Fertilizer Trials on Small Coffee Farms in Uganda

In about 70 trials on small African Robusta coffee farms and 100 similar trials on small Arabica farms mean responses to nitrogen fertilizers were profitable and became increasingly so with continued applications. A soil treatment with magnesia in 21 Robusta trials gave no increase in yield when applied in addition to the nitrogen fertilizer. The other Robusta trials compared urea with ammonium sulphate and showed that there was no significant difference in their effects on yields. The ratio of dried to fresh cherry weight was not decreased by the nitrogen fertilizer.

Over 100 correlations of yields or responses with agronomic details and soil and leaf analyses were tested for significance. Routine soil analyses proved disappointing indicators of coffee yield or response to nitrogen, which confirms other evidence that nitrogen is the only major deficiency in Ugandan coffee. Agronomic practices seemed to have only minor effects on yield. Leaf nutrient contents were often significantly correlated with yields, but none with sufficient regularity to indicate a real and common association. Correlations between leaf nitrogen and yield were never significant even though the treatments significantly affected the levels of leaf nutrients. This is contrary to the results in earlier station experiments.