Soil Phosphorus Availability Differences Between Sprinkler and Furrow Irrigation.

Water flowing in irrigation furrows detaches and transports soil particles and subsequently nutrients such as phosphorus.  To reduce the risk of erosion and offsite phosphorus transport, producers in south-central Idaho have been converting from furrow to sprinkler irrigation on the >200,000 acre Twin Falls irrigation tract.  We completed research on soil phosphorus dynamics in furrow versus sprinkler irrigated soils from four paired-fields in the region.  Surface soils (0-2.5 inches) were obtained from fields in September following barley harvest.  Furrow irrigated soils contained 38 parts per million of plant-available phosphorus (i.e. Olsen-extractable), on average, as compared to 20 parts per million under sprinkler irrigation.  These results are important as 20 parts per million extractable phosphorus can be construed as the point where soil phosphorus is considered low to medium in soil testing; extractable phosphorus values over 40 parts per million limit sites to phosphorus application based on crop uptake only.  These soils were also analyzed using a sequential extraction technique, and total and amorphous iron were determined to identify inorganic phosphorus pools.  Soils under furrow irrigation had greater concentrations of inorganic phosphorus in the soluble/aluminum-bound/iron-bound and occluded phases, and in the amorphous iron phase.  Phosphorus concentrations in all other soil phases were similar between the two irrigation practices.  Findings suggest that iron redox chemistry plays a large role in phosphorus release under furrow irrigation, even in aridic systems.  In terms of soil phosphorus, results support the use of sprinkler irrigation as a best management and conservation practice for landscape-scale agriculture.