Use of manures

Take home messages

• Poultry litter and other manures are a valuable fertiliser resource if used sensibly.
• They can be an environmental and health hazard if not used sensibly.
• Decisions to apply manure or poultry litter require soil test results to identify which nutrients are needed and cost comparisons with other fertiliser options to ensure the most profitable option is considered.
• Manures and poultry litter contain organic matter (carbon), trace elements and other nutrients which may add to their fertiliser value. 

Background

Bulk manures and poultry litter can be the basis of very productive and valuable crop, pasture and agricultural production when used wisely. They also have potential to create human and animal health risks, and cause dust, smell and water pollution if not stored, spread and managed in an appropriate manner.

Poultry litter and manures may contain human and animal pathogens, so good hygiene has to be practised when handling manure. Animals should be prevented from gaining direct access to manure or litter. Care should be taken to prevent manures contaminating fresh produce that would not be cooked prior to eating.

Manures and poultry litter contain a range of nutrients and organic matter. Their value as a fertiliser will depend on their cost effectiveness at supplying required nutrients. For example if a soil test shows that a paddock is deficient in major nutrients such as, nitrogen (N), phosphorus (P), potassium (K) and sulphur (S) then manure or litter may be a very cost effective option.  However if the soil test result shows that most nutrients are adequate and only nitrogen is required then it may be more profitable to use a fertiliser such as urea. The value of organic matter, calcium, magnesium and trace elements contained in manures is often not costed unless they are grossly deficient in the soil.  Wise fertiliser use combines nutrient budgeting (i.e. knowing what nutrients are being used and removed from a paddock) with soil testing (to check that the desired soil nutrient levels are being achieved).

Take care when storing and spreading manure or poultry litter to prevent it entering watercourses or moving to off-target areas. It may be possible to incorporate by ploughing; however, this destroys established pasture and increases soil erosion risk.

Spreading litter or manure onto a grazed or harvested pasture or crop with 5–10 cm of stubble will help to hold the litter in place and reduce risk of movement due to wind or water. To further protect waterways, maintain an unfertilised vegetated buffer 10–30 m wide around all boundaries and next to any watercourses. The width of the buffer will depend on slope, groundcover and sensitivity of the waterway to pollution.

Composting manures

Composting reduces the weight and volume of the original material by 50 to 60%.  It will also remove the majority of animal and human pathogens, however it does not eliminate the risk of botulism.

The composting of poultry litter or manure should be managed so that the process is even and effective. Ideally it should have:

• A carbon to nitrogen ratio of 30:1; and
• a moisture content of 40%–50%.

A low carbon to nitrogen ratio will result in extensive loss of nitrogen, which would be a problem if straight poultry manure was used. Manures need to be mixed with a carbon source such as straw for efficient composting. Manure or litter should be heaped in rows approximately 1.2 m high and 2.4 m wide to achieve temperatures of 60°C to 70°C.

This temperature is enough to kill most human and animal pathogens except Listeria monocytogenes, Clostridium perfringens and Clostridium botulinum.  If litter is stacked too deep, temperatures can exceed 95°C and result in fire.  In practice, manure or litter is often partially composted during storage in heaps before it is spread onto crops or pastures. This may result in 45%–55% of the manure nitrogen being lost during storage.  Although composted manure may be a more valuable fertiliser than fresh, it has the disadvantages of increased cost and time required for processing, and reduces the total amount of nitrogen and organic matter available for land application.

Farmers who use poultry litter on crops and pastures need to be aware of the effect of the composting process on nitrogen, sulphur and organic matter availability. They need to be aware that losses will occur during storage and that these losses will increase if the litter is stacked too deep. In industries that are very cost-sensitive, it may be more profitable to apply fresh manure or poultry litter, which minimises cost and maximises total nutrient input.

What nutrients are in manure?

Nutrient content of manures will vary depending on animal type, what they are fed, stocking density, bedding if any, and position in yard or shed. It is always valuable to have a nutrient analysis to decide on fertiliser value of the manure and rates to be used. If this is not possible Tables 1-3 provide a guide to nutrient content of common manures. Generally poultry litter will have a higher percentage of nitrogen and phosphorus and a lower percentage of potassium and sodium than cow or feedlot manure.

Table 1. Nutrient content of organic fertilisers.

(Source:  Agfact  Organic fertilisers : An introduction    J. Burgess, 1992)

Table 2. Feedlot manure.

(Source:  Powell, 1994)

Table 3. Poultry litter nutrient survey 2010.

Changes to litter analysis

Compared to previous analyses the 2010 poultry litter survey shows that broiler litter now contains less phosphorus due to changes to broiler feed formulation.  Litter also contains trace elements including copper, zinc, manganese, boron and chloride.

