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.