P.J. Grant
Grasslands Research Station, P Bag 3701
Marondera, Zimbabwe
Trials were conducted for 6 years at two granite sand sites in Zimbabwe to study to responses of Siratro (Macroptilium atropurpureum) and Oxley fine-stem stylo (Stylosanthes guianensis) to single superphosphate applied at levels of 0 to 2400 kg/ha, applied either split (annual and biennial) or all in the first year of the trials.
Siratro yields were increased by P applications throughout the trial. Applying P initially increased stylo yields, but in the third season negative responses occurred at high levels of P application, and this progressively affected all P treatments at all sites. By the fifth season only sections of one replicate at Marondera that received extra Zn in the fourth year responded positively to P application.
There were significant correlations between Zn content of the herbage, level of P application and yield decreases from the second to the fourth season at one site. Application of Zn in the fourth season apparently prevented yield declines in plots not previously affected, and other plots adversely affected by high initial P application recovered in the sixth season, probably due to the release of immobilised Zn by weathering.
The decline in stylo yield was probably linked with Zn immobilisation in a Zn/P interaction ratio which has been referred to by a number of authors.
