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Nitrogen application rates should not be based just on crop yield goal alone or on habit. Part of (or all of) the nitrogen requirement can be filled by carryover soil nitrates, nitrogen in manure or lagoon water, previous legume crops, or the nitrates in irrigation water.
Tests run on surface soil samples usually include an analysis for carryover nitrate-nitrogen. This surface nitrate is important and can be subtracted from the total nitrogen requirement. Much of the carryover nitrogen may be found in the subsoil, especially in when fields are irrigated or when crops are destroyed or yields are affected by weather conditions or other stresses.
Subsoil nitrates can accumulate in hot, dry years, even when yields are good. Above-normal temperatures may also mean above-normal conversion of organic nitrogen to nitrate, because of increased microbial activity driven by warm soil temperatures. If this occurs late in the season when plant nitrogen uptake rates start to drop off, nitrates may accumulate in the subsoil. This also occurs when fields are fallowed or idled.
Although deep sampling for nitrate is a good idea in any field, it may not be necessary to deep sample every field. Knowing the field history can give a good idea of whether or not it contains residual nitrates. The following guidelines can help you decide if a field should be deep sampled for soil nitrates.
¶ Table 1. |
|||
| Should this field be sampled for subsoil nitrate? |
Definitely |
Possibly |
May not be |
| General field situation | |||
| Yields of corn, grain sorghum, or wheat have been reduced by one-third or more by drought or hail: |
✓ |
✓ |
|
| Manure or lagoon water was applied in the past one to two years, but the nitrogen rate has not been adjusted: |
✓ |
||
| Nitrogen applicator ran out on part of the field and it looked the same throughout the season: |
✓ |
||
| Crop rotation program is corn/soybeans or grain sorghum/soybeans: |
✓ |
||
| An alfalfa or sweet clover crop was destroyed a year ago and nitrogen rate has not been adjusted: |
✓ |
✓ |
|
| Corn | |||
| Sugarbeets, cotton, potatoes, or malting barley will be grown next year: |
✓ |
||
| You calculate that you have used more than 1.2 lb. of nitrogen per bushel of yield produced for a five-year yield average and have a fine-textured soil: |
✓ |
✓ |
|
| You calculate that you have applied 1.0 lb. or less nitrogen per bushel of yield produced for a five-year yield average: |
✓ |
✓ |
|
| Corn has more stalk rot and above-normal breakage of lower stalk at harvest time: |
✓ |
✓ |
|
| The very lowest corn leaves stay green after silks are dry and brown: |
✓ |
||
| Corn shows some lighter green areas in the field: |
✓ |
✓ |
|
| Grain sorghum | |||
| Grain sorghum heads out at different times in the field: |
✓ |
||
| You calculate that you have used more than 1.2 lb. of nitrogen per bushel of yield produced for a five-year yield average and have a fine-textured soil: |
✓ |
||
| You calculate that you have used 1.0 lb. or less nitrogen per bushel of yield produced for a five-year yield average: |
✓ |
✓ |
|
| Grain sorghum shows some lighter green areas in the field: |
✓ |
✓ |
|
| Wheat | |||
| Wheat has darker green spots in the field: |
✓ |
||
| Field has been fallowed during the previous season: |
✓ |
✓ |
|
| You have been using more than 100 pounds of nitrogen and have a five-year wheat yield average of less than 50 bushels: |
✓ |
||
| Weeds have not been controlled during fallow period: |
✓ |
✓ |
|