Estimating ‘effective’ levels of phosphorus applied as superphosphate and rock phosphate
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The effectiveness of different phosphates can be measured in field experiments in two ways: (i) different levels of each fertilizer are applied once only at the start of the experiment and the effectiveness of the fertilizer residues is measured in subsequent years relative to the effectiveness of freshly-applied fertilizer; (ii) different levels of each fertilizer are applied so that the same level is applied cumulatively to the same plot each year. Both methods can be used to compare rock phosphates with superphosphate. The first method can be used to measure the residual value in kg ha−1 of each fertilizer from the relationship between yield and the level of P applied. The residual value is calculated by dividing the amount of P as freshly-applied superphosphate needed to produce a given yield by the amount of P from the fertilizer residues required to produce the same yield. This ratio is then used to estimate the proportion of the P applied, as kg P ha−1, that is effective for plant growth each year. Provided the experiment is big enough to have sufficient plots for the freshly-applied superphosphate, the residual value of rock phosphates applied in any number of years can be estimated. While this method allows an estimate of how the availabilty of the residues from any one application changes with time, it gives no information on whether there is any additive benefit from residues from cumulative applications. The second method more closely resembles fertilizer practice on farms but it can not be used to measure the residual value of the fertilizers because of the problem of knowing what to plot on the x axis, the amount of P applied. It could be kg P ha−1 applied each year, or kg total P ha−1 applied up to that year. This problem may be resolved by using the first method to calculate the residual value of each individual application of fertilizer and adding the individual residual values, in kg ha−1, for the appropriate number of years. That is, the sum of the residual values measured using the first method could be used to determine the “effective” level of P for the second method. This approach was tested in a field experiment in Western Australia that measured the residual value of two rock phosphates and superphosphate. When the residual value measured using the first method was used to calculate the ‘effective’ level of P applied as each P fertilizer, the relationship between yield and the ‘effective’ level of P applied could be described by the same equation for the three P fertilizers for P applied at the start of the experiment only or cumulatively each year.
Key wordssuperphosphate rock phosphate Calciphos C-grade ore fertilizer effectiveness residual value
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