Soil testing labs generally follow recommended analytical procedures to test soil for nutrients, but the way they recommend fertilizer can be quite variable.
Some labs have a set of crop response data to generate their fertilizer recommendations. Others include nutrient removal, which is the amount of nutrients it takes for a given target crop yield. Another method is to use base saturation or basic cation saturation ratios to recommend fertilizer.
Is there a need to test soil for base saturation? In a recent paper, Adrian Johnston with the Potash and Phosphate Institute of Canada and Rigas Karamanos with Western Co-operative Fertilizers Ltd. cautioned growers about using base saturation as a way to measure soil fertility or to estimate fertilizer requirements.
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The most common basic cations are calcium, magnesium, potassium and sodium. Base saturation is the percentage of the soil particle surface occupied by these bases. Base saturation is most commonly used to determine if lime needs to be added to soil to reduce pH. It does not necessarily imply the nutrient fertility of a soil.
The term basic cation saturation ratio, or BCSR, is used to describe the ideal proportions of the major exchangeable cation nutrients.
Fertilizer recommendations made using BCSR do not consider whether any particular nutrient is in sufficient or deficient amounts in the soil.
Most labs in Western Canada use the sufficiency philosophy: if a nutrient is deficient, a crop response is likely, but if it is sufficient a crop response is less likely.
With the BCSR philosophy, if the ratio of base cations is wide or low, then a fertilizer recommendation for the appropriate nutrient is triggered. This poses a problem in calcareous soils found on the Prairies where calcium levels are high, but potassium and magnesium levels are more than sufficient.
Karamanos’s research concluded that while a calcium-potassium ratio can be wide, the chances of a yield response to potash application are low. The results proved that using a BCSR to make potassium recommendations, while ignoring the level of soil test potassium, can be an expensive mistake.
Potash responses on potassium-sufficient soils can occur occasionally and are more often related to providing a source of potassium early in the growing season when the soil is cold, when there’s variability of soil potassium supply across the field or when there is a chloride effect rather than the ratio of potassium to other bases.
Using BCSR for highly weathered soil in the southern United States, where base levels are low, is a common and accepted process, but using the BCSR to generate fertilizer recommendations doesn’t work in Western Canada.
Using BCSR may ultimately recommend nutrients and cause expenses that producers do not require.
Brent Flaten, P.Ag., CCA, is an agronomic crop enhancement specialist for Agricore United in Regina.
