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The amount of nitrogen found in the soil test result can be directly deducted from the nitrogen requirement. Nitrogen "credits" refer to adjustments made to the crop nitrogen requirement that are not included in laboratory analysis. The estimated nitrogen credits are assumed to occur during the first growing season that follows the time that the soil sample is collected.
Mineralization is the conversion of plant-unavailable organic nitrogen by microbial activity into plant-available inorganic nitrogen, like nitrate or ammonium. This occurs most rapidly during the growing season when organic materials or waste products - like manure or compost - are applied to the soil and incorporated before planting. A certain fraction of the total nitrogen can be deducted from the nitrogen fertilizer requirement, depending on the material characteristics. The MC credit is only an estimate. The exact credit depends on microbial activity which is affected by soil temperature and soil moisture content.
Laboratory analysis of a manure or compost sample is the most accurate way to identify the nutrient content. Crop Files 2.05.005 through 2.05.244 contain survey values of many different types of manure with typical ranges that can be used to estimate nutrient content for planning purposes. The table below contains generic default values for nutrient content, if no other information is available.
¶ Table 1. |
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Nutrient Values |
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| Manure Type |
N |
P2O5 |
K20 |
| Beef manure, lb/ton |
12 |
18 |
25 |
| Dairy manure, lb/ton |
8 |
25 |
27 |
| Dairy manure, lb/1000 gallons |
17 |
25 |
17 |
| Swine manure, lb/1000 gallons |
54 |
54 |
33 |
| Poultry manure, lb/ton |
28 |
106 |
54 |
A certain amount of nitrogen may be deducted from the nitrogen requirement in certain situations if a legume was grown during the previous crop season, then destroyed by tillage.
The nitrogen "replacement credit" results from the mineralization of the legume root system and nodules. This past crop credit can be applied to annual crops that follow the legume in the rotation. PC credits are different for warm-season crops (e.g., corn, sorghum) or cool-season crops (e.g., winter wheat, oats).
¶ Table 2. |
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| Previous Crop | Replacement Credit, lb N/ac | |
| Warm Season | Cool Season | |
| Soybeans | 35–50 | 0 |
| Dry beans | 20–30 | 0 |
| Other grain legumes (lupins, etc) | 15–25 | 0 |
| Cover crop | 20–30 | 0 |
¶ Table 3. |
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| Leaf Color at Harvest | Credit, lb N/ac |
| Yellow | 0 |
| Yellow/Green | 30 |
| Green | 60 |
¶ Table 4. |
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| Alfalfa | Replacement Credit, ll N/ac | ||
| % of Stand | Plants/sq. ft | Warm Season | Cool Season |
| >80% | >4 | 100 | 60 |
| 60–80% | 3–4 | 75 | 40 |
| 40–60% | 2–3 | 50 | 20 |
| <40% | <2 | 0 | 0 |
¶ Table 5. |
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| Legume Crop | Replacement credit, lb N/ac |
| Sweet clover, harvested | 3-–50 |
| Sweet clover, unharvested | 60–100 |
| Red clover | 30–60 |
Nitrogen can be mineralized from soil organic matter so can be deducted from the nitrogen requirement. Mineralization is the conversion of organic nitrogen forms (e.g., "protein") by microbial activity to inorganic nitrogen forms (e.g., nitrate or ammonium) thus becoming available for uptake by the plant root system. Microbial activity depends on soil temperature and moisture content, so is difficult to predict accurately. Thus, the SOM credit is only an estimate and should only be used for certain cropping situations.
The formula below is based on a "standard" SOM of 2%. Nitrogen is added to soils with less than 2%, assumed to be sandy or eroded soils with limited microbial activity. Nitrogen is deducted from soils with greater than 2% SOM up to a maximum of 6% OM. Soils with greater than 6% SOM are assumed to be poorly drained, so much of the mineralized nitrate-nitrogen can be lost by denitrification.
¶ Table 6. |
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¶ OMadj as lb N/ac = [(OM% x A) x (OM% x (B + (OM% x C)))] - D |
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| Factors for: | Factor A | Factor B | Factor C | Factor D |
| Corn | 230 | 0.048 | -0.00650 | -32.2 |
| OMadj_2 | ||||
| OMadj_3 | ||||
¶ Table 7. |
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| If OM% is……………… | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 | 4.5 | 5.0 | 5.5 | 6.0 | > 6.0 |
| OM_adj using factors for: | |||||||||||||
| Corn …............................... | +30 | +23 | +12 | 0 | -13 | -27 | -39 | -49 | -55 | -57 | -53 | -42 | 0 |
| OM_adj_2 .......................... | |||||||||||||
| OM_adj_3 …....................... | |||||||||||||
Also see the following Crop Files: