Managing Livestock Manures 4: Managing Manure on Organic Farms

Nutrient supply to soils and crops

Soil biological activity and soil conditioning benefits

The soil environment is the habitat of a diverse array of bacteria, fungi, protozoa and invertebrate animals (including earthworms), which contribute to the maintenance and productivity of soil-crop systems. These organisms are responsible for the cycling of nutrients through the soil. The majority of this cycling is undertaken by the soil microbial biomass (bacteria and fungi). Organic matter additions, as manure or compost, are a source of energy (carbon) for the soil organisms, helping to sustain their populations and their activity.

The organic matter added in solid livestock manures and composts also acts as a soil conditioner and improves soil structure, partly because of improved biological activity and partly because of its interaction with soil minerals.

Improvements in biological activity and soil condition are most likely to be achieved in soils receiving regular applications of solid animal manure or compost.

Nitrogen availability

Analysis of manure for the total nutrient content does not necessarily indicate the amounts of nutrients that may be available during the subsequent growing season. In organic systems, a better understanding of manure nitrogen availability will help develop an environmentally sound and sustainable system.

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The total N content reflects two N pools within the manure (Figure 3):

readily available (primarily the ammonium-N content and, for composted manures, the nitrate-N content plus, for poultry manures, uric acid-N)

slowly available (organic N), relying on breakdown and recycling by soil biological activity.

Slurries and poultry manures have a large readily available N content (40-60% of the total N), compared with FYM (10-25% of total N). The rest of the total N contained in  manures is organic-N, which will be released (mineralised)  slowly (months to years) following manure application. Composted manures tend to have lower available N contents compared with the fresh, source manure.

Readily available nitrogen allows a rapid crop response, whereas the organically bound N is a slow release form. The availability to the growing crop will also be affected by losses via ammonia volatilisation and nitrate leaching. Thus, the amount of crop-available N is affected by many factors including:

  • manure type
  • slurry DM content
  • duration of storage
  • application timing
  • method of application
  • soil type

Good manure management can increase the potential for N supply from applied manures. This may be important for organic systems where nitrogen shortage is limiting crop production.

Phosphate and potash

Short-term availability of P and K from animal manures is greater than from rock sources. Over a rotation, all of the P and K in manures should become available. On soils with a high reserve of P, the phosphorus from manures should not exceed that removed by crops to reduce the risk of P loss to the environment.

Sulphur and magnesium

Manure also provides a useful source of sulphur and magnesium, which may be deficient in some soils. For cattle slurry, 50% sulphur availability has been measured in the season following application. The remaining organically bound sulphur will become slowly available as the manure is mineralised. Magnesium inputs from manures provide maintenance of soil reserves.

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