Agriculture Reference
In-Depth Information
Soil Test Sample
Nutrient
Units
Ideal
Actual
K
%
2-5
2.2
Ca
%
60-70
36.6
Mg
%
10-20
11.6
Na
%
0.5-3
0.2
CEC
mEq/100g
7.7
The soil test gives us a CEC number. A sandy soil will usually have a CEC of between
0-10, while a clay soil will often be from 20-50. You can see from the equation that a lower
CEC soil is going to require a smaller application of fertilizer. This makes sense because a
sandier soil can't hold as much nutrient, so if we add too much it will just leach through.
The value from our soil test is 7.7.
Let's break down the rest of the equation using calcium as an example. We would need
400 pounds per acre of calcium to occupy 100% of the cation exchange sites in a soil with
a CEC of 1. That number never changes. The math that went into determining this number
is beyond the scope of this topic, but it involves looking at the weight of the soil and the
weight and molecular structure of the calcium ion. The second number in our equation,
therefore, is 400.
To do this part of the calculation for the other main cations, we would use 240 pounds
for magnesium, 780 pounds for potassium and 460 pounds for sodium. The phosphorus to
potassium ratio should be 1:1 in most soils, so phosphorus is 780 pounds. The sulfur num-
ber is often put at half of phosphorus, so 390 pounds. You don't have to memorize these
numbers, as they're right here.
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