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I had my soil tested last fall. As part of the submission I checked the box that said I use manure as a soil amendment / fertilizer.
In the results from the lab, they included a note that said that manure is a good source of many necessary nutrients, but it is not a good idea to rely on it as the sole source of nutrients over the long term.
Unfortunately they didn't say why. What's wrong with adding a bit of manure to my garden every year? (I have vast amounts of horse manure at my disposal.)
You are misinterpreting what the lab was telling you. They are not telling you that it is bad or harmful to use manure over a long period of time. They are telling you that it is bad to use manure as the only thing you add to your garden.
Think of soil building as composting (because that's what it is). You want to maintain the same sort of C:N ratio as you do when composting. When you rely on manure as your only additive, you're unbalancing your soil. You're adding way too much nitrogen. It will have a similar effect to spraying a ton of NPK fertilizer. It will burn out the soil life and eventually make your soil less fertile. You can add manure to your garden every year, but you need to be adding carbon as well to off set all that nitrogen you're adding. You should be adding lots of mulch (hay, wood chips) and compost in addition to the manure.
Finally, manure is a good source of many nutrients. But it is by no means the source of all the nutrients your garden needs. Soil needs a hell of a lot more than just NPK and Mg/Ca to be healthy. In fact, we don't fully understand all the chemical reactions going on down there, let alone have the ability to optimize them. Your best bet is to -- as previously mentioned -- add manure and then add carbon. Compost any extra green matter on your land and add it back in. Plant nutrient accumulators and then use them for mulch. Also you can add other soil additives (like green sand, or kelp) to add various trace elements and nutrients.
Summary: There is nothing wrong with adding manure every year. But you should be mulching/composting on top of the manure to add carbon. You should also add other soil amendments to add what manure lacks.
In looking around to respond to comments, I found that some manure sources have harmful levels of salt. Since mine is homegrown vs from a CAFO, it's well-rotted, and I've got good water penetration, I don't think it's an issue. But for people buying bagged manure that may originate at CAFOs, it's worth keeping in mind and following guidelines from the article above:
- Use well-aged manures rather than fresh manures taken directly from feedlots;
- Apply up to 5 tons per acre of dry matter per year or 10 tons per acre every other year;
- Use supplemental nitrogen fertilizers only as required based on tissue tests, plant performance and previous experience;
- Plow or rototill manure into the soil, irrigate and wait at least 30 days before planting;
- Do not apply manure where water penetration is poor; and
- Monitor soil salinity and sodium levels by periodic soil tests.
(5 tons per acre works out to about 200 pounds per 1000 square feet.)
Update: I just saw this article about manure applications on pastures:
However, if manure is applied to the same pastures over many years, phosphorus can build up. Excessive phosphorus levels in soils and the threat of phosphorus-saturated soils leaching soluble phosphorus are serious concerns in some parts of the country.
I believe the lab folks mean to say that you shouldn't only depend on manure, there are other alternatives. I do not believe they meant it as being harmful to your garden. In fact, it is very normal to use as an a fertilizing agent to a garden, there are tons of nutrients that garden plants benefit from manure. I would say as long as it isn't a meat eating animals manure you have nothing to worry about.
This question has been answered very fully. I entirely agree with most of the comments and there is little to add; however, there is one important aspect of long-term/ over-manuring that has not been mentioned: soil pH. Regular use of manure, whether the cow or horse variety, will slightly lower the pH, and over-use will over-acidify your soil, just as too much fertilizer would, although to a lesser extent. Hence the need for regular soil testing. The increased soil acidity can, of course, be corrected by liming.
Lowered pH. Regular manure application lowers soil pH. The acidifying effect of manure is less than that of inorganic fertilizers.
Soil Scientist: What is Manure? - University of Minnesota Extension - a very full and interesting article.
