Phosphorus Critical Value and Requirement Factor Determination for Tef (Eragrostis tef) Production in Ethiopia.

Tef [Eragrostis tef (Zucc.)  Trotter], is a cereal crop that receives significant percentage of chemical fertilizer application in Ethiopia. Hitherto, the existing phosphorus fertilizer recommendation is based on field research results that give recommendation for wide area. Very recently, attempts have been made in the country to establish and furnish soil testing laboratories to give fertilizer recommendation based on soil test result. However, so far, the critical value and requirement factor of phosphorus for tef production in the country are not established. Critical value and requirement factor are two important decision criteria that decide either to apply phosphorus or not and how much to apply if application is indispensable, respectively. Therefore, field experiment was executed in major tef producing areas of the country to determine these criteria. The experimental design was RCBD with two replications. Treatments were factorial arrangement of four levels of nitrogen (0, 46, 92 and 138 kg N ha^-1) and phosphorus (0, 10, 20 and 40 kg P ha^-1) applied on 50 different farmers’ fields with plot size of 8 m by 5 m. Intensive composite soil samples were collected from each plots and sites and phosphorus was analyzed using Olsen method. Critical value was determined following the Cate-Nelson diagram method and quadratic plateau model at 138 kg N ha^-1 application rate. The Cate-Nelson diagram method results in 10 ppm P as a critical value and 6.72 kg P ha^-1yr^-1 as a requirement factor. But quadratic plateau results in 20 ppm P as critical value with the same requirement factor. Both of these results were verified and the Cate-Nelson diagram method results in economical grain yield advantage. Therefore, 10 ppm P as a critical value and 6.72 kg P ha^-1yr^-1 as a requirement factor can be used to render phosphorus fertilizer recommendation for tef production based on soil test result in Ethiopia.