One person who called had a source of free horse manure (lucky guy!). The manure was mixed with sawdust (this is not an unusual circumstance as sawdust and wood shavings are often used as bedding in stables) and he wanted to know if this would affect his vegetable crop.
Yes, it will!
Sawdust and wood shavings are very high in carbon content. When the microorganisms start breaking down the manure and sawdust they need energy. They get this energy from carbon and nitrogen. Because of the very high level of carbon in the sawdust, the microorganisms have an unbalanced diet and are in search of more and more nitrogen. Because these microorganisms can collect nitrogen more efficiently than plants due, the result is stunted plants. If you are using sawdust and manure or sawdust that has not been aged in anyway, you must add supplemental nitrogen.
The nitrogen that is recommended, ammonium nitrate (34-0-0) or ammonium sulfate (21-0-0) is chemically-based and I don't know of any organically available sources. If you are not taking an organic route, the nitrogen can be added to the manure/sawdust before land application or it can be worked into the soil after the manure has been applied.
To read more about this visit the Fact Sheet on Horse Manure Management: The Nitrogen Enhancement System and A Horse Owner's Guide to Good Stewardship, both are authored Randall James at Ohio State University Extension.
Another solution to this problem is to COMPOST the manure and sawdust. This is a great option for organic producers. Composting will also help break down weed seeds and other possible pathogens that the horses cannot do by themselves. Compost is also a great way to add both fertility and organic matter to your soil and to reduce the days to harvest restrictions that come with using raw manure.
The major components of compost are:
- "Brown stuff"- dried and dead stuff like leaves and pine needles. This is your CARBON.
- "Green stuff"- green leaves, vegetable scraps, leaf clippings, etc. This is your NITROGEN.
- OXYGEN. We need aerobic activity!
- WATER. Not too much, not too little.
Balance is Key to composting. Balancing water and oxygen and carbon and nitrogen is key to your composting success!
There are many designs out there for compost systems and you will have to choose what is right for you in your system. There are two types of composting styles that are recognized by the National Organic Program (NOP). They are the windrow method (long piles of materials that are turned regularly) and the aerated static pile method (pile is not turned, rather air is supplied through perforated pipes on the bottom of the pile). If you are an organic grower there are many regulations you need to follow for making your own or even purchasing compost, so check with your certifying agency.
To learn about the ins and outs of composting and how to construct your own composting system visit the following resources:
USDA composting site
Composting on Organic Farms
Extension: Making and Using Compost in Organic Agriculture
Debbie Roos comprehensive list of composting links
If you have questions, please come by to talk or stop by your local Cooperative Extension office to see your local Agriculture Agent!