I have been asked a number of times: what does the value of %base saturation (%BS) mean on soil test reports? And can I apply K to my soil even though the soil test is high but the %BS of K is low?

Base Saturation (%BS): It is the percentage of soil exchange sites (Cation Exchange Capacity or %CEC) occupied by the basic cations Ca²⁺, K⁺, Mg²⁺, and Na⁺ (calcium, potassium, magnesium, sodium ions) in soil. Base saturation increases with soil pH.

Cation Exchange Capacity (%CEC): A soil’s ability to attract, retain, and supply cations to plants. These cations include the basic cations and acidic cations, aluminum (Al³⁺) and hydrogen (H⁺). Other soil cations include micronutrients as well. In soil, the surfaces of the clay minerals and organic matter are surrounded by negatively charged ions. Like a magnet, these negative charges attract positively charged ions (cations) like Ca⁺, K⁺, etc., which bind onto the surfaces and prevent nutrients from leaching ( i.e. carried by rainwater moving down into the soil, and away from plant roots). Most of the SOM in soil is within the top 6 inches and retains many of these cations.

Should I apply K to my soil even though the soil test is high but the %BS of K is low?...No.

The %BS concept suggests fertilizer application should be made of a given nutrient, e.g. K, if the soil test ratio of K (meq/100 g) to the sum of positively charged nutrients in the soil exchange site (CEC) is < 5% K. That is, an ideal soil CEC should contain 65% Ca, 10% Mg, and 5% K or a 13:1 Ca:K ratio. There is no justification of this concept which has been refuted time and again since it was suggested in the 1940’s. This concept will tend to overestimate fertilizer needs for North Dakota producers because the agricultural soils here have CEC’s mostly in the 10 -17 meq/100 g range and above (i.e. sandy loam, loam, silt loam, and clay loam soils).

The NDSU fertilizer recommendation is based on the concept of sufficiency level of available nutrient. This means there are optimum levels of a given soil nutrient (or narrow range) above which plants will not respond to fertilizer application. In a wheat field for example, if your K soil test is >100 ppm (200lbs/a) applying K₂O will likely be uneconomical. For corn in North Dakota, a soil test above 160 ppm K is very high. Below 160 ppm, modest application of 20 – 30 lbs. K₂O may be beneficial especially in cool spring.

Conclusion: In ND, do not make fertilizer application decisions based on the % base saturation. 

North Dakota fertilizer recommendation tables accessible at:

http://www.ndsu.edu/fileadmin/soils/pdfs/sf882.pdf

On-going Research

We are evaluating wheat response to K rates of two wheat varieties of high (Sy Soren) and low (Barlow) resistance to lodging. The %BS of K was below 3.4 at both locations, and soil tested in the medium to high range in Carrington and Dazey, ND. Preliminary evaluations show that the variety with less resistance is lodging more following heavy rainfall events and irrespective of amount of K applied at both sites. Biomass dry weight is showing little response to K rates as well.

Wheat field (soil test in medium range) in Dazey, ND treated with 0, 25, 50, or 75 lbs. K. No lodging response to K.

Note: Low CEC soils indicate low clay and low OM. You can at least manage your soil OM. To increase CEC, apply manure to raise OM content.

Application of total required K and other fertilizer nutrients to a sandy soil with low CEC entails high risk of nutrient loss. Split application of fertilizer is recommended in these soils.

Jasper.Teboh@ndsu.edu
Soil Scientist