Why analyze manure?
Manure is a valuable source of nutrients for crops. Testing manure for nutrient content helps meet crop nutrient needs efficiently. This leads to increased profit and decreased risk of pollution. Nutrient estimation tables give a general idea of nutrient content, but they tend to differ from actual values due to factors like storage type and animal diet. Therefore, we don’t recommend you rely on those tables. The most accurate way to manage manure for nutrients is to analyze for nutrient content.
Nutrient content in manure varies from one area to another. Solids tend to settle to the bottom of liquid storage systems. Solid storage systems vary based on bedding content and time of stacking. It is important to make the manure as uniform as possible so that the applied rate is accurate.
Use a pit agitator to mix liquid manure to make it more uniform. The solid portion of the manure will begin to settle to the bottom right after agitation. Solid manure is more difficult to make uniform. When piling the manure, alternate between areas with large amounts of bedding and areas of small amounts of bedding.
How to sample manure for nutrient analysis
Manure sampling timing can change the accuracy of a manure test. We recommend taking the sample at manure application. The main drawback of this method is that you cannot use the test results to adjust application rates for the current year. However, the results will help with future fertilizer rate calculations. Keeping detailed records of manure test results will allow for accurate rates in the following years.
Sampling in storage and before manure application allows time to receive results and adjust rates in the current season. However, nutrients are lost with further storage and handling, so it may not give an accurate picture of the nutrients applied. This is particularly true for farms with large amounts of livestock and manure since collecting a representative sample of the manure may not be easy to do.
Manure should be tested each year for the first three years of operation; then every three or four years. Also test whenever management practices change that could alter nutrient content, like the storage system or feed. The Minnesota Pollution Control Agency (MPCA) requires farms with over 100 animal units to test manure at least once every four years. Visit the MPCA website for more information.
Sample liquid or semi-solid manure when manure is applied. Sampling in storage can be dangerous due to harmful fumes. Here are the recommended methods for sampling:
- Collect many samples from the spreader or during pit pumping, and combine them in a clean plastic bucket.
- Stir the manure ensuring that any solids are suspended.
- Subsample from this mixture quickly before solids settle out.
- Place subsample in a well-labeled, plastic, wide-mouth container with a screw-top lid. Do not use glass containers as they can break in response to gas expansion.
- If the sample must be mailed to the testing laboratory, freeze the sample and place in a sealed plastic bag.
Solid manure varies in nutrient content more than liquid manure across storage. Therefore, the key to accuracy is sampling at many sites.
- Collect multiple samples from various places in the manure pile, and place them in a clean plastic bucket. If collecting during application, sample from each load to represent the beginning and end of application. If collecting from storage, avoid sampling from the top or edges of the pile where a crust has formed.
- Mix the manure samples well.
- Place a subsample in a one-gallon size sealable plastic bag. Remove as much air from the bag as possible before sealing.
- Double-bag the sample with another sealable plastic bag.
- Label well, and freeze the sample if not immediately brought to a testing laboratory.
During sampling, wear gloves and appropriate personal protective equipment. Send all manure samples to a testing laboratory as soon as possible after sampling to minimize nutrient losses. Visit the Minnesota Department of Agriculture website for a list of certified manure testing laboratories.
Understanding a manure analysis output
Choosing which manure tests to do depends on individual needs and concerns. At the very least, measure total nitrogen (N), total phosphorus (P), total potassium (K), and moisture content. We also highly recommend having your manure tested for ammonium nitrogen (NH4-N) content. Please note that none of the nutrients in manure are 100% available the first year. To calculate the amount that is plant available, see Manure Characteristics.
- Total nitrogen is reported as TN (total nitrogen) or TKN (total Kjeldahl nitrogen). “Kjeldahl” refers to the analysis type. Either test will measure the total amount of inorganic N (plant-available) and organic N (not plant-available) at the same time.
- Ammonium-N is a form of nitrogen that is immediately plant available. Having it tested can give you a better idea of how to handle your manure. For example, manures with high ammonium content should be injected or incorporated into the soil as quickly as possible to preserve that nitrogen for plants. When exposed to air, ammonium can quickly convert to a gas (a process called volatilization) and be lost.
- Phosphorus: Total phosphorus is reported as P or P2O5. Since nutrient recommendations use P2O5, make sure this is the form you are using for calculating plant available phosphorus (PAP). To convert from total P to total P2O5, use this equation: P2O5 = P * 2.29.
- Potassium: Total potassium is reported as K or K2O. Much like phosphorus, nutrient recommendations use K2O. If you need to convert from total K to total K2O, use this equation: K2O = K * 1.2.
- Moisture content and total solids: These are reported as percentages and represent solids present in the sample.
- Micronutrients: Manure contains the micronutrients that plants need for growth, so they usually do not need to be tested. An exception may be made if there is concern of a micronutrient deficiency.
- pH: If manure will be surface applied, it might be valuable to test for pH. Higher pH will increase the amount of ammonium lost to the atmosphere as ammonia.
- Electrical conductivity: This tests the soluble salts in the manure. This might be a helpful test if the manure application site has saline or sodic soil.
- Carbon to nitrogen ratio: Use this test for manures that are composted or that have a lot of bedding. It tells whether the nitrogen will be plant-available immediately after application.
Reviewed in 2018