Nitrogen Slide Rule Instructions

On the slide across from the 3 at the bottom of side 1, the nitrogen recommendation is read opposite the average ppm. When there soil test results aren't available, use 3 ppm for fine textured soils and 1 ppm for coarse textured soils. For example, with 2% organic matter and 200 bu/ac yield expectation, the N rate should read 195. If the soil nitrate ppm is 9, the N rate should read 147. Other credits will need to be subtracted from this value, such as for soybeans (45) and irrigation water nitrates.

Note: if the soil nitrates are given in lbs N/ac, convert by dividing the lbs N/ac by the number of feet in the sample x 3.6. For example, if there is 100 lbs of N in 2 ft of soil, the 100 lbs is divided by 7.2 and there is an average ppm of 14 ppm (N rate is now 107); if the sample was from 3 ft, the number is 10.8 and there is an average of 9 ppm.

Side 2. Determining the corn:nitrogen price ratio (C:N).

Set the price per pound of applied N below the price expected for corn. The C:N ratio is shown at the black down arrow. For example, at $4.00 corn and $ 0.50/lb N the ratio is 8.

Set slide so that the up facing black arrow in window 6 is under the desired C:N ratio.

The N rate on the moving slide across from No. 6 is the N rate determined on Side One. Find the value on the moving slide and determine the value on the nonmoving scale. For example, at a C:N ratio of 8, both scales should line up so that the N rates are the same.

If the ratio is 5 ($4 corn: $0.50 N) the 195 lb N recommended N rate for 3 ppm soil nitrates from Side 1 is changed to about 152 lbs N. If the C:N ratio is 10, the same 195 lb N recommendation is now 215.