Testing Determines Fertilising Rate; Yes, You Can Put on Too Much Fertiliser

FOR most farmers, fertiliser comprises a large part of their seasonal budget.

In an effort to help farmers gain optimum results from their use of fertilser, and their investment, the following questions were put to Greg Sneath, the technical manager for the New Zealand Fertiliser Manufacturers’ Research Association.

What is the value of soil testing in order to get the right fertiliser mix for the property?

Answer: Soil and herbage tests can help with deciding on nutrient requirements and the best rate of application. This is particularly the case if there is a good history of testing, conducted at the same time of the year, on the same blocks of land, using reliable sampling methods. It is important to identify the trends in changes in soil nutrient levels which occur over time. Annual monitoring of soil nutrients, and the use of nutrient budgeting with the help of an accredited fertiliser advisor using models such as Overseer, can help ensure the right amount of nutrients are applied at the right time.

Under today’s intensive farming systems, it is even more important than ever to ensure fertiliser applications are based on accurate information and sound advice from accredited advisors.

What are optimum Olsen P Levels?

Answer: Olsen P is a measure of what is believed to be `plant- available’ phosphorus, and the optimum Olsen P level for an individual property will depend on both the soil type, the crop being produced, the type of livestock enterprise and the economic objectives of the farmer.

Phosphorus in the soil is either bound to soil colloids (clay and organic matter particles) immobilized in organic matter or exists in soil solution.

As plants use soil solution P, more P is released from the soil colloids and becomes plant available.

Fertiliser P is applied to build up the amount of P on soil colloids and inorganic forms (capital P), so that the soil can act as a store of P for plants throughout the growth cycle.

Once built up to the optimum P level for the individual farm, further P fertiliser is used to keep the soil store `topped up’ (maintenance P) by replacing P used up in the production of meat, milk or wool.

On average, for pasture on ash soils (such as you might find in dairying in Taranaki) and for sedentary soils, target Olsen P levels for near maximum pasture production range from 20 to 30, while on pumice and peat, target ranges are 35 to 45.

What are the best times for fertiliser application?

Answer: Timing can be very important if you are to achieve the best results and the most efficient use of applied fertiliser. It needs to be applied under the right conditions, and at a time to match crop growth. For nutrients which are bound tightly to soil colloids — phosphorus, for example — single applications may suffice, and for routine maintenance applications onto pasture, the timing is not critical. For fertilisers which are easily leached — eg, nitrogen, potassium and sulphate sulphur — split applications during the growing season can be important, particularly if the soil type is course and there is high rainfall.

For nitrogen, if pasture growth is restricted by cold weather, waterlogged conditions, or even very dry conditions then a response to the fertiliser will be limited, wasting much of the fertiliser applied. The timing should match anticipated demand for pasture growth.

Pasture growth in mid-spring is generally more consistent than autumn, when responses are often smaller. Summer responses are aided by irrigation or good reliable summer rainfall. Winter applications have the greatest risk of nitrogen losses from leaching and de- nitrification but with care can provide valuable feed for different stock classes.

Is there such a thing as over-application?

Answer: Fertiliser applications might be applied with two different goals in mind.

satisfactory soil nutrient levels in a desirable range. These applications replace nutrients which are `harvested’ when produce is sold off the farm or nutrients are transferred around the farm to non-productive areas as dung and urine, or lost to plants by soil processes.

Capital applications of fertiliser are provided to gradually build up the nutrient levels where some nutrients might be deemed to be deficient. It is important that fertiliser applications are well considered and the ratios of appropriate nutrients are kept in balance. Some nutrients can interact and suppress uptake of another.

Nutrient availability is also affected by factors such as soil type and pH. If measurements and guidelines are not followed, it is possible to apply more of a nutrient than is required — ie, over- application of fertiliser.

Too much fertiliser can not only waste a lot of money, but excesses can create serious nutritional problems for the crop / pasture and contribute to serious degradation of the environment. Efficient and responsible use of fertiliser requires careful consideration, which is aided by soil, plant and animal tissue testing and by the principles and guidelines found in the Code of Practice for Fertiliser use.

What are the possibilities of more stringent regulation due to environment concerns (following the international trend)?

Answer: The Resource Management Act requires everyone to avoid, remedy or mitigate any adverse effects which their activities have on the environment. With increasing urban pressures and intensification of agricultural practices, there has been growing awareness of the resultant impact on waterways and sensitive environments.

While on the whole, fertiliser use is a `permitted’ activity, it is important that fertilisers are used responsibly and in keeping with the guidelines provided by the “Code of Practice for Fertiliser Use”.

Increasingly, the code is being included in regional plans for land management practices, to help farmers to comply with their obligations under the Resource Management Act. In sensitive areas, regulations are being considered for controlling the levels of nutrient discharge into waterways (whatever the source).

It is increasingly important to be environmentally aware, and ensure responsible land management practice is employed at all times. Indeed, demands from overseas markets such as those in Europe, are beginning to require evidence of environmentally sustainable practices — that’s not to underplay, of course, our own motivation for safeguarding our land and waterways for New Zealanders.

Are there any new or technological breakthroughs related to fertilisers?

Answer: Most recently, the benefits of nitrification inhibitors have received a lot of attention for their potential to reduce the environmental impact of nitrogen, and increase efficiencies in fertiliser use.

They are in themselves not new, but the way they are now being applied is new, and they show great promise in reducing greenhouse gas emissions, and for reducing the leaching losses of nitrate from pasture.

The new interest relates not so much to applied fertiliser, but to the very localised high nitrogen levels in the form of urea deposited in livestock urine patches in intensive farming systems.

These urine patches receive far more nitrogen than the pasture can use, and nitrogen is lost in the form of nitrous oxide, (a greenhouse gas) or as nitrates which are leached into ground water. Nitrification inhibitors slow the conversion of ammonium to nitrate, and have been shown to greatly reduce greenhouse gas emissions, and to reduce leaching losses from urine patches.

With today’s more intensive farming systems these new products, along with more sophisticated nutrient modeling and nutrient budgeting processes, farm mapping and GPS application systems, are helping to `fine tune’ the efficiencies in fertiliser application and use.

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