Manure has tremendous nutritive value but only if it is available to the crop and that crop is efficient at taking up those nutrients. Under conventional farming practices, manure is spread, injected, or worked into a field after harvest as the standard method of “applying” it to the soil for later use with a future crop. Can we make more efficient use of manure under a sustainable farming approach? Nutrients in manure, especially nitrogen, will vary in how stable they are in the soil and therefore how much is available for plants. Soil temperature, root structure and stage of plant maturity are critical factors that determine nutrient uptake and use by the plant. Let’s consider Nitrogen, Phosphorus, and Potassium (N, P, K) specifically in our conversation about manure management and how it fits into the future of an evolving, more sustainable crop plan.
Let’s define sustainable farming as one that has reduced tillage, maintains a living roots system year-round with some sort of cover cropping, and plants more diverse crops at one time rather than mono-cropping through interseeding practices, adding mixed species or just intensive crop rotation within the growing season. Also, for the sake of our discussion, we should understand how N, P, K function within the plant to appreciate how the application of manure can affect plant uptake and use.
- Nitrogen is an essential element of all the amino acids in plant structures and important in the growth and development of vital plant tissues and cells. Nitrogen is also critical to photosynthesis and is very mobile in the soil and can move down in the soil or be lost to the air.
- Phosphorus is used in root development. It is needed for ATP, the energy molecule of plants, which converts sunlight into plant growth. Phosphorus is not mobile in the soil, so roots must reach it to get it.
- Potassium is easily taken up by the plant and is required later in plant life for fruit and grain fill as it transports sugars into the fruit. Potassium can interfere with calcium uptake during vegetative growth if calcium levels are not high enough in the soil.
Manure, as a liquid, is high in ammonia which needs to be absorbed right away or risk being lost to volatilization. Nitrates in manure are subject to leaching from rain or irrigation and passing through soils without enough plant roots for efficient uptake. Alfalfa is slower than grass to recover after mowing and take up nitrogen from manure. The alfalfa tap root does great for mining water but is poor at grabbing nitrogen from manure. Also, legumes with an active nitrogen producing system of their own, which may signal not to take up manure nitrogen because it has its own system of fixing nitrogen in place. Grasses, on the other hand, have sprawling surface roots that grab nitrogen from near the surface and leave very little passing through to the subsoil. Grasses will regrow immediately after mowing and this helps stimulates the uptake of nitrogen.
Phosphorus uptake in plants is dependent on having a broad, hairy root system that can come in contact with phosphorus in the soil. Corn and other grasses can be very efficient provided the phosphorus is placed within reach of the roots. Placement of phosphorus close to the seedling is crucial for early root development that will provide nutrient and water uptake for the plant later in life.
Potassium levels in manure are often very high. Crops like alfalfa are luxury consumers of potassium and will take in excess amounts if it is available. It is not uncommon to see high levels of potassium in soils from manure applications. We can use alfalfa to help lower levels if they become excessive.
Temperatures below 50 degrees will reduce ammonia volatilization and if we have a growing crop in the field, nitrogen will be taken up by the crop. If there is some, even modest plant growth happening, manure nutrients will be taken into the root system and held for future needs, such as spring regrowth. Open fields cannot grab and hold manure within such a stable system. This is one of the values of having a cover crop.
Having grasses in with alfalfa ensure more effective and timely uptake of manure after application. Less nitrogen is lost due to volatilization and leaching with a mix of roots systems from grasses and legumes.
Open bare soil, after harvest does a terrible job at absorbing manure. We then have to till the soil to help incorporate the manure, either before, during or after application depending on the system. Having a living crop between the rows at harvest, even if very small and immature at harvest time, will take off and grow after harvest. Within 30 days after corn harvest, given the right mix of inter-seed species, we can grow over 1.5 tons dry matter of nutrient absorbing, soil holding, crop feeding bio-mass for next year or next crop. This becomes a great time, a month after harvest of corn, to apply manure and have it there to help your next crop in spring.
There is tremendous potential to make better use of the valuable nutrients in our manure. Given some of the benefits of more diverse roots structure and moisture saving capacity of cover crops, we can use some of the sustainable cropping ideas in a modified conventional system to help us. Manure is rich in nutrients and in today’s market, the potential savings in fertilizer from making better use of capturing those nutrients should be impetus enough to explore subtle changes in how we care for the land.
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