Soil Fertility Guide
Appendix
Developing a Fertilizer Recommendation Rate
The same principle can be used in reverse to estimate a fertilizer N recommendation as follows:
Step 1: Determine crop need — based on expected yield and removal rate
Step 2: Use the estimate of soil nitrate levels
Step 3: About half of this available nitrate-N is retuned to the organic pool
Step 4: Amount mineralized from organic matter (as before in Step 2 and 3)
Step 5: N immobilized by crop residue (as before in step 6)
Step 6: Calculate N need (difference from above)
Step 7: Calculate N fertilizer need based on efficiency (usually about 50%)
Adjustments may need to be made to these estimates based upon environmental conditions that may cause greater than expected nitrogen losses. These conditions would be leaching of nitrate-N on sandy soils and denitrification when soils are saturated. Adjustments may also be required where release of N from soil organic matter may be higher or lower than average.
For further information, contact your MAFRD GO Representative.