Summary:
- The BCSR, or Basic Cation Saturation Ratio, is the idea that a particular (“balanced”) ratio of three basic cations (Ca, Mg, and K) is best for soil health and plant growth.
- The ratio defined by the BCSR was developed from ca. 1892-1979 based on historic studies with flawed conclusions and methodologies.
- The scientists who developed it claimed that the BCSR indicated the proper “balanced” or “ideal” soil, and deviations from this ratio could result in reduced plant growth.
- While soil base saturation data can provide useful information, the idea that there is an “ideal” soil or that soils are either “balanced” or “unbalanced” as a result of their basic cation ratios is not supported by evidence.
Outline:
- What is the BCSR, and how does it compare to other nutrient management approaches?
- What data were used to inform the development of the BCSR, and do they hold up to the scrutiny of later studies?
- Is the BCSR helpful in informing fertilizer application rates and nutrient management strategies?
The Basic Cation Saturation Ratio, or BCSR, has been used in soil science for decades to inform fertilizer recommendations and nutrient management strategies. However, soil scientists have long questioned its accuracy and usefulness. A 2007 review by Kopittke and Menzies found flaws in the methodologies used to develop the BCSR, and they also determined that recent studies to not back up the claims made about the BCSR and its usefulness in agriculture. In this lesson, we explore their findings and the role of the BCSR (if there even is one) in modern nutrient management decision-making.
What is the BCSR, and how does it compare to other nutrient management approaches?
The BCSR, or Basic Cation Saturation Ratio, is the idea that a certain ratio of three basic cations–Ca2+, Mg2+, and K+–is “balanced” and therefore essential for maximum plant growth. In fact, the scientists who developed the BCSR claimed that it was only at this particular ratio (usually defined as 65% Ca, 10% Mg, and 5% K) that maximum plant growth could be achieved.
The basic cations are essential to plant growth, as they are also plant nutrients. Deficiencies in any one nutrient can lead to decreased plant quality or yield. The goal of soil testing, in general, is to quantify the amount of plant-available nutrients in the soil to help inform nutrient management strategy choices.
After the nutrients have been quantified, the data must be interpreted in order to determine fertilizer needs and application rates. One common method of interpretation is called the “sufficiency method.” With this method, soil test results are compared to the nutritional needs of the plants or crops being grown on the soil.
The other method of interpretation is, of course, the BCSR. To use this method, the ratio of the concentrations of Ca, Mg, and K are compared to the “ideal soil” as outlined by the BCSR. From there, fertilizer is added to the soil to make the ratio of basic cations in the soil match the “ideal soil” ratio. For example, if the ratio of Ca:Mg:K is 45:20:8, fertilizer is added to the soil to make the ratio match the 65:10:5 ratio of the BCSR.
In short, the sufficiency method is based on matching the nutrients available in the soil to the nutritional demands of the plants growing in it, whereas the BCSR is based on matching the nutrients in the soil to a theoretically “ideal” ratio. Some people like to say that, because the BCSR isn’t based on the nutritional needs of plants, this method “feeds the soil, not plants.” The BCSR, however, has not been supported by research, which is something we’ll get into in the next section.
What data were used to inform the development of the BCSR, and do they hold up to the scrutiny of later studies?
The history of the BCSR begins as far back as 1892, when a scientist named Loew suggested that there was an optimal ratio between Calcium an Magnesium ions in soil. Loew, along with a scientist named May performed a study in 1901 and concluded that the optimum ratio of Ca:Mg was 5:4. This garnered a lot of attention and led to a lot of research.
In 1916, a scientist named Lipman found that, though many scientists had promoted the idea of an “optimal” ratio of the two elements in soil, maximal growth was achieved not at one ratio, but many. He concluded that there was no evidence to support an optimal Ca:Mg ratio with regards to plant growth. Similarly, Moser reviewed the literature again around 1933 and concluded that the amount of Ca in the soil was important–not the ratio of Ca:Mg.
In the 1940s, a man named Bear and his colleagues determined that, “for the ideal soil, … 65% of the exchange complex should be occupied by Ca, 10% by Mg, 5% by K, and 20% by H.” In 1948, Bear and Toth confirmed these preliminary findings in another study, but were careful to say that, while this was the “ideal soil,” maximum plant growth was achieved across a range of Ca:K ratios. In developing the “ideal” ratio, their aim was to limit the amount of luxury K consumption by providing a high saturation of Ca. Potassium is a much more expensive element than calcium, making it important to reduce the amount of unnecessary K consumption by plants.
