As we all know, too much of a good thing is not necessarily good. 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 is sometimes 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 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 extra 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.

When manures are applied regularly to a field to provide enough nitrogen for a crop like corn, phosphorus and potassium may build up to levels way in excess of crop needs (see table 12.2). When ammonium is properly conserved, the manure rate necessary to meet crop nitrogen requirement is substantially reduced. Correspondingly, phosphorus and potassium applications are moderated, reducing or eliminating the accumulation of these nutrients in soil.

When manure is applied based upon needed or allowed P additions, as required by some nutrient management plans, N-conserving management means that less fertilizer N will be needed. Erosion of phosphorus-rich topsoils contributes sediments and phosphorus to streams and lakes, polluting surface waters. When very high phosphorus buildup occurs from the continual application of manure at rates to satisfy crop nitrogen needs, it may 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, such as alfalfa—that do not need manure for N—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 nitrogen and phosphorus management, see chapter 19.)

Farmers that purchase much of their animal feed 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 remain on the farm as manures. If they apply all these nutrients on a small area of land, nitrogen and phosphorus pollution of groundwater and surface water is much more likely. It is a good idea to make arrangements with neighbors for use of 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.

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 ppm 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 high-metal-content manure 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.

E. COLI 0157:H7

The bacteria strain known as E. coli 0157: H7 has caused numerous outbreaks of severe illness in people who ate contaminated meat and a few known outbreaks from eating vegetables—once when water used to wash lettuce was contaminated with animal manure and once from spinach grown near a cattle farm. This particular bacteria is a resident of cows’ digestive systems. It does no harm to the cow, but—probably because of the customary practice of feeding low levels of antibiotics when raising cattle—it is resistant to a number of commonly used antibiotics for humans. This problem only reinforces the commonsense approach to manure use. When using manure that has not been thoroughly composted to grow crops for direct human consumption—especially leafy crops like lettuce that grow low to the ground and root crops such as carrots and potatoes— special care should be taken. Before planting your crop, avoid problems by planning a three-month period between incorporation and harvest. For short-season crops, this means that the manure should be incorporated long before planting. Although there has never been a confirmed instance of contamination of vegetables by E. coli 0157: H7 or other disease organisms from manure incorporated into the soil as a fertility amendment, being cautious and erring on the side of safety is well justified.