By Ross H. McKenzie PhD P. Ag.
Soil testing labs and agronomists use various philosophies and techniques to make fertilizer recommendations to their clients. Many soil testing labs and agronomists utilize regional field crop response data on which to base their fertilizer recommendations. Some agronomists estimate crop nutrient uptake and removal to make fertilizer recommendations. Ideally, using regional crop response research data calibrated with soil test nutrient levels has worked very well across Western Canada. This “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.
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.
To understand the pros and cons of this approach we need to review a bit of soil chemistry. First, cations are positively charged elements that occur naturally in soil. Common cations in soil that contribute to higher soil pH (basic soil) are: potassium (K+), calcium (Ca++), magnesium (Mg++) and sodium (Na+). 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, K has one positive charge, but calcium and magnesium each have two positive charges.
Soil colloids are particles in soil that are mostly made up of clay and highly decomposed soil organic matter. Soil colloids tend to have a negative charge and hold positively charged cations on the negative exchange sites. Soil colloids are very important because most of the positively charged soil nutrients are held on the surface of soil colloids. 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) of soil. It is important to note there are two main types of clays in soil. Smectite clays have a relatively high CEC, hold water well, and will expand and contract when wetted and dried. Kaolinite clays have a very low CEC, and do not swell and contract when wetted and dried. Smectite clays are typically the dominant type of clay in western Canadian soils. It is for this reason that medium to fine textured soils (loam - clay loam - clay soils) typically have moderate to high CEC, and have a good capacity to hold positively charged soil nutrients. Soils that tend to have mostly kaolinite clay are typically very old, highly weathered. Soils with higher kaolinite clay are found in areas such as the southern areas of the United States, areas in Australia and tropical soils.
The term “base saturation” is used to indicate the amount of negative sites occupied by base cations (K, Ca, Mg and Na) on the soil colloids. The acidic cations (Al and H) occupy the remaining exchange sites. Therefore, 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 is above 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 per cent declines.
The BCSR term is used to indicate the ideal proportion of the exchangeable cation nutrients K, Ca, Mg and H. This approach was proposed by Bear and co-workers in 1945, based on a greenhouse experiment in New Jersey. Bear et al. (1945) suggested that in the ideal soil, Ca, Mg, K, and H should occupy 65 per cent, 10 per cent, five per cent and 20 per cent of the cation exchange capacity, respectively. The approach was intended to make K, Ca and Mg fertilizer recommendations. The BCSR approach was not intended for recommending nitrogen (N), phosphorus (P), sulphur (S) or micronutrient fertilizers. Their approach might apply to soils that are dominated with kaolinite clay, have a very low cation exchange capacity, very acidic soils and soils that are highly weathered. Soils with all these characteristics would be extremely rare in Western Canada.
The sufficiency approach to fertilizer recommendations
Over the past 30 years, there has been considerable field research to study and compare the BCSR to the sufficiency level approach to making fertilizer recommendations. Below are a few examples.
In Nebraska, fertilizer recommendations using BCSR were compared with those from the sufficiency level approach. Over nine years, Olson et al. (1982) compared BCSR and sufficiency level approaches for making fertilizer recommendations. They concluded that cation balance in soil is not an essential consideration in estimating crop nutrient needs. They also stated: “The nutrient sufficiency approach to soil testing, when adequately calibrated, promised the surest method of achieving most economic yields while conserving non-renewable resources and preserving environmental integrity.”
Rehm and Sorensen (1985) reported that adjusting K:Mg ratios did not affect corn yields in Nebraska.
In Western Canada, Johnston and Karamanos (2005) reported results in 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, published in the Soil Science Society of America Journal by Kopittke and Menzies (2007), 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.”
In summary, after review of research comparing the BCSR and sufficiency level approaches to making fertilizer recommendations, it appears that plants are more sensitive to actual K, Ca and Mg levels in soil versus the cation ratios present in soil. Generally, a review of scientific research would suggest the sufficiency level approach for plant available K, Ca and Mg in soil is superior to base cation saturation ratios for predicting crop response to fertilizer.