Extractable soil phosphorus threshold values for dryland maize on the South African Highveld

Abstract

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.

Maize fertilizer research over the last 25 years – an overview

“…(1994). Recent work by Schmidt et al (2007) indicated that threshold values are related to the degree of leaching and silt-plus-clay content of the soil. For practical purposes they proposed threshold values ranging from 33.5 mg kg -1 for 13% silt-plus-clay to 14.6 mg kg -1 for 60% silt-plus-clay.…”

Hardpan and maize root distribution under conservation and conventional tillage in agro-ecological zone IIa, Zambia

“…Expected relative maize yields in unfertilised soils may be around 40-50 % of potential yield according to data for critical levels from Nigeria (Adeoye and Agboola, 1985). Even lower relative yields may be expected based on data for Oxisols in Brazil (Smyth and Cravo, 1990) and data from South Africa (Schmidt et al, 2007). Below the 10 to 20 cm soil depth, the extractable P concentrations were generally around 2 mg kg -1 for both conservation and conventional tillage (Fig.…”

Simulation of land use impacts on sediment and nutrient transfer in coastal areas of Western Cape, South Africa

“…Therefore, P concentration was supposed to be low on the natural vegetation types with 250 mg kg -1 [38], and 500 mg kg -1 on cultivated land [39]. However, these assumptions are rather uncertain, because of the site specific high variability of soil phosphorus [40].…”