soil 2

Title: soil 2




Mike,
The fertility level and other associated chemical characteristics
depend, in part, on soil texture and type.  Crop to be grown also
influences what level of nutrients are considered 'optimum'.  However, soil
testing and associated fertility recommendations originated to solve local
crop problems and how/what tests are done as well as how they are
interpreted depend on local/regional experience/research data.  Numbers
from the UW Soil and Plant Analysis Lab would not be comparable to labs in
distant locals such as CA or FL.  The UWEX Publication A2809 fully
describes the fertility recommendations and philosophy of the Soil Science
Dept, UW-Madison.  I encourage you to contact either the UW Soil and Plant
Analysis Lab, 262-4364 or the UWEX Publications office, 262-3346 or your
county extension office for a copy.

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Mike,
Your question is simple enough, but the answer is not quite so clear cut.
Our laboratory is located in Washington state.  Different methods of
extraction and different units for reporting results are used in different
parts of the world.  Also, we in the western US have a little different
approach to soil fertility than you would encounter in, for example, the
Midwest or eastern US.  If you could tell me to what part of the country
your questions are related, my answer would be a little easier and possibly
make more sense to you.


Hi,

     Your request about soil test levels was forwarded to me so I'll
try and give you some numbers. First, it's difficult to come up with a
number for some of these things but here are a few numbers that could be
considered optimum for corn or soybean production:

P Bray1 (weak Bray)  ~20 ppm
Ammonium acetate extractable K ~ 100 ppm
CEC ~ 20-30 meq/100g soil
Ca ~ 60% of CEC. Ca is usually in the 2000-3000 ppm range
Mg ~ 20% of CEC. Mg is usually in the 200-300 ppm range
Many organic growers, for example, may be more interested in the Ca/Mg
ratio than absolute values
K ~ 5% of CEC
Na, I don't have a number but in Iowa soils probably less than 10 ppm

N, too many factors go into N availability. For the late spring nitrate
test for corn we consider 25 ppm adequate for the crop.

pH ~ 6.3-7 depending on the crop. Some crops need acid pH. Nutrient
availability is generally best at pH 7 for all nutrients.

Organic matter in the 5%-10% range is normal for Iowa soils.

Micronutrients:
B by the hot water method ~ 1ppm
Mn by dilute phosphoric acid method ~ 11-20 ppm
Zn by DTPA-TEA ~ 1 ppm
For Fe, Cu, and SO4 we don't have reliable numbers.

These numbers are really ballpark. I'm sure you would get different
opinions from different people. Anyway, I hope this helps. Let me know if
you have any questions.

Don


Donald G. Wetterauer
Iowa State University
Soil and Plant Analysis Laboratory
G411 Agronomy Hall
Ames, IA  50011
Phone: 515-294-6757
FAX: 515-294-5567

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