Submitted to: Soil & Tillage Research
Publication Type: Book / Chapter
Publication Acceptance Date: 3/19/1998
Publication Date: N/A
Interpretive Summary: Semi-arid regions are very susceptible to a loss in soil quality when farmed because of low crop yield, removal of crop residues for use as feed or fuel, and frequent fallowing to improve water storage and control weeds. What crops are grown, the frequency of tillage, the amount of fertilizer applied, the amount of residue returned to the soil, and the degree of erosion influence the level of organic matter in soil. It is important to quantify each effect to determine what agricultural practices are necessary to prevent a deterioration in soil quality. If treated correctly, soil can also act as a sink through better plant growth to absorb and tie-up some of the carbon dioxide in the atmosphere and thereby reduce adverse effects of global warming. This study was part of a national effort to obtain information on sustainable agricultural practices from long-term experiments, because they are possibly the only method for determining any changes. We analyzed carbon and nitrogen (which are a measure of organic matter) in semi-arid soils where experiments with differing crop management treatments have been conducted for 30-65-years. Growing a crop every year, minimizing tillage, and adding fertilizer to increase crop growth were very favorable for maintaining soil organic matter. Fallowing intensified organic matter loss in every experiment. It appears possible through analysis of long-term experiments to develop sets of practices that will maintain soil quality and prevent the loss of crop productivity in semi-arid regions. Our analysis was only possible because long-term research experiments had been conscientiously maintained for many years by dedicated individuals.
Technical Abstract: Soil quality is difficult to maintain at equilibrium in cultivated soils of semi-arid regions because of low crop residue production, frequent removal of residues, frequent fallowing, and often excessive wind and water erosion. Identification and quantification of practices that help to maintain soil organic matter (SOM) is necessary to delineate management systems for sustaining crop production without degrading the environment. Long-term experiments are considered the primary method to assess changes in SOM due to soil and crop management. We analyzed carbon and nitrogen change in semi-arid soils of the Pacific northwest where crop management experiments have been conducted for 30 to 65 years. Most systems lost soil organic matter when rotations included summer-fallow. Reducing tillage or adding nitrogen fertilizer to increase crop growth decreased soil C and N loss. It is projected that SOM can be maintained or increased only if crop residues are returned to soil, erosion is kept below 2-4 t ha-1, moldboard plowing is avoided, and fallowing is eliminated or kept to less than one year in for or five. Development of crop management criteria to maintain soil quality and achieve maximal C sequestering capability appear possible, but only because there are long-term experiments with periodic measurements of SOM components.