In my last column, I discussed soil pH and its effect on plant nutrients and fertilizers. In this article, more about basic, or alkaline, soils and how some agronomists are using base saturation to make fertilizer recommendations.
Remember that soils with a pH of 6.6 to 7.4 are in the neutral range, pH 7.5 to 7.8 are slightly alkaline and pH 7.9 to 8.4 are moderately alkaline.
Soils with a pH above 7.5 generally have a higher saturation of base cations (positively charged elements) that contribute to higher soil pH. These are potassium (K+), calcium (Ca++), magnesium (Mg++) and sodium (Na+).
Soils with a pH >8.4 usually have excess levels of Na and are sodic. Remember, the two cations that contribute to soil acidity are: hydrogen (H+) and aluminium (Al+++). The number of positive charges varies depending on the element. For example potassium has one positive charge but calcium and magnesium each have two positive charges.
Cations and the CEC
Cations are not very mobile is soil. This is because positively charged cations will attach to negative sites on clay particles and humus (highly decomposed organic matter). The combination of clay particles and humus are referred to as soil colloids.
Soil colloids have negative charges and hold positively charged cations on the negative exchange sites. Soil colloids play an important role by holding cations. Soils with higher levels of clay and organic matter usually have a greater capacity to hold more cations and these soils will be more fertile.
The amount of cations a soil can hold is termed “cation exchange capacity (CEC)”. CEC can be determined by a soil testing lab, but generally is not very helpful in developing fertilizer recommendations in Western Canada. Further, CEC level is mostly affected by the amount of clay in soil. Therefore CEC does not change from year to year, so there is no point in testing CEC each year.
The term “base saturation” is used to indicate the amount of negative sites on the soil colloids that are occupied by the base cations (K, Ca, Mg and Na). The acidic cations (Al and H) occupy the remaining exchange sites. Base saturation describes how completely the soil colloid surface is filled with the basic cations (Ca, Mg, K, and Na) versus all cations. Normally, base saturation is expressed as a percentage:
When all soil colloid exchange sites are occupied with base cations, the base saturation is 100 per cent, which occurs when soil pH > 7.0 (alkaline or basic soil). As soil pH declines below 7.0, Al and H occupy a percentage of exchange sites on soil colloids and base saturation percent declines.
Base-cation saturation ratio
In recent years, some agronomists have been using the “base-cation saturation ratio (BCSR)” approach to interpret soil test results to develop fertilizer recommendations. This approach attempts to balance specific soil cations according to varying ratios to balance nutrient levels in soil.
The “base-cation saturation ratio” term is used to indicate the “ideal proportion” of the exchangeable cation nutrients K, Ca, Mg and H in soil. This approach was proposed by soil scientist Dr. F. E. Bear and co-workers in in 1945 based on a greenhouse experiment in New Jersey. They suggested that in the ideal soil Ca, Mg, K and H should occupy 65, 10, five and 20 per cent of the cation exchange capacity, respectively. The approach was to make K, Ca and Mg fertilizer recommendations. The BCSR approach was not intended for recommending nitrogen, phosphorus, sulfur or micronutrient fertilizers.
Over the past 30 years there has been considerable field research studying and comparing the BCSR to the sufficiency level approach to making fertilizer recommendations.
In 2005, western Canadian researchers Dr. Adrian Johnston and Dr. Rigas Karamanos reported results of six trials on soils with sufficient plant available soil K concentrations, but low K saturation percentages, with no significant yield increase with wheat and barley from adding K fertilizer.
A very extensive review paper on BCSR was published in the Soil Science Society of America Journal in 2007. In the article, researchers Kopittke and Menzies reviewed the published research from numerous research studies on BCSR, including the early work by Bear. They concluded that: “within the ranges commonly found in soils, the chemical, physical, and biological fertility of a soil is generally not inﬂuenced by the ratios of Ca, Mg, and K. The data do not support the claims of the BCSR, and continued promotion of the BCSR will result in the inefﬁcient use of resources in agriculture and horticulture.” (Find the whole article online here.)
Research comparing the BCSR and “sufficiency level” approaches to making fertilizer recommendations has shown that plants are more sensitive to actual soil test K, Ca and Mg levels in soil versus using BCSR. A review of scientific research shows that the sufficiency level approach for plant available K, Ca and Mg in soil is superior to BCSR for predicting crop response to fertilizer.
Many soil testing labs and agronomists base their fertilizer recommendations on regional field crop response data. Ideally, using regional crop response research data calibrated with soil test nutrient levels has worked very well across Western Canada. The “sufficiency level” approach focuses on keeping plant-available soil nutrient levels within an optimum range so that each nutrient is above a deficiency level in soil. This has proven to be the best approach to making fertilizer recommendations for most western Canadian farms.