- Animal manures are a valuable source of nutrients for crop growth.
- Each manure has unique characteristics based on the farm operation.
- Regular laboratory manure analysis is an important step in manure and nutrient management planning.
- The total nutrient content in manure is not available the first year and some nutrients may be lost depending on management practices.
Nutrients in manure are valuable resources, but not all manures are created equal. Manure nutrient content depends on many factors, including:
- Animal species.
- Livestock diet.
- Livestock housing and bedding.
- Manure storage and handling system.
- Dilution from water (wash water or rain water).
Since these factors are different on each farm, the most reliable way to determine the nutrient content of manure is to collect a thorough sample and send it to a laboratory for analysis.
Some people may use what are called “book values” (averages published by various organizations) to estimate the nutrient content in their manure, but we do not recommend this practice. Small differences between the actual nutrient content of manure and the average “book value” can cause significant over- or under-application of nutrients that may affect crop yields and water quality.
The graphic above summarizes the variability in manure nutrient characteristics for several different livestock species. The data was collected from three different commercial labs that operate in Minnesota from 2012 to 2018.
The x-axis of the graph shows the livestock species and whether the manure is stored as a liquid or solid, as well as the number of samples that the data summarizes for each species.
The y-axis shows the nutrient content. If the manure is stored as a liquid, then the nutrient content is reported as pounds of nutrient per 1,000 gallons of manure. If the manure is stored as a solid, then the nutrient content is reported as pounds of nutrient per ton of manure.
Within each species, four nutrients were measured: total nitrogen, ammonium-nitrogen, total phosphorus (described as P2O5), and total potassium (described as K2O).
Tips for collecting a thorough manure sample for laboratory analysis
Always use caution and proper safety measures while sampling manure.
Liquid and semi-solid manure
- The best and safest time to sample liquid manure is after the pit or lagoon has been agitated and is being pumped out.
- Collect 15 to 25 samples as the pit is pumped out from beginning to end. Dump into a 5-gallon bucket.
- Mix the manure thoroughly.
- Take a subsample, usually about a quart, and place in a plastic container.
- Freeze the sample prior to sending to the lab.
- This method will not allow you to get the analysis results back prior to application for the current year. But the analysis can be used to determine whether appropriate amounts of manure were applied to meet crop needs and for estimating the Year 2 and 3 nutrient credits.
- Manure can be sampled from the stockpile or during hauling.
- Stockpile: Using a pitchfork or shovel, collect 15 to 25 samples from many different depths in the pile but avoid the crust.
- During hauling: Collect several subsamples from each load.
- Place samples into a 5-gallon bucket and mix very well.
- Take a subsample and place in a sealable plastic bag, then double up the bag.
- With the stockpile method, you may be able to have your manure analyzed prior to application.
- If sampling during hauling, you will not get the analysis results back prior to application for the current year.
Nutrients are not entirely available for crop use the first year after application. This is because nutrients can change forms, and only some of these forms are available for plants to use.
When nutrients are bound to carbon they are in an organic form. If not bound to carbon, they are in an inorganic form.
Typically, plants can only use the inorganic form of nutrients, but manure supplies both organic and inorganic forms. Microbes can break down organic forms of nutrients and mineralize them into inorganic forms. However, this can take several years and depends on soil moisture and temperature conditions.
Nitrogen (N) availability in manure is more challenging to estimate than phosphorus (P) or potassium (K).
- “Plant Available N” (PAN) is the amount of N available from manure. We can determine this on a yearly basis.
- “Nitrogen credits” or “N credits” are for second year PAN, multiplied by the application rate. Use this N credit when planning fertilizer or manure applications in the second year after manure application.
- Typically, additional N is not released beyond three years.
Raw manure supplies two forms of N to the soil: organic-N and ammonium, an inorganic form.
Ammonium is immediately available to plants, while organic-N needs to mineralize into ammonium to be useful. Ammonium can easily convert to ammonia gas in a process called volatilization, and be lost to the atmosphere if not incorporated into the soil.
A third form, nitrate, is also inorganic and immediately available to plants. It may be present but is usually negligible in raw manures.
The overall availability of the combined forms of N depends on two main factors:
- Animal species – different types of animals will have different proportions of organic-N and ammonium in their manure. For example, swine manure has a higher proportion of ammonium while dairy manure has a higher proportion of organic-N.
- Manure application method – injecting liquid manure is the preferred method to minimize ammonia gas losses. When broadcasting either liquid or solid manure, incorporate them as soon as possible to reduce gas losses.
See the Table 1 for the percent of total N available per year and how animal type and method of application affects it.
To calculate PAN, multiply the Total N value from the manure analysis by the N availability factor (percent total N available) for each specified year from the table.
To determine N credits for year 2, calculate the PAN for that year and multiply by the application rate. Remember to account for N credits in these years.
- PAN (Year 1) = Total N (from manure analysis) x N availability (first year)
- PAN (Year 2) = Total N (from manure analysis) x N availability (second year)
Table 1. Manure nitrogen availability and loss as affected by method of manure application and animal species.
|Year||Broadcast + incorporate
|Broadcast + incorporate
|Broadcast + incorporate
(including double disks)
|Injection with sweeps||Injection with knife or coulter|
|N available/ year||N available/ year||N available/ year||N available/ year||N available/ year|
In Minnesota, 80% of the P and 90% of the K in animal manures is available the first year.
Calculation for the first year “Plant Available P” (PAP):
PAP = Total P as P2O5 (from manure analysis) x 0.8
Calculation for the first year “Plant Available K” (PAK):
PAK = Total K as K2O (from manure analysis) x 0.9
Reviewed in 2021