The Contribution of Phosphorus Leached from Crop Canopy to Losses in Surface Runoff

Simulated rainfall (6 cm/hour) was used to leach P from growing cotton (Gossypium hirsutum), sorghum (Sorghum sudanense), and soybean (Glycine max.) plants as a function of soil type and time interval between rainfall events. The relative contribution of plant material and surface soil to the transport of soluble P in surface runoff was estimated. The amount of soluble P in plant leachate, collected before contacting the soil surface, was found to increase with plant age and soil‐water stress when changes in leaf area index were accounted for. A period of at least 1 day between rainfall events was needed for P to reaccumulate on the leaf surface. The contribution of soluble P in plant leachate to that transported in surface runoff was estimated as the difference in runoff concentration between planted and bare soil. When plants were subject to a soil P stress, canopy leachate contributed the major proportion (90%) of that transported in surface runoff. With an increase in plant age from 42 to 82 days, the contribution of plant leachate to surface runoff increased from approximately 20–60%. The possible sorption of leached P is discussed along with estimating leaching losses in improving the predictive ability of nutrient models.