The nutrients in poultry litter are in both mineral and organic forms. This means a proportion of the nitrogen, phosphorus and potassium is immediately available to plants while the remainder (organic) must react in the soil to change into a form which is available for plant use.

Most of the nitrogen in poultry litter is available soon after spreading. Approximately 10% (range 6%–30%) is in the ammonia form which will be lost to the atmosphere unless cultivated or washed into the soil within a few days of spreading. If washed or incorporated into the soil by rain or irrigation, the nitrogen will soon become available for plant growth.  Most of the other nitrogen in poultry litter becomes urea within a short time of spreading, and from then on acts similarly to urea fertiliser.

Approximately 92% (range 81%–95%) of the phosphorus in poultry litter is available for plant use. On average 38% is in a water soluble form, which means it is immediately available for plant use. The remainder is slowly released as organic fractions of the litter decompose, usually within a year of application. Leaching is not normally a problem because most soils bind phosphorus.

Potassium and sulphur in poultry litter is readily available to plants. Some may be lost by leaching down into the subsoil if excessive rain or irrigation occurs. Poultry litter will often maintain existing potassium and sulphur levels but additional fertilisers will be required to overcome deficiencies.

Litter is normally alkaline with a pH around 8; however, compared with lime, it has a lower neutralising effect on acid soil. (The high pH is mostly caused by the ammonia contained in the litter.) On soils that are very acidic, applying lime to the soil will improve pasture or crop growth.

Moisture content in litter is variable, which is one reason why it is sold by volume (cubic metres) rather than weight (tonnes). Moisture content will affect the amount of dust and overlap required when spreading. It is also important when calculating the rate of nutrient being applied to a paddock. Wet litter will have a more offensive smell and lose more ammonia than dry litter.

Poultry litter is mostly organic matter (carbon 42%). Organic matter helps hold moisture, improves soil structure, encourages organisms such as earthworms and microbes, and will hold and supply nutrients. Any increase in organic matter will normally improve your soil.

Comparing the cost of poultry litter with other fertiliser costs

Comparing the cost of poultry litter (with its variable nutrient analysis and wide range of components including organic matter) with other fertilisers will always require some approximation.  The method suggested here is to compare the best available estimate of nitrogen, phosphorus and potassium contained in the litter with what it would cost to buy the same nutrients in commercial fertilisers.

The organic matter, calcium, magnesium, sulfur and trace elements are not normally given a value; however, this technique could be used to value any nutrients which are required.

The following example compares the cost of poultry litter at Maitland with the commercial fertilisers; urea, superphosphate and muriate of potash:

Table 4. Example cost of nutrients.

Example

Cost of fresh litter: $30/m³ delivered and spread

2.5 m³ fresh litter = 1 t dry litter

Therefore, cost of 1 t dry litter =  $75   ($30 × 2.5)

Assuming average analysis of 3.9% N, 1.1% P and 1.5% K on a dry weight basis, 1 t dry litter contains 39 kg N, 11 kg P and 15 kg K.

If the same nutrients were supplied using urea, superphosphate and muriate of potash, the cost (using the prices in Table 4) would be as shown in Table 5.

Table 5. Example value of nutrients in 1 tonne of dry litter.

However, not all nutrients in the litter will be available in the year of application. Assuming 70% of N and 80% of P and K are available, then the equivalent fertiliser would cost $122.  These calculations could be repeated using different fertilisers and prices. Some allowance may also be made for nutrient availability, losses, and spreading cost when commercial fertilisers are used. 

If litter is being spread on land that has a high phosphorus and potassium level, the price comparison changes dramatically.  This situation can arise after repeated applications of poultry litter because plants do not require all the phosphorus that poultry litter can provide if it is applied in quantities that will meet the plants’ nitrogen requirements.

In this case a growth response is not expected as a result of the additional phosphorus and potassium; therefore it is given no value.

Hence 2.5 m³ of litter costing $75 is compared with:

39 kg N from urea  @ $1.73 = $67.47

In this case, litter would be more expensive than alternative fertilisers. It would also not be recommended due to the risk of excess phosphorus contributing to environmental pollution.

Conclusion

Used efficiently, manures can be very cost-effective; however, if used in inappropriate situations it is expensive and wasteful.

Contact Details

Neil Griffiths

Technical Specialist Pasture Production,

NSW Department of Primary Industries, Tocal Agricultural Centre, PATERSON, NSW, 2421

(02) 49398948; 0427 007 425

neil.griffiths@dpi.nsw.gov.au