Soil Analysis and cation ratios (BCSR) - a review

Background

This note is intended to clarify the value of cation measurement of soils and the use of Cation Exchange Capacity values (CEC) and Base Cation Saturation Ratios (BCSR).

Regular soil analysis is an indispensable requirement of nutrient management and fertiliser recommendations. The complexity of the soil means that this is a practical and not an exact science. New ways of improving our understanding and ability to predict nutrient requirements should continually be sought. On the one hand, analytical technology is developing to permit new assays and greater accuracy of analysis whilst global positioning and mechanised coring are advancing opportunities of sampling. But these opportunities are limited by the agronomic ability to interpret results into meaningful and economic practice. It is important that technology is not adopted "just because it is there", rather than because it is useful.

BCSR

Recently a new direction of analysis has been promoted in the UK involving measurement of the cations, calcium and sodium, as well as the normal potassium and magnesium and examining the ratio of each of these with the total cation exchange capacity (CEC) of the soil. This approach was developed in the US in the 1940s and taken up extensively by US commercial labs in the 1970s. The concept is that an optimum ratio of basic cations (calcium, magnesium, potassium and sodium), and an optimum total base saturation (the % of the CEC of a soil covered by base cations) exists for optimum plant growth and soil conditions.

The American Society of Agronomy (ASA) reviewed methods for soil testing and analysis in 1977. They compared the BCSR technique with the more traditional Index system of available nutrients. This is the system used in the UK in which fertilisers and manures are applied to maintain a desired level of each nutrient in the soil, set by many years of field experiments and response curves. The ASA concluded that there were insufficient results to confirm either that a best base cation saturation ratio or a best total base saturation exists. Results from trials in the US in the early 1980s found no evidence to support the validity of the BCSR concept as a basis for managing nutrients for maximum crop yield. The best approach was to maintain sufficient, but not excessive, levels of each cation. There are no published results relating the BCSR concept to yields of crops in the UK or justifying the economics of the approach for our conditions - BCSR analysis tends to be expensive.

The US assessors regarded the BCSR concept as most applicable to highly weathered soils of low pH requiring relatively major adjustments in fertility and where high Mg levels need to be maintained - soils on which the technique was first developed. As there are few such soils in the UK, it is questionable as to how applicable this is to the majority of UK soils.

Cations in UK soils

Most UK soils contain sufficient calcium for optimum growth of the major crops especially where pH is maintained at satisfactory levels by calcitic materials. On many chalk and limestone soils where the very high calcium saturation ratio is indicated by very high pH, the Ca cation balance cannot be economically altered because of the nature of the parent material.

Potassium and magnesium are measured by normal soil analysis as available concentrations which are well correlated with crop yield from many response trials. Requirements are thus derived from measurements of concentrations but imbalances of these two nutrients which may have adverse effects on crop growth can be identified and responded to from these values. Yield response to sodium is very crop specific and soil concentrations where response may be expected are also well documented for UK soils. Excess levels of sodium are very rarely found in UK soils - normally only as a result of sea flooding.

Soil Structure

Those promoting the BCSR approach place considerable emphasis on the importance of soil structure with claims that adverse cation balance can be the cause of poor structure. It is known that excess sodium such as in sea flooding causes structural damage but not in the quantities used for crop response or where it is used to modify herbage mineral content for better balance in the grazing animal. Claims that magnesium can have similar adverse effects are not supported by evidence. Addition of calcium (lime) generally improves structure but must not be allowed to increase pH to levels at which trace elements availability is reduced. Whilst potash levels vary widely in soils there is no documented evidence linking soil K with structure. Soil structure does not always receive the attention it deserves in practice and certainly needs to be emphasised as an integral part of soil management whatever system of nutrient measurement is being followed. However structural problems are normally the result of inappropriate cultivations and physical handling of the soil. Cation balance is not a key factor for most UK soils.

Biological activity

It is also proposed that cation ratio management can improve biological activity in soil leading to an overall improvement in nutrient availability and a decreasing dependence upon fertiliser additions. This is a difficult concept to prove as the measurement of biological activity is far from straightforward. The proof of conventional nutrient management exists in the ability to maintain and improve crop yields and quality in the long term and avoid wasteful loss of cations when using the established "index" system. Soil biology is of course affected by a large number of factors - notably rotation and crop residue management and these are of much greater importance than nutrient balances. In respect of soil structure and biological activity it should be noted that the importance of these factors is already reinforced by the established integrated approaches. Though there is room for improvement, these factors have previously been ignored. Some advocates of BCSR also claim that the chloride form of potash damages biological content or activity in the soil. It is suggested that chlorine gas, hydrochloric acid or hypochlorite are formed in the soil when potassium chloride is added but this is categorically refuted by experienced soil scientists. In view of the fact that this form of potash represents 90% of all potash used throughout the world with no signs of adverse effects it seems an unlikely proposition. No factual evidence has been produced in support.

Interpretation

A recent authoritative reference book "Soil Analysis - Handbook of reference methods" published in the US by the Soil and Plant Analysis Council states -"the actual method of soil analysis is relatively unimportant, compared to the interpretation of the obtained analysis result...........If the laboratory analysis result is well correlated with crop response or yield, the method of obtaining the analysis result is of miner concern......." This clearly places the emphasis on the correlation of analysis results with crop yield and performance. In the absence of such information for new analytical systems in the UK, such approaches cannot be recommended for general adoption to replace conventional soil analysis or even to supplement it.

Conclusion

The decision facing farmers offered alternative analytical systems is whether there is proof that the expenditure on analysis and the cost of the recommended treatment is worthwhile in the long term. If such proof is available, new techniques which supplement and improve on existing approaches are to be welcomed.

update: March 2001