Soil testing is a valuable management tool that can be used to make
fertilizer recommendations. One approach used to make
recommendations based on soil testing is the "sufficiency level"
approach. This approach is built on the concept that there are
certain levels of plant nutrients in the soil that are defined as
optimum. This is the approach that the Manitoba Provincial Soil
Testing Lab used. Usually a crop will respond to a fertilizer
application when the soil test value is below the defined level for
a particular nutrient, but response will be limited if above the
critical level.
Another approach that you may have heard of and is not commonly
used in most soil testing labs is the "basic cation saturation
ratio" (BCSR). In this approach, it is thought that maximum yields
can only be achieved by creating an ideal ratio of calcium (Ca),
magnesium (Mg), and potassium (K) in the soil. Little emphasis is
placed on other nutrients. These ratios have changed over the years
but the first recommendation was that 65% calcium, 10% magnesium, 5%
potassium, and 20% hydrogen should occupy the exchange complex.
These percentages calculate to 13 parts of Ca to 2 of Mg to 1 of K.
Fertilizer recommendations are very different between the two
approaches.
The basic cations (Ca, Mg, K) are known to have antagonistic
effects on each other. This means that if there is a very high level
of one of these nutrients it may reduce the availability of one of
the others. However, these interactions between nutrients are only
important when one of them is approaching deficiency.
The concept of optimum cation ratios has been debated over time,
but there is very little research evidence to show that these ratios
have either a positive or negative effect on crop production. There
is evidence that common field crops will tolerate a wide range of
cation ratios, but has no effect on yield as long as one of the
nutrients was not limited.
A major drawback of the BCSR approach is that a nutrient
deficiency may exist even though the ratio of cations is considered
optimum. For example, the total amount of exchangeable cations for
sandy soils that are low in organic matter is small. Although these
soils may have the ideal BSCR, the actual total amount of nutrients
available for plant growth is not sufficient, and therefore a
deficiency may result. Another important point is that on soils that
have a high level of carbonate minerals, an unrealistically high
potassium recommendation may result. This results from the fact that
if these minerals are dissolved by the soil test extractant, this
will release calcium and magnesium into the extractant, thereby
inflating the calculated CEC and the Ca and Mg percentages.
The most important consideration in making economic fertilizer
recommendations is to ensure that there is an adequate supply of
available cations in the root zone.
Here is some typical base saturation ratios from Manitoba:
Soil |
% Ca |
%Mg |
% K
|
% Na |
% H |
Total CEC |
Characteristics |
"Ideal" |
65 |
10 |
5 |
0 |
20 |
- |
- |
1 |
37 |
51 |
1.4 |
11 |
0 |
46 |
Heavy Clay |
2 |
66 |
28 |
5.7 |
0.36 |
6 |
16.8 |
Loamy Sand |
3 |
74 |
24 |
1.5 |
.86 |
0 |
39.9 |
Clay |
4 |
39 |
34 |
3.67 |
22.8 |
0 |
27 |
Clay Loam (Sodic) |
It is likely that soils 1 and 3 would receive a recommendation to
apply K if the BCSR approach was used. However, if the sufficiency
level approach were used, K would not be recommended, as the level
of K in these soils was over 200 ppm or 400 lb./acre (very high). |