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Field Crop Response to Potassium on Soils with High Extractable Potassium and Varying Soil Supply Rates of Potassium

Crop responses to K on many western Canadian soils, even on soils testing high in K, have many people questioning the soil testing procedures used and/or the method by which K varies within fields. Using the Plant Root Simulator (PRS) probe, an ion exchange membrane system for measuring soil nutrient supply rate, a project was initiated in the semi-arid region of Saskatchewan to evaluate crop responses to fertilizer K on soils which are regarded as non-responsive using traditional K extraction methods.

IPNI-1999-CAN-SK23

Overview Related Article
15 Mar 2001

Justification

    Response trials from as far back as 1962 indicate that some soils would respond to potassium (K) despite high soil extractable K test levels. "Response to potash is sometimes obtained on soils not deficient in K on the basis of soil test, but at the present time, there is no method of predicting response on these soils" (Western Canada Fertilizer Association, p.12.)

    Soils (0-6" samples) extracted using ammonium acetate (NH4OAc) are considered to be very K deficient with 0-150lb ac-1 (0-6") extractable K2O, moderately deficient with 151-250lb ac-1 extractable K2O and marginal to adequate in K fertility with 251-400lb ac-1 extracted K2O. For most crops in Western Canada, no K2O would be recommended when soil HN4OAc extractable K2O exceeds 250 lb ac-1. Using conventional soil extraction methods, more than 80% of Saskatchewan would be considered unresponsive to K.

    On the other hand, recent widespread use of Plant Root Simulator (PRSTM) probes as an assessment of soil K supply rate have shown that only 50 to 60% of soils would be completely unresponsive to K fertilizer. When soil supply rates are used to formulate nutrient management plans with the PRS Nutrient ForecasterTM, supply rates lower than 50 ug K 10 cm-2 24 h-1 are considered to be responsive to K fertilizer. Those between 50 and 100 ug K 10cm-2 24 h-1 have the potential to respond to K fertilizer depending on other factors influencing plant growth and nutrient demand. Using the Crop Nutrient Forecasting system with the PRSTM probes clearly indicates that environmental conditions greatly impact the probability of a K response.

    In conjunction with the Western Ag Innovations Precision Farming study, a wide range of soil types and K supply rates have been measured in Southern Saskatchewan. This area is considered to be generally unresponsive to K fertilizer.