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The Garden Plot for September 26, 2008
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Manures As Fertilizers”
With the rising cost of fertilizers, agriculture is taking a new look at manures as a plant food. How much fertilizer does manure provide? Manures provide nitrogen, phosphorus, potassium, sulfur, calcium, and micronutrients in varying amounts. Nitrogen is the limiting nutrient and ranges from 0.5 to 4%. If it is untested, manure contains approximately 1% nitrogen. So, it takes a lot of manure to add nitrogen if you have a low fertility soil. For example, a low fertility soil in which nitrate-nitrogen levels are less than 10 ppm and organic matter levels are less than 2% needs about 4 pounds of nitrogen per 1000 square feet. To provide this much nitrogen from manure, you would have to apply about 400 pounds of manure per 1000 square feet. One cubic yard of manure (about a pick-up bed load) weighs approximately 1400 pounds. So, you would need a bit over one third of a pick up load of manure for a 1000 square foot garden on low fertility soil. A high fertility soil with an organic matter content of at least 3% would need only 100 pounds of manure per 1000 square feet, or about ¼ as much as the low fertility soil.
These are approximate calculations, since the amount of nutrients available depends on the type of manure and how it is handled. Poultry manure contains the most nutrients, while horse manure contains the least. Manure that contains lots of bedding (such as straw or sawdust) will have fewer nutrients per pound than pure manure. Composted manure contains less nitrogen than fresher manure. But be careful! Fresh manures have the potential to leach nitrate into the ground water and to transmit human pathogens, including E. coli. However, manures have been used in gardens and on farms for centuries without reports of widespread food poisoning. The bottom line is that manures and composts are not all created equal and they should not be OVERUSED or OVERAPPLIED. There is a difference between kinds of manures, how long the manure has been aged or composted, and the temperatures the piles of manure or compost have reached during storage or composting.
In a 10-year compost trial at the Rodale Institute, researchers found that composting manures reduced nitrate losses into the ground water by 60 % when compared with chemical fertilizer, and 70 % when compared with fresh dairy manure. The Rodale researchers also found that the more stable nitrate in the compost provided little food to support the E. coli pathogen. E. coli was eliminated from the water that ran off compost piles after 6 and 12 weeks, while fresh manure piles still leached E. coli after 6 months.
So, should gardeners be afraid of using manures in their gardens? Absolutely not! But, use manure with care. It’s best if you can compost or age manures before using them directly on your garden. Most pathogens and weed seeds will be killed if the manure pile reaches 145 F for several days. If you can’t compost, apply fresh manure to the garden at least 3 months prior to harvest. This means you need to apply in very early spring. Fall applications are an option, but you may loose nitrogen to leaching. When you add manure to a garden, do not leave it on the soil surface. Cultivate the manure into the soil to a depth of 6-8”.
Another thing that makes manures worth the work it is to apply them, is that manures add much more than just fertilizer. Besides providing plant nutrients, manures increase soil organic matter content, improve soil structure, and provide a food source for beneficial soil microorganisms.
For more information on using manures as fertilizers, contact your local county extension agent.
Helen Atthowe's new short program of gardening tips
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