Soil Testing, Interpretation, and Stretching Fertilizer Dollars

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Soil Testing, Interpretation, and Stretching
Fertilizer Dollars Eastern Extension Agent
Training, November 14, 2007
by Clain Jones, Extension Soil Fertility
Specialistclainj_at_montana.edu 406 994-6076
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Your Requested Topics(from Larry)

• Accurate soil testing and interpretation of test
  results. This should take into consideration
  fallow vs continuous crop, crop rotations,
  legumes, etc.
• Making your fertilizer dollars count

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Developing a Sampling Plan

• What sampling strategy (ex random vs zone vs
  grid) should I use?
• What depths should I sample?
• How many samples should I collect?

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Sampling Goal

• Determine an accurate average nutrient content of
  a field (or a zone) by collecting sub-samples and
  compositing them.

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Sampling Strategies

• Random Zone Grid

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Advantages or disadvantages of each?
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Sampling Strategies

• How determine zones?
• Group by
• Yield
• Remotely sensed indices
• Electrical Conductivity
• Nutrient levels
• Soil series
• Soil texture
• Topography

• Which of these are fairly easy/cheap to obtain?

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Accuracy

• Tight grid sampling (lt300 ft grid) is more
  accurate and produces higher yield than soil
  series sampling.
• Overall, grid and zone sampling are more accurate
  than random sampling.
• Topographic sampling is less accurate than tight
  grids, but more accurate than 5 acre grids.
• Recommendation First determine field variability
  with grid sampling. If field variability is low
  (lt 2-3 fold range), use grid sampling. If field
  variability is high, break into zones.

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How deep should I sample?

• 0-6 in. and 6-24 in. for nitrate (and chloride
  and sulfate if deficiencies in your region)
• 0-6 in. for phosphorus, potassium, zinc, organic
  matter, salinity, and pH.

Who has access to a soil probe or auger?
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What will decision on of subsamples to collect
per field depend on?

• 1. Desired accuracy and 2. Desired precision

If your desired accuracy is 25 and your desired
precision is 80, how many subsamples would you
collect per field?
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Does size of the field affect numbers of
subsamples to collect?
Field size does not matter much!
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What else do you want to know about soil
sampling/testing?
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Soil Test Interpretation

• New Montguide (MT200702)

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(No Transcript)
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What do Low, Medium, and High mean?
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Table 1. Use this table to compare results with
MSUs low, medium, and high levels.
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Fertilizer Recommendations
New Montguide (MT200703AG)
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Converting ppm to lb/ac (usually lab does this
for you, but not always)
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Fertilizer Recommendations

• Use EB 161.
• Nitrogen Need yield potential.
• Spring wheat 3.3 lb N/bu
• Winter wheat 2.6 lb N/bu
• Malt barley 1.2 lb N/bu
• Grass 25 lb N/ton
• Fertilizer N
• Available N (from table)
• - spring soil N (lb N/ac) (-25 lb N/ac if fall
  soil N)
• - 10 lb N/ac if previous crop was an annual
  legume, 40 lb N/ac if previous crop was alfalfa
• - 20 lb N/ac if gt 3 O.M. 20 lb N/ac if lt 1
  O.M.
• - ½ N between 2 and 3 ft.
• 10 lb N/ac per 1000 lb of residue/ac if N
  will be topdressed on stubble (up to 40 lb N/ac)

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How estimate straw residue?

• Need previous yield (bu/ac) and test weight
  (lb/bu)
• Grain yield (lb/ac) Yield x test weight
• Straw yield (lb/ac) Grain yield x 1.67
  (ww) Grain yield x 1.33 (sw) Grain
  yield x 1.13 (barl)
• Residue straw yield straw baled

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What will change for crop-fallow instead of
continuous crop?

