Managing manure: A balancing act
By Ross H. McKenzie PhD P. Ag.
Society is becoming increasingly concerned about surface and groundwater quality degradation from agricultural operations. We are becoming more aware that environmental problems can occur with over-application of manure applied to land repeatedly over a period of years. Overloading soil with nutrients, poor manure handling or poor timing of manure applications can lead to contamination of surface or groundwater.
However, if good manure management practices are followed, animal waste can be utilized as a valuable nutrient resource rather than treated as a waste that becomes an environmental concern.
It is important to realize it is not possible to apply manure to match crop requirements for all nutrients. When manure is applied based on one nutrient, other nutrients will be either over- or under-applied. For example, if feedlot manure is applied to meet the nitrogen (N) requirements of a wheat crop, phosphorus (P) will be applied at approximately three to six times the rate of crop requirements, depending on the P level in manure. Repeated applications over a period of years will result in build-up of high levels of P in soil. To complicate the situation, nutrients such as N and P are contained in a number of different organic forms in manure. The unavailable nutrient compounds break down and release at different rates over a period of years.
Many producers who have applied manure to their land based on N content have noticed a gradual build-up of soil P. Many fields exceed 400 lb/ac of available P in the top six inches of soil, while some fields are in the range of 800 lb/ac of available P. This is equivalent to 1770 lb/ac of P2O5 and 3540 lb/ac of 12-51-0 phosphate fertilizer, respectively. To further complicate things, the soil test doesn’t measure the unavailable inorganic or organic forms of P in the soil. Soil tests just show the plant available fraction of inorganic P in the soil.
When soil test P is very high, it can result in reduced crop yields due to nutrient imbalances and it can interfere with plant uptake of other nutrients. Further, high soil P can result in greater potential for runoff into surface water. The USDA says one pound of P in surface water can result in about 500 lb of cyanobacteria (bluegreen algae) production in water.
Producers with high soil P should take serious steps to draw down soil P levels. They should try to avoid manure application on fields testing over 200 lb P/ac and not apply manure on fields testing over 400 lb P/ac. Efforts should be made to grow crops with high P requirements to draw down soil P levels. For example, growing crops such as alfalfa or cereal silage to draw down P soil levels would be a good management practice.
Developing best management practices
To prevent problems from developing, intensive livestock operators should adopt best management practices (BMP). When developing a long-term manure management plan, producers have to decide whether to apply manure based on N or P. In the long-term, it is advisable to use P as the nutrient to match with crop removal, rather than N.
It is important to remember BMP will vary from farm to farm, depending on the climatic zone of the farm, the type of animals in confinement, number of animals in confinement, total amount of manure produced, and how the manure is handled, stored and applied. It will also depend on the amount of land available to apply manure, soil types on the farm, types of crops grown and the yield potential of each crop. BMPs should be tailored to match the needs of each farm, and following a step-by-step approach is essential.
Determine where and how much manure can be applied
The factors of where to apply the manure and what rates to apply require the greatest attention. These two factors go hand-in-hand and involve identifying the fields where manure should not be applied, determining the acreage of each field and soil testing each field.
Soil sample each field to a depth of at least two feet and take depth samples at 0-6, 6-12 and 12-24 inches. Have samples analyzed at each depth for nitrate-nitrogen, phosphorus, potassium, sulphate-sulphur, pH and electrical conductivity (EC – a measure of salt in the soil). Take at least 20 random sampling sites across each field to make up a composite soil sample for each depth. Ideally, each field should be analyzed each year, either in late fall or early spring, to track soil nutrient changes and for manure and fertilizer planning.
This information is needed to identify which nutrients are excessive, high, adequate or deficient in each field. This allows you to determine how much of each nutrient must be added to the soil to ensure adequate nutrient levels for crop growth, and which nutrients are high or excessive in a field.
Representative manure samples should be taken and analyzed for total and available nutrients, specifically N, P, potassium (K) and sulphur (S). “Available” nutrient refers to an element that is in plant available form or one that can be rapidly released for uptake by growing plants in the year of application. “Total” nutrient refers to an element in both plant available and unavailable forms. When determining manure application rates, available nutrients and a portion of the unavailable nutrients that can be released from manure in the year of application must be matched to crop nutrient requirements in each field. Book values of typical nutrient levels in manure can be obtained from provincial agriculture departments.
Some operations may find they don’t have a sufficient land base for matching the total P in manure with crop removal. This may mean expansion of their land base or working with adjacent farms to apply manure. In addition, commercial N fertilizer may have to be added to make up the difference between what the crop requires and what is contained in the manure.
All information gathered can be put together to develop a manure management plan. Each provincial agriculture department has work sheets and computer based programs to help work though this exercise and specialists are available to assist with the planning process. Although it seems like a daunting exercise, it is essential in planning your manure management program and to optimize crop production.
DEVELOPING A MANURE MANAGEMENT PLAN
This is a simple case study outlining how development of BMP might work:
- Small feedlot in southern Alberta with 1600 acres (10 quarter sections) of irrigated land.
- Feedlot produces 10,800 tons of manure annually.
- All fields are soil tested and found to have between 50 and 80 lb of N/ac in the top 24 inches of soil.
- Two fields have soil P levels above 400 lb P/ac, three fields between 200 and 400 lb P/ac, five fields level 400 lb P/ac. Therefore, the two fields high in P should not receive manure.
- Manure was analyzed and found to have about 5 lb of N/ton and 15 lb of P2O5/ton that will be available to a crop in the year of application..
- If plant available N content of manure = 5 lb N/ton x 10,800 tons of manure, then 54,000 lb of plant available N is produced annually in the manure.
- If plant available P2O5 content of manure = 15 lb/ton x 10,800 tons of manure, then 162,000 lb of P2O5 is produced annually in the manure.
Manure management based on N:
If crop removal averages between 150 and 180 lb N/ac with various irrigated crops grown, and 50 to 80 lb of N comes from the soil, the remaining 100 lb N/ac would come from the manure. Thus, 54,000 lb of N from manure ÷ 100 lb of N/ac = 540 acres of land needed annually for matching manure application with crop removal.
Manure management based on P:
Assuming average irrigated crop removal is about 60 lb P2O5/ ac, then 162,000 lb of P2O5 in manure ÷ 60 lb of P2O5/ac = 2700 acres of land needed for matching manure application with crop removal.
In this very simplified example, a 540 acre land base would be needed for matching N in manure with crop removal, but would result in serious over-application of P to land in the long term. However, a 2700 acre land base would be needed for matching P2O5 in manure with crop removal. Keep in mind the farm has several fields with high soil P that should not be manured.
In this simple example, the farm has a land base of 1600 acres, minus 320 acres that should not be manured, leaving 1280 acres available for long-term sustainable manure management. The farm would require about 1420 additional acres for managing manure based on P.
In situations where intensive livestock producers do not have a large enough land base to balance manure application with crop removal of P, there is an opportunity to work with a neighbour. Indeed, the potential to purchase manure by paying for delivery and spreading has a double benefit. The confinement operator can dispose of extra manure, and the nearby neighbour has the advantage of an excellent source of fertilizer. In addition, manure applied to eroded fields will benefit from improved soil physical quality and productivity.