Is Base Saturation more than just BS?

Many soil test reports include a section on % base saturation, and some consultants and fertilizer dealers use these numbers in making fertilizer recommendations even though they are not used in the OMAFRA recommendations. Should we be following suit?

What is base saturation?

Base saturation refers to the proportion of the cation exchange sites in the soil that are occupied by the various cations (hydrogen, calcium, magnesium, potassium). The surfaces of soil minerals and organic matter have negative charges, that attract and hold the positively charged cations. Cations with one positive charge (hydrogen, potassium, sodium) will occupy one negatively charged site. Cations with two positive charges (calcium, magnesium) will occupy two sites.

Base saturation does not have any relation to fertilizer recommendations for the negatively charged nutrient ions, like phosphate, nitrate and sulfate.

To determine the base saturation, you must first know the cation exchange capacity, which is a measure of the amount of negative charge in the soil. To determine this precisely, the soil is saturated with an unusual cation like barium, to displace all of the other ions, and then the amount of barium sticking to the soil is determined.

How is it used?

Fertilizer recommendations using base saturation are aimed at "balancing" the cations in the soil to meet a particular range of base saturation percentages for each cation. This approach goes back to the work during the 1940's of Dr. F. Bear of New Jersey. He found that an acid, infertile coastal plain soil, which had been limed and fertilized for optimum crop production, had a base saturation percentage of 5% hydrogen, 3% potassium, 15% magnesium and 70% calcium.

What are the limitations of Base Saturation?

There are three key limitations to using base saturation percentages for fertilizer recommendations. The first is that the proportion of cations in the soil is not easily changed, particularly in soils with high cation exchange capacity. It takes huge additions of calcium or magnesium to raise the contents of either.

The second limitation is the lack of evidence of any change in crop yield or quality in response to a particular percent base saturation. The usual justification for using base saturation is anecdotal evidence that soils at particular base saturations grew good crops. In trials where direct comparisons were made between treatments intended to produce different base saturation percentages, there were no significant differences between treatments as long as the soil pH was adequate, and there was adequate supply of each nutrient.

The third, and possibly the greatest practical limitation, is that a normal soil test does not actually measure the cation exchange capacity. The lab adds up the amount of calcium, magnesium and potassium extracted from the soil sample, and adds a factor for hydrogen ions based on the buffer pH. The approach provides approximately the same answer as a direct determination of cation exchange capacity in acid soils, but errors appear in alkaline soils. Most alkaline soils contain calcium and magnesium carbonate compounds, which dissolve in the extracting solution. The net result is a much higher measured amount of calcium and magnesium, and apparently larger cation exchange capacity. If you are trying to raise the potassium level to 3% of the CEC, you would be adding fertilizer to balance the amount of undissolved limestone in the soil.

Is % Base Saturation Useful at All?

There are some situations where the percent base saturation provides useful information. Where the amount of magnesium in the soil is very high (equal to or greater than the amount of calcium), for example, there can be serious problems with soil structural stability. It is also a good check on the lab, since any sudden changes in % base saturation from previous samples likely indicates a problem in the lab or with the sampling.

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