Manure always has been an excellent source of organic and inorganic nutrients. The challenge is knowing how to effectively manage the host of factors that affect the nutrient content of manure.

According to the USDA’s Natural Resource Conservation Service (NRCS) it starts with knowing what nutrients the soil already contains. This can only be determined by using a good home testing kit or sending a sample away to a certified soil testing laboratory. A quality laboratory will be able to assess the present levels of major plant nutrients, micronutrients, soil pH, soil textural analysis, soil organic matter as well as contaminants.

Soil testing prices can vary depending on your state, the lab that is used and the items being tested. The NRCS states that most labs charge from a range of $7-10 for a basic test but the cost increases as more nutrients are analyzed. Soil tests are normally only needed every three to five years but the NRCS recommends yearly testing whenever manure is applied or there is a need for large nutrient or pH changes to the soil.

A recent study conducted by the University of Kentucky found that soil nutrient values will vary seasonally. For this reason, it is necessary to take soil samples at the same time each year. The study determined fall as the optimum time for sampling because this is when soil tends to have an ideal moisture range and nutrient values are at their lowest. Also, if there happens to be a pH problem there is ample time to apply fertilizer or lime to fix it whereas in the spring there is limited time before planting.

Manure matters

Knowing the soil nutrient content is just one half of the equation. The other half is knowing the manure makeup. Manure is made up of organic matter, which improves soil structure, aeration, moisture holding capacity and water infiltration. It also supplies micronutrients such as calcium, magnesium and sulfur, which are beneficial to the soil should a deficiency exist. Both calcium and magnesium also create an added value by producing a liming effect when added to the soil.

Among the many plant nutrients found in manure none are more crucial to farmers then nitrogen, phosphorus and potassium. The over or under application of these nutrients can have adverse effects on crops, which is why proper sampling and analysis is necessary. Nitrogen is the primary building block for plant protoplasm, which is needed for flower differentiation, speedy shoot growth, the health of flower buds and the quality of fruit set. Nitrogen also acts as a catalyst for the other minerals. Phosphorus is needed for energy transfer and storage in plants. It helps plants to mature and promotes root, flower and seed development. Potassium is needed to activate enzymes, form sugars and build up essential oils. It also improves cold weather tolerance. The values of these key nutrients vary based on factors such as animal species, feed, temperature, precipitation, bedding, handling and storage.

According to the Cooperative Extension at Penn State University, for best results, manure should be sampled at the time of application or as close as possible to application because this ensures that samples will be well-mixed and representative of the manure being applied. Manure samples should not be collected directly from the storage facility because of the difficulty in collecting a representative sample.

Iowa State University Extension and Outreach adds that collecting manure samples for nutrient analysis should not be a one-time event. If factors such as feed, management, storage or land application change frequently then samples should be taken on a yearly basis. If there have been no significant changes to these factors for at least three years, then sampling frequency can be reduced. The university extension also points out that much like soils, manure sampling should be taken near the same time every year to account for any seasonal changes.


When choosing manure, determine how much nitrogen is in it. Too much nitrogen can burn plant leaves and roots, whereas too little nitrogen will stunt plant growth. It is helpful to know that most of the nitrogen in manure is in the organic form and must be converted to ammonium or nitrate forms before it can become available to plants. This occurs naturally when microbes in the soil physically break down the organic matter found in manure.

Microbes operate most effectively at temperatures between 70 to 100 degrees Fahrenheit. Microbe activity slows considerably once temperatures fall below 40 degrees Fahrenheit. Soil at 60 to 80 percent water holding capacity provides the most efficient moisture content for microbes. Too much moisture deprives the microbes of the oxygen that they need to survive. A soil pH between 4 and 9 is necessary to support different microbe varieties. There is a sharp drop-off in microbe activity any time the pH goes above or below this range.

