Fertliser Value Of Dairy Soiled Water

Fertiliser N prices, having decreased in 2009 and 2010, increased again in 2011. Fertiliser accounts for roughly 15-20 per cent of the total variable costs on dairy farms.
calendar icon 2 January 2012
clock icon 7 minute read


  • Dairy soiled water offers a substitute for synthetic fertilizers that can cut costs and reduce environmental impacts.
  • Soiled water can achieve 80 per cent of the grass DM yield response of CAN fertilizer at equivalent rates of total N application.
  • Apply soiled water from May to August at 30,000l/ha (20kgN/ha) to get the optimal grass yield response.
  • Soiled water also contains significant P and K and can be considered as a more balanced 15-2-14 NPK compound fertilizer.
  • Managing soiled water effectively to replace synthetic fertilizer N, P and K could potentially save €1300 a year on a 100-cow farm.


Fertilizer N prices, having decreased in 2009 and 2010, increased again in 2011. Fertiliser accounts for roughly 15-20 per cent of the total variable costs on dairy farms. With prices being closely linked to the price of oil, and global fertiliser demand set to increase, it can be expected that fertiliser prices will increase further. The price instability seen in recent times also poses a challenge to farmers as it adds to uncertainty in costs. For these reasons, we need to make the best of all available nutrient sources on-farm.

Low-cost, high return substitutes for fertiliser can help decrease fertiliser use, offering cost savings to farmers and reducing environmental impacts. One such substitute that is widely available on dairy farms is soiled water. Dairy soiled water is a dilute mixture of dung, urine, spilt milk and detergents produced from the washing down of parlours and holding areas. It contains N, P and K that can be used as a fertiliser to increase grass yield. However, soiled water is often seen as a problem rather than an opportunity and is often applied to land as a waste, without trying to get the most out of the nutrients contained in it.

Nutrient content of soiled water

As part of a research programme funded by the Research Stimulus Fund of the DAFM, a survey of 60 dairy farms over a 12 month period was carried out to assess the volumes of soiled water produced and its nutrient content. Approximately 10,000l (10m3) of soiled water were produced per cow per year. On average, this contained around 590mg/l N (0.6kg/m3) (Table 1). For comparison, cattle slurry is assumed to have a N content of 5kg/m3. Roughly a third of the N in soiled water is rapidly plant- available ammonium-N and the balance is mostly organic N. This organic N would probably not be immediately plant-available but can become available over a growing season following mineralisation. Soiled water also contains significant quantities of P and K. The average P content of soiled water was 80mg/l and the K content was 570mg/l. Therefore, soiled water can also meet some of the P and K requirements on-farm.

Table 1. Average nutrient content of dairy soiled water from 60 farms

  kg/m3 Units/1000 gallons
Total N 0.6 5.4
Rapidly Available N 0.2 1.8
P 0.08 0.7
K 0.6 5.4

Fertiliser replacement value

So dairy soiled water contains significant quantities of N. But, is it effective as a fertiliser to increase grass yield? And how much soiled water is needed to replace a given amount of fertiliser N? To answer these questions, plot experiments were carried out with soiled water and fertiliser N (CAN) applied at different rates (zero, 15, 22, 30kg total N/ha) to two different soils (poorly drained and well drained).

Figure 1. Grass yield response to soiled water and CAN fertilizer N (average of soiled water or CAN at 15, 22 and 30kg N/ha).

On average, soiled water applied during the growing season (February-September) gave 80 per cent of the grass DM yield response of CAN applied at the same level of total N content (Figure 1). Soiled water applied at 22kg N per ha (roughly 35m3/ha) could replace 17kg N per ha of CAN fertiliser while maintaining the same grass production (Figure 2). The soiled water produced on a dairy farm of 100 cows could replace 480 kg of fertiliser N, or 1.7 tonnes of CAN, 570 kg of K and 80 kg of P. Assuming costs of €330 a tonne for CAN, €450 a tonne for muriate of potash (50 per cent) and €425 a tonne for superphosphate (16 per cent), this gives cost savings of €575 per year in N, €513 in K and €212 in P; a total cost saving of €1300 per year. In recent years, P and K fertiliser usage has decreased markedly and high N, low P and K compound fertilisers such as 27-2.5-5 NPK have come to dominate, causing concerns about P and K deficiencies. Soiled water can be considered as equivalent to a more balanced 15-2-14 NPK compound fertiliser.

Figure 2. The fertiliser value of soiled water compared to CAN and the grass yield over eight weeks from soiled water applied under optimal grass growth conditions.

Soiled water had a high N fertiliser replacement value on both a well-drained acid brown earth soil and a poorly drained gley soil, so soiled water should have a high N fertiliser replacement value across a range of soil types. Soiled water also maintains high replacement values through the summer and autumn. In contrast, slurry has typically been found to have a replacement value of only 15 to 50 per cent, decreasing through the growing season. What makes soiled water a more effective substitute for fertiliser N? Soiled water is more dilute than slurry and infiltrates better into the soil. This means that less N is lost as ammonia emitted to the air and that N is delivered effectively to the grass roots.

Roughly two thirds of the N in soiled water is in the organic form and not immediately available to plants. It was surprising, then, to find such high N fertiliser replacement values. This may be because soiled water spreads N more evenly compared to fertiliser pellets which concentrate N in the area around the fertiliser pellet. Soiled water delivers N to a larger area of the sward. Soiled water application may also cause additional plant-available N to be released form the soil.

Strategies to maximise value

The best yield response to both soiled water and fertilizer N can be got from May to August- the time of peak grass growth potential and N requirement. To get the most out of the N in soiled water, it should be applied during this period. This is also a period when soil moisture deficits and water stress can become an issue and soiled water can be used to alleviate that.

Because two thirds of the N in soiled water is in the organic form and not immediately available to plants, it is better to apply soiled water early in the growing season so that this organic N can be mineralised and become available to the grass over the growing season. If you have the capacity to store soiled water through the winter period for application in the spring or early summer, in a clay- or plastic-lined lagoon for example, this can help you get the most out of the N in your soiled water. Rates of application are limited by the Nitrate Regulations to 50,000l/ha (4,500gallons/acre or 5mm with an irrigator) every six weeks. This amounts to roughly 30kgN/ha. In this experiment, soiled water was applied at 15, 22 and 30kgN/ha but there was little difference in yield between the two higher rates. Application at approximately 20kgN/ha (approximately 30,000l/ha or 2,700gallons/acre) may be optimum.

At present, most dairy farmers use a vacuum tanker to spread their soiled water. A pump and irrigation system or an umbilical system can save on spreading costs and time. If fertiliser N is to be applied to a paddock in the same rotation, we recommend applying soiled water before the fertiliser N. This is to avoid the risk of leaching of N from the fertiliser when soiled water is applied.

Care should be taken to avoid over-application of P and K, which can have environmental and herd health impacts (e.g. milk fever, grass tetany). As always, the correct balance of nutrient supply is what is needed. The composition of soiled water on dairy farms varies a lot. This presents a challenge for effective nutrient management. We recommend sampling and analysing your soiled water to get an idea of its N content. When sampling the soiled water tank, it is important to sample from the liquid part and not the crust at the surface or the sediment at the bottom.


Dairy soiled water is a valuable source of nutrients on dairy farms with surprisingly high N availability and N fertiliser replacement value and significant P and K content. If managed correctly, soiled water can help to replace some of the synthetic fertiliser use on-farm, improving nutrient use efficiency, saving on costs and reducing environmental impacts.

December 2011

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