Amendment Effects on Soil Test Phosphorus

Applications of animal manures have increased soil test P values in many parts of the USA and thus increased the risk that soil P will be transferred to surface water and decrease water quality. To continue farming these areas, landowners need tools to reduce the risk of P losses. A field experiment was conducted near Kurten, TX, on a Zulch fine sandy loam (thermic Udertic Paleustalfs) with Bray‐1 P values exceeding 3000 mg P kg −1 soil (dry wt.) in the A p horizon to evaluate the effectiveness of soil amendments for reducing soil test P values. Soils were amended annually from 1999 to 2001 with 1.5 and 5.0 Mg gypsum ha −1 , 1.4 Mg alum ha −1 , or 24.4 Mg ha −1 of waste paper product high in Al alone or in combination with 1.5 Mg gypsum ha −1 and/or 1.4 Mg alum ha −1 These treatments supplied a maximum of 225 and 1163 kg ha −1 yr −1 of Al and Ca, respectively. Soil Bray‐1 P and dissolved reactive P levels were monitored from 1999 to 2004. None of the soil amendment treatments affected Bray‐1 P values. Only annual additions of 5.0 Mg gypsum ha −1 from 1999 to 2001 significantly reduced soil dissolved reactive P. Dissolved reactive P levels reached minimal levels after two applications of 5.0 Mg gypsum ha −1 but increased in 2003 and 2004. These results indicate that soil dissolved reactive P levels can be reduced if sufficient amounts of gypsum were added to supply Ca in amounts similar to the soil test P values.