Nitrate loss is a big risk in wet weather
OPINION: It’s been a wet one all right – Metservice is already describing 2017 as ‘‘the year it didn’t stop raining’’ with Waikato rainfall at record levels for January to September.
It is a legacy farmers are taking into spring as they prepare to fertilise paddocks in coming months with those saturated soils starting to warm up.
Generally speaking, getting the best bang for buck out of fertiliser while protecting economic and environmental bottom lines is a key goal for farmers.
Finding that balance can be tricky and requires advice from qualified farm consultants and nutrient management advisors.
There are a range of risks when applying fertiliser. I recommend all farmers have a nutrient budget and a nutrient management plan for their properties and discuss their situation with their fertiliser representative.
It’s also a requirement of our current regional plan to have such a budget and plan if nitrogen (N) use exceeds 60 kilograms per hectare per year.
Nutrient budgeting is widely accepted as the appropriate first step in managing nutrient use and it’s also the preferred tool for evaluating the environmental impacts of farm management practices.
Overseer, a computer decision support model, is used to advise on nutrient management and green house gas emissions.
It predicts what happens to the nutrients that are brought on to the farm in the form of fertilisers and supplementary feed in the same way that a financial budget can track money. A new version of Overseer was recently released by Agresearch.
An important issue to consider is nitrate leaching.
Plants need N for healthy leaf growth, but N is an extremely mobile nutrient. If more nitrogenous fertiliser is applied than plants can take up most of the unused nitrogen ends up leaching down through the soil into groundwater.
Sometimes N will also be lost to waterways as run-off and some is always released back into the air as gas.
Time fertiliser application to avoid periods when plant uptake of N is low, such as when soils are saturated, during heavy rain, cold periods and low soil temperatures.
Applying N fertiliser in split dressings.
Irrigating farm dairy effluent to a large enough area.
Adjusting fertiliser policy for effluent irrigated areas to account for the nutrient value of effluent. Use fenced wetlands and wellmanaged open drains as nutrient traps. The nutrient phosphorus (P) behaves very differently to N because it binds with the soil and only dissolves slowly in water over time.
❚❚❚❚This means it doesn’t readily leach to groundwater.
But it can damage the health of waterways through soil erosion and surface run-off into water. Typically, 80 per cent of P loss can sometimes come from about 20 per cent of the farm area.
Farmers can reduce the amount of P run-off by keeping Olsen P to optimum agronomic levels and by:
Following the NZ Fertiliser Manufacturers’ Research Association Code of Practice for Nutrient Management. Applying fertiliser when grass is in an active growing phase.
Leaving a grassed buffer strip between paddock and waterway – the strip filters the P before the run-off reaches the water.
Controlling run-off from tracks, races, feed and stand-off pads.
Bala Tikkisetty is a sustainable agriculture advisor at Waikato Regional Council.
Handling a wet winter.