Factors in the molybdenum and phosphorus status of soils on the Dorrigo Plateau of NSW

Abstract

Samples of 50 virgin surface soils were collected from the Dorrigo Plateau of New South Wales within the 90-220 cm rainfall zone. The parent rocks were granites, sediments, basalt, and trachyte. The soils were used in a pot trial in which clover plants were grown with and without added molybdenum or phosphate. Laboratory measurements were made of the soils' ability to adsorb molybdate and phosphate and of extractable molybdate and phosphate. Regression equations, which included terms both for adsorption, and for extractable molybdate or phosphate, gave a good description of the yield of clover plants grown on the soils without added molybdate or phosphate, respectively. The ability of the soils to adsorb molybdate and phosphate increased with annual rainfall. The apparent influence of rainfall on adsorption was greater for soils derived from basalt or trachyte than for soils derived from granite or sedimentary rocks. Oxalate-extractable molybdate was lower in soils derived from sedimentary rocks than from basaltic or trachytic rocks. Soils derived from granite varied widely, two having very high contents. Fluoride-extractable phosphate was very low in soils derived from trachyte, granite, and sedimentary rocks, reflecting the phosphorus levels in the parent rocks. Fluoride-extractable phosphate was also low on the krasnozems despite large variations in the phosphate content of the parent rock. On the other basaltic soils the extractable phosphate was directly related to the phosphate content of the parent rock.