PLANT AVAILABILITY OF PHOSPHORUS IN ASH, CALCIUM PHOSPHATE AND DIFFERENT TYPES OF SEWAGE SLUDGE / Fosforns växttillgänglighet i olika typer av slam, mineralgödsel samt aska
Different types of phosphorus fertilizer were evaluated in a three-year field experiment with oars and barley in mid-western Sweden. The treatments were, except a control, mineral fertilizer (calcium phosphate): ash; sewage sludge precipitated with iron, aluminium or lime; and biologically treated sewage sludge. To ensure that only the effect of the phosphorus source was evaluated, all differences in nitrogen, potassium and lime were compensated for. The treatment plot size was 8 x 40 m2 and the experiment was repeated in three blocks.
During dry conditions (first experimental year) the easily soluble phosphorus in calcium phosphate (mineral-P) gave better crop growth than all other treatments. In the two following years, there were no significant differences between the treatments. The results show that successful phosphorus fertilization involves keeping the soil in a state in which it can supply the crop with phosphorus when other conditions are optimal.
Although three years of field experiments is too short a time for evaluation of fertilizers, the results indicate that in the long run, crop supply of phosphorus is more dependent on a correct comprehensive evaluation of soil type – crop – fertilizer than on the chemical form of the phosphorus in the fertilizer.