RELATIVE ABILITY OF A RANGE OF CROP SPECIES TO USE PHOSPHATE ROCK AND MONOCALCIUM PHOSPHATE AS P-SOURCES WHEN GROWN IN SOIL

Glasshouse experiments were conducted to compare the ability of a range of crop species to use the medium reactive Jordan phosphate rock (JPR), the highly reactive North Carolina phosphate rock (NCPR) and the highly soluble monocalcium phosphate (MCP) as P sources in a P-deficient silt loam when supplied with nitrate-N. Growth of wheat and barley resulted in excess anion uptake, a consequent rise in rhizosphere pH and yield increased in the following order: control < JPR < NCPR < MCP. Growth of buckwheat resulted in excess cation uptake, and as a result it used NCPR as effectively as MCP; the yield order was as follows: control < JPR < NCPR = MCP. Lupin, when supplied with nitrate, raised its rhizosphere pH and yield followed the same order as that for wheat and barley: control < JPR < NCPR < MCP. In contrast, when not supplied with nitrate and relying substantially on biological N2 fixation, lupin absorbed a cation excess, lowered rhizosphere pH and the yield order was control < JPR = NCPR = MCP. Although rape and kale absorbed an anion excess and raised rhizosphere pH they were surprisingly effective at using phosphate rock as a P source; the yield order was control < JPR = MCP < NCPR. This was attributed to the known ability of rape to acidify its rhizosphere in a localised area just behind the root tip in response to P deficiency. The opportunity appears to exist for the strategic use of phosphate rock at times within arable rotations when species are present that can effectively use such P sources.