Planning fertiliser use in spring – what you need to think about
By Bala Tikkisetty
With a refreshed National Policy Statement for Freshwater Management around the corner, there is increasing pressure for
farmers to improve nutrient management in their farming systems.
As the soil starts warming up over the next few weeks, farmers will be preparing to fertilise their paddocks. Finding
that balance between getting best bang for buck while protecting economic and environmental bottom lines is critical for
farmers and requires advice from their fertiliser reps and consultants.
That’s because healthy soils are a balance of biological, physical and chemical properties, and are a dynamic mixture of
minerals, organic residues and living micro and macro organisms – all of which support farm production and provide
various ecosystem services.
As there are a range of risks when applying fertiliser, and strategies to help you avoid them, it’s recommended all
farmers have a nutrient budget and a nutrient management plan for their properties and discuss their situation with
their fertiliser rep.
There are a range of tools to help practice sustainable nutrient management.
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 impact of farm management practices.
Overseer, a computer decision support model, is being used to advise on nutrient management and greenhouse gas
emissions. It predicts what happens to the nutrients that are brought onto the farm in the form of fertilisers and
supplementary feed in the same way that a financial budget can track money.
When doing nutrient budgets in Waikato, bear in mind recent soil quality monitoring results that reveal high fertility
and compaction remain problems on dairy and some dry-stock sites.
Another issue to consider is nitrate leaching. Plants need nitrogen (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.
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 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.
Farmers can reduce the amount of phosphorus run off by keeping Olsen P to optimum agronomic levels. Other tips include:
following the Codes of Practice for FertMark and SpreadMark
applying fertiliser when the grass is in an active growing phase
leaving a grassed buffer strip between paddock and waterway – the strip filters the phosphorus before the run off
reaches the water
controlling run off from tracks, races, feed and stand-off pads.
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 nitrogen and phosphorus.
In New Zealand, the common nitrogenous fertilisers are urea (46 per cent N), ammonium sulphate (21 per cent N), DAP (18
per cent N) and calcium ammonium nitrate (27 per cent N). The form of nitrogenous fertiliser best used depends not only
on the cost per unit N, but also on the overall efficiency of the fertiliser N.