Assessing the plant minimal exchangeable potassium of a soil
Schneider, André | Augusto, Laurent | Mollier, Alain
The plant minimal exchangeable K (EPₗ,ₘᵢₙ) defines the lower accessible limit of the most available pool of soil K to plants. It is also an index of long‐term K reserve in soils. However, its estimation by the classical method of exhaustion cropping is laborious. This study aimed at comparing EPₗ,ₘᵢₙ values obtained by the exhaustion cropping method with EPₗ,ₘᵢₙ values estimated by an alternative approach based on the cationic exchange capacity (CEC) of the infinitely high selective sites for K (i.e., always saturated with K) in the K‐Ca exchange (EK‐Cₐ,ₘᵢₙ). A set of 45 soil samples, corresponding to the various fertilization K treatments of 15 long‐term K fertilization trials, was used in this study. The selected soil samples presented a wide range of texture, CEC, and exchangeable K. The plant minimal exchangeable K was found more or less independent of the K treatment, whereas EK‐Cₐ,ₘᵢₙ increased when the soil exchangeable K content increased. The plant minimal exchangeable K was systematically lower than EK‐Cₐ,ₘᵢₙ, showing that EK‐Cₐ,ₘᵢₙ is at least partially available to the plant. Hence, EK‐Cₐ,ₘᵢₙ is not a surrogate of EPₗ,ₘᵢₙ. Conversely, the plant minimal exchangeable K was strongly, positively correlated to soil CEC (measured at soil pH; r² = 0.90***). This soil property can consequently be used as a proxy of EPₗ,ₘᵢₙ.
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