In the Garden
Ralph Purkhiser, Purdue University Master Gardener
It was probably by chance thousands of years ago that mankind learned that adding the manure of animals to their fields resulted in larger plants. It was the first fertilizer, and has been used for that purpose ever since. However, now there are some who are asking all gardeners to refrain from using manure and manure products to fertilize gardens where edible plants are being gown. The situation is complicated and each gardener must make up his or her mind about using animal wastes to fertilize fruits and vegetables.
There is no doubt that animal waste is a good source of nitrogen that is needed by plants. The bio-mass also improves the tilth of the soil. Initially, the only problem was when there was more nitrogen than the plants could use. You can get too much of a good thing. Not only can excess nitrogen burn the plants, it washes into streams and contaminates the water, resulting an algae. Too much manure may also make the soil too acidic for many plants. Manure may also contain viable seeds that will sprout in the garden as weeds.
Composting the manure with leaves and other brown matter will take care of most of these problems. One should use a hot composting method, allowing the pile to heat sufficiently to destroy seeds and pathogens. Once the compost has reduced to a crumbly mass and has lost the methane smell of manure, it is ready to be added to the garden.
However, composting does not destroy everything that may be in the manure. If the animals ate plants grown on land that was contaminated with heavy metals, those contaminants passed from the soil there to the plant, then to the animal and finally to the animal waste. Composting, even with high heat, will not destroy this type of contamination. There may also be residuals of pesticides, hormones and antibiotics in the manure.
E. coli and salmonella may also be present in manure. In recent years, there have been disease outbreaks that have been traced to the use of manure on food crops. The situation here is quite complicated. The Tonka trucks in the crawl space that occupied my leisure time as a child have been replaced by electronic devices for many children today. Playing in the dirt actually helped build up a resistance to E. coli and salmonella, but in the antiseptic environment of today, we fail to produce those antibodies.
For these reasons, some health-care professionals have called for a ban on the use of manure as a fertilizer on food crops. So far, there has been no official ban, but growers who use manure could face lawsuits if people become sick after eating food produced in this way.
If you are not selling your produce, and you are reasonably sure that the animals whose manure you are using have not been given hormones or antibiotics and have not been pastured on contaminated land, you may go ahead and use the manure after composting or by making manure tea. I have tried to present the situation here, but it really is very complicated, and I urge you to look into the matter further if you use manure-based fertilizer. Safety of the food supply is the ultimate goal.