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—especial y 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.
capacity causes farmers to apply it at other times. In the
Plant growth is sometimes retarded when high rates
fall, manure can be applied to grasslands that don’t flood
of fresh manure are applied to soil immediately before
or to tilled fields that will either be fall-plowed or planted
planting. This problem usually doesn’t occur if the fresh
to a winter cover crop. Although legal in most states, it is
manure decomposes for a few weeks in the soil and can
not a good practice to apply manures when the ground
be avoided by using a solid manure that has been stored
is frozen or covered with snow. The nutrient losses that
for a year or more. Injection of liquid manure sometimes
can occur with runoff from winter-applied manure are
causes problems when used on poorly drained soils in
both an economic loss to the farm and an environmental
wet years. The extra water applied and the extra use of
concern. Ideally, winter surface applications of manure
oxygen by microorganisms may mean less aeration for
would be done only on an emergency basis. However,
plant roots, and loss of readily plant-available nitrate by
research on frost tillage has shown that there are win-
denitrification may also be occurring.
dows of opportunity for incorporating and injecting
When manures are applied regularly to a field to
winter-applied manure during periods when the soil has a
provide enough nitrogen for a crop like corn, phospho-
shallow frozen layer, 2 to 4 inches thick (see chapter 16).
rus and potassium may build up to levels way in excess
Farmers in cold climates may use those time periods to
of crop needs (see table 12.2). When ammonium is
inject manure during the winter (figure 12.2), although
properly conserved, the manure rate necessary to meet
the windows of opportunity may be limited.
crop nitrogen requirement is substantially reduced.
Correspondingly, phosphorus and potassium applica-
POTENTIAl PROblEMS
tions are moderated, reducing or eliminating the accu-
As we all know, too much of a good thing is not neces-
mulation of these nutrients in soil.
sarily good. Excessive manure applications may cause
When manure is applied based upon needed or
plant-growth problems. It is especially important not to
allowed P additions, as required by some nutrient man-
apply excess poultry manure, because the high soluble-
agement plans, N-conserving management means that
salt content can harm plants.
less fertilizer N will be needed. Erosion of phosphorus-
136
Building SoilS for Better CropS: SuStainaBle Soil ManageMent