Manure is a great source of fertilizer in today’s agricultural industry. It supplies a healthy dose of the primary nutrients nitrogen (N), phosphorus (P) and potassium (K). I will refer to P2O5 (phosphorus oxide) as P and K2O (potassium oxide) as K going forward. There are also plenty of secondary and micronutrients, but for the sake of simplicity N-P-K will be the focus. Table 1 outlines a typical composition and yearly availability of dairy manure ranging from 4% to 10% dry matter when broadcasted and incorporated within one hour or injected.
It is important to understand that there are many forms of manure, which can have different nutrient profiles. This inconsistency is one of the major problems encountered when trying to use manure as a fertilizer source. The totals, in Table 1, can vary by as much as 50% from one farm’s pit to the next.
This variability can also be seen on the same farm from one year to the next. For this reason, manure sampling is a very important tool. When using manure, the ability to predict how much N-P-K will be available to a crop in a given growing season can be fickle. Application method, timing of incorporation and weather can all affect the availability of various nutrients. One last concern is the manure’s ability to fit the needs of the crop.
When looking at manure’s ability to fulfill a crop’s need, N is a great place to start. Corn has a need for 1 pound of N for every bushel of grain produced. If the goal is 200 bushels, 200 pounds of N must be made available. When calculating N, starting with credits from the previous year makes the most sense. In this example, the yield goal will be 200 bushels in a continuous corn rotation. This means there is not a credit from last year’s crop, but there is an N credit from last year’s manure application. If the manure application was based solely off N requirements, 18,000 gallons of manure would have to be applied last year and this year to meet the 200-pound N requirement. If 18,000 gallons of manure were applied both years, 169 pounds of N would be made available from this year’s application and 34 pounds from last year. However, variability in the manure or inevitable losses from uncontrollable environmental factors have not been accounted for, meaning the available N will likely be short of 200 pounds.
When growing soybeans, it is also important to remember that soybeans have the ability to fix their own N. If too much N is applied, there is a potential to limit yield. The main reason for this is: If beans have more than 30 pounds of N provided to them, the bacteria that fixes N may become lazy and produce less “free” N. Excessive amounts of N also tend to cause beans to overgrow, become more susceptible to diseases like white mold and lodge, causing more potential for yield loss. If applying manure to beans, it may be better to apply pen pack (mix of dry manure and bedding). Pen pack will have less readily available N in the first year compared to other manures.
When figuring how much manure to apply to reach optimum N levels, P must also be taken into consideration. The reason being that if manure is applied to a corn crop at an N rate, P is likely being applied at double the recommended rate. If the field has a low P value this is not a concern and can actually benefit soil health. However, if there is a history of spreading manure, or just high P values in general, it does become a concern for a couple of reasons. Phosphorus is prone to polluting surface water through topsoil erosion, which is the main reason P concentrations in the soil are so heavily monitored. High P concentrations can also negatively affect the uptake of other nutrients through antagonism. One nutrient that can be affected in this scenario is zinc. Continuing with the previous example, 18,000 gallons of manure will supply 112 pounds of P this year. That is an overapplication of roughly 37 pounds of P, which equates to a 50% surplus. A better application rate, if applying manure by P guidelines, would be 12,000 gallons. These 12,000 gallons would meet the P requirements of 200-bushel corn (approximately 75 pounds of P), but there would be a need for an additional 74 pounds of N.
Once P needs are determined, the last primary nutrient to look at is K. When harvesting a corn crop for grain, there is not a huge drain on the soil in regard to K. If corn silage is being harvested, then that is another story. Most of the K a corn plant takes up goes to the stalk and leaves. That is why corn silage has such a high demand for K. That is also why whenever hay or straw is harvested, the soil must be replenished with K. For example, 1 bushel of corn harvested for grain requires .22 pound of K. One bushel of corn harvested for silage requires 1.1 pounds of K. Continuing with the 12,000 gallon example, a 200-bushel corn crop harvested for silage is going to require at least 220 pounds of K. The 12,000 gallons of manure that were applied are only going to supply 204 pounds of K. This difference of 16 pounds may not seem like a lot, but it will result in lowering soil K values 2 to 3 ppm. A constant drain on the soil like this outlines the importance of other K sources besides manure.
At the end of the day, manure does provide a significant amount of nutrients to a crop; however, manure does not simply fit all of a crop’s needs. The amount and type of manure is crop-dependent – and even within a certain crop, manure recommendations will vary based off of aspects including yield and soil fertility. It is also unlikely that a farm will have the perfect amount of manure. Excess manure will need to be applied responsibly and, if a shortage occurs, the manure should be handled in the most efficient way possible.
A fertilizer plan for a corn crop will require more than just manure. Along with manure, urea and other fertilizer sources should be used to help balance the equation efficiently. As a reminder, examples discussed in this article will not hold true to every situation. That is why seeking help from an agronomist to balance the equation is a great idea – and giving the agronomist the tools they need like manure, soil and even tissue samples will lead to a balanced equation and a bumper crop.
