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Soil risk map - soil risk for farm dairy effluent application

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Caveat - please read first

The information used to create the soil risk map for farm dairy effluent (FDE) exists at the regional scale of 1:50 000. The soil risk map information is therefore indicative only. It is important to undertake a site-based assessment of the soil risk for FDE application specifically for the soils on your farm using the Pocket Guide provided by DairyNZ.

Note: the soil risk map for farm dairy effluent application is currently under review.

Important

Soil risk for farm dairy effluent application

Soil risk for farm dairy effluent application

About the soil risk map and soil risk categories

There are over 400 soils in the Waikato region.

Each soil type behaves slightly differently when liquid is applied, and this means the risks vary for each soil when applying effluent in different weather conditions. For example, the risk could be a result of saturated soil resulting in effluent run off across the surface, or could be a result of soil conditions that create cracks allowing effluent to bypass the root zone to the water network below.

Soil risk types

DairyNZ has produced a guide to determine soil risk for farm dairy effluent application (see the document link on this page) in the field that has helped to group the soils into simple High risk and Low risk categories. This information has been integrated into the soil risk for farm dairy effluent application map.

The table below summarises the soil types:

High risk Low risk
Artificial drainage or coarse soil structure  Well drained flat land (<7°) *
Impeded drainage or low infiltration rate  Other well drained but very stoney flat land (<7°)*
Sloping land (>7°) or land with hump and hollow drainage  Low water table (>50 cm) peat soils with good structure.

High risk soils

High risk soils have an increased risk of effluent loss to waterways. This is because the preferential or overland flow water transport mechanism that reduces contact time with the soil within the root zone where nutrient utilisation takes place. Careful management of effluent through deferred and low application rates will minimise the risk.

Low risk soils

Low risk soils drain liquid more evenly through the soil profile as a result of their porosity and fine soil structure. Therefore these soils have a greater frequency of when effluent can be applied, with very small or zero water deficits, because the liquid rarely runoffs, ponds or moves preferentially through the soil when draining. They cannot however receive effluent when they are already saturated. Most of these soils also have a large water holding capacity that allows greater volumes to be applied; however, shallow and stoney varients can only handle small volumes of liquid safely in one event. 

*Whole farm nutrient risk situations

Low risk soils can still be at risk of nutrient loss to ground water because of their free draining characteristic which makes them ‘leaky’.

If nutrients are applied in quantities greater than can utilised by the plant, then there is risk that these will move past the capture area of the roots with their value lost to the farmer. This is accelerated in periods of high rainfall when the soils are saturated and through drainage occurs. In the cooler months the plants requirement for nutrients is reduced as the growing rate is slower. Therefore, it is important to adjust nutrient loading rate in late autumn and early winter when these situations are likely to occur.

For effluent management it is important to understand the nutrient content of your effluent by having it tested, as due to farm system variations the concentrations vary considerably. For example, effluent derived just from dairy shed wash down may only be 0.5 kg/m3 total nitrogen, so applying 5mm of effluent would apply—22.5 kg/ha of nitrogen. In comparison a feedpad derived effluent may have a concentration of 1.35 kg/m3 total nitrogen so applying 5mm of effluent would apply 67.5kg /ha of nitrogen.

Understanding your farm soils and the content of your effluent can help plan applications and by adjusting rates and timings accordingly can to help minimise nitrogen leaching as well. Your nutrient budget can assist with this.