MINNEAPOLIS — I’m sure all of us at some point in childhood were told, “You get what you get, and you don’t make a fit.” Oddly enough, these words also ring true for manure! Unlike commercial fertilizers that can be mixed and manipulated to give you the desired nutrient content, manure nutrient ratios are fixed. It is what it is and you get what you get. The fixed nutrient ratios of manure don’t always line up with the ratios that crops need, which means you’ll almost inevitably over- or under-apply some nutrients. Overapplication of a nutrient can lead to pollution, while underapplication can lead to nutrient deficiencies or the need to pay for additional commercial fertilizer. What can you do to meet this tricky challenge of manure?
Nitrogen-based vs. phosphorus-based application rates
I often hear “The agronomist told me to spread my turkey litter and then put on more nitrogen with commercial fertilizer. Why can’t I just put all my nitrogen on with manure?” In general, manure tends to over-apply phosphorus, especially poultry litter. For example, turkey manure applied to corn, based on the nitrogen requirements of the crop (i.e. nitrogen-based rate), applies five times as much phosphorus as is needed by the crop. If turkey litter is added each year to this same field at that nitrogen-based rate, soil phosphorus will build up to very high levels quickly.
In contrast, when application is based on the crop’s phosphorus needs (i.e. phosphorus-based rate), the application rate will generally be lower than a nitrogen-based rate and not cause phosphorus buildup. And in most cases, this will underapply nitrogen, so more needs to be added through commercial fertilizer. In the situation above, the agronomist recommended applying at a phosphorus-based rate to avoid soil phosphorus buildup. Since that underapplied nitrogen, he recommended adding commercial fertilizer nitrogen to avoid nutrient deficiency.
Should you apply based on nitrogen needs or phosphorus needs? Well, that depends on your soil and manure tests. Compare the crop nutrient needs to the nutrients you already have available in the soil and what you plan to add with manure. If you have high-phosphorus soils and adding manure will add excess phosphorus, you might consider applying at a phosphorus-based rate to avoid further phosphorus buildup. If your soil test phosphorus is low, you might consider using a nitrogen-based rate so long as it won’t build up soil phosphorus levels to a high level.
Avoiding soil phosphorus buildup
In conjunction with the previous section, I often hear, “What’s wrong with overapplying phosphorus? It sticks in the soil. Can’t I just bank it for future years?” While it is true that phosphorus is far less mobile in the soil than nitrogen, it can still be lost through runoff and erosion. Phosphorus leaching can also be a concern in soils with very, very high phosphorus levels.
There are two main methods to avoid phosphorus buildup. The first is to apply at a phosphorus-based rate (as described above). The other method is to apply manure at the nitrogen-based rate, and then not apply any manure to that field until the excess phosphorus has been used up. For example, my family applies turkey manure at a nitrogen-based rate once every three years. The first year’s application supplies all the necessary nutrients for the crop (including nitrogen), but it overapplies phosphorus. It takes two subsequent years for that excess phosphorus to be drawn down by the following crops. In those two years, no manure is applied, and nitrogen or other nutrient needs are supplied by commercial fertilizer. Of course, this method only works well for those that have enough land to support it. Keeping a field manure-free for years may not be feasible for those with large amounts of manure, or few acres.
Overall, manure’s fixed nutrient content is challenging. But with a little forethought and planning, you can balance those tricky nutrients to make an accurate application without being nutrient deficient.
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— Chryseis Modderman, University of Minnesota Extension manure educator
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