Give Credit Where Credit Is Due!
Nitrogen sources other than fertilizer can provide a significant contribution to crop N requirements.
Have you applied manure or grown alfalfa on a field within
the last three years? Does your irrigation water contain nitrate? Any
of these situations contribute nitrogen (N) to the growing crop. The crop
doesn’t care if the N comes from legumes, irrigation water, or fertilizer!
In some cases, these N credits may entirely satisfy crop needs and no
additional manure or fertilizer is required. Table 2
shows various N credits. A starter fertilizer may be the only supplemental
fertilizer needed to enhance seedling vigor. Be sure your crop advisor
or soil testing lab knows whether any of these N credits apply when requesting
a recommendation for a field.
Table 2. Estimates of N credits from various sources.
|
N Credit |
|
|
Soil organic matter* |
30 lb N per % OM |
|
Residual soil nitrate* |
3.6 lb N per ppm NO3-N (1 ft. sample) |
|
Previous alfalfa crop** |
|
|
>80% stand |
100 - 140 lb N/acre |
|
60 – 80% stand |
60 - 100 lb N/acre |
|
<60% stand |
30 - 60 lb N/acre |
|
Dry Beans |
25 - 30 lb N/acre |
*These credits are often factored in N fertilizer rates recommended by
soil testing labs and should not be deducted twice.
**For the second year, use ½ of the first year N credit.
Legumes
Legume crops can be a very significant source of plant available N due to bacterial N2 fixation in root nodules (see Legumes Fix Nitrogen). Plowing down a good stand of alfalfa may release more than 100 lbs. of N per acre in the first year after plow down, and 50 lbs. the second. If you have an older stand of alfalfa, you may want to consider rotating it to a high N requiring crop and moving your alfalfa acres to another field. Using the above N credits for alfalfa, the difference between projected price of the rotating crop and alfalfa, and your market price for N, you can decide whether to invest these N credits in another crop or keep the field in hay. Either way, don’t underestimate the contribution of this N source.
Manure
We cover this topic in more detail in the next article, but Table
3 provides approximate manure credits for Colorado. Keep in mind that
a manure application to a field within the past three years will still
be supplying some N to this year’s crop. A rule of thumb for N release
from manure is that 40% of total N is available in the first year, 20%
in the second, and 10% in the third year.
|
Manure (solid) |
% H2O | Total N | Available 1st Year |
| ---- lb N/ton Manure ---- | |||
|
Beef |
32 |
23 |
9 |
|
Dairy |
46 |
13 |
5 |
|
Swine |
82 |
10 |
4 |
|
*Credit based upon average values for Colorado. |
|||
Irrigation water nitrate
Irrigation water containing nitrate can supply considerable amounts of
N because it is applied during the growing season and is immediately available
for crop uptake, thus potentially reducing fertilizer required. Fields
irrigated with water that has nitrate-nitrogen (NO3-N)
concentrations greater than 10 ppm are most likely to benefit from crediting.
The South Platte River, the Arkansas River, and the San Luis Valley include
several locations where enough NO3-N has
accumulated over time in the groundwater to benefit crop production. Producers
using this nitrate enriched groundwater to supply most of a field’s water
will profit by crediting this N source when determining their fertilizer
rate. Crediting water nitrate also improves water quality by removing
it from the groundwater through crop uptake while reducing their fertilizer
needs.
How to calculate irrigation water nitrate credits:
- Determine the NO3-N content of the irrigation well water by field test kits or laboratory analysis. Initially, sample water twice a year to account for seasonal variability, and then annually.
- Estimate how much water to credit. Most N uptake is during a crop’s vegetative growth stage. Only credit water applied before the crop begins reproductive growth (boot stage, pollination, tuber growth) The upper limit for these credits are: 15 inches for corn; 12 inches for small grains; and 6-10 inches for potatoes. Use Table 4 (chart or equation) to determine the N credit.
Remember that reducing a fertilizer rate by crediting irrigation water nitrate should not be practiced without using soil testing to initially determine a crop’s N needs.
EXAMPLE SITUATION:
- Crop: corn
- Well test results: 18 ppm NO3-N
- Seasonal consumptive use for area: 21 inches of water
- Inches of water to credit = 21 inches ET x 70% of seasonal (.70) = 15 inches
- Water Credit = 18 ppm x 0.23 x 15 inches/acre = 60 lb N /acre
Saves $19/A (@$0.32/lb N)
Total all available N sources from irrigation water, legumes and manure determine the total N credit. The “free” N available to your crop might surprise you!
