In the warmth of summer, plants and microbes are growing vigorously and use ammonium and nitrate quickly. Losses of nitrate to leaching is greater in spring and fall when fewer plants and microbes can turn it into organic matter. More complex ecosystems (e.g., a pasture with many plant species, a rotation that includes cover crops, or a weedy field,) are more likely to have some plants and microbes active at all times of the year, preventing the loss of nitrogen from the root zone.

What Determines the Nutrients in Manure?

• Type of animal and age of herd/flock
• Feed and feed supplements
• Bedding
• Collection and storage system
• Application method
• Timing of application
• Soil texture and weather

Anytime you change one of these factors, you can expect your manure nutrient test to change. Each of the factors are described below.

Type of animal

Different animal species produce different types of manure. A ton of fresh manure from most species contains roughly 10 to 20 pounds of nitrogen, 5 to 10 pounds of phosphorus, and 10 to 15 pounds of potassium. However, nitrogen and phosphorus levels are even higher in poultry manure, and sheep manure contains greater potassium levels.

Feed

Typically, 75% to 90% of the nutrients in feed is not used by animals for growth and is excreted in urine and feces. So it is not surprising that the nutrient content of manure changes when a farmer changes feed sources or when pasture plants change over the seasons. The level of protein and inorganic salts in feed (sodium, calcium, potassium, magnesium, phosphate, and chloride) will be reflected in the characteristics of the manure, and the proportion of available versus organic nitrogen may change.

Innovations in feeding practices are dramatically affecting the nutrient value of manure. As you carefully balance feeds for optimal nutrient use by the animals, the nutrients excreted in manure will change. For example, the use of phytase in feed and the use of low phytate corn varieties will improve animal use of phosphorus and will substantially lower the level of phosphorus in manure.

Bedding

Bedding material absorbs urine, somewhat reducing nitrogen losses. Bedding is more important for changing the rate at which nutrients are available than for changing the nutrient content of manure. Compared to manure alone, the higher carbon content of manure-plus-bedding makes less nutrients available in the first year after application (but more in subsequent years).

Collection and storage system

Manure exposed to sun and wind will lose nitrogen through evaporation. Rain and runoff will leach soluble nitrogen. Phosphorus and potassium losses are negligible in storage except in open systems in which runoff and leaching can cause losses of 20 to 50 percent.

Application method

Incorporating manure into the soil immediately after application minimizes nitrogen loss to the air and allows soil microorganisms to start decomposing the organic matter, making nutrients available more quickly to the crop. In manure in which half of the nitrogen is in the form of ammonium, about 25% of the nitrogen is lost within the first 24 hours after surface application. Another 20% may be lost in the next 3 days. Nitrogen loss to the air (as ammonia) is greater on dry, windy days, and from the manures of poultry and veal calves. Losses are reduced if it rains shortly after application, and if application is done as the air temperature is dropping (such as in the late afternoon).

Phosphorus and potassium are not lost to the air, but they can be carried away in runoff.

Solid manure that is allowed to settle out of the liquid may contain 50%-80% of the phosphorus. This may be important if the liquid is applied separately from the solids.

Soil and weather characteristics

Decomposition occurs faster under warm, moist conditions, making nutrients available to plants more quickly. Rain after application reduces volatile losses of nitrogen.

On coarse textured soils, decomposition is rapid, so nutrients are available more quickly. Coarse soils have a low CEC (cation exchange capacity, or nutrient holding capacity) and low water-holding capacity, so it may be necessary to limit the amount of manure applied to coarse soils to prevent nutrient leaching.

Fine-textured soils retain nutrients longer in the rooting zone. Infiltration is slower than on coarse-textured soils. To prevent runoff, the amount of liquid manure applied to fine soils may have to be limited.

Intensive Grazing and Other "Direct Deposit" Approaches

One important manure management system that is often neglected is direct deposit of manure onto land by grazing or herded animals. Because animals manage the storage and application components of this system, labor savings are significant. However, nutrients can be lost to runoff and to nitrogen evaporation. These losses can be minimized by mechanically incorporating manure into the soil after herding animals on a plot, or by maintaining a vigorous biological community (e.g., beetles, earthworms, chickens, wild turkeys) that can chop, bury, and decompose the manure so it is quickly fixed into forms that are not easily leached or volatilized. The greatest nitrogen losses are from urine spots because the nitrogen is in an inorganic form and overwhelms the soil’s ability to buffer and immobilize the nitrogen.

A "direct deposit" approach to manure management can be used on pastures, hayland, cover crops, and crop fields where livestock clean up residues, weeds, sprouted grains, etc. This strategy is especially suited to areas that will be tilled after the manure is deposited.

Two important factors to consider when planning a direct deposit system are irregular distribution and the freshness of the manure. Inevitably, more manure is deposited near watering, feeding, and bedding areas. Urine (with its different mix of nutrients), is typically separated from dung. For these reasons, grazing will greatly increase the variation of nitrogen in the soil across the field. The salt and acid content of fresh urine and manure can have a temporarily dramatic effect on soil and plants. Consider that a urinating cow is applying nitrogen at a rate of 1100 pounds per acre; sheep are applying nitrogen at 250 pounds per acre. Most urine nitrogen is lost to the atmosphere or leaching, in part, because plant growth is reduced by urine scorch.