Organic Fertilizers: The Power of Animal Manures

Once inexpensive fertilizers became readily accessible after World War II, many farmers, extension agents, and scientists began to underestimate the value of manure. The focus shifted more towards how to dispose of manure rather than how to utilize it effectively. In fact, some scientists endeavored to determine the absolute maximum quantity of manure that could be applied to an acre without diminishing crop yields. Some farmers who were reluctant to spread manure actually piled it next to a stream, hoping that the floodwaters of the following spring would carry it away. We now understand that manure, much like money, is better distributed than concentrated in a few places. The economic contribution of farm manures can be substantial. Nationally, the manure from 100 million cattle, 60 million hogs, and 9 billion chickens contains approximately 23 million tons of nitrogen. Valued at 50 cents per pound, this equates to a value of about $25 billion for just the nitrogen contained in animal manures. The value of the nutrients in manure from a 100-cow dairy farm may exceed $20,000 per year; manure from a 100-sow farrow-to-finish operation is worth about $16,000; and manure from a 20,000-bird broiler operation is worth about $6,000. The additional benefits to soil organic matter buildup, such as improved soil structure and enhanced diversity and activity of soil organisms, may double the value of the manure. If you’re not reaping the full fertility benefit from manures on your farm, you may be wasting money.

Animal manures can exhibit very different properties, depending on the animal species, feed, bedding, handling, and manure-storage practices. The quantities of nutrients in the manure that become available to crops also depend on when the manure is applied and how quickly it is incorporated into the soil. Additionally, the impact of manure on soil organic matter and plant growth is influenced by soil type. In other words, it’s impossible to provide blanket manure application recommendations. They need to be customized to each situation.

We’ll begin the discussion with dairy cow manure but will also provide information about the handling, characteristics, and uses of some other animal manures.

The type of barn on the farmstead often determines how manure is managed on a dairy farm. Dairy cow manure that contains a significant amount of bedding, usually around 20% dry matter or higher, is spread as a solid. This is most common on farms where cows are kept in individual stanchions or tie-stalls. Liquid manure-handling systems are common where animals are housed in a “free stall” barn and minimal bedding is added to the manure. Liquid manure usually ranges from 2% to 12% dry matter (88% or more water), with the lower dry matter if water is flushed from alleys and passed through a liquid-solid separator or large amounts of runoff enter the storage lagoon. Manures with characteristics between solid and liquid, with dry matter between 12% and 20%, are typically referred to as semi-solid.

Composting manures is becoming an increasingly popular option for farmers. By composting manure, you help stabilize nutrients (although considerable ammonium is usually lost in the process), have a smaller volume of material to spread, and have a more pleasant material to spread—a significant advantage if neighbors have complained about manure odors. Although it’s easier to compost manure that has been handled as a solid, it does require a lot of bedding to get fresh manure to a 20% solid level. Some farmers are separating the solids from liquid manure and then irrigating with the liquid and composting the solids. Some are separating solids following digestion for methane production and burning the gas to produce electricity or heat. Separating the liquid allows for direct composting of the solids without any added materials. It also allows for easier transport of the solid portion