Another key point when it comes to nitrogen availability is whether manure is in solid or liquid form. A four-year study done by the University of California found that liquid manure provided almost five times more available nitrogen than solid manure per ton of dry weight. The study also found that 75 percent of the nitrogen in liquid manure was available the first year after application, whereas only 45 percent of the nitrogen from the solid manure was available in the first year. This is good to know because certain crops require more nitrogen than others at certain growing stages.

One way to ensure that plants get the proper amount of nitrogen is by using certain animal manures. Nitrogen content in manures depends in large part on what an animal is fed. Animals that are herbivores like cows, horses and rabbits tend to produce manure that has a low nitrogen content. Manures that are low in nitrogen already have an ideal carbon to nitrogen ratio so they can be tilled directly into the soil without fear of causing plant damage.


Animals that are omnivores like chickens and pigs turn out manure with a higher nitrogen content. A good way to offset the high nitrogen content in manure before applying it to the soil is by composting it with carbon-rich materials like leaves or straw. Decomposition of organic materials is greatly increased when there is a proper balance between carbon and nitrogen materials in manure. Decomposition slows down when carbon is too high and speeds up when nitrogen is too high. Maintaining a carbon/nitrogen ratio of around 30:1 is ideal.

For decomposition also consider that different soil textures play a part in the decomposition rate of manure. Due to less surface area and a lower water holding capacity coarse-textured soils allow for a more rapid rate of manure decomposition. On the other hand, fine textured soils such as clay do not provide adequate space for air to collect, causing less biological activity and a slower breakdown of organic. Also coarse-textured soils have a greater potential to lose nitrate from leaching when compared with fine-textured soils.

Nitrogen can also be lost through a process known as volatilization. It occurs when nitrogen is converted from ammonium to ammonia gas and released to the atmosphere. Volatilization losses increase when manure is located near the soil surface in warm, moist and high pH conditions. Losses from volatilization can be reduced by effectively tilling manure under the soil and applying it in the spring when soil and air temperatures are cool.

Proper application

Once the manure content is determined and the proper application rate is established the next step is deciding when to apply manure to the soil. Jose Hernandez, an Extension educator at the University of Minnesota, said that the timing of manure applications can make a significant difference in nutrient availability to the crop. According to Hernandez, the ideal time to spread manure is in the spring because that is when crop nutrient uptake will be at its peak and losses due to runoff and leaching are reduced.

Fall is another acceptable time for manure application, according to Hernandez, because it allows more time for the organic portions of the manure to break down before the plant needs the nutrients. The only negative to a fall application is that it provides more time for nitrogen leaching to occur. Hernandez said that if fall application is necessary then it should be done later in the season when soil temperatures are below 50 degrees Fahrenheit because low soil temperatures prevent the nitrogen from leaching.

Douglas Beegle, a professor of Agronomy at Penn State University, said all field types can utilize manure as long as nutrient levels aren’t too high to begin with and proper application management practices are taken.

For example, manure that is distributed into pastures without proper management can lead to weed seed problems and parasite eggs. This can be effectively managed by composting manure prior to application. The high temperatures generated in a compost greatly reduce parasite eggs, weed seeds, parasite eggs and other pathogens.

There are other benefits to composting including raising manure pH, reducing odor and reducing bulk, which makes it easier to handle. When composting it is important to note that the longer manure can decompose the lower the nitrogen availability will be.

A study at the University Wisconsin Extension highlighted several types of fields that may be inappropriate for manure application. According to the study manure should not be applied to fields with thin soils over fractured limestone or fractured sandstone bedrock because it can cause significant groundwater problems due to nitrate leaching. Manure should not be applied to fields that have greater than a 12 percent slope because nutrients can be carried off to lakes and streams during thaws or during early spring rains. Manure shouldn’t be added to fields that are within 200 feet of streams, 1,000 feet of lakes or on wet soils and other areas that are periodically flooded unless it is incorporated within three days. Manure application should be avoided on fields with high phosphorus levels.