Effect of cropping and low-chemical input systems on soil phosphorus fractions

The adoption of alternative management practices has been shown to increase soil organic matter. However, the effect of adopting these practices on so il phosphorus (P), especially organic P, is not clear. We evaluated the eff ect of such practices-mainly no-tillage, zero and low-chemical input, organ ic-based, row crop agricultural systems-on soil P and compared them with co nventional agriculture and perennial farming systems. We also compared soil P under conventional agriculture to an adjacent forest site and a never-ti lled native successional community site in southwest Michigan. Sequential f ractionation analysis of soil inorganic and organic P fractions showed that long-term conventional row crop agriculture resulted in a 79% reduction of NaOH-extractable organic P compared with adjacent forested sites. The calc ium phosphate pool and the residual P fraction, however, increased under co nventional agriculture compared with the forest site, probably because of f ertilizer inputs. Adoption of no-tillage and low-chemical input systems wit h a winter leguminous cover crop in the rotation for 7 years did not increa se organic P significantly in any of the fractions extracted from the annua l cropping systems. However, organic P extracted by NaOH increased to 22.1% after adoption of continuous alfalfa for the same period compared with 11. 4% extracted under conventionally tilled annual cropping systems. We conclu de that continuous alfalfa. can help restore soils to their native P fertil ity levels by taking up P from the stable residual fraction and transformin g it into moderately labile organic P through root death. We found no evide nce that low chemical input organic based systems are sustainable with rega rd to P as there is no evidence that organic P is accumulating as a result of the use of cover crops. Further investigation is warranted after these s oils become P limiting and more years have passed under the same treatments .