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.
AgResearch recently released the new Overseer® nutrient budget model version, which can be downloaded from www.overseer.org.nz.
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 un-used 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.
The amount of N leaching from pastures can be reduced by:
• timing fertiliser application to avoid periods when plant uptake of N will be low, such as when soils are saturated, during heavy rain, colder periods and times of low soil temperatures
• applying N fertiliser in split dressings (as many split doses as possible)
• irrigating farm dairy effluent to a large enough area
• adjusting fertiliser policy for effluent irrigated areas to account for the nutrient value of effluent
• using fenced wetlands and well-managed 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. Other tips include:
• 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.
There is increasing pressure for farmers to improve their nutrient management because of the effects that N and P can have on water, and because improving nutrient use efficiency is equally important for farm profitability.
So a clear assessment of fertiliser requirements will both improve economic returns from pasture and help avoid contamination of ground and surface water with nutrients, particularly N and P.
