Phosphorus reserves and availability in soils receiving long-term manure applications

In agricultural areas where intensive farming practices are common, applications of phosphorus (P) in amounts larger than crop requirements have led to an increased incidence of soils with a high P concentration. Accumulation of P in soil generally results in a greater potential for P enrichment of surface runoff and P losses during drainage. Estimating the size of the available P pool and the P supplying capacity of such soils is useful for calculating P-use efficiencies as related to agronomic practices and can be useful for assessing soil P characteristics that may have a negative impact on the environment. In this work, the available P content of 29 soils from farms with intensive confined animal operations was determined using eight traditional and three P-sink methods. The results obtained are discussed in relation to the degree of P saturation (DPS) of the same soils. The residual P availability status was evaluated via successive anion exchange resin membrane extraction of the soils. The cumulative P removed after eleven extractions fitted well a first-order equation, the asymptote of which represents the potential amount of desorbable P (Ultimate Releasable P). Relationships between the soil P test values and both the Ultimate Releasable P and the DPS lead to the conclusion that about 501,,c of the soils examined have it high potential for releasing P to surface and ground waters.