Manure is usually applied in the off-season. This is so you don’t have fresh manure around when you are about to harvest low-growing produce. It’s also just when most people have the time to think about it. Most manure applications are in the fall, but anytime in spring or winter is usually a decent time to spread it around the garden. Manure has some initial available fertility, but most of its benefit comes after it has been broken down by the various biological and microscopical agents in the soil web.
Manure will have an initial boost to fertility, then the rest of it becomes food for your fertility-creating microbes. That’s where the real magic lies. Those microbes, it in healthy populations, will produce far more fertility than the manure originally contained. Here’s my practical advice.
The simplest way to use manure in the garden is to apply it on top of the soil, up to two inches thick, in the fall and leave it for the worms and bugs to work into the soil. With decent soil, that’s arguably the most effective and longest-lasting way to use it. I do this in the fall and winter. It works well.
My second preferred way to use manure is to lay it in a trench. This is most applicable when planting things such as potatoes or asparagus crowns. I dig a trench 9 inches deep and lay three inches of somewhat dry manure (rabbit is my favorite) in the bottom. Then sprinkle just a little straw, and place my spuds or root crowns. This, combined with a little mulch, grows better potato yields than our local commercial producers.
My third preferred way to use manure is to lay it on the soil up to two inches deep, and mulch over it with 4-6 inches of hay. This has the benefit of inflicting the most improvement in soil quality and natural fertility, and it retains the vast majority of the initial fertility in the manure. When covered or buried, the little bit of fertilizer contained within is preserved. When left bare on the top of your soil, it will lose most of its nitrogen fertilizer. That accounts for a small part of the fertility potential of manure.
It will not retain much of its nitrogen content, but it will allow the soil to produce a greater-gained amount of nitrogen through biological activity. All manure practices have that aspect, not just this one. Nitrogen is the most needed plant fertility agent, and most of it should come through biological capture from the atmosphere. In other words, bugs, bacteria, amoebas n’ stuff will feed on the manure and increase their population. That is how you start the engine to the most powerful fertility manufactory ever conceived.
Manure does not need to sit and age for fear of it being too strong. Horse manure is the only one I wouldn’t use fresh, and that’s because horses fed hay or pasture are often full of weed seeds. A horse’s digestion doesn’t break down seed coatings well. Other farm animals do not share that concern. Poultry and swine manure has a higher likelihood of microbial pathogens like E.coli or Coccidiosis. Swine are most often quite unhealthy in their gut, but a healthy hog has a cleaner manure that doesn’t stink much. Healthy animals make cleaner, more fertile manure.
In general, 90 days is considered a safe timeframe from applying manure to harvesting low-growing produce or root crops. By then, even the filthiest of manures have been broken down and are rendered ‘safe as soil’. Personally, I generally go with 60 days. That’s because I can ensure my animals are incredibly healthy and clean.
It is common practice to rototill manure into garden soil. I can’t recommend that unless you are starting with barren soil that has virtually no organic matter present. You are causing more harm than good by tilling highly or moderately fertile soil. Spring tilling causes massive fertility loss and has harmful effects for the rest of the growing season. It disrupts the fertility manufactory of the soil. That fertility system, when healthy and strong, will outperform any fertility application we can attempt.
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