INFLUENCE OF MANAGEMENT-PRACTICES ON SOIL ORGANIC-MATTER, MICROBIAL BIOMASS AND COTTON YIELD IN ALABAMAS OLD-ROTATION

The ''Old Rotation'' cotton experiment was designed to aid farm manage rs in implementing rotation schemes that not only increase yield, but also improve soil quality. Six different crop rotation treatments were imposed since 1896. Rotations were: IA, cotton (Gossypium hirsutum L. ) grown every year without a winter legume and without N fertilization ; IB, cotton grown every year with a winter legume and without N ferti lization; IC, cotton grown every year without a winter legume and with 134 kg N as NH4NO3 ha(-1) year(-1); IIA, 2-year cotton-corn (Zea mays L.) rotation with a winter legume and without N fertilization; IIB, 2 -year cotton-corn rotation with a winter legume and with 134 kg N ha(- 1) year(-1) as NH4NO3; and III, 3-year cotton-corn- (alternating soybe an [Glycine max (L.) Merr.] or rye (Secale cereale L.) rotation with a winter legume and with 134 g N as NH4NO3 ha(-1) year(-1). Crimson clo ver (Trifolium incarnatum L.) was the winter legume cover crop. The 2- year cotton-corn rotation with a winter legume and with 134 kg N ha(-1 ) year(-1) (IIB) and the 3-year cotton-corn soybean/rye rotation with a winter legume and with 134 kg N ha(-1) year(-1) (III) had higher amo unts of soil organic matter, soil microbial biomass C and crop yield t han the other four treatments. The cotton grown every year without a w inter legume or N fertilizer (IA) had a lower amount of soil organic m atter, soil microbial biomass C and N and cotton seed yield than all o ther rotations. In 1988 and 1992 cotton seed and legume yield were cor related in positive, curvilinear relationships with soil organic matte r (r(2) ranged from 0.72 to 0.87). In most months, soil microbial biom ass C and N was lower in the cotton grown every year without winter le gumes or fertilizer (IA) than the other five rotations. In 1994, micro bial biomass C and the C-mic:C-org ratio correlated in positive, curvi linear relationships with seed cotton yield (r(2)=0.87 and 0.98, respe ctively). After 99 years of management the ''Old Rotation'' cotton exp eriment indicates that winter legumes increase amounts of both C and N in soil, which ultimately contribute to higher cotton yields. Microbi al biomass C and the CmicCorg ratio are poor predictors of annual crop yield but may be an accurate indicator of soil health and a good pred ictor of long-term crop yield.