Manures, like other organic residues that decompose easily and rapidly release nutrients, are usually applied to soils in quantities judged to supply sufficient nitrogen for the crop being grown in the current year. It might be better for building and maintaining soil organic matter to apply manure at higher rates, but doing so may cause undesirable nitrate accumulation in leafy crops and excess nitrate leaching to groundwater. High nitrate levels in leafy vegetable crops are undesirable in terms of human health, and the leaves of many plants with high N seem more attractive to insects. In addition, salt damage to crop plants can occur from high manure application rates, especially when there is insufficient leaching by rainfall or irrigation. Very high amounts of added manures, over a period of years, also lead to high soil phosphorus levels (table 12.2). It is a waste of money and resources to add unneeded nutrients to the soil, nutrients that will only be lost by leaching or runoff instead of contributing to crop nutrition.
A common per-acre rate of dairy-manure application is 10 to 30 tons fresh weight of solid, or 4,000 to 11,000 gallons of liquid, manure. These rates will supply approximately 50 to 150 pounds of available nitrogen (not total) per acre, assuming that the solid manure is not too high in straw or sawdust and actually ties up soil nitrogen for a while. If you are growing crops that don’t need that much nitrogen, such as small grains, 10 to 15 tons (around 4,000 to 6,000 gallons) of solid manure should supply sufficient nitrogen per acre. For a crop that needs a lot of nitrogen, such as corn, 20 to 30 tons (around 8,000 to 12,000 gallons) per acre may be necessary to supply its nitrogen needs. Low rates of about 10 tons (around 4,000 gallons) per acre are also suggested for each of the multiple applications used on a grass hay crop. In total, grass hay crops need at least as much total nitrogen applied as does a corn crop. There has been some discussion about applying manures to legumes. This practice has been discouraged because the legume uses the nitrogen from the manure, and much less nitrogen is fixed from the atmosphere. However, the practice makes sense on intensive animal farms where there can be excess nitrogen—although grasses may then be a better choice for manure application.
For the most nitrogen benefit to crops, manures should be incorporated into the soil in the spring immediately after spreading on the surface. About half of the total nitrogen in dairy manure comes from the urea in urine that quickly converts to ammonium (NH4+). This ammonium represents almost all of the readily available nitrogen present in dairy manure. As materials containing urea or ammonium dry on the soil surface, the ammonium is converted to ammonia gas (NH3) and lost to the atmosphere. If dairy manure stays on the soil surface, about 25% of the nitrogen is lost after one day, and 45% is lost after four days—but that 45% of the total represents around 70% of the readily available nitrogen. This problem is significantly lessened if about half an inch of rainfall occurs shortly after manure application, leaching ammonium from the manure into the soil. Leaving manure on the soil surface is also a problem, because runoff waters may carry significant amounts of nutrients from the field. When this happens, crops don’t benefit as much from the manure application, and surface waters become polluted. Some liquid manures— those with low solids content—penetrate the soil more deeply. When applied at normal rates, these manures will not be as prone to lose ammonia by surface drying. However, in humid regions, much of the ammonia-N from manure may be lost if it is incorporated in the fall when no crops are growing.
Other nutrients contained in manures, in addition to nitrogen, make important contributions to soil fertility. The availability of phosphorus and potassium in manures should be similar to that in commercial fertilizers. (However, some recommendation systems assume that only around 50% of the phosphorus and 90% of the potassium is available.) The phosphorus and potassium contributions contained in 20 tons of dairy manure are approximately equivalent to about 30 to 50 pounds of phosphate and 180 to 200 pounds of potash from fertilizers. The sulfur content as well as trace elements in manure, such as the zinc previously mentioned, also add to the fertility value of this resource.
Because one-half of the nitrogen and almost all of the phosphorus is in the solids, a higher proportion of these nutrients remain in sediments at the bottom when a liquid system is emptied without properly agitating the manure. Uniform agitation is recommended if the goal is to apply similar levels of solids and nutrients across target fields. A manure system that allows significant amounts of surface water penetration and then drainage, such as a manure stack of well-bedded dairy or beef cow manure, may lose a lot of potassium because it is so soluble. The 20% leaching loss of potassium from stacked dairy manure mentioned above occurred because potassium was mostly found in the liquid portion of the manure.
Timing of Applications
Manures are best applied to annual crops, such as corn, small grains, and vegetables, in one dose just before soil tillage (unless a high amount of bedding is used, which might tie up nitrogen for a while—see the discussion of C:N ratios in chapter 9). This allows for rapid incorporation by plow, chisel, harrow, disk, or aerator. Even with reduced tillage systems, application close to planting time is best, because the possibility of loss by runoff and erosion is reduced. It also is possible to inject liquid manures either just before the growing season starts or as a side-dressing to row crops. Fall manure applications on annual row crops, such as corn, may result in considerable nitrogen loss, even if manure is incorporated. Losses of nitrogen from fall-applied manure in humid climates may be as much as 25% to 50%—resulting from conversion of ammonium to nitrate and then leaching and denitrification before nitrogen is available to next year’s crop. It was determined in modeling studies that fall applications of liquid manure posed the greatest risk for nitrate leaching in a dairy system in New York.
Without any added nitrogen, perennial grass hay crops are constantly nitrogen deficient. Application of a moderate rate of manure—about 50–75 pounds worth of available nitrogen—in early spring and following each harvest is the best way to apply manure. Spring applications may be at higher rates, but wet soils in early spring may not allow manure application without causing significant compaction.
Although the best use of manure is to apply it near the time when the crop needs the nutrients, sometimes time and labor management or insufficient storage capacity causes farmers to apply it at other times. In the fall, manure can be applied to grasslands that don’t flood or to tilled fields that will either be fall-plowed or planted to a winter cover crop. Although legal in most states, it is not a good practice to apply manures when the ground is frozen or covered with snow. The nutrient losses that can occur with runoff from winter-applied manure are both an economic loss to the farm and an environmental concern. Ideally, winter surface applications of manure would be done only on an emergency basis. However, research on frost tillage has shown that there are windows of opportunity for incorporating and injecting winter-applied manure during periods when the soil has a shallow frozen layer, 2 to 4 inches thick (see chapter 16). Farmers in cold climates may use those time periods to inject manure during the winter (figure 12.2), although the windows of opportunity may be limited.