Soil Phosphorus Dynamics under Sprinkler and Furrow Irrigation

Furrow irrigation detaches and transports soil particles and subsequently nutrients such as phosphorus (P). To reduce the risk of erosion and offsite P movement, producers can convert from furrow to sprinkler irrigation. We completed research on soil P dynamics in furrow versus sprinkler irrigated soils from four paired fields in south central Idaho. Surface soils (0-2.5 inches) were obtained from fields in the fall following harvest. Furrow irrigated soils contained 38 parts per million (ppm) of plant-available P (i.e. Olsen-extractable) on average, as compared to 20 ppm under sprinkler irrigation. These results are important as 20 ppm Olsen-extractable P may be considered the concentration where soil P is considered low to medium in soil testing; extractable P values over 40 ppm limit sites to P application based on crop uptake only, based on the Idaho One Plan. Soils were additionally analyzed using a sequential extraction to identify inorganic soil P pools, and an amorphous Al and Fe technique was used to help further explain differences in extractable soil P under furrow and sprinkler irrigation. Soils under furrow irrigation had greater concentrations of inorganic P in the soluble/Al-bound/Fe-bound and occluded Fe phases (i.e. Fe coated P), and in the amorphous Fe phases. These findings suggest that Fe reduction chemistry plays a large role in P availability under furrow irrigation, even in calcareous soil systems. INTRODUCTION Surface water enrichment of nutrients (e.g. P), due to anthropogenic activities such as production agriculture, is a serious problem in the United States (USEPA, 1996). The primary pathway for P loss from agricultural soils is through surface runoff (Vadas et al., 2004). Once in irrigation return flow waters, P may be transported distances greater than 11 miles (Ippolito and Nelson, 2013). Thus, managing irrigation practices may help influence runoff and reduce P losses from agricultural production systems. Production agriculture utilizes two major irrigation practices: furrow and sprinkler. Sprinkler irrigation has been shown to improve irrigation use efficiency as compared to furrow irrigation (Al-Jamal et al., 2001), making sprinkler irrigation an attractive water conservation practice. Conservation efforts also suggest that increasing sprinkler and reducing furrow irrigation will reduce sediment and thus P loss. For example, the 200,000 ac Twin Falls irrigation tract in south central Idaho realized a decrease in suspended sediment losses from 400 lbs/acre in the early 1970s to 9 lbs/acre in the mid 2000s by implementing several management techniques which included replacing furrow with sprinkler irrigation (Bjorneberg et al., 2008). Changes in soil P dynamics may also occur due to the difference is soil water status between the two irrigation methods. Thus, we investigated how irrigation practices alter soil P chemistry. MATERIALS AND METHODS Fields from four southern Idaho producers were identified; each producer grew barley on relatively adjacent fields using either furrow or sprinkler irrigation. All producers used somewhat