A comparison between plant response and chemical measurements of the dissolution of reactive phosphate rock in soils of different pH and phosphorus retention

Abstract

Chemical measurements of the dissolution of reactive North Carolina phosphate rock (PR) in a lateritic podzolic soil adjusted to different pH values or amended to different P retention capacities were compared with plant response data. Soil pH adjustment consisted of incubating soil mixed with dilute HCl or solid SrCO3. The P retention capacity was adjusted by adding different amounts of synthetic goethite. Clover was grown as a test crop under glasshouse conditions on soils treated with no P and 800 µg Pg-1 soil as PR. The dissolution of PR, measured as the increase in soil exchangeable Ca (ÄCa) at harvest time, increased by 46% for a decrease in soil pH from 5.78 to 4.55. Both dry weight and P content of clover tops showed no response to the greater PR dissolution at low soil pH, although twice as much P was bicarbonate-soluble at pH 4 55 compared with pH 5.78. This increase in bicarbonate-soluble P was only partly due to the greater dissolution of PR. Increasing the goethite content of the soil by 9% resulted in a 107% increase in PR dissolution but induced a 54% decrease in the bicarbonate-soluble P level compared with the control soil and consequently reduced both dry weight yield and P content of clover tops by 19% and 34% respectively.