By Charles White
The answer depends on a lot of factors, including actual manure nutrient content and the timing of manure application and incorporation relative to crop need.
Manure is an excellent source of nutrients, including nitrogen (N), phosphorus (P), potassium (K), and many other secondary and micro nutrients. While the P and K contained in manure are generally equivalent to the availability of P and K in fertilizers, the availability of N contained in the manure will depend on how quickly it is incorporated into the soil and the time of year it is applied relative to when the crop will need it.
One of the most important aspects of managing nutrient availability from manures is to recognize that manure has a highly variable nutrient analysis across different farms. While the Agronomy Guide and other information sources do provide average values for different manure types, these averages only get you in the ballpark. Using these ballpark figures risks either underestimating or overestimating the actual nutrient content of your manure. Underestimating manure nutrient content means you will probably apply too much supplemental fertilizer, wasting money and increasing the risk of polluting the environment. Overestimating manure nutrient content means you may not apply enough supplemental fertilizer, leading to yield losses and lower profitability. A much more accurate way to quantify the nutrient content of your particular manure is to have it analyzed by a lab. Most labs that offer soil testing or forage testing also offer manure analysis.
Sampling manure for analysis at the time it is being spread is usually the most convenient option. Special care should be taken to obtain a representative sample. For liquid and solid manures that are well mixed, subsamples of manure should be collected periodically throughout the process of unloading a manure storage structure and combined in a bucket. Mix the samples well in the bucket and then take a final sample from the bucket to send to the lab for analysis, usually a volume of about 2 cups. For manure that is not well mixed in the structure, it is advisable to collect separate samples for analysis as the manure consistency changes during unloading because nutrient concentrations are also likely changing. If you do this, be sure to keep track of which spreader loads correspond to each manure sample and what fields those loads are spread in.
A big challenge with sampling manure at the time it is being spread is that the nutrient content from the analysis will not be known before determining the application rate. This is not a big problem if application rates are low, because supplemental fertilizer rates can always be adjusted afterwards based on the analysis results. However, it becomes a more pressing issue when manure rates are high enough that they might exceed regulatory requirements for either N or P balanced application rates. These cases call for keeping track of manure analyses over time with a running average value. If the historical running average is relatively stable, it can be used to plan current application rates. New manure nutrient analyses should be added to the running average when they are obtained. Values that are substantially different from the running average should be evaluated carefully. If they are the result of a temporary phenomenon, such as more or less rainfall than normal diluting a liquid manure storage, then the results should not be included in the running average. If they are the result of a management change that will be consistent into the future, such as a new manure handling system, a change in bedding, or a livestock ration change, then a new running average should be started.
Calculating the amount of N in manure that will be available to a given crop requires adjusting for ammonia volatilization losses, wintertime leaching losses (for manure applied in the fall before the next summer’s crop), and the amount of organic nitrogen that will mineralize during the period of crop growth. These adjustments are made by multiplying the N content of the manure by availability factors obtained from look-up tables in the Penn State Agronomy Guide. The “total N” method of calculating N availability uses a single availability factor multiplied by the total N content of the manure (Agronomy Guide Table 1.2-14). This method relies on an assumption about the relative content of ammonium and organic N in different manure types. The “N fractions” method is a more accurate approach that uses separate availability factors for the ammonium content and the organic N content of the manure (Agronomy Guide Table 1.2-15). The ammonium availability factor depends on the time of year the manure is applied and how quickly it is incorporated into the soil. Longer periods of time before incorporating the manure allow for more volatilization losses, so the ammonium N availability factor declines. The availability factor for organic N depends on the time of year the target crop is growing, with summer crops having a greater availability factor because of warmer soil temperatures and faster mineralization rates.
Some labs report a column on the manure analysis for “Available N” based on assumptions about how the manure will be applied in the field. However, the assumptions made by the lab may not accurately reflect your own manure management practices, therefore it is always best to calculate your own manure N availability using factors in the Agronomy Guide that correspond to your actual management. You should be especially wary when “Available N” quantities are listed on an analysis report for which you did not submit detailed information about how quickly you would be incorporating the manure, what time of year the manure would be applied, and what time of year the crop that needs the N would be growing.