particles in a soil, overall soil fertility is reduced. Indeed, eroded soil contains up to twelve times the concentration of nutrients as the original soil." With a view to better assessment of the effects of erosion and the incorporation of that knowledge into conservation planning and agricultural development, he makes the following recommendations: - greater monitoring of nutrient and organic carbon losses - calculation and reporting of enrichment ratios in relation to soil type, erosion rate, runoff, crop, and management - monitoring of plant-available water and other effects of erosion - improvement of the data base for quantifying the impact of erosion - economic modeling of the costs of erosion.
Maureen B. Fant