PO 18. Compare and contrast the following approaches for making fertilizer recommendations:

Cation saturation ratios are not commonly used as the basis for nutrient recommendations.  The process assumes there is an ideal distribution of exchangeable cations (Ca, Mg and K) on the CEC.  These nutrients then can be drawn off the CEC.  When they are in balance, the soil solution ensures adequate nutrient availability to plants.  The most commonly applied ratio is: 65% Ca, 10% Mg, 5% K, and 20% misc.

However, past studies suggest no relationship between %K and %Mg saturation and yield.  Especially for calcareous (high pH) soils, adjustments to particular ratios can be very expensive, and saturation estimates (particularly for K) are not reliable.  Generally, if the soil pH and the soil test K and Mg levels are optimum, the balance of cations on the CEC will be acceptable for optimum crop production.  Further adjustments to cation saturations on the CEC are unnecessary.  Several studies suggest that despite increasing % saturation for K and Mg, crop yield shows no response.

• Liebhardt, W.C. 1981. Soil Science Society of America Journal 45:544-549.
• McLean, E.O 1977. ASA Special Publication no. 9.
• McLean, E.O., and M.D. Carbonell. 1972.  SSSA Proceedings 36:927-93.