>In the 70's and 80's many commercial labs were using this philosophy or the
>soil build up and maintenance philosophy or a combination of both which
>almost always resulted in fertilizer being added even when it was not
>needed. This was also true with some land grant soil testing labs. Many
>land grant labs used the sufficiency level approach which basically says you
>test for a nutrient and if lacking you apply that nutrient.
This business of SLAN versus BCSR is an area of special interest to me as I
have been intimately involved in the development of a soil testing system in
New Zealand and was forced to take a serious look at the pros and cons of
the different methods. I favor the balance view, as a concept, even though I
claim to be reasonably au fait with its dubious background. Let me elaborate.
The question really is what happens if you DONT use balance as your basis?
In my opinion the assessment of individual cations by the sufficiency
approach can lead to problems of interactions between, for example,
potassium and other essential cations, such as magnesium and sodium;
particularly in grazed psture systems where the concern is as much for
animal health as it is for simply plant yield.
The argument I feel has unfortunately been one of EITHER the "sufficiency
level" approach OR the "balanced cation ratios" approach, when really a
combination is a much better solution. We have ample evidence of the
troubles caused when only the SLAN is used on our grasslands and we
therefore educate our students about the need to consider balance between
cations. Hence the BCSR is an attractive concept. It points our fertiliser
use in the right direction and forces a consideration of the whole complex
of ions. We also, for the same reasons, embrace the Sumner DRIS method which
attempts to take balance even further by providing a conceptual framework
for incorporating soil and plant analysis. DRIS and BCSR have many common
features. Neither have been thoroughly tested in this country, as yet
Obviously bringing the animal into the equation is a new issue, but in the
NZ situation we are often dealing with soil tests for pastures destined to
be eaten by a grazing animal; so you cant leave the animal out. Approaching
this with the notion of cation balance in the soil and in the plant does
suggest we are cognisant of the interactions and implications of our
fertilizer programs to all parts of the system.
I find the SLAN methods rather sterile and not so integrative. The idea of
studying each element in isolation is old science that has not really done
much for animal welfare in this country. I should add however, that SLAN is
still the basis on which many scientists and laboratories operate here; even
in the grassland situation.
I concur however that more research along these lines is needed. In our
case, it is unlikely to eventuate as funding for soil testing research is a
very low priority under current economic circumstances.
You asked for feedback so I have obliged. I will be interested to hear what
others contribute either for or against.
Dr Max A. Turner
Department of Soil Science
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