Guideline Index

Chapter 13: Using Dairy Effluent

13.5 Applying dairy effluent and sludge to pastures and crops

Numerous production studies have shown that the application of dairy effluent and sludge to either pasture or crops is an effective way of increasing forage production on-farm.

These studies have demonstrated that this is a desirable way of returning and reusing increasingly valuable nutrients back onto the farm. Further, economic analyses of the results strongly support the costs associated with installing an effective effluent system to contain, store and apply effluent back to farm land. Payback periods are very dependent on the effluent system installed and the various components selected.

13.5.1 Underlying principles for application of dairy effluent and sludge

A number of key principles need to be followed to effectively utilise and safely return effluent and sludge back onto the farm. These include:

  • Allow sufficient land area to apply effluent at an agronomically sensible rate to meet the crop or pastures nutrient requirements.
  • The main nutrients in effluent are not ‘balanced’ and each must be considered individually. The nutrient that requires the largest reuse area sets the application rate – see Section 13.5.4.
  • Total potassium per application should be no more than 60 kg K/ha, and no more than 120 kg K/ha per year. This applies for both liquid effluent and sludge.
  • For liquid effluent, the total nitrogen application should be no more than 60-80 kg N/ha. However, nitrogen application rates as sludge can be much higher due to the slow release nature of much of the organic N in the sludge.
  • Heavier applications increase the risk of problems such as nitrate poisoning , mineral imbalances and make less efficient use of the applied N as well as increasing the risk of losses to the environment.
  • Treat effluent and sludge as a nutrient source rather than just something that needs disposing of.
  • A lighter rate over a larger area is preferable to overloading a small area. If no chemical analysis is available, effluent should be spread at a rate of 1 megalitre per 12 hectares. This is based on typical nutrient concentrations found in surveys of farm treatment systems.
  • Apply effluent or sludge to paddocks when there is no likelihood of runoff from the property.
  • Rotate effluent applications around at least three or four different areas if possible to avoid excessive build-up of nutrients in the soil.
  • Conduct regular soil testing of the areas where effluent is being applied to monitor nutrient levels and soil health.
  • Isolate the paddock and restrict cattle grazing for at least 21 days after the application of effluent to pasture or crops. This withholding period will overcome any palatability or fouling issues, reduce the risk of any pathogens and allow the plants time to respond to the nutrients. For direct application of sludge to pasture, up to 6 – 8 weeks may be required due to the solids content.

13.5.2 Strategies to reduce problems

Any effluent or sludge applied to land should not leave the farm boundary or pollute any surface waterway or ground water. In addition, steps should be taken to reduce the risk of odours. Strategies to minimise these risks include:

  • Where possible, effluent should be applied to land during the drier months. Applications in the wetter months increase the risk of runoff to streams or leaching to ground water when soils are saturated.
  • Apply effluent or sludge on areas well away from watercourses or drainage lines.
  • Apply effluent at such a rate that the liquid does not remain ponded for more than one hour after application.
  • For all spray applications, use sprinkler nozzles that produce large droplets rather than a fine spray. Note, the lower the nozzle height, the lower the odour potential.
  • Where possible apply effluent, or spread sludge or manure during the day rather than in the early morning or late evening when odours can travel further before being dispersed.
  • Consider the wind direction and velocity on days when applying effluent or spreading sludge or manure. Adjust application times to suit.
  • If the spreading of sludge or manure could result in odour, consider direct injection into the soil, soil incorporation soon after spreading or applying only light rates.

13.5.3 Where and when to apply effluent and sludge

13.5.2.1 Effluent
  • Dairy effluent typically contains relatively large amounts of readily plant available nutrients, particularly nitrogen (N) and potassium (K) in addition to a range of other essential plant nutrients. Depending on the farm, the effluent can also be quite saline.
  • Best plant growth responses are obtained when effluent is applied to actively growing crops or pastures in the warmer months of the year. This is due largely to responses to the N content and to a lesser extent the water content. Forage crops, such as turnips or rape, have been found to give excellent responses to effluent. Responses to actively growing pastures are good, but considerably lower for drought stressed or dormant pastures.
  • Due to its usually high salt content, effluent should not be regarded as an irrigation water. While plants do respond to the water content, application rates should be based on the nutrient (especially N and K) content of the effluent rather than water requirements. Effluent can be shandied with irrigation water to reduce the salinity levels.
  • Again due to the salt content, effluent should not be applied to young seedlings or to irrigate a crop up due to the risk of burning.
  • The application of effluent is also an effective way of increasing the K content of soils. This can be a useful method of correcting induced low soil K levels in paddocks that have been repeatedly cut for hay or silage.
  • Where possible effluent applications should be synchronised with herd and paddock rotations to allow sufficient time between application and grazing. A standard practice is for at least a 21 day interval to allow time for the pasture to respond and reduce pathogen risks.
13.5.2.2 Sludge
  • Dairy sludge extracted from the bottom of treatment ponds is physically and chemically quite different to the liquid effluent. Its high solids content, typically 6-8% DM as spread, requires specialist handling equipment which influences when and how it can be used.
  • Importantly, while a proportion of the nutrients such as ammonium-N in the sludge are in readily plant available forms, the majority of the nutrients are in various organic forms. These need to be mineralised to convert them to plant available forms. As a result, applied sludge acts as a long-acting, slow-release nutrient source for pasture and crop growth.
  • Sludge can be applied directly to established pasture, preferably in the drier months, to enable the water content to drain and evaporate off leaving the nutrient rich solids on the soil surface. These solids then act as an effective, long-acting nutrient source. Direct applications at wetter times of the year run the risk of rainfall washing these solids off the pasture.
  • Application rates are usually limited by trafficability over the spread area and typically are not more than 5 – 10 mm (50,000 – 100,000 L/Ha).
  • Sludge can also be applied to cultivated ground and incorporated into the soil prior to sowing of a crop. As with direct application to pasture, this is best done in the drier part of the year.
  • Overseas, sludge is often injected directly into soil using specialist equipment. This minimises odours and N losses by ammonia volatilisation. However, suitable equipment is not commonly available in Australia.
13.5.2.3 Manure

See the following links for information on the re-use of manure:

13.5.4 Worked example; calculating application rates when applying effluent.

Scenario; effluent is to be applied to grazed pasture with the nutrient application to not exceed 60 to 80 kg N each application (see Section 13.5.1 ) or 120 kg K annually. The nutrient concentration in the effluent was tested to be 200 mg total Nitrogen per Litre, 30 mg P/L, and 400 mg K/L.

Often, testing laboratories report nutrient concentrations in effluent using units of mg/L (milligrams per litre). Fortunately, this is equivalent to expressing nutrient concentration as kg/ML (kilograms per megalitre) which is a more useful measure.

It is also useful to know that an application rate of 1 ML/Ha is equivalent to a depth of 100mm. Therefore a 10 mm application of effluent represents and application rate of 0.1 ML/Ha.

The target application depth can be calculated according to:

Equation 13.1

Refer to Chapter 15.6  to 15.8 for more information about P requirements
Refer to Chapter 15.6 to 15.8 for more information about P requirements

Clearly, potassium is the limiting constituent in this example – it limits the effluent application to a total of 30 mm in any year. While that could occur in two applications of 15mm, or even three of 10 mm each, it is well less than the limit imposed by nitrogen of 30 to 40 mm per application or 100 mm per year for phosphorus.

The application rate should be set by the limiting constituent and shortfalls in supply of the other nutrients can be made up by applying conventional fertiliser.