THE EFFECTS OF VARIOUS N-SOURCES ON THE A VAILABILITY OF PHOSPHATE FROM ROCK PHOSPHATE AND MONO-CALCIUM PHOSPHATE

The effects of (NH4)2SO4, CO(NH2)2, Ca(NO3)2 and blood meal on the amo unt of available P released to barley (Hordeum vulgare L.) and maize ( Zea mays L.) from Marocco rock phosphate (RP) and monocalcium phosphat e (MCP), applied to slightly acid soils from Holetta, Jima (Ethiopia) and Pettenbrunn (Germany) were studied under greenhouse conditions. Wh en the sparsely soluble P-source (RP) was used, application of (NH4)2S O4 gave the highest dry matter yield and P-uptake. When the soluble P- source (MCP) was used, no significant effect of N-form on P-uptake or dry matter yield of plants was observed, although (NH4)2SO4 and CO(NH2 )2 applications decreased the pH of the soils as when applied with RP. The increased P-uptake due to (NH4)2SO4 application appears to have r esulted both from soil acidification and increased plant growth. When N was applied as (NH4)2SO4 together with either RP or MCP remarkable p H decreases on all the soils were observed. Although not so pronounced as (NH4)2SO4, CO(NH2)2 and blood meal when applied with either P-sour ces had also a pH decreasing effect on all the three soils. Unlike to the other N-sources, Ca(NO3)2 increased the pH of each of the soils wh en applied with either P-sources. The residual P including root residu es had no effect on the dry matter yield and P-uptake of maize (second crop), except on Pettenbrunn soil, which contained less P-fixing comp onents (Fe- and Al-oxides) as compared with that of Holetta or Jima. F ractionation of phosphorus carried out at the end of the experiment sh owed that the major part of the applied phosphorus, that was not taken up by the plant, was transformed to a larger extent to difficultly so luble Fe- and Al-P and to a smaller extent to Ca-P forms.