Phosphorus Movement in the Landscape

Phosphorus movement in runoff often promotes algal growth in lakes. Thus, agricultural soils and management practices that enhance the potential for P movement must be identified. The main factors controlling P movement are transport (runoff and erosion potential) and source factors (surface soil P and method, rate, and timing of fertilizer and animal manure applications). Implementation of management that minimizes runoff and erosion will reduce P transport in runoff, although total algal availability can increase. The continued application of P has increased surface soil test P contents in excess of levels sufficient for optimum crop yields. Although increases in soil P have been related to P enrichment of runoff in plot and watershed studies, information for given management systems still is needed to reliably quantify critical soil P levels above which excessive P enrichment of runoff will occur. Clearly, P applications must be carefully managed, in addition to minimizing transport potential, to efficiently reduce P movement in landscapes. This may be achieved with regular soil testing, P incorporation, application during times of low runoff probability, and irrigation management. Research QuestionThe movement of P in runoff from agricultural land can accelerate the eutrophication of surface water. Due to the easier identification and control of point sources and a lack of direct human health risks associated with eutrophication, nonpoint sources of P now account for a larger share of the nation's water quality problems than a decade ago. Thus, agricultural soils and management practices that are vulnerable to P loss must be identified before economically viable management systems that minimize P movement can be developed. A P indexing system is proposed to identify agricultural soils and management practices that have the potential to excessively enrich the P content of runoff waters. The main factors controlling P movement are described, as well as associated remedial measures. Literature SummaryLong-term field studies shows that P movement occurs in dissolved and particulate (sediment-bound) forms and, for cultivated land, 75 to 90% of the P moves with eroded soil. Dissolved P, however, is more available to algae than particulate P. Thus, remedial measures must not only consider total P loss, but its algal availability. Quantifying P movement by long-term studies is costly, time consuming, and labor intensive, and the results are site specific. Many computer models can simulate P movement in runoff. However, their use by field personnel, such as farm advisors, extension agents, and consultants is often restricted by complex data and computer requirements. Thus, an indexing system is needed to identify agricultural soils and management practices that might adversely affect surface waters. Applied QuestionsWhat are the major factors controlling P movement in the landscape? The main factors controlling P movement can be divided into transport and source factors (Fig. 1). Transport factors include the mechanisms by which P moves within a landscape and are rainfall and irrigation induced erosion and runoff. Factors which influence the source and amount of P available to be transported are soil P content and rate and method of P applied in either mineral fertilizer or organic forms. Fig. 1Transport and P source factors involved in development of a P movement index. How do the transport factors determine P movement? The factors controlling dissolved and particulate P movement within a landscape are conceptualized in Fig. 2. The first step in the movement of dissolved P in runoff is the removal of P from a thin layer of surface soil (0.04 to 0.12 in.) and plant material. Fig. 2Transport factors involved in the movement of P within the landscape. Since P is tightly sorbed by soil clays, erosion determines particulate P movement. As P moves to a lake by stream flow, there is generally a progressive decrease in P load by water dilution and sediment deposition. Phosphorus often becomes more algal available as it moves, however, as a result of chemical and physical processes. Full scientific article from which this summary was written begins on page 492 of this issue. How do the source factors determine P movement? Decades of P fertilization at rates exceeding those of crop removal have increased soil test P in areas of intensive agricultural and livestock production. This increases the potential for P movement if runoff or erosion occur within a landscape. The loss of fertilizer or manure P in runoff is influenced by the rate, time, and method of application; form of P added; amount and duration of rainfall; irrigation practices; and vegetative cover. Generally, P losses are less than 5% of those applied. Even so, they often exceed levels limiting algal growth. RecommendationsRunoff and erosion control measures include conservation and contour tillage, cover crops, terracing, buffer and riparian zones, and sediment detention reservoirs. Most of these practices, however, are more efficient at reducing particulate than dissolved P losses. In fact, the movement of algal available P under no till systems can increase. Thus, effective control measures must address both transport and source factors. Clearly, P applications must be carefully managed, and P transport potential minimized, to efficiently reduce P movement in landscapes. This may be achieved with regular soil P testing, incorporation, and application during times of low runoff probability. Overall, the cost and difficulty of control measures increases as the distance between P source and treatment increases. Thus, identification of sites vulnerable to P movement, via the P indexing system, will aid development of sustainable agricultural management systems that are agronomically and environmentally sound.