|
Nitrogen Credits Reduce Fertilizer N Need
Consider all nitrogen sources to reduce N fertilizer
requirements.
If you have applied manure or grown alfalfa on a field within
the last three years, or your irrigation water contains nitrate,
you may get by with less N fertilizer this year. The crop does
not 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 fertilizer is required. A starter fertilizer
may be the only supplemental fertilizer needed to enhance seedling
vigor. Table 3 shows various N credits. Be sure your crop advisor
or soil testing lab knows whether any of these N nutrient credits
apply when requesting a recommendation for a field.
Legumes
Legume crops can be a significant source of plant available N
due to bacterial N2 fixation in root nodules. 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. in the second
year. Crops following dry beans will benefit also from a small amount
of N fixed. Don’t underestimate the contribution of this
N source.
Manure
We cover this topic in more detail in the next article, but Table
4 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.
Irrigation water nitrate
Nitrate enriched irrigation water 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. Consider calculating a credit if your irrigation water
contains more than 10 ppm nitrate-nitrogen (NO3-N) concentration.
Crediting water nitrate also improves water quality by removing
it from the ground water through crop uptake. Six years of trials
at several locations in Weld County showed that reducing the N rate
(by up to 120 lbs N per acre) to account for the irrigation water
nitrate did not significantly reduce corn grain yield. The economic
return on this practice was favorable in most years.
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. Refer to the 2001
From the Ground Up Agronomy Newsletter on Nitrogen Fertilizer at
http://www.colostate.edu/Depts/SoilCrop/extension/Newsletters/2001/Nitrogen/index.html
for more information on how to use irrigation water nitrate crediting.
| Table 3. Estimates of
N credits from various sources. |
|
| N Source |
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. |
| Table 4. Manure N credits.* |
|
| 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
Sample manure for actual nutrient content when feasible. |
by Troy Bauder
Extension Specialist
Water Quality
|
