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Commonly used fertilizers among farmers in Nigeria come in compound form (N:P:K; 15:15:15). Usually, there are two main sources of nitrogen in compound fertilizer: nitrate N and ammonium N. The two nitrogen sources are combined in most compound fertilizers. This study was carried out to evaluate the possible impacts of chemical interactions between K+ and NH4+ on potassium bioavailability in soil when fertilizers containing both cations are applied. Soil samples were collected from existing cocoa plantation and subsequently equilibrated in solution containing only potassium ions (20, 40, 60, 80 and 100 mg L-1K) in a batch adsorption manner. Since the study was a comparative type, subsamples of the same soil were equilibrated in solution containing ammonium and potassium ions (20, 40, 60, 80 and 100 mg L-1(K/NH4). Result shows a 64.6, 35, 18, 18 and 14% reduction in K adsorption in soil treated with solutions containing ammonium and potassium when compared with soil treated with solution that contained only potassium. Among the various adsorption models tested, the Freundlich model gave the highest value of coefficient of determination R2. The study showed that, application of N: P: K fertilizer that has NH4+ as its N source could result in the reduction of potassium storage capacity of the soil for plant use.
Keywords: Potassium, Ammonium, fertilizer, adsorption, cocoa