• Yield potentialWill increase (30-60)
• Stubble correction
• Assume 50 of stubble has decomposed during
  fallow year (NRCS).
• Fall sampling N correctionDecrease by 50
  (Miller et al. 2006)
• (were studying Fall to Spring nitrate
  differences currently)

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Your turn How does fallow affect N rate
calculation?
Calculation Box 3 (assume lt 1 O.M.)
Assume also fall sampled, 44 lb N/ac below 2 ft.
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Your turn How does fallow affect N rate
calculation?
Calculation Box 3 (assume lt1 O.M.)
Assume also fall sampled
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Phosphorus and Potassium Fertilization Strategies

1. Sufficiency Approach Do you want to apply
   minimum necessary to maximize yield in most
   years? If so, use Table 18 (P) and Table 19 (K).
2. Maintenance Approach Do you want to replace the
   nutrients removed at harvest? If so, use Table
   21.
3. Build Approach Do you want to build your soil P
   and K, to minimize yield losses and save on
   fertilizer in future years? If so, add amounts
   from 1 and 2.

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Effect of Approach on P Rate
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QUESTIONS?
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How should grower adapt to high fertilizer costs?

• Subsurface band whenever possible
• Avoid applying urea during high risk conditions
  (see EB 173)
• Use sufficiency approach or maintenance approach,
  whichever is smaller
• Fertilize for maximum economic return
• HOW? Use economic calculator from Griffith and
  Jones, hopefully released by late winter.

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Effect of Available N on SW Yield
R2 0.19
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Yield and Protein Response Models

• Based on almost 100 data points from 1993 2006
  collected in Montana (mainly Golden Tri)
• Out of 12 independent variables (ex Jul Max T,
  Palmer drought severity index for May, Jun,
  July), only 4 explained most of variability
• Apr July Precipitation
• Avg Jun T How?
• O.M.
• Total Available N
• R2 0.69 for yield
• R2 0.68 for protein

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Effect of N on Spring Wheat Grain Yield following
Fallow
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Effect of N on Spring Wheat Grain Protein
following Fallow
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Effect of N on Marginal Return of Spring Wheat
following Fallow
How much would you fertilize?
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Effect of N on Marginal Return and Yield of
Spring Wheat after Fallow
Average conditions
Need 215 lb N/ac for Max Yield 3.6 lb N/bu Need
175 lb N/ac for Max Return 3.0 lb
N/bu Difference 40 lb N/ac or 0.6 lb N/bu
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Effect of N on Marginal Return and Yield of
Spring Wheat after Fallow
Average conditions
Low Discount
Need 215 lb N/ac for Max Yield 3.6 lb N/bu Need
165 lb N/ac for Max Return 2.9 lb
N/bu Difference 50 lb N/ac or 0.7 lb N/bu
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Effect of N on Marginal Return and Yield of
Spring Wheat after Fallow
Average conditions
High Discount
Need 215 lb N/ac for Max Yield 3.6 lb N/bu Need
175 lb N/ac for Max Return 3.0 lb
N/bu Difference 40 lb N/ac or 0.6 lb N/bu
Are there conditions/prices you would like to
test?
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Total Available N for Maximum Return on SW
following Fallow (lb N/bu)
Protein Discount (/0.25) Dry Average Wet
4 2.4 2.9 2.9
8 2.6 3.0 3.1
12 2.8 3.0 3.3
Based on 5/bu. At 7/bu, rates increase from 0
to 0.3 lb N/bu.
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Conclusions

• Grid and management zone sampling appear to be
  more accurate than random sampling.
• At least 8 subsamples should be collected per
  field to form a composite.
• N, P, and K recommendations can be made by
  knowing how to use EB 161 AND giving the grower
  some options (e.g. sufficiency vs. build)
• Rates of N fertilizer should be backed off from
  the recommended rate (e.g. 3.3 lb N/bu SW) when
  fertilizer price is high, commodity price is low.
• Fertilizer N rates should be about the
  recommended rate when commodity and protein
  premiums are high.

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For more information on soil testing and nutrient
cycling http//landresources.montana.edu/nm For
Duane Griffiths economic calculator that does
not yet include weather or protein premiums
http//landresources.montana.edu/soilfertility
Go to Fertilizer Economics on right hand bar,
then next-to-last link.
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QUESTIONS?