Soil Properties Critical when Applying Effluent

Dairy effluent is a great source of nutrients for growing pasture. But if not managed properly effluent can also be a significant source of contaminants which harm our waterways. Understanding how soil properties affect nutrient loss is a key to maximising the benefits of effluent on farm and minimising its impacts on waterways.

Soil 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. So effluent irrigation systems should be matched to soil properties to minimise runoff and leaching. The rate at which effluent can be applied to the land for maximum production benefit is determined by the soil’s properties including structure, porosity and infiltration rate.

The nature of the effluent and cattle treading on soils can affect the infiltration rate. Treading damage, which occurs most when the soils are wet, significantly reduces the infiltration rate. For some soils this can result in accumulation of effluent below slopes and in hollows. It can then enter surface waterways.

Movement of water through soil pores is generally 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 or if application rate exceeds infiltration rate.

Low rates of hydraulic conductivity are found in soils that are poorly drained, and ponding and runoff often occur with high rainfall. Many of these soils are artificially drained to reduce the incidence of ponding and water-logging, and this carries a risk that effluent can bypass the soil and be directed rapidly into waterways

Leaching occurs as excess water moves through the soil. So soils with lower water holding capacity are more susceptible to leaching, while soils with high water holding capacity (deep silt loams) can store significant quantities of effluent.

The soils that have low available water holding capacities, are the shallow to moderately deep soils, as well as sandy or stony soils. Effluent irrigation on these soils is likely to result in leaching unless it is applied at low rates and in small doses. The irrigation system on these soils must be capable of low rates of application to gain the maximum nutrient benefit.

Drainage and the level of biological activity of the soil at the application site are important. Aim to apply effluent at a rate that keeps it in the root zone so that the nutrients can be utilised by pasture.

Permeable soils with a deep water table and no drainage limits are best for putting effluent on. However, on stony soils the risk of effluent draining directly to ground water would be an issue to consider. In such situations, application depths and rates should be adjusted to account for this risk.

Another issue is "bypass flow". When effluent application rates are higher than infiltration rates, 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. This means little opportunity for the water to be retained within the root zone and high leaching of nitrate is 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.

Efficient effluent storage provides flexibility when it comes to application and helps maximise nutrient uptake (image: DairyNZ)

A key to avoiding over application can be having adequate effluent storage so that irrigation can be deferred if conditions aren’t right. DairyNZ has released a new smart-phone app to help farmers apply effluent more efficiently. The Dairy Effluent Spreading Calculator app provides dairy farmers and effluent spreading contractors with guidance around nutrient application rates based on the depth and type of effluent they apply.

H2Grow would like to thank Bala Tikkisetty for this blog post. Bala is a sustainable agriculture advisor at the Waikato Regional Council.

If  you are keen for further information about best practice for applying effluent you will find a raft of useful information on the Waikato Regional Council website.

Or alternatively contact Bala directly, email bala.tikkisetty@waikatoregion.govt.nz or call (freephone) 0800 800 401.