To determine the coefficients of c1 and c in Mitscherlich-Bray equation, critical level and fertilizer recommendation for potassium (K) as by Mitscherlich-Bray (MB) equation, an experiment in a randomized complete block design of 5 treatments and 3 replications was carried out in 12 irrigated wheat (Triticum aestivum L.) fields in Sarpol-e-Zohab region during the 2007-08 growing season. The treatments included T1 = Farmer's conventional fertilization practice= NP); T2 = control (all the nutrients included except K);
T3 = T2 + 100 kg ha-1 muriate of potash (MOP); T4 = T2 + 200 kg ha-1 MOP and T5 = T2 + 300 kg ha-1 MOP in three stages of: preplanting, tillering, and stalk bearing. Following harvest, grain yield was determined, and MB Equation's coefficients for K calculated. The value of c1 coefficient for the 11 wheat fields was found as 0.0020. Potassium critical level was 115 mg kg-1 for 85% of the maximum yield under non-subsidized, i.e., free market fertilizer prices condition. However, under existing subsidized fertilizer prices, potassium critical level achieved for 95% of maximum yield was 180 mg kg-1. Therefore, based on the information obtained, the managers of soil test laboratories will have a suitable tool in predicting the farmers' relative yields and economic return on wheat with each incremental increase in K-fertilizer application just only by analyzing the available K in the soils. There is need for such information also for other regions and for other different field crops.