“How much manure do I need?”

The answer depends on:

• the crop you are growing and the expected yield;
• existing soil nutrients as determined by a soil analysis;
• desire to build soil nutrients for future years or mine what’s currently in the soil;
• the type of manure you wish to use and a laboratory analysis of it; and
• any other fertilizer nutrients you expect to apply.

Note that manure characteristics vary significantly among species, so choosing a rate based on the coverage it provides on the soil surface is not a recommended method of determining an appropriate application rate. For instance, poultry litter is much more nutrient dense than beef feedlot manure so the soil coverage resulting from a recommended application rate of litter will be much less than what would result from a recommended application rate of beef manure.

What are your goals?

Because fertility needs differ among crops, a good starting point is figuring out what nutrients are needed. Fertility recommendations can come from trusted advisors (like a crop consultant) or can be calculated yourself using guidance and resources provided by UNL, such as CropWatch. Remember, when determining nutrients needed, account for all sources of nutrients (i.e., existing soil resources, irrigation water, starter fertilizers, etc.).

Once you know what nutrients are needed for your crop, you can choose a manure application rate to meet agronomic crop needs while accounting for excess soil nutrients or building soil fertility. Usually, agronomic rates are determined on nitrogen (N) need and/or phosphorus (P) removal. In either case, these need-based rates are chosen on just one nutrient and, therefore, may over- or under-apply other nutrients, so they’re many times used as guideposts. For this reason, someone may choose a rate that deviates from these. NEVER apply more N than necessary as doing so increases the risk for nitrogen leaching. Also, if your soil already has enough phosphorus (P), manure may be better utilized on acres that need phosphorus and can benefit from additional intrinsic benefits of manure. N-based and P removal rates can be calculated as:

What does manure provide?

Many people are unfamiliar with reading manure lab analysis reports, but this skill is critical when choosing a proper manure application rate based on manure nutrient availability. This starts with taking a manure sample and sending it to the lab for a manure analysis. Example manure analyses can be seen in figure 1 to the right. Once that information is received, calculations can be made to determine available nutrients.

Nitrogen availability in manure

Nitrogen in manure is typically found in two forms, ammonium N and organic N. While nitrate N is occasionally found in small amounts in some composts, it is not usually significant for most manures. For this reason, if you know total N (sometimes called Total Kjeldahl Nitrogen or TKN) and either organic or ammonium N, the other nitrogen form can be calculated.

Ammonium N is available the first year but is prone to loss through volatilization into the atmosphere when manure is applied to the soil surface. Volatilization, however, can be reduced by quick incorporation with tillage, irrigation, or precipitation.

Organic N is not all available the first year. It becomes available slowly – and over multiple years – as microbes in the manure and soil mineralize it into plant-available forms.

Availability of nitrogen differs regionally due to differences in climate as well as application practices on the farm, so lab-provided availability may not match nutrient availability for your particular situation. Read more about Nebraska availability factors.

Phosphorus Form and Availability

Phosphorus in a manure analysis is reported as P₂O₅, which is likely what would be used for any fertility recommendations, so no conversion is necessary. Even though P₂O₅ in manure is only about 70% available the first year, manure is generally applied at a rate that exceeds needs for P₂O₅ in a single year, so one can assume that 100% will be available over time.

Calculating N-based and P-based Rates

N-based Rate

Remember, an N-based rate is always a maximum possible rate. It should never be exceeded due to the risk of nitrogen leaching. To calculate a manure application rate that meets all crop N needs, all that is required is knowing what a crop needs and how much N is available from manure and other sources (e.g. residual organic N, irrigation water, etc.). The following example uses the availability from the previous example.

Rate based on 4-year Phosphorus Removal

A phosphorus removal rate will meet the P needs for the planned crops in the next few years but is designed to maintain current soil test values. Using a rate in excess of the P removal rate will build soil P values, which can cause environmental concerns if soil P values become excessive and/or the field has a high risk for P loss (as determined by the P-index). Using a rate below P removal rate will allow the crop to mine excess P from the soil.

To calculate a 4-year removal rate, one must know crop removal rates, which will vary based on expected yield. Table 1 gives estimated removal rates of P₂O₅ for various crops.

What rate should be chosen?

• If your goal is to build soil nutrients, an N-based rate may be the best option as it may provide additional nutrients in excess of what your crops need, thus allowing for soil fertility building.
• If your soils have plenty of P or other nutrients, you may want to choose a lower rate to mine excess nutrients from the soil.
• A lower rate also might be preferable when fertilizer prices are high as short term fertility needs can be met on more fields by complementing the manure with smaller amounts of commercial N. This also maximizes the number of acres that may receive soil health benefits from manure application (such as reducing soil bulk density and compaction, as well as increasing soil aggregate stability, water infiltration and retention).

How do I know I applied at the intended rate?

Choosing an application rate is important, but if you don’t know what you actually applied, what is the point? As with any fertilizer spreader, a manure spreader should be calibrated. Learn more about calibration.