App to guide effluent application
Taking different soil types into account is a key aspect of the dairy farming sector’s ongoing efforts to improve effluent management and associated issues.
Farm dairy effluent can be a significant source of water contaminants, either by direct run-off or through nitrogen leaching.
The amount of water (or ‘‘hydraulic loading’’) in soil and various soil properties influence these types of effects.
For example, soil texture and structure determine the amount of water that can enter and be retained in a particular soil, as well as the transmission rate of excess water through soil.
So these various soil properties should be weighed up when deciding on effluent irrigation systems, and when applying effluent to pasture, to maximise nutrient benefits and waterway protection.
When discussing such decisions with effluent advisers, it’s worth bearing in mind a number of factors.
Leaching occurs in response to movement of excess water from the soil, meaning soils with lower water-holding capacity are more susceptible to leaching.
Conversely, soils with high water-holding capacity (deep silt loams) can store significant quantities of effluent.
Soils with low available water-holding capacity are the shallow to moderately deep soils, as well as sandy or stony soils.
Generally speaking, lower water-holding capacity is the result of restricted rooting depths because of the shallow nature of the soils and high water tables.
Effluent irrigation on shallow soils with high water tables is likely to result in leaching.
Permeable soils with a deep water table and no drainage limitations are best for putting effluent on.
However, on stony soils the risk of effluent draining directly to ground water must be considered. In such situations, application depths and rates should be adjusted to account for this risk.
Meanwhile, it’s important to take into account the rate at which effluent can infiltrate soil. If too much is put on too quickly, this can lead to runoff into waterways.
The nature of the effluent and cattle treading on soils can affect the infiltration rate.
Treading damage by stock, which occurs most when the soils are wet, significantly reduces infiltration rate. For some soils this can result in effluent accumulating below slopes and hollows. It can then enter surface waterways.
Water transmission through soil pores is generally described as hydraulic conductivity. When soil hydraulic conductivity is low, effluent irrigation 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 high rainfall. Many of these soils need to be artificially drained to reduce the incidence of ponding and water-logging.
For pugged soils affected by treading, effluent application rates which are above 10mm an hour are likely to result in ponding if soil water content at the time of irrigation is high.
When effluent application rates are higher than infiltration rates, water can enter macro-pores open at the soil surface, and then move very rapidly through socalled ‘‘bypass flow’’ through a relatively dry soil matrix.
This gives little opportunity for the water to be retained in 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.
One of the keys to avoiding overapplication can be having adequate effluent storage so farmers can defer irrigation 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.
The app allows for nutrient application rates for dairy effluent to be easily calculated, based on a number of customisable inputs so effluent nutrients can be applied with greater precision.