Phosphorus and potassium balance of long-term fertilizer trials on two Belgian loamy soils

In field experiment on two different loamy soils, plant response and soil enrichment or depletion with increasing P and K doses were studied during 16 consecutive years. No response to P addition was observed in this period, except for three crops which gave a higher yield at the first P rate only. Six crops, among which ray-grass, potatoes, sugar beets and maize, responded towards the first K fertilizer rate, indicating that the K quantity of the soil is nog as strongly buffered as the soil P quantity. In order to avoid a too strong depletion of the soil as well as luxury consumption by the plants, P and K inputs should roughly compensate for the P and K removal with the harvests at optimal yield for all the crops of the rotation. Applying linear relationships between excess fertilizer K and amount of extractable K, it is shown that the mode of K fixation is different for both soils.