Influence Of Alternative Amendments On SOIL Phosphorus In Organic Farming Systems.

For the past decade, organic production has been one of the most rapidly growing sectors of U.S. agriculture.  This growth has led to a boon in alternative soil amendments because many conventional products are prohibited for use in organic production systems.  The mechanisms and efficacy of some of these alternative amendments are not thoroughly understood, particularly with respect to organically farmed soils in the Northern High Plains region.  Furthermore, the costs compared to the benefits of these amendments have not been thoroughly examined.  Field trials were conducted on a 14.5-hectare half-pivot at the University of Wyoming’s Sustainable Agriculture Research and Extension Center (SAREC).  Four amendments allowed for use under the National Organic Program requirements were applied at the manufacturer’s recommended rate and replicated four times on 2.4 x 6 m plots. Amendments utilized in this study included humic acid, compost tea, bone meal, and rock phosphate (alone and in specific combinations).  Liquid amendments (humic acid, compost tea) were applied using a 3.78 l sprayer with 2.4 m spray boom while granulated amendments (bone meal, rock phosphate) were hand-cast.  Soil cores (0-15 cm) were collected in late spring, mid-summer and post-harvest during a three-year rotation of organic alfalfa, corn and dry beans.  Soil pH and electrical conductivity were not affected (P>0.05) by application of any of the fertilizers or amendments during the study.  Extractable P (Olsen Na-bicarbonate extraction) was significantly higher (P<0.05) in plots that received conventional fertilizer and composted manure versus plots treated with any of the alternative amendments.  No significant differences existed in total P across all treatments but there was a general trend of increasing total P for plots amended with all treatments except conventional fertilizer.  The 2012 dry bean yield did not differ among treatments (P > 0.05) but plots receiving conventional fertilizer were the lowest yielding plots on average.  For organic farmers, there does not seem to be a short-term advantage to crop yields or soil extractable P pools with the application of humic acid, compost tea, bone meal or rock phosphate.  However, we suspect that bone meal and rock phosphate may be beneficial in building soil total P over the long-term, which may then impact extractable P pools.