The optimum K2O application rate for starch potatoes in relation to K-DL-contents in sandy soils in North-Western Germany

The results of 35 K fertilizer experiments on potatoes were evaluated. The double lactate soluble K contents in soil varied between 1,7 mg and 15 mg/1 00 g, the starch yield between 22 dt/ha (drought) and 115 dt/ha, and the ec onomically optimum K2O application rate be tween 0 kg/ha and 295 kg/ha. The optimum K2O rate decreased significantly with increasing K contents in soi l. The regression equation was: K2O [kg/ha] = 191-10,8 K-DL [mg/100 g]. The K uptake and rainfall had no significant influence. Fertilizer recommendations with acceptable accuracy can be deducted on aver age, though not for each single harvest, because of the large deviation. The transformation of the experimental results into fertilizer recommendati ons for ail sandy soils in Lower Saxony was discussed. As compared to the r egression the current recommendations are by 50, 40 and 30 hg/ha K2O higher in soil test classes A, B and C, respectively. All recommended K rates are essentially lower than the K uptake.