PO 45: Determine crop nutrient needs by using:

  1. Yield potential
  2. Crop rotation/sequence
  3. Soil nutrient supply
  4. Soil test information
  5. Field history
  6. Pre-sidedress N test

N recommendations in the Northeast are not based on a soil nitrate (NO3-) test, as nitrate is very mobile in the soil and is subject to leaching.  Crop needs are usually derived from information about the yield potential of a field (ideally based on historic yield data), its place in the crop rotation, estimated soil N supply from mineralization of organic matter, and management history of the field, especially when manure has been applied in previous years.  The final recommendation needs to be adjusted for N fertilizer uptake efficiency, recognizing that not all N applied will end up in the crop.

For instance, here is a calculation for N requirements for corn in New York state.

  • N (lbs/acre) = ([yield potential (bu/acre) x 1.2] – soil N supply – sod N supply)/N uptake efficiency

  • For example: third-year corn after alfalfa; no manure in previous three years.  Tiled and drained lima soil; yield potential = 140 bu/acre; N uptake efficiency = 70%; soil N = 75 lbs/acre; sod N = 10 lbs/acre.

N = ([140 x 1.2] – 75 – 10) / 0.70 = 119 lbs/acre

 


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