1 April 2000

Challenging views on soil

Soil scientist Johnny Jonhstons views on base cation saturation ratios under this title (Crops, 4 March) generated several letters. This selection covers most of the points raised.

The Plumb line

MUCH of what Mr Johnston says is of course pertinent as there are indeed a number of people offering to do a cation test without the correct knowledge of:

  a) How to do the analysis and

  b) How to interpret the results

However, I would like to raise the following points:

1. The current UK Nutrient Index System test method gives no indication of the soils ability to hold nutrients, no indication of the amount of each nutrient in the soil and no indication of the reason for the pH value of a soil.

2. The results from a cation test may indicate the need for additional fertilisers. However, in the ISS soil audit, we would not rely on this information solely to make recommendations for plant nutrient requirements (this is considered separately).

3. As Mr Johnston rightly points out, there are a number of "proponents jumping on the bandwagon" and I agree that many of them do not understand the science behind what makes a soil fertile and do not understand the principles involved in the cation exchange process.

4. As for the proof of how cations work, there is plenty of documented evidence to support the principles involved. In our case, some 3,000-plus samples from all around the UK have given us an excellent insight into how to use the analysis as a management tool to predict nutrient requirements, identify the correct type of fertiliser to apply and how and when to apply liming materials.

5. Mr Johnston is also right to warn of the difficulties involved in reducing excesses and in the case quoted, it is very unlikely that the calcium at 86% base saturation would be reduced by using sulphate of ammonia – nor would we expect it to.

6. As for the comment of high yields in a high magnesium soil, there is no doubt that high yields can be obtained. What is interesting is the challenge of achieving higher yields on lower inputs. This can only be achieved if a full soil analysis is conducted that identifies not only the cations and anions but also all of the micronutrients, the biology and the plant soluble nutrients. It depends on the results of this full audit as to what types of product we recommend.

Regarding the criticism of the proposed "ideal" ratio, this bears no relationship to the composition of the plant. This statement indicates a complete lack of understanding of the science behind the ratios and what the whole process of soil nutrient balancing is about.

Robert Plumb

Independent Soil Services, Hall Farm House, Back Street, Gayton, Kings Lynn, Norfolk

Antipodean angst

until June last year I spent 10 years advising over 500 farmers in New Zealand as an independent soil fertility consultant using the cation ratio approach.

The largest independent soil testing laboratory in New Zealand which started 15 years ago, has used CEC/BCSR as its standard procedure for measuring soil nutrients, and this lab is now used in preference by a large number of independent agronomists and fertiliser industry personnel compared to the Quick-tests offered by the traditional research institutions. This is because, when fully understood, the additional information from BCSR can be an invaluable tool for farmers to get more from their soil.

Mr Johnston claims that there is no modern research showing cation ratios affect plant yields. In New Zealand, soil scientist Dr Andrew Carran (AgResearch Grasslands) recently found that in pasture, white clover production was suppressed when calcium:magnesium ratios became too wide.

I also found the same thing occurring on a clients continually cropped maize ground (700 acres), where pH had previously been the only criteria used to determine lime requirements and the soil became saturated in calcium ions to the extent that yield was being reduced by calcium competing out magnesium. Once magnesium was applied to the crop, yields improved.

Mr Johnston says the BCSR system is not relevant to UK soils which are different to US soils. This same argument has been used by NZ boffins over the years to criticise