Soil Fertility Guide


Developing Fertilizer Recommendations without a Soil Test

Growers may choose not to soil test every field every year and yet still need to develop a fertilizer recommendation. They can consider the following approach in making a recommendation 101 . General fertilizer recommendations without a soil test are provided in Appendix Table 23.

The approach is based on drawing a balance between inputs and outputs during the previous growing season. Any positive excess in the balance can be considered as a soil test value for next year. This method works only for nitrogen, since soil phosphorus and potassium soil tests are meant to measure both 'available' and 'potentially available' levels and, in any event, change slowly; therefore, the same soil test can be used for 2-3 years.

Consider all inputs and outputs:

 General fertilizer recommendations without a soil test.

Focus on those that are most important and cannot be controlled (for example, volatilization is gaseous loss of ammonia/urea and can be controlled by banding the N fertilizer). This example does not involve legumes/pulses. The previous years soil test goes under inputs. So, now we have:

Starting with the inputs:

Step 1: You need to start from last year's soil test. This soil test must be for a 0-24" depth; if not, you need to estimate a 0-24" depth soil test, so multiply results from 0-6" by 2 or 0-12" by 1.5 (remember this is only an estimate!)

Step 2: We now need an estimate of N mineralized (released) from soil organic matter during the growing season. Mineralization amounts are dependent upon quantity and quality of soil organic matter, crop residues and microbial activity driven by soil heat and moisture. For organic matter levels less than 8%, an average estimate can be made by multiplying the % organic matter from the soil test by 14.

Step 3: The plant roots do not reach 100% of the mineralized N and whatever the plants roots don't reach the microbes do; some of it could be potentially lost out of the system. In any event, on average, 20% of mineralized N is normally left in the soil as "available" N.

Step 4: The final input is the "actual" N that was applied as fertilizer to the previous crop.

Step 5: On average, a crop consumes 50% of the fertilizer and microbes immobilize or consume about 20-25% of this applied N. Approximately 25% of fertilizer N is available for the following crop.

Now, let's examine the outputs:

Step 6: Unless a crop is to be seeded on fallow, soil microbes will utilize some of the "available" N in the soil to break down the straw from the previous crop, a process that is known as immobilization. An average estimate of 30 lb N/ac is reasonable.

Average leaching and denitrification losses are very low (normally less than 7%) under normal conditions. If "abnormal" conditions prevailed during the previous year, it is strongly recommended that an "actual" soil test be taken.

Step 7: You finally need to account for the amount of nitrogen removed in the crop. You can use crop removal tables ( Table 1) to arrive at an estimate of N uptake and removal.

Step 8: Estimated soil test

Now, let's put all of these inputs and outputs together (same as you would write cheques and deposits in your chequing account):

Example in Estimating a Soil Test.

For further information, contact your MAFRD GO Representative.