A high percentage of the nutrients in feeds passes right through animals and ends up in their manure. Depending on the ration and animal type, over 70% of the nitrogen, 60% of the phosphorus, and 80% of the potassium fed may pass through the animal as manure. These nutrients are available for recycling on cropland. In addition to the nitrogen, phosphorus, and potassium contributions given in table 12.1, manures contain significant amounts of other nutrients, such as calcium, magnesium, and sulfur. For example, in regions that tend to lack the micronutrient zinc, there is rarely any crop deficiency found on soils receiving regular manure applications.
FORMS OF NITROGEN IN MANURES
Nitrogen in manure occurs in three main forms: ammonium (NH4+), urea (a soluble organic form, easily converted to ammonium), and solid, organic N. Ammonium is readily available to plants, and urea is quickly converted to ammonium in soils. However, while readily available when incorporated in soil, both ammonium and urea are subject to loss as ammonia gas when left on the surface under drying conditions— with significant losses occurring within hours of applying to the soil surface. Some manures may have half or three-quarters of their N in readily available forms, while others may have 20% or less in these forms. Manure analysis reports usually contain both ammonium and total N (the difference is mainly organic N), thus indicating how much of the N is readily available—but also subject to loss if not handled carefully.
The values given in table 12.1 must be viewed with some caution, because the characteristics of manures from even the same type of animal may vary considerably from one farm to another. Differences in feeds, mineral supplements, bedding materials, and storage systems make manure analyses quite variable. Yet as long as feeding, bedding, and storage practices remain relatively stable on a given farm, manure nutrient characteristics will tend to be similar from year to year. However, year-to-year differences in rainfall can affect stored manure through more or less dilution.
|Table 12.1 Typical Manure Characteristics|
|Dairy Cow||Beef Cow||Chicken||Hog|
|Dry Matter Content (%)|
|Liquid (fresh, diluted)||7||8||17||6|
|Total Nutrient Content (Approximate)|
|Phosphate, as P2O5|
|Potash, as K20|
|Approximate amounts of solid and liquid manure to supply 100 pounds N for a given species of animal*|
|Solid manure (tons)||10||7||4||10|
|liquid manure (gallons)||4000||2500||1500||3600|
|*Provides similar amounts of nutrients. |
Source: Modified from various sources.
The major difference among all the manures is that poultry manure is significantly higher in nitrogen and phosphorus than the other manure types. This is partly due to the difference in feeds given poultry versus other farm animals. The relatively high percentage of dry matter in poultry manure is also partly responsible for the higher analyses of certain nutrients when expressed on a wet ton basis.
It is possible to take the guesswork out of estimating manure characteristics; most soil-testing laboratories will also analyze manure. Manure analysis should become a routine part of the soil fertility management program on animal-based farms. This is of critical importance for routine manure use. For example, while the average liquid dairy manure is around 25 pounds of N per 1,000 gallons, there are manures that might be 10 pounds N or less OR 40 pounds N or more per 1,000 gallons. Recent research efforts have focused on more efficient use of nutrients in dairy cows, and N and P intake can often be reduced by up to 25% without losses in productivity. This helps reduce nutrient surpluses on farms using only needed P.