Changes in the phosphorus status of soils and the influence on maize yield

[摘要] The principle objective of this study was to obtain P fertilizer guidelines for large maize producing areaswest of the Drakensberg mountains according to a similar approach previously used in KwaZulu-Natal based on the sufficiency concept of soil extractable P.Data from 14 different P fertilizer trials at various localities in the Free State, Gauteng, Mpumalanga andNorth West provinces was used. Different phosphorus treatments were applied for all trials in order toestablish differences in extractable soil P levels which were expected to have corresponding effects onmaize yield. Long-term rainfall varied from 990 mm per annum for the Athole trial in the eastern maizeproducing region to 494 mm per annum for the Wolmaransstad trial in the western region. The duration of trials varied between one and nine seasons.Firstly, simple regression equations with high R²-values were obtained for relationships between Ambic1 and Bray 1 extractions over soils, but since it was demonstrated that relationships for different soilsdiffered significantly from each other the use of these equations may result in a very high degree ofinaccuracy with respect to P fertilizer recommendations. Soil properties had a significant effect on theefficacy of the two extractants. Furthermore, slopes of relationships between Ambic 1 and Bray 1 couldbe predicted by using exchangeable Ca in simple regression relationships (R²-values of between 80 and 83%).Secondly, it was established that P requirement factors (PRF's) cannot be obtained over soils (R²-valuesvaried between 10 and 54 %), but rather for different soils separately (R²-values varied between 75 and 99%). Differences between the PRF's in total soil volumes (1.7 to 63.2 for Ambic 1 concentrations and 0.8to 27.3 for Bray 1 quantities) indicated that the soils used in this study differed in their behavior to appliedP. Phosphorus requirement factors could be predicted by a simple regression equation using degree ofleaching based on the clay content as input parameter (R²-values between 60 and 78 %) as well as sixmultiple regression equations using either one of exchangeable Ca, Mg, K, silt content or degree ofleaching based on the clay content (R²-values between 52 and 99 %) as input parameters. Theimplementation of any of these regression equations should be practical since all the parameters are usuallyincluded in standard analysis. However, the simple regression with degree of leaching based on the clay content appears to be an obvious option above the multiple regression equations since it is based on fiveinput variables, i.e. exchangeable Ca, K, Mg, Na and clay content.Lastly, threshold extractable P values were derived for 10 out of the 14 localities that have been includedin this study with varying R²-values. These threshold extractable P values were related to soil propertiesand it was found that the degree of leaching and silt-plus-clay content were the parameters that explainedmost of the variation. However, it was decided to explore only the relationships between thresholdextractable P values and silt-plus-clay contents in more detail. By excluding data from two localities ofwhich the topsoil contained lime, the R²-values of the mentioned relationships improved substantially sothat threshold extractable P values could be derived from the silt-plus-clay content range of the other eightlocalities. For example the threshold extractable soil P concentrations based on Bray I for the total soilvolume 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 (Bray 1) thresholds are much higher on the sandy soils than the value of 19 mgP kg:' (Bray 1) for 95 % relative yield currently in use according to existing guidelines. This may notnecessarily imply that more P fertilizers will be sold according to higher soil P thresholds obtained in thisstudy, since the corresponding soil sampling procedure also measures more residual P from enriched zonesover rows where P fertilizer was band placed. The soil sampling procedure according to existing guidelinesexcludes sampling from these zones.

[发布机构] University of the Free State