Interpreting Soil Test Reports and Making Fertilizer Recommendations

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Interpreting Soil Test Reports and Making
Fertilizer Recommendations MT Assoc of Ag
Educators 2007
by Clain Jones, Extension Soil Fertility
Specialistclainj_at_montana.edu 406 994-6076
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Objectives

• Give a very brief overview of nutrients needed by
  crops
• Explain how to interpret a soil test report
• Determine fertilizer needs for a given soil test
  report

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There are 14 mineral nutrients that have been
found to be essential for growth of most plants
The macronutrients are simply needed in larger
amounts by the plant than the micronutrients.
Nutrient deficiencies of the bolded nutrients
have been observed in Montana
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Why do farmers have their soil tested?
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What should you or your students first look for
on a soil test report?

• Depth should have at least a 0-6 in. section
• Nitrate-N Is it in lb/ac or ppm? If in ppm, you
  need to convert to lb/ac 2 x ppm x depth/6 in.
  and add up separate depths.
• Is phosphorus measured as Olsen P or Bray P? (MSU
  guidelines are for Olsen P and there are not good
  conversions between the 2).

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What else should I and my students look for?
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Any red flags here?
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Fertilizer Recommendations

• Use EB 161. Point out Guidelines
• 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)

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• Example
• Winter wheat
• Yield potential 40 bu/ac
• Soil test N 54 lbs/ac (top 2 ft.)

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If 50 lbs per acre of N needed, how much urea
(46-0-0) is needed?

• The 46-0-0 means this fertilizer is 46 N, 0
  P2O5, and 0 K2O. So the fraction of N in urea is
  0.46 (46/100).
• N fertilizer (50 lbs/acre) 0.46
• 109 lbs urea/acre

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Phosphorus
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Phosphorus
Example Winter wheat Olsen P 10 ppm P2O5 needed
42.5 lb/ac
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How much MAP (11-52-0) do you need to get 42.5 lb
P2O5/ac?
The 52 means MAP is 52 P2O5 so fraction is 0.52

• MAP 42.5 lb P2O5/ac 0.52

MAP needed 85 lb/ac
Potassium table (Table 19) and calculations are
essentially identical to P
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Questions so far?
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Your turn!!!

• Use Fertilizer Guidelines-Keep in mind these are
  guidelines-may need to adjust for your region and
  field history. (If you know you wont use again,
  please return)

Crop Spring wheat Yield pot. 50 bu/ac Soil N
35 lb/acOlsen P 14 ppmSoil test K 200 ppm
130
25
40
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QUESTIONS?
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Your turn againHow much anhydrous ammonia
(82-0-0) do you need to meet your N requirement?
Fertilizer needed (130 lb N/acre)/0.82
159 lb/acre
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Conclusions

• Given some criteria, soil test reports can be
  quickly evaluated for potential problems.
• N, P, and K recommendations can be made by
  knowing how to use EB 161 (or similar
  guidelines).
• Hopefully, this overview will help you better
  assist your students.

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For more information on soil testing, nutrient
cycling, and fertilizer calculations
http//landresources.montana.edu/nm MSU Soil
Fertility webpage http//landresources.montana.ed
u/soilfertility