Consider soil factors when applying effluent

As farmers know well, dairy effluent can be a great resource for nutrients. But if application is not managed properly it can also be a significant source of bacteria and nutrient leaching and run-off that can hurt the health of waterways.

Soil properties are one of the things to weigh up when managing effluent so that maximum benefit is achieved from the available nutrients and water is best protected.

Texture and structure determine the amount of water that can enter and be retained within a particular soil, and the rate of transmission of excess water through that soil.

Water balances, drainage characteristics and consequent leaching losses of effluent constituents are strongly dependent on those properties. Effluent irrigation systems should be matched to soil properties to minimise runoff and leaching.

Leaching occurs in response to movement of excess water from the soil. Therefore, soils with lower water holding capacity are more susceptible to leaching. For example, 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.

The majority of soils have moderate available water holding capacities. Soils that have low available water holding capacities, are the shallow to moderately deep soils. In both cases, low available water holding capacities are the result of restricted rooting depths due to the shallow nature of the soils and high water tables. Effluent irrigation on these soils is likely to result in leaching.

Drainage and the level of biological activity of the soil at the application site are important. Permeable soils with a deep water table and no drainage limitations are preferable. However, on stony soils the risk of effluent draining directly to ground water would be an issue to consider. In such situations adjust application depths and rates accordingly.

The rate at which effluent can infiltrate soil must be adequate to avoid runoff of effluent and prevent contamination of surface waterways. The nature of the effluent and cattle treading on soils can affect the infiltration rate.

Treading damage occurs by compaction and plastic deformation when the soils are wet, significantly reducing infiltration rate. For some soils this can result in accumulation of effluent below slopes and in intersecting hollows. It can then enter surface waterways.

Transmission of water in liquid form through pores is described as hydraulic conductivity. When hydraulic conductivity of the soil 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 hydraulic conductivity are found in soils that are poorly drained, and ponding and runoff often occur with rainfall events. Many of these soils are artificially drained to reduce the incidence of ponding and waterlogging.

The effect of treading on hydraulic conductivity within the top layers of soil is another issue. For pugged soils, application rates of effluent which are above 10 mm per hour are likely to result in ponding of effluent,if soil water content at the time of irrigation is high.

When the rate of application of water is higher than the infiltration rate, water can enter continuous macro-pores that are open at the soil surface, and then move very rapidly via so-called “bypass flow” through a relatively dry soil matrix. There is little opportunity for the water to be retained within the root zone. If irrigation of effluent occurs in such circumstances, deep penetration of solutes present on the soil surface or in the effluent can occur in a short time. Plants cannot, therefore, take up those solutes and high leaching losses of nitrate are likely to occur. Bypass flow of farm dairy effluent can occur in soils that undergo shrinkage and fissuring during drying, especially when these soils have been previously compacted by treading.

To summarise, soils can only hold a limited amount of water and that varies between soils. Also soils vary with the rate at which water can pass through the soil. For this reason, it is important to know your soil and ensure the rate that effluent is applied at does not exceed the ability of the soil to hold the water and transmit the water.

As effluent needs to be managed to avoid any adverse impact on natural and physical resources, the following rules operate in the Waikato region:

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

· 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 on to any part of the irrigated land shall not exceed 25 millimetres depth per application and 150 kilograms of nitrogen per hectare per year.

· Effluent must not pond on the land surface and must not create any odour or cause nuisance outside of your property boundary.

Having adequate storage to allow for wet periods is essential to enable farmers to manage effluent for maximum benefit and within the rules.