Key points to managing fertiliser in spring

As the soil starts warming up over the next few weeks and farmers prepare to fertilise their paddocks, finding the right balance is best done with advice from fertiliser reps and consultants.

Healthy soils are a balance of biological, physical and chemical properties, and are dynamic mixture of minerals, organic residues and living micro- and macro-organisms, all of which support farm production and provide various eco-system services.

As there are risks when applying fertiliser, and strategies to help avoid them, I recommend all farmers have a nutrient budget and a nutrient management plan for their properties.

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 used to advise on nutrient management and greenhouse gas emissions. It predicts what happens to the nutrients brought onto the farm in the form of fertilisers and supplementary feed in the same way that a financial budget can track money.

Fertiliser reps or consultants can help with nutrient budgets for those who don’t have them, while AgResearch recently released the new Overseer nutrient budget model version 6.2.2, which can be downloaded from http://www.overseer.org.nz 

One factor to bear in mind when doing nutrient budgets is that you may not have to fertilise if soils already have enough nutrients.

Another 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 N 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 from N because it binds with the soil and 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 NZ Fertiliser Manufacturers’ Research Association code of practice for nutrient management

- 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.

In summary, a clear assessment of fertiliser requirements will improve economic returns from pasture and help avoid contamination of ground and surface water with nutrients, particularly nitrogen and phosphorus.

Under moves to protect the health of key rivers, there is increasing pressure for farmers to improve their nutrient management because of the effects that nitrogen and phosphorus can have on water. Improving nutrient use efficiency is equally important for farm profitability.

• Bala Tikkisetty is a sustainable agriculture advisor at Waikato Regional Council