Available organic nitrogen
Organic nitrogen is not available to the crop until it has been mineralized to the ammonium form by microbial action. The speed of mineralization increases with warm temperatures and adequate moisture, which promote microbial activity and will slow down to almost no activity when soil temperatures approach freezing.
Nitrogen from solid manure applied just before planting may not be available to a crop in time to meet the crop requirements. The influence of soil microbial populations and weather conditions in mineralizing nitrogen is important.
A combination of manure nutrient analysis, organic carbon (OC) and C:N ratio is used to calculate the available organic N. It is generally assumed that organic matter is about 58% organic carbon (% OC x 1.72 ~= OM). The equation used to estimate the available organic nitrogen in Ontario is shown in Table 1 with separate equations for liquid and solid manure. estimates the portion of organic N that will be available for crop uptake. The availability is estimated by determining how much nitrogen the soil microbial populations would require to mineralize all the N in the manure. If the manure contains more N than is required by the microorganisms, then the surplus is released. If there is less N in the organic material than required by the organisms, the additional N required will be immobilized (borrowed) from the soil, which would (temporarily) decrease the plant-available N.
Based on a University of Guelph evaluation (2013) using data from 180 field studies that measured crop yield response to manure. The equations assume the average carbon content of manure is 42% of manure dry matter and that the retained carbon is 37.4% for liquid manure and 31% for solid manure and that the C:N ratio of soil life is 8:1.
| Table 1. Estimated Percentage of Organic Nitrogen Available in Year of Application (Spring – as-applied) | |
| % Organic N = % Total N – % NH4-N (ppm NH4-N/10,000 = % NH4-N) | |
| Liquid: [% organic N – (% DM ÷ 50.93)] | Solid: [% organic N – (% DM ÷ 61.44)] |
| X 100 = lbs/1000 gal X 10 = kg/1000 L or kg/m3 | X 20 = lbs/ton X 10 = kg/tonne |
| Example: Liquid Dairy Manure 4.5% Dry Matter (DM) 0.25 % Total N 0.12% NH4-N 0.04% P 0.19% K Available Organic N = [ (0.25 ‒ 0.12) ‒ (4.5 ÷ 50.93)] = (0.13 ‒ 0.09) = 0.04 % 0.04 % x 100 = 4 lbs/1000 gallons 0.04 % x 10 = 0.4 kg/ m3 | Example: Solid Broiler Manure 70% Dry Matter 3.12 % Total N 0.6% NH4-N 1.4% P 1.8% K Available Organic N = [ (3.12 ‒ 0.6) ‒ (70 ÷ 61.44)] = (2.52 ‒ 1.14) = 1.38 % 1.38% x 20 = 27.6 lbs/ton 1.38% x 10 = 13.8 kg/T |
| Dr J Lauzon, K Janovicek, U. of Guelph 2013 (based on an evaluation of data from 180 field sites that measured crop yield response to manure) | |
| The Equation: Available manure Organic N = Organic N x (carbon content of manure x carbon retained by soil life x C:N ratio of soil life) x Conversion factor (% to lbs per ton or per 1000 gallons) Assumptions: Organic N (%,) = Total N (%) ‒ Ammonium N (%)Average carbon content of manure is 42% of manure dry matterRetained carbon is 37.4% for liquid manure and 31 % for solid manureThe C:N ratio of soil life is 8:1 | |
Previous manure nitrogen credits
Most of the available N in manure is used by the crop, the soil microorganisms, or is lost during the first growing season following application. Depending upon the C:N ratio of the manure and frequency and rate of application, the organic-N release may be extended for several years after application. Many factors affect availability, however for planning purposes, an estimated 10 percent of the organic N portion becomes available in the second cropping year after application; 5 percent then 2% for subsequent cropping seasons.
Example:
Solid chicken (broiler) manure was applied at 5 ton/ac to a field after wheat harvest. The analysis was 65% DM, 2.85% total N and 0.51% NH4-N. The organic N portion is 2.34% (2.85 – 0.51 = 2.34). The available nitrogen for the following spring crop would be estimated between 25 to 35 lbs/ton (125 – 175 lbs/ac) depending on timing and management. The following years:
2nd crop after application: 2.34 x 0.10 x 20 = 4.7 lbs/ton or 23 lbs/ac
3rd crop after application: 2.34 x 0.05 x 20 = 2.3 lbs/ton or 12 lbs/ac
4th crop after application: 2.34 x 0.02 x 20 = 0.9 lbs/ton or 5 lbs/ac
Since the organic N portion of liquid manure is smaller, nitrogen credits are more significant with solid manure, and especially when frequently applied to the same field.