Nitrate-N and Manure Nitrogen Index
NH4-N, whether applied directly or from the mineralization of organic-N, is further converted to nitrate-N (NO3-N) by microbial action in the soil. This process occurs rapidly (weeks) during the growing season. Unlike NH4-N, which will adhere to soil particles, the nitrate ion can move freely with soil water.
Loss of NO3-N through leaching or denitrification will occur if manure, especially liquid manure that has high NH4-N, is applied to bare soil in the late summer or fall. Nitrate loss can also occur from N remaining in the soil when more nitrogen has been applied to a crop than what the crop needed/used. During the growing season, nitrogen moves with water, upward to plant roots. During the non-growing season, when water tables generally recharge, water (and nitrogen) moves with gravitational flow. Leaching occurs when the nitrogen moves below the root zone, and potentially into ground or tile water. Denitrification occurs in water-logged soils that have low oxygen levels. The soil microbes “steal” oxygen from the nitrate and the remaining nitrogen is released to the atmosphere, usually as N2 but sometimes as nitrous oxide (N2O), a potent greenhouse gas.
The amount of loss will depend on how much NO3-N is produced which in turn depends on the time required for NH4-N and organic-N to be converted to NO3-N. Late summer application of manure have a greater chance of NO3-N losses (especially from leaching) than manure applied just before freeze-up, or when soil temperatures are cold with minimal microbial activity.
Cover crops can help retain the nitrogen from leaching with late summer or fall applications of manure. Nitrogen scavengers, such as brassicas (radish), cereals (oats or cereal rye) and legumes (red clover) will take up nitrogen and hold it in the roots and plant biomass.
Phosphorus Risk Assessment
Phosphorus is an important plant nutrient for crop production, but it can also contribute to environmental problems when it ends up in rivers, lakes, and streams. Phosphorus in surface-water sources increases eutrophication or aquatic plant growth, (algae blooms), which leads to oxygen fluctuations and decreased ability for the water source to support aquatic life. Risk of P loss is a function of source and transport. A field with high runoff potential, but not near a watercourse has low risk as does a field near a watercourse but with low risk of erosion or runoff. However, even with a low transport risk, any movement of manure or fertilizer P in the field (e.g., areas with ponding) can result in disproportionate distribution in the field.
Phosphorus binds tightly to soil particles with little movement, which is why P placement near the seed at planting is important. In fields with moderate P fertility levels, the biggest risk of comes from P movement with the soil by water or wind erosion. With erosion and runoff, the risk of surface-water contamination by phosphorus cannot be based on a soil-test phosphorus level alone.
The risk of P contamination to surface water increases when soil test results indicate that no additional phosphorus is required to achieve maximum economic yield, but when manure nutrients are still applied. At higher soil test phosphorus levels there are fewer binding sites in the soil therefore the risk of phosphorus moving with soil water and through field tiles also increases.
Manure and fertilizer application timing and placement also affects risk. Maximum risk occurs from surface application of manure and fertilizer P, especially:
- during the non-growing season when frozen soils restrict infiltration and/or when snow-covered fields increase risk of runoff during snowmelt,
- during summer dry periods when large cracks in clay-textured soils increase preferential flow to files,
- when application occurs on fields with topography, situated near watercourses.
To address the environmental risk of additional phosphorus application when soil test levels are adequate, a phosphorus index has been developed. The Phosphorus Loss Assessment Tool for Ontario (PLATO) is a calculator used to estimate the risk of phosphorus loss from fields receiving manure or fertilizer and to consider and compare management practices that impact phosphorus availability and losses.
The phosphorus index considers:
- soil erosion potential
- water runoff potential
- distance to water sources
- phosphorus soil fertility levels
- tile drainage
- manure phosphorus solubility
- fertilizer and manure application timing, method, and rate
4R phosphorus management decisions that consider right source, rate, timing, and placement will help minimize risk of P-loss.
