South African maize farmers apply only two thirds of the potassium (K) which is annually removed in the grain, causing a deficit of more than 13 000 tons K per season. It is expected that these maize fields will gradually become depleted and more K will be required to maintain or improve yields. Research goals were set for sandy soils in the main maize producing areas of South Africa to determine the lower & upper threshold values and sufficiency levels for soil K, the K status of these soils and to evaluate the sufficiency level concept for K at the various localities in the Free State Province. Field experiments were conducted at nine localities during four consecutive seasons. The experimental data included the soil K extracted with NH 4 OAc and the relative maize yields (RY) of the treatments, +K which received macro, secondary and micro nutrients and 0K which received the same nutrients but K was omitted. The results indicated a lower threshold value of 125 mg K kg -1 below which maize is expected to respond positively and an upper threshold value of 190 mg K kg -1 above which maize is expected not respond to additional K. The extractable K from 25 000 sandy soil samples received from maize farmers indicated that approximately 50% of these samples contained less K than the lower threshold value, approximately 30% was between the lower and upper threshold values and the remaining 20% was above the upper threshold value. The sufficiency levels obtained with the Mitscherlich model at 98% RY, ranged from 109-340 mg K kg -1 . The difference between the sufficiency values and the extractable soil K at any of these localities, gives an unbiased measurement of the K fertilizer to be recommended.

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There is still no consensus in South Africa on the parameters used to advise maize farmers on the quantity of N needed for optimum yields. In order to determine the lower and upper threshold values for nitrogen (N), the biological optimum and the mineralization of organic N in the main maize growing areas of South Africa, field trials were carried out at seven localities over three years. The treatments were full fertilization as reference (Ref), with the comparative treatment being where N was omitted (0N). Relative maize yields (RY) were determined and composite soil samples were taken. Bulk density (t m -3 ) and the NO 3 -N extracted with K 2 SO 4 -solution were determined in the laboratory and the kg NO 3 -N ha -1 calculated. The RY responded to N with a lower threshold value in the soil stratum (0-600 mm) of 70 kg NO 3 -N ha -1 (8 mg NO 3 -N kg -1 ), an upper threshold value of 161 kg NO 3 -N ha -1 , a biological optimum of 280 kg NO 3 -N ha -1 and an estimated N mineralization during the growing season of 40 kg NO 3 -N ha -1 .