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South African analytical laboratories use different extractants for P in soils of which the most important are Bray 1, Ambic 1, Bray 2, Truog and Olsen. The fertilizer industry prefers to use Bray 1 extractions for fertilization recommendations and the ARC-Grain Crops Institute, Ambic 1 extractions for calibration research. Therefore it became necessary to establish relationships between the P extracted by the two methods in specific soils and if possible to relate differences in these relationships to soil properties. Soil samples collected to a depth of 150 mm from phosphorus fertilization trials at 12 localities were used for this study to ensure a large gradient in extractable P. The clay content of the 150 mm topsoil at these localities ranged between 8.4 and 47.0%. More than 3000 soil samples were analyzed for extractable P using both the Ambic 1 and Bray 1 methods. Relationships between Ambic 1 and Bray 1 extractable P for each locality and for 12 localities in total were determined. In addition, multiple and simple regression analyses were carried out to establish relationships between the slopes of Ambic 1 versus Bray 1 relationships and some soil properties of the different localities. Excellent relationships between Ambic 1 and Bray 1 extractable P with R2-values ranging from 92 to 98% have been established at nine localities. At the remaining three localities the R2-values ranged from 75 to 88%. Although the relationship between Ambic 1 and Bray 1 extractable P over all 12 localities was statistically significant with a R2-value of 91 %, the slopes of the different localities differed significantly from each other. Most of the variation between these slopes was attributed to exchangeable calcium (R2 = 80 and 83%), degree of leaching based on clay content (R2 = 80 and 71%), pH(KCI) (R2 = 70 and 69%), effective cation exchange capacity (R2 = 68 and 74%), degree of leaching based on silt-plus-clay content (R2 = 68 and 63%), pH (H 2 0) (R2 = 62 and 68%), Mnoxides (R2 = 60 and 72%) and the exchangeable Fe content (R2 = 60 and 54%) of the soils. It is suggested that at least some of these soil properties should be considered in the development of an universal equation for the conversion of extractable P values from one to the other.

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The principle objective of this study was to establish P fertilizer guidelines for dryland maize on the South African Highveld according to the sufficiency concept of soil extractable P. Data sets from nine different P fertilizer trials at various localities in the Free State, Gauteng, Mpumalanga and North West provinces were used. Different P treatments were applied for all trials in order to establish differences in extractable soil P levels, which were expected to have corresponding effects on maize yield. Long-term rainfall varied from 765 mm per annum for the Dirkiesdorp trial in the east to 494 mm per annum for the Wolmaransstad trial in the west. The duration of trials varied between one and nine seasons. The clay content of the top 150 mm soil at these localities ranged between 8.4 and 47%. Extractable P threshold values with varying R2 values were derived for all localities. These values were related to soil properties and it was shown that the degree of leaching and silt-plus-clay content were the parameters that explained most of the variation. However, it was decided only to explore relationships between threshold P values and silt-plus-clay contents in more detail. By excluding data from two localities of which the topsoil contained free lime, the R2 values of the mentioned relationships improved substantially so that P threshold values could be derived from the silt~plus-clay content range of the other seven localities. The extractable soil P threshold concentrations based on Bray 1 for the top 150 mm soil layer, to obtain 90% relative yield varied from 33.5 mg kg-1 at 13% silt-plus-clay to 14.6 mg kg-1 at 60% silt-plus-clay. These P thresholds were much higher on the sandy soils than the value of 19 mg P kg-1 (Bray 1) for 95% relative yield according to the ARC-Grain Crops Institute (1994) guidelines over all SOils. This may·not necessarily imply that overall more P fertilizers should be applied, since the corresponding soil sampling procedure also measures residual P from enriched zones over rows where P fertilizer was band-placed. Research results used to establish the ARC-Grain Crops Institute (1994) guidelines excluded sampling from enriched zones over rows.