ROCK PHOSPHATES ARE NOT EFFECTIVE FERTILIZERS IN WESTERN AUSTRALIAN SOILS - A REVIEW OF 100 YEARS OF RESEARCH

The 1990s mark the centenary of the earliest work to identify the value of rock phosphate fertilizers for Western Australian agriculture. This review summarizes this and subsequent work. We arrive at a simple conclusion: rock phosphates are ineffective fertilizers because they do not dissolve rapidly in Western Australian soils. The effectiveness of different types of rock phosphate fertilizers has been compared with the effectiveness of superphosphate in several long-term field experiments on a variety of non-leaching soils in south-western Australia. These experiments have consistently shown that, all types of rock phosphate fertilizers are between one twentieth to one third as effective as freshly applied superphosphate both in the year of application and in subsequent years. Glasshouse experiments produce similar results. Laboratory studies of soils from these experiments have shown that the poor effectiveness of the rock phosphates is primarily due to the small extent of dissolution of these fertilizers in Western Australian soils. Several factors are responsible for the inability of adequate amounts of rock phosphate to dissolve in these soils. The soils are only moderately acid (pH in water > 5.5) and generally have low pH buffering capacities so can not rapidly contribute a large supply of protons to promote extensive dissolution of rock phosphate. The soils also have low capacities to adsorb the P and Ca released during dissolution of rock phosphate. They also have low water-holding capacities, and in the field under the Mediterranean climate the soil near the surface rapidly dries between rains thereby restricting dissolution of rock phosphates. In the laboratory it has been shown that rock phosphate dissolution is considerably enhanced in permanently-moist, acid soil with high pH buffering capacity, and high P and Ca buffer capacities. Thus the low extent of dissolution of rock phosphate fertilizers in Western Australian soils is responsible for the poor agronomic effectiveness of these fertilizers measured in the field experiments. © 1990 Kluwer Academic Publishers.