In 1959, Graham stated that “the balance soil scientists recommend… is 75% Ca, 10% Mg and from 2.5 to 5% K,” but he also suggested that there were possible ranges for each of these elements at which plant growth was good. However, Graham did not make it clear how he determined the “balanced” ratio of elements, though Liebhardt published in 1981 that it was through modification of data collected from Bear et al. (1945), Bear and Toth (1948), and Hissink (1925).
While studying growth and N2 fixation of legumes, a colleague of Graham, Albrecht, concluded around the same time as Bear that it is important to maintain a high Ca saturation in soil. Later, in The Albrecht Papers (1975), Albrecht wrote that, for a “balanced” soil, 65% of the CEC must be loaded with Ca and 15% must be loaded with Mg.
Though decades of research led to the development of the BCSR, recent analysis of historical data finds significant flaws in the methodologies and conclusions made by past scientists. In many of the studies, pH is rarely controlled, making it a significant confounding variable. Application of calcium fertilizers (namely, lime) also increases the pH of the soil. Plants do exhibit sensitivity to pH, something that Albrecht deliberately ignored: in a 1975, Albrecht wrote “plants are not sensitive to, or limited by, a particular pH value of the soil.” Later analysis of Albrecht’s own data showed that this was false, and potentially led him to other incorrect conclusions.
In 1951, Giddens and Toth, in a study of ladino clover, determined that maximum yield could be obtained at any ratio of Ca:Mg:K provided that Ca was the dominant cation in the soil. Studies on various plants examining the effects of specific ratios of Ca:Mg also showed that there was no single optimum ratio for plant growth; it was only important that the Ca:Mg ratio be <1.
In 1948, Bear and Toth stated that the K:Mg ratio was more important than the Ca:Mg ratio. While it is true that large applications of K can limit Mg uptake in plants in certain circumstances, reexamination of the data collected by Bear et al. (1951) as well as more recent studies have concluded that the K:Mg ratio can vary as long as the absolute amount of K and Mg in the soil is sufficient to meet the needs of plants.
It was also suggested that “unbalanced” soils can lead to nutritional deficiencies and reduced quality in crops. However, the studies used to develop this idea did not track other potential limiting factors. As yield increased, so did crop quality, suggesting that the reduced crop quality was likely due to another factor that also limited growth. Some studies, in applying enough lime to achieve a “balanced” ratio, actually observed a decline in livestock productivity. We now know that “overliming” can cause declines in soil structure, limit P availability, and cause deficiencies in micronutrients, all of which can limit crop growth.
The idea of a “balanced soil” also led to the belief that the “ideal” BCSR would also cause improved plant growth as a result of improved soil structure. While the high Ca content of the “ideal” soil is beneficial to maintain good soil structure, good soil structure can occur across a range of ratios–not just 65:10:5 Ca:Mg:K. More important to soil structure is the amount of the basic cation sodium (Na) in the soil, as soils with two much Na (also called sodic soils) exhibit clay dispersion and sharp declines in soil structure and plant productivity.
Similar to soil chemical and physical properties, soil biological properties seem to be unaffected by the ratio of Ca:Mg in soil. Still, there is relatively little data on this topic, and more research would help determine whether or not there is one specific saturation ratio that would promote soil biological productivity and weed resistance.
In summation, the idea behind the BCSR–that maximal plant growth is only achieved at a very specific ratio of Ca:Mg:K–is a myth that does not hold up to the scrutiny of later studies and newer investigations of historical data. The conclusions used to develop the BCSR often did not take into account other important variables affecting plant growth.
Is the BCSR helpful in informing fertilizer application rates and nutrient management strategies?
Some people have been led to believe that the BCSR focuses on “feeding the soil” by replenishing the cations that were removed from exchange sites in the soil according to a specific ratio. In reality, this leads to overapplication of fertilizers, especially calcium, and can cost more money to obtain the same results as other nutrient management strategies.
When compared to the sufficiency level concept, utilization of the BCSR to inform fertilizer application rates often leads to far higher nutrient application recommendations. In one, 8-year-long study, the cost of fertilizer recommended according to the BCSR was double that recommended according to the sufficiency level concept.
There is little evidence to support the idea that the BCSR is better for soil health or plant growth. Matching the nutrients available to the needs of plants is a much more cost-effective strategy for fertilizer application, and avoids the risk of overapplication.
View Source: Kopittke and Menzies – 2007 – A Review of the Use of the Basic Cation Saturation
The Science of Soils 