Getting value for fertiliser dollars

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 for farmers and requires advice from their fertiliser reps and consultants as it’s a pretty technical area.

That’s because soils are a very dynamic mixture of minerals, organic residues and living micro and macro organisms – all of which support farm production.

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 nitrogen and phosphorus.

There are a range of tools for helping make this assessment.

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 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 nutrient management plan builds on the budget and identifies what the farmer will do to improve the efficiency of the farm nutrient system and minimise losses to the environment. There’s a wide range of options for reducing losses, depending on individual situations, and many of them can be found in the ‘‘menus of practices to improve water quality’’ on the Waikato Regional Council website.

An understanding of the behaviour and fate of nutrients in the soil-plantanimal system helps in preparing a nutrient management plan, as does a plain English knowledge of some important terminology when using the Overseer model.

Mineralisation involves conversion of soil organic nitrogen into plant available forms.

Mineral N stands for those nitrogen fertilisers used to directly supplement the nitrate and ammonium pools in soil.

Ammonification is the breakdown of organic nitrogen (from dung, urine and dead plant material) into ammonium ions. These ions are subsequently nitrified in the presence of a good oxygen supply to nitrate ions. Ammonification is carried out by a variety of soil micro-organisms.

Nitrification involves the biological conversion of ammonium to nitrate. Nitrate concentrations in aerobic, warm, and moist cultivated soils are normally higher than ammonium concentrations, so in such soils the main source of nitrogen taken up by plants is nitrate. This is because in warm aerobic soils the rate of nitrification is rapid. In acidic soils nitrification is slow and ammonium is probably the main plant nitrogen source.

Immobilisation is the conversion of plant available nitrogen into organic form. If the mineralisation rate is less than the immobilisation rate then net-immobilisation occurs and pasture and crops growing in such a soil will become nitrogen deficient. Whether net-mineralisation or net-immobilisation occurs depends mostly on the carbon to nitrogen (C:N) ratio in the organic substrate added to the soil.

De-nitrification involves microbial reduction of nitrate to nitrous oxide and nitrogen gas. This process requires, in addition to soil micro-organisms, a source of nitrate and decomposable organic matter, and anaerobic conditions such as water logged soils.

Volatilisation is conversion of ammonium to ammonia gas. This commonly occurs in animal urine spots and after urea application during high temperatures.

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).

There is increasing pressure for farmers to improve their nutrient management because of the effects that nitrogen and phosphorus can have on water, and because improving nutrient use efficiency is important for farm profitability. So it’s important for farmers to work closely with their qualified nutrient management advisor and their farm consultant to ensure that, as much as possible, the nutrients in their farm system are used productively.

The result will be optimum production and cleaner water.