There are two types of manures: brown and green.
Brown manures, the manure we’re all more familiar with, come from animals.
Green manures come from plants (the result of covercropping).
Brown manures, or animal wastes, are a great organic matter source but it does have some interesting characteristics that ought to be understood before using it. For one, brown manures are fairly concentrated in nutrients. At first that sounds like good news, and it is great news, but concentrated nutrients can be harmful if you’re not careful.
Have you ever put too much fertilizer on a potted plant in the house, or spilled fertilizer on your lawn? What’s the result? The plants burn up and die because the nutrients in fertilizers are technically salts, so instead of helping the plants they burn them up and kill them. When this happens, the fertilizer exerts a greater osmotic pressure on water than plants do, and they end up drawing water away from the plant.
Using excessive amounts of fertilizer, or tons of rich brown manure, can be, in a lot of ways, like dumping a cup of salt on your dinner plate. A little salt may enhance the flavor of your meal but too much and it can ruin the food and be dangerous, if not deadly, to your health.
You can avoid burning your plants when using brown manures in two ways. First, don’t use too much. Take it easy on brown manures. With composts and green manures you can be a little less particular. But with brown manures, because they’re more concentrated, you have to use them in moderation.
Another thing to keep in mind with respect to concentration, is the type of brown manure you’re using. Brown manures that come from meat animals such as beef cattle or pigs, for example, are typically even more concentrated. This is because meat animals are often fed high salt diets, which causes the animal to hold more water in its body.
Some farmers do this on purpose because water is heavy and meat animals are sold by the pound; that is, the heavier the better, or in other words, a salty diet increases the water content and thus the weight of the animal which in turn increases profits. And, as everybody knows, what goes in must come out.
Brown manures from meat animals are much more likely to cause plants to burn. Brown manures that come from non-meat animals such as dairy cows or horses, are, though still fairly concentrated, not nearly as concentrated as meat animal brown manure.
While we’re on the subject of manure sources, just don’t ever get your manure from a factory farm. Ever. Because the animals are packed together and they’re don’t roam freely in a pasture, the manure they produce is just part of their everyday living quarters, where they eat and sleep. The animals are usually very unhealthy, and as a result, the feed that they’re given contains antibiotics. You don’t want antibiotics in your compost or on your garden, and you certainly don’t want the drug-resistant diseases that are resulting from this practice.
(2) The second way to avoid burning plants with brown manure is to only use older or aged brown manure. Any brown manure that you use on your garden should have sat around for six months or more. Aged brown manure is less concentrated and it has the added benefit of being less smelly so you don’t attract lots of flies, suffocate every time you go outside, or annoy the neighbors. In fact, it really shouldn’t stink at all… it should smell like a nice, rich dirt.
If you have access to fresh manure that you would like to use but don’t want to apply it to your garden and potentially burn your plants, you can age the manure yourself. You can put fresh manure directly into your garden at the end of the growing season in late October to early December depending on where you live, and it will age over the winter. Perfect for planting in about 6 months later, in mid spring.
Another way to handle fresh manure is to create a compost pile, particularly a brown compost pile. Brown compost piles are made from the brown parts of plants, such as branches and dead leaves. These substances are nitrogen poor and can therefore greatly benefit from a brown manure contribution that is nitrogen rich. Mix the compost and manure really well and let it sit for a month or so. The major drawback to aging manure on site is the smell that you’ll inevitably get when the manure is fresh. Unless you live in a rural setting, just get pre-aged manure.
Just a couple more cautions on brown manure. Agricultural studies have shown that continuous and prolonged use of brown manure over many years leads to a phosphorus build up in the soil to the point of phosphorus toxicity. Phosphorus is a nutrient that plants need and use all the time. But too much of anything can be dangerous. And so it is with phosphorus build-up from brown manures. If you’ve been heavily using brown manures for years as an organic matter source you might need to use other organic matter sources for a while to prevent phosphorus toxicity. Generally speaking, this is rarely a problem, especially for home gardeners, because it takes years of repeated brown manure use for phosphorus to accumulate to dangerous levels, but I just thought I’d mention it here so you’re aware of the potential hazard.
The other caution about brown manures is the potential for weed seeds. Remember the phrase I used above, “what goes in must come out”? Well, if the cow eats a ton of weed seed while she’s foraging, a lot of that weed seed will pass straight through her system and come out in the manure unharmed and ready to germinate. If the manure is turned often so that the aerobic activity in the pile heats the pile up well, then most weed seeds will have been killed. If not, then the weed seeds are still viable. If you apply that brown manure to your garden you will, in effect, be planting weeds in your garden. That’s assuming that the cow has eaten weed seeds which she may not have done at all. In this case, whether or not a brown manure has weed seeds in it or not can sometimes be a bit of a gamble. So just be aware of that. If you buy brown manures that are packaged and sold commercially, they are often sterilized to kill any weed seeds that may be present after the aging process.
Brown manures are an excellent organic matter source that is full of plant nutrients. Use brown manures wisely and your garden will flourish.
Be sure to check out our podcasts on different types of organic matter hyperlinked below.