12.11: Potential Problems Using Manures

As we all know, there can be too much of a good thing. Excessive manure applications may cause plant growth problems. It is especially important not to apply excess poultry manure because the high soluble-salt content can harm plants. Plant growth can also be retarded when high rates of fresh manure are applied to soil immediately before planting. This problem usually doesn’t occur if the fresh manure decomposes for a few weeks in the soil ,and it can be avoided by using a solid manure that has been stored for a year or more. Injection of liquid manure sometimes causes problems when used on poorly drained soils in wet years. The extra water applied and the additional use of oxygen by microorganisms may mean less aeration for plant roots, and loss of readily plant-available nitrate by denitrification may also be occurring.

Nutrient Imbalances and Buildup

When manures are applied regularly to a field to provide enough nitrogen for a crop like corn, phosphorus and potassium build up to levels way in excess of crop needs (see Table 12.2). It is often mistakenly believed that it results from the fact that manure nutrient ratios are out of balance with crop uptake requirements (especially more P). Nutrient ratios for most manures (Table 12.1) are actually equivalent to a crop’s needs (roughly a 2:1 ratio for nitrogen and phosphorus). If most nitrogen was conserved through good timing and application methods—applied immediately prior to the growing season or in a standing crop, and injected or incorporated—the manure rate necessary to meet crop nitrogen requirement can be substantially lowered and the accumulation of P and K in soil reduced!

Erosion of phosphorus-rich topsoils contributes sediments and P to streams and lakes, polluting surface waters. When soil P levels have already built up and manure applications are restricted based on allowed P additions, as required by some nutrient management plans, N-conserving management means that less fertilizer N will be needed. When very high P buildup occurs, it may also be wise to switch the application to other fields or to use strict soil conservation practices to trap sediments before they enter a stream. Including rotation crops that do not need manure for N, such as alfalfa, allows a “draw-down” of phosphorus that accumulates from manure application to grains. (However, this may mean finding another location to apply manure. For a more detailed discussion of N and P management, see Chapter 19.) When P buildup is a concern, the Phosphorus Index is a tool used to assess the potential for P to move from agricultural fields to surface water. It considers soil and landscape features as well as soil conservation and P management practices in individual fields. These include so-called source factors such as soil test P, total soil P and rate, method and timing of P application. It also considers transport factors like sediment delivery, relative field location in the watershed, soil conservation practices, precipitation, runoff and tile flow/subsurface drainage. This allows a nutrient planner to estimate whether P movement risk is low, medium or high, and to suggest appropriate mitigation measures.

Nutrient Imports and Exports

On integrated crop-livestock farms it is commonly possible to produce all or nearly all the feed needs for the livestock. This helps to keep nutrient imports and exports close to balanced, one of the advantages of integrated farms. But there are different kinds of combinations of cropping and livestock. One extreme is farms that import all the feeds for their animals and then have to somehow get rid of the accumulating manure. More commonly, farms produce most of their own feed but animal numbers exceed the production from the farm’s own land base. These farmers purchase additional amounts of animal feed and may have too much manure to safely use all the nutrients on their own land. Although they don’t usually realize it, they are importing large quantities of nutrients in the feed that then remain on the farm as manures. If they apply all these nutrients on a limited area of land, nutrients start to build up and nutrient pollution of groundwater and surface water is much more likely. It is a good idea to make arrangements with neighbors to use the excess manure. Another option, if local outlets are available, is to compost the manure (see Chapter 13) and sell the product to vegetable farmers, garden centers, landscapers and directly to home gardeners. Even when manure is exported from the farm, if there is just too much manure in a given local region, shipping long distances will become very expensive. New manure treatments (like different types of drying and mass reduction methods) may offer ways to make it more transportable to areas of nutrient and carbon deficits.

Poultry and hogs are routinely fed metals such as copper and arsenic that appear to stimulate animal growth. However, most of the metals end up in the manure. In addition, dairy farmers using liquid manure systems commonly dump the used copper sulfate solutions that animals walk through to protect foot health into the manure pit. The copper content of average liquid dairy manures in Vermont increased about fivefold between 1992 and the early 2000s, from about 60 to over 300 parts per million on a dry matter basis, as more farmers used copper sulfate footbaths for their animals and disposed of the waste in the liquid manure. Although there are few reports of metal toxicity to either plants or animals from the use of animal manures, if large quantities of manure with a high metal content are applied over the years, soil testing should be used to track the buildup.

Another potential issue is the finding that plants can take up antibiotics from manure applied to soil. About 70% of the antibiotics used in animal agriculture ends up in the manure. Although the amounts of antibiotics taken up by plants are small, this is an issue that may be of concern when using manures from concentrated animal production facilities that use considerable amounts of these substances.

Nutrient Losses with Grazing

In grazing systems the animal excrements are directly deposited on the surface (in the case of cattle, colloquially referred to as “cow pies”). Some of the ammonium/urea is lost to the atmosphere as the manure dries, similar to non-incorporated manure from confined animal systems. Overall, this reduces concerns with N leaching, and runoff tends to be low due to high vegetative cover on pastures. But because these cow pies are unevenly distributed, they generate small areas with concentrated nutrients while areas in between the “pies” have less nutrients and may still benefit from additional fertilizer. For this reason, nitrate leaching may still be a concern with intensively managed and fertilized dairy pastures.