By Bala Tikkisetty

Farm dairy effluent, an important source of nutrients for pasture, can also be a significant source of contamination of waterways.

Therefore a good understanding of soils on individual farms helps prevent nutrient value being lost via leaching and runoff, and helps to protect rivers, streams, lakes and groundwater from nutrients and faecal contamination.

A number of soil properties, including the water holding capacity of soils, determine the potential for these negative effects.

So effluent irrigation systems should match soil properties to minimise runoff and leaching of the nutrients and contaminants effluent contains.

Properties such as texture and structure determine the amount of water that can enter and be retained within a particular soil, and the rate at which excess water goes through that soil. Water retention, drainage characteristics and consequent leaching losses of material in effluent are strongly dependent on these properties.

Leaching occurs in response to movement of excess water through the soil. Soils with high water holding capacity (deep silt loams) are able to store significant quantities of effluent compared to those that are shallow, sandy, or stony. These soils with lower water holding capacity are therefore more susceptible to leaching and runoff.

Water or effluent can also be transported to depth in relatively dry soil that contains large pores open at the soil surface. When ponding occurs at the soil surface during effluent irrigation, material present in the effluent can be rapidly transported below the root zone through a phenomenon known as bypass flow. High leaching losses of nutrients can result from this. That’s one reason why Waikato Regional Council has rules limiting effluent application depths.

Many soils that are used for dairying in the Waikato region are relatively shallow and have limited water holding capacity. Effluent irrigation on these soils is likely to result in more significant leaching compared with other soils.

Adequate soil infiltration rates are also important to avoid runoff of effluent and prevent contamination of surface waterways. For effluent irrigation, the effects of effluent and of cattle treading on the infiltration rate can be negative.

During irrigation, suspended solids in farm dairy effluent accumulate on, and just below, the soil surface, creating an organic layer that temporarily acts as a surface coating. Although infiltration rates are then reduced to very low levels, the effect is transient, lasting about a couple of days.

Also, some soils are susceptible to treading damage during grazing for a large part of the milking season and this, too, can affect the effluent infiltration rate. Therefore, the relationship between grazing and irrigation is another important factor in developing sound effluent irrigation systems.

To summarise, when the ability of soil to absorb water is low, irrigation of effluent will result in ponding and run-off once the total water capacity of the soil is exceeded. Low rates of absorption are found in soils that are poorly drained, and where ponding and runoff often occur with rainfall events. Many of these soils need to be artificially drained to reduce the incidence of ponding and waterlogging. In those cases extra care is needed to ensure that effluent doesn’t bypass the soil and move directly into the drainage system and into streams.

Farmers need to schedule effluent irrigation to suit soil and climatic conditions. To achieve this storage facilities with the required capacity are needed. To increase the flexibility in the scheduling programme, and to minimise the risks of drainage and runoff, small depths of effluent should be applied. Irrigators with low application rates will help reduce the risk of drainage and runoff and maximise the opportunity to make use of the nutrients in the effluent.

When the rate of application of water is higher than the infiltration rate, water can either run off into surface water or enter large pores open at the soil surface and move very rapidly through bypass flow. During this bypass flow, there is little opportunity for the water to be retained within the root zone and high leaching losses of nutrients are likely. Bypass flow of farm dairy effluent can particularly occur in soils that undergo shrinkage and fissuring during drying, especially when these soils have been previously compacted by treading.

To help avoid negative impacts on the environment, there are some important rules to follow over land application of farm dairy effluent.

Untreated effluent should not be discharged into any drain, stream or river.

Have an effluent storage system so it can be possible to defer spreading effluent until conditions are suitable.

Effluent ponds, storage facilities, feed pads and stand-off pads must be sealed to prevent seepage.

Feed pads and stand-off pads must be at least 20 metres away from surface water.

The maximum loading rate of effluent to irrigated land shall not exceed a depth of 25 millimetres per application and 150 kilograms of nitrogen per hectare per year.

Effluent must not pond on the land surface or create odour or nuisance outside a property’s boundary.

In my experience, farmers have been taking a real interest in understanding their soils better to help them manage effluents so as to get the best nutrient value out of them and protect the environment. The council is very keen to help with this and recently put ‘high and low-risk’ soil information on-line at www.waikatoregion.govt.nz/soilmapinfo.

Bala Tikkisetty is a sustainable agriculture coordinator at Waikato Regional Council. For more information contact him on 0800 800 401 or bala.tikkisetty@waikatoregion.govt.nz.