Manure and no-till
The goal with no till is to minimize disturbance of the soil and seedbed. The goal with manure application in a no-till system is a combination of nutrients to feed the crop and soil microorganisms and organic matter contributions that help improve soil health. Manure application is most effective when nutrients are incorporated. When manure is utilized in a no-till system, there must be a compromise. Some tillage will be required, or some loss of nutrients will occur. Advances in application technology and in-crop application opportunities (including dribble bar application of manure under a crop canopy or slurry-seeding cover crops after cereal harvest) allow manure application to occur in no-till systems with minimal nutrient loss.
A few points to consider when applying manure in a no-till system:
- Plan manure application to consider crop rotation and in-crop applications, especially in a true no-till system where nutrients are not incorporated through tillage.
- Manure applied after wheat or spring cereal harvest is the best option. Shallow tillage using vertical tillage or disc when soils are dry will help incorporate cereal straw and manure and could also incorporate cover crop seed. During warm, dry conditions earthworms reside deep in the soil where minimal shallow tillage will not destroy their channels, and nitrogen will help with straw breakdown.
- Apply manure to corn when soils are dry to avoid rutting and compaction. Consider side-dress applications to standing corn.
If nitrogen, is the important component, decide which is more important: limited pre-planned tillage or some nitrogen loss. Manure type will influence how much nitrogen is potentially lost. Solid manure has a smaller portion as ammonium N, therefore less total nitrogen will be lost. The higher the organic N component is the less available N is, but it results in the release of nitrogen over a longer period of time.
Where manure is surface applied to bare soil, a majority of the ammonium portion of the manure will be lost. Rain (10-15mm gentle, infiltrating) shortly after application will incorporate some of the ammonium.
- Although white mould is less problematic in no-till systems, manure applied ahead of soybean planting often results in a denser canopy where risk of white mould is higher. Choose a variety with some resistance to white mould.
- Liquid manure applied to forages is a good option. Apply manure as soon as possible after harvest, before re-growth. Application to older grass-alfalfa stands will give the largest nutrient benefit.
- Surface applied phosphorus from manure in no-till scenarios will increase risk of movement (runoff) to water courses, especially when application occurs outside the growing season. Increased residue cover or cover crops will limit movement, however where manure is applied on sloping land, it is best to have a buffer (vegetated or separation distance) from a water courses or in-field area of concentrated flow.
- Never plan to apply manure to frozen or snow-covered land since nutrient (especially phosphorus) runoff will occur. Avoid applying liquid manure to snow-covered perennial forages, since ice can form under the manure, resulting in increased winterkill and runoff.
- A nutrient analysis is important, regardless of which crop the manure is applied to, so that commercial fertilizer application can be balanced with what was provided in the manure.
- High volumes of surface applied manure can lead to slower soil warm-up in spring or sealing of soil. Calibrate application equipment to ensure an accurate rate.
Manure and Strip Till
Incorporating manure into a strip till system provides several advantages relative to including manure in a no-till system.
Strip tillage confines tillage to narrow zones corresponding to the rows where the crop will be planted. This allows strips of soil to be loosened and cleared of residue while leaving the rest of the field covered with protective crop residue. The advantages include:
- reducing soil erosion since the remaining residue helps to slow water movement and runoff.
- improving soil health with higher soil microorganisms and earthworms and maximizes water infiltration and moisture retention.
- the tilled strips help to warm and dry the seedbed ahead of planting.
- providing nutrients (manure or fertilier) to be placed and incorporated where the crop needs it and reducing envrionmental risk of runoff or volatilzation loss.
Manure can be injected into the tilled strip where nutrients will be placed in or near the seed zone. Often strips will be tilled in conjunction with manure injection toolbars. With the advent of autosteer and GPS guidance systems, strips can be formed followed at a later date with manure or fertilizer application. Producers spring applying manure into strips just ahead of planting should consider the ammonia and total salts (electrical conductivity) of the manure. High ammonium-N and high total salts can result reduced emergence due to “root burn” when planting occurs to quickly after application.