Manure: a complete, but not balanced fertilizer

 We all know that manures can serve as valuable soil amendments due to its potential to improve soil quality and tilth while proving nitrogen, phosphorus, potassium, sulfur, and numerous other plant nutrients, but in not properly managed and land applied it can also result in negative environmental consequences. Overall, the vast majority of farmers do a great job thinking about how they can best use manure in their farming enterprise and implement techniques and practices to help capture the most benefit as they can. However, as I’ll discuss here, this isn’t always as easy as it sounds.

A comment I often hear about manure is that it is a complete fertilizer. When people say this they usually mean that manure has all the essential nutrient needed for crop growth; however, just because it has the right nutrients in it, doesn’t mean that they are available at the ratio our plants want. This adds some unique challenges to manure management that just don’t exist with other fertilizer options. Take for example a field that needs 150 units of N. If we go down to the co-op and pick up some anhydrous ammonia we can go to our field and put on the needed nitrogen; however, if we use manure to provide the nitrogen we’ll also get some phosphorus and potassium along with it. That is manure is a packaged deal, we can just pick out a chunk of nitrogen and say I’ll take this and leave the rest. The nutrients come as a package deal.

This in itself isn’t an issue, but rarely is the nutrient ratios (N:P:K) in manures such that it matches with crop need. This can occur for a variety of reasons, but the most common have to do with losses of nitrogen during storage. This results in a situation where extra phosphorus ends up getting applied to reach the crops nutrient need. This may not be an issue if we rotate fields that receive manure and wait until our crops have reviewed the extra phosphorus we ended up applying, but this might require using alternative nitrogen sources for several years as we wait for our crop to remove some of the phosphorus.

Alternatively, sometimes when manure is stored outside soluble nutrients, like potassium, may be lost from the manure to a greater extent than more stable nutrients like phosphorus. If the manure is then applied at a phosphorus limiting rate we might end up removing more potassium with the harvested crop than we applied. Repeat this a few times and our soils could even become potassium deficient.

So the next time you hear someone mention manure is a complete fertilizer, just remember that that may not mean it is a balanced fertilizer. Soil testing can be a great tool to help you track what’s happening to phosphorus and potassium levels in your soil, and can serve as a great piece of information when you are planning the best ways to get the most from your manure.