Agriculture Reference
In-Depth Information
the M3 test, approximately 1/2 as many.This soil also has an excess of K and Mg
and a deficit of Ca.
Note that the Ideal Soil ratios in the third column are changed significantly from
the "ideals" for a soil with a pH <7. Rather than a ratio for Ca:Mg:K:Na of
68:12:4:1.5 or 70:15:4:1.5, with 10 to 15% exchangeable H+, 100% of the CEC is
accounted for by the base cations. Our working "ideal" BCSR ratio for a soil >7
pH is
83.5% Ca 10% Mg
5% K
1.5% Na
To calculate the desired amounts of P, B, Fe, Mn, Cu, and Zn, we use the amount
of K at 5% of CEC. Ideal K for a soil with a CEC of 10.26 is 200ppm, so Ideal Pwill
be 200ppm, Fe 100ppm, Zn 20ppm etc. The amount of Sulfur desired can be
higher than K x 0.50 in order to help displace any out of balance cations (the
excess K and Mg in this case), from the exchange sites.
If the CEC number calculated from the Mehlich 3 test had been used, the "ideal"
amounts of secondary minerals would have been too large. Ideal K at 4% of
19.06meq would be 297ppm, at 5% of 19.06meq, 372ppm.
At first glance this soil appears to have enough Ca potentially available. The M3
test found 2256ppm Ca, more than enough to bring the Ca up to the desired
1713ppm (83.5%), but apparently that Ca is not soluble or available in this soil, so
we would want to raise the level of soluble Ca in the soil with the aim of replacing
some of the excess exchangeable K and Mg with Ca.
The Soil Rx written for the Walnut Grove soil called for 1525# of gypsum CaSO 4 .
At 22%Ca and 16% S, 1525# = 336#Ca and 224#S, or 168ppm Ca and 112ppm S
added to the plow layer.
According to the Ideal Soil Chart, B should = Ideal Ca x .001. Boron in the Walnut
Grove soil was measured at 1.36ppm by the M3 test. The Rx called for 10# of
borax 9%B, which adds 0.45ppm B and raises the total B to 1.81ppm, a little more
than 1/1000 th of our desired Ca saturation of 1713ppm.
Example 2: Mehlich 3 vs AA8.2, Center Point Texas
The next example is a limestone-derived soil that is common in centralTexas. The
grower wanted to put in a new USDAOrganic orchard and vegetable garden. The
Mehlich 3 test extracted 13815ppm Ca. 13815 / 200 = 69.08meq of Ca.TheAA8.2
test found only 1932ppm of exchangeable Ca. 1932 /200 = 9.66meq of Ca.
Total CEC dropped 84% from 71.35 to 11.15.
 
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