Crop residues returned to the soil can maintain or enhance soil quality and productivity through favorable effects on soil properties and life-support processes. In the U.S., crop residues produced by 19 principal crops are estimated at 400 million Mg/yr, compared with 2952 million Mg/yr produced in the world. Crop residues returned to the soil contribute to the maintenance of and an increase in soil organic carbon content and add plant nutrients and reduce fertilizer requirements. Total amount of principal nutrients in crop residues range from 40 to 100 Kg/Mg. Consumption of fertilizers in 1991 was 116 million Mg of N, P and K in the world and 16 million Mg in the U.S. In comparison, nutrients (N + P + K) contained in crop residues amount to 9 million Mg/yr in the U.S, and 74 million Mg/yr in the world. Therefore, a considerable quantity of fertilizers can be saved by returning the residues produced to the soil. Residue management is also important in soil and water conservation. About 2.5 Mg/ha of crop residue applied as mulch can reduce soil loss by 60%. Water conservation is an equally important benefit of returning crop residue, especially for rainfed agriculture in arid and semi-arid regions. Water stored in the root zone increases with increasing amount of residue returned to the soil. There are also snow retention benefits of crop residue mulch. Fuel value is estimated at 18.6 x 10(9) J or 2 bbl of oil per Mg of crop residues. Total fuel value is estimated at 7 EJ/yr (1 EJ = Exa Joule = 1 x 10(18) Joules) in the U.S, and 64 EJ/yr in the world. Management of crop residues can also be a useful strategy for sequestering carbon into the soil, and reducing emissions of radiatively-active gases into the atmosphere: About 1.5 Pg (1 Pg = 1 peta gram = 1 x 10(15) g) of carbon Is stored in the crop residues produced annually in the world. Crop residues influence agricultural sustainability by enhancing productivity. Productivity increases with residue returned are greater with low rates of fertilizers than with high or recommended rates. Generally there is a synergistic effect between residue returned and fertilizer use. Innovative cropping systems should be developed to produce the desired amount of crop residue for alternative uses and returning surplus to the soil. In this regard, cover crops and conservation tillage systems can play an important role in designing appropriate cropping/farming systems.
