In the post-World War II rush to the “Green Revolution,” farmers and gardeners alike looked to inexpensive synthetic fertilizers to keep their soils producing, while largely abandoning methods of old. In less than a single generation, manure became more of a disposal issue than the blessing earlier agriculturists knew it to be. Today, with the rising costs associated with producing food, manure is once again looked upon as a valuable resource. For the home gardener and market gardener, it remains one of the best options.
Most gardeners know, or should know, that compost is like black gold for the garden. Being essentially organic matter — once alive and drawing nutrients from the ground — when the material in a compost pile has been broken down into rich humus, it can then return those nutrients to the soil to add fertility and structure to the growing medium.
Now, what if we could let animals participate in the process? When ruminant animals consume their feed, they break down the cellulose structures in the plant to release the nutrients within. Only a part of those nutrients are used by the animal, the rest pass through and are contained in the waste products along with plenty of organic matter. For instance, in cattle, approximately 75 percent of the nitrogen (N), 80 percent of the phosphorus (P), and 85 percent of the potassium (K) found in forages pass through the animal to remain in the manure and urine. This manure can be spread in fields or turned into garden soil, or simply added to a compost pile to simmer a while before nurturing the garden.
As with all soil amendments, there are things to keep in mind and precautions to take when fertilizing with manure — all in order to ensure the best results and food safety. Most manures, and particularly those from poultry, are too strong or “hot” to be put directly into use in the garden, and all manures have the potential to carry harmful pathogens from the animal that produced it. The methods below allow anyone with a few chickens or even a few head of cattle to produce safe and rich fertilizer for their gardens.
Composting livestock manure
For most small-scale applications, thoroughly composted manure that has decomposed in a hot compost pile is the best and easiest way to make sure your manure is safe for the garden.
When I say a hot compost pile, I mean one where a good 2-to-1 ratio of dry brown materials — such as wood shavings from bedding or dry straw — to green organic material — in this case the manure — has been evenly mixed, kept at proper moisture levels, turned to aerate regularly, and allowed to reach its maximum internal temperature for a number of days. This is often called cooking, as the temperatures in an active compost pile can regularly reach 120 to 150 degrees Fahrenheit. That high, prolonged period of heat in the manure compost will serve to both sterilize any weed seeds that may be in the manure and to kill off potentially harmful pathogens, such as E. coli or Salmonella spp, thus producing a bounty of well balanced and biodynamically rich fertilizer.
Another benefit to composting manure before use, at least when more browns than greens are used, is that the aerobic microbes that actually do the work of breaking down manure and carbon-rich brown materials also will help neutralize the soluble salts of ammonium, potassium, sodium, calcium and magnesium that naturally occur in manures. If not completely broken down (as when applying raw manure to the soil), these salts can accumulate in the soil and may cause plant burning, impact seed germination, and reduce soil water availability.
Another word of caution: Composting fresh manure should never be done in conjunction with any of the molasses teas that are often used to increase microbial action in the compost heap, lest you stimulate the growth of pathogens.
The use of raw manure
While a hot compost pile will quickly break down the organic material in manure, raw applications of manure can take significantly longer to completely break down in the soil. It will still happen, but this type of application should only be used dependent on the current crop rotation scheme. If, for instance, a small farmer or home gardener is planning on planting a mixed crop of veggies, root crops and the like, an application of raw manure just before planting or during the season is not appropriate and could even be dangerous. Food safety dictates that raw manure should never be applied to root crops or crops where the actual plant leaf is eaten, such as spinach or lettuce.
However, if the farmer or gardener were looking to increase soil structure and fertility for the next season, or as part of a cover crop or grain production rotation, then there can be great benefit to using uncomposted manure.
When the raw manure is added to the garden, the same biological function takes place as does in the compost pile, only at a much slower pace. The microbes and bacteria naturally occurring in the soil break down the manure and convert the nutrients in it to a soluble form that plants can easily access.
The rich organic material in the manure encourages worms, other beneficial insects and bacteria to flourish and aids in building soil structure and water storage ability. Because fresh manure contains a high level of soluble nitrogen in the form of urea — one of the oldest and safest sources of nitrogen known to man — it is often too rich for many growing plants and can cause either burn or excessive foliar growth with little fruit production. If incorporated into a crop rotation schedule that includes a heavy nitrogen-feeding cover crop, forage grass or some grains, the soluble nitrogen can be easily consumed, and it is then added back to the soil as green manure (turned into the soil to further improve structure and fertility), or used as feed to continue the manure cycle, while the bulk of the nitrogen and organic material is broken down in the soil and readied for the next crop rotation.
Using raw manure on the soil will not ‘heat kill’ weed seed. However, applying manure to a growing cover crop has the added benefit of allowing weed seeds to germinate and be culled before setting more seed.
How to apply
Whether you decide to use raw manure or composted manure in your garden, the method of application has a significant impact on the amount of available nutrients actually added to the soil.
In a 1996 paper published by the Cooperative Extension Service of the University of Kentucky, it was shown that the number of days between the day of application and when the manure is actually incorporated into the soil can cause a significant decrease in the amount of nitrogen available to the following crop. For instance, when manure applied in the spring for a summer annual crop is incorporated into the soil within one day, between 50 and 75 percent of the available nitrogen is found to be available for the summer crop. After just six days post application, that available percentage is reduced to as low as 15 to 20 percent. Additionally, if autumn or winter applications are incorporated with no cover crop, the available nitrogen will be in the 15 to 20 percent range, while applications with a cover crop retain as much as 40 to 50 percent until the following season.
Consider the source
When making a plan to introduce manure fertilizer to your garden or small farm operation, whether from animals you raise yourself or from sources outside your production cycle, remember that all manures are not created equal.
Manure from cattle is one of the “coolest” as far as available nutrients for growth and production. It’s not always going to be readily available to most home gardeners, but if you do have access to it, it makes a great addition to any garden, and, when composted, it produces a rich, dark soil amendment. While not excessively high in nitrogen, 3 to 4 pounds per ton, it is generally considered to have the best nutrient balance of the livestock manures.
Probably the most common manure for use in small farms or garden situations is poultry manure because of the ease of raising these animals and their limited space requirements. At nearly 19 pounds per ton of available nitrogen, manure from poultry is considered to be one of the “hottest,” or highest in immediately available soluble nitrogen. If adding it to your compost pile, a little can go a long way. It’s generally not a good idea to use this in large quantities because the high levels of nitrogen can cause burning or too much growth with little production. With some high nitrogen feeders like sweet corn, you will generally be fine as long as you are monitoring plant growth.
A number of other commonly kept animals such as sheep and goats also produce good quality manure, and rule-of-thumb nutrient quantities are easily searchable for any of them. However, to ensure you are applying the proper nutrient levels to the proper crops, a soil test and available nutrient test for the manure to be used should be done when possible. Over application of manure can lead to leeching and other nutrient imbalances, which can cause significant environmental damage. Testing services are most often available through local land grant college extension service offices.
With thoughtful use and careful planning, manure can be incorporated into both the home garden and the small-scale farm to close the loop between the crops of the field, the livestock in the barn, and the farmer in the garden. It’s a timely solution that never really went away, and certainly deserves consideration.
GRIT blogger Paul Gardener and his family enjoy the fruits of their labor throughout the year at their suburban Utah home.