Maize response to Langebaan rock phosphate—superphosphate mixtures under glasshouse conditions

Abstract

A previous investigation showed that Langebaan rock phosphate (Langfos) was not an effective substitute for superphosphate. However, in the light of reports that P availability is enhanced when phosphate rock (PR) and monocalcium phosphate are combined, an improvement in Langfos effectiveness was considered possible if mixed with superphosphate. To investigate this, an Avalon sandy loam and a Balmoral clay were treated with a total of 0, 50, 100, 175, 250 and 350 mg P kg⁻¹ and.0, 200, 400, 800 and 1600 mg P kg⁻¹, respectively, applied as a mixture of superphosphate and Langfos to provide P from the two sources in the ratios of 1:0, 4:1, 2:1, 1:1, 1:2, 1:4 and 0:1. Soils were incubated for four weeks before planting to maize. Plants were harvested after six weeks and yield data obtained were examined in relation to total P, water soluble P and citric acid soluble P applied, and to the quantities of P extracted by the Bray 1, Ambic, Mehlich 3 and modified Truog extractants. The water and citric acid-soluble P contents of the various mixtures provided a more reliable index of plant available P, than did their total P contents. The relative agronomic effectiveness of the superphosphate—Langfos mixtures increased in close association with an increase in the proportion of superphosphate in the mixture, and there was little evidence to suggest that an interaction between Langfos and superphosphate had occurred. Both the Ambic and Bray 1 extractants provided reliable indices of plant available P.

Keywords:

• Agronomic effectiveness
• interaction
• P extractants
• phosphate rock
• superphosphate