Nutrients excreted by cows in the forms of manure and urine are directly related to nutrients in what they eat. What matters more than what nutrients are excreted, however, is how available they are to plants grown in soil to which the manure has been applied. The method of application and storage of manure before application also play roles.
According to a study published by two Departments of Natural Resources and the Environment in Australia and Australian Laboratory Services, dairy cows in particular return as much as 65 percent of the phosphorus and 11 percent of the potassium present in whatever they have eaten to their environment in their manure. The University of Arkansas Cooperative Extension further adds that exact nutrient amounts are also affected by how manure is stored and how much it is handled. Generally, the more it is handled in between the cows excreting it and its application to garden soil, the more nutrients it loses. Cow manure does not have a lot of nutrients compared to other animal manures, such as from chickens and horses. Exact nutrient analysis can be performed by soil-testing laboratories. Gardeners should contact their local cooperative extension office for more information on local manure analysis options.
While cow manure's nutritional value is comparatively low, the true benefit lies in its use as an all-purpose soil builder. Its physical characteristics as organic humus (partially decomposed organic matter) help to improve soil structure. Due to its low nutritional content, it is safe for use in all planting situations, even those that are sensitive to over-application of phosphorus.
The University of Arkansas Extension advises gardeners to consider not only the individual nitrogen, phosphorus and potassium (NPK) values in manure, but also what percentage of those nutrients are available to plants. As time passes, the natural process of decomposition and nutrient conversion changes what percentages of nutrients are actually available to plants. If gardeners apply cow manure regularly each year, they can assume 100 percent availability of nutrients every year after the first year, because nutrients from the first year's application become available in addition to nutrients from each subsequent year's application.
Of available nutrients, nitrogen is the most volatile component of cow manure, and the most dependent on time and handling for its availability to plants. Applications of cow manure in the spring mean more nitrogen makes its way into the soil. However, it is also more likely to be temporarily unavailable to plants because it must be converted into an inorganic form by breaking down further in the soil. Applications of cow manure in the fall make it more likely nitrogen will be available for plants that are planted the following spring, because the manure will already have broken down.
If gardeners have easy access to a ready supply of aged cow manure, or keep cows themselves, cow manure may seem like a logical, economically sound garden amendment. Depending on storage methods, amount of time stored, application and nutrient analysis, it may not be as useful for nutrients as gardeners expect. It is undoubtedly great at soil conditioning and textural improvement, and more organic matter worked into soil is not a bad thing. However, due to the potential need for additional nutrients, gardeners should not assume it is the only fertilizer they will need.