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Soil Fertility and Sampling
Ron Meyer, Area
Extension Agent - Golden Plains Area
Date: 3/2/2011
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Soil fertility is important
for high yield potential as well as maintaining plant health and integrity.
Pre-plant is a good time for soil testing for the immobile nutrients,
phosphorus (P) and potassium (K) and one of the mobile nutrients N. Adequate P
and K and N are key to maximizing yield potential for 2011 and
beyond.
Soil Tests. If there is concern about fertility, especially due
to very high or very low yields in 2010, soil test to aid with fertility
decisions.
Key items to consider when soil sampling, are timing, depth,
and tillage systems. To increase consistency, try and sample fields the same
time each year. But testing could be done anytime. Take soil cores from the top
6 inches minimum down to 2 feet.
Fertilizer applications should be based
on the values received from the soil test (Table 1). Fertility test results
that show adequate for the crop to be grown may only require a maintenance
level of fertilizer. Lower test results will require “build up”
amounts of fertilizer along with the maintenance amount. Always consider
residual fertility from previous crops and manure applications when determining
fertilizer application amounts.
Soil pH. Soil pH can greatly impact crop
growth. High pH is difficult to lower, but awareness allows for management of
associated risks. High soil pH can reduce availability of iron (Fe), manganese
(Mn), boron (B), copper (Cu), and zinc (Zn). High pH can also increase
carryover potential of sulfonylurea (SU) and triazine herbicides.
Cation
Exchange Capacity (CEC). CEC is the ability of the soil to attract, hold, and
release cations or positively-charged particles. Plant nutrients in soil
solution exist as cations that can bind to soil particles that are negatively
charged anions. The CEC in a soil varies by the amount of clay and organic
matter present. Higher amounts of clay and organic matter produce higher
CEC’s, which have a greater capacity to hold nutrients. The CEC is
expressed in milligram equivalents per 100 grams of soil
(meq/100g).
Phosphorus and Potassium. Soil fertility levels for P and K
are greatly affected by the inherent availability in the soil and by crop
removal. Each bushel of corn harvested per acre, removes the approximate
equivalent amounts of 0.37 pounds per acre of P2O5 and 0.27 pounds per acre of
K2O (Table 1). These amounts are important when calculating the amount of
maintenance or build-up fertilizer to apply. Crops cut for silage and stalks
that are chopped and bailed remove more nutrients because the majority of the
above ground tissue is harvested.
As yield potential increases, managing
soil fertility is crucial. Low P levels can result in stunted, purple corn. Low
K levels can result in yellow leaf margins in corn and soybean. Even if soil
levels of P and K are adequate, deficiency symptoms may appear due to soil
compaction and restricted root growth causing a temporary nutrient deficiency.
Additionally, low K levels can increase stalk lodging potential and affect
drought tolerance, as K is critical to plant cell structure and plant water
pressure.
As yield potential increases, it is critical to manage soil
fertility.
Sources: Growing Knowledge.Monasnto. Corn & Soybean
Field Guide. 2007 Edition. Purdue Crop Diagnostic Training & Research
Center. Purdue Pest Management Program. Purdue University.
Hoeft, R.
and T. Peck, 2002. Soil testing and fertility. Illinois Agronomy Handbook
2001-2002. P 90. University of Illinois.
J. Saywer. 2003. Drought
impacts on soil fertility. Integrated Crop Management. Iowa State University.
Soil Fertility Manual. The Potash & Phosphate Institute.
Sampling
soils for nutrient management. 10/05. SD-NRCS-FS-50. USDA NRCS & South
Dakota State University. http://denr.sd.gov. (verified 9/13/10).