Dynamics of potassium fractions in the soil‐plant system after the application of an acid potassium‐rich effluent

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Abstract
We did a greenhouse experiment focused on the application of olive oil mill wastewater on potassium dynamics in the soil‐plant system. Two doses of wastewater and two of K fertilizer containing amounts of potassium equivalent to those in the wastewater treatments were tested and compared with the results of control and control plus mineral fertilizer lacking K. The application of wastewater and of K fertilizer significantly increased initial exchangeable K in soil in comparison with control values, although the effect of wastewater was weaker than that of fertilizer. Treatments with wastewater or K fertilizer, but not with mineral fertilizer lacking K, kept K concentrations in ryegrass above the critical level during all three harvests. In both control treatments the K requirements of the crop, represented by cumulative K uptake, led to the mobilization of assimilable K from the reserve of nonexchangeable K in soil. This was not observed in treatments with industrial wastewater or K fertilizer: initial exchangeable K in soil was more than adequate to cover this requirement. Treatments with wastewater and K fertilizer had residual effects, i.e., a usually significant increase in water‐soluble K, exchangeable K, mobile K reserve and potential K reserve in soil, in comparison with both control treatments. In contrast, the increases in K firmly bound in minerals and total K were not significant.