THE ROLE OF SOIL ORGANIC-MATTER IN SUSTAINING SOIL FERTILITY

MANY tropical soils are poor in inorganic nutrients and rely on the re cycling of nutrients from soil organic matter to maintain fertility. I n undisturbed rainforests such nutrients are recycled via the litter(1 ); 'slash-and-burn'agriculture, meanwhile, depends on the mineralizati on of organic nutrients from the plant remains(2,3) or on (short-lived ) inputs from ash(4). This dependence on organic nutrients in tropical soils has the result that tests of soil quality which only give isola ted measures of inorganic nutrient status are unreliable(5), and that the effects of fertilization can be inconsistent because of leaching o r fixation of inorganic nutrients. Here we attempt to quantify the rol e of organic matter in sustaining the fertility of soils from three di fferent climate zones. We estimate rates of carbon turnover from ecolo gical measurements and C-14 dating. and determine its relation to the soil carbon and nutrient budgets. We find that agriculture without sup plementary fertilization was economical for 65 years on temperate prai rie and for sis years in a tropical semi-arid thorn forest. An extreme ly nutrient-poor Amazonian soil showed no potential for agriculture be yond the three-year lifespan of the forest litter mat, once biological nutrient cycles were interrupted by slash-burning. These observations suggest that quantification of organic-matter cycling may provide an important guide to the agricultural potential of soils.