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Six factors affecting fertiliser rates

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Achieving maximum yields from your land requires healthy and fertile soil. Here are six tips from the Department of Environment and Primary Industries on getting the balance right.

Factors which Influence Fertiliser Rates 

1. Soil fertility - A soil test is a good guide in assessing nutrient status and fertiliser requirements. A common approach where soil nutrient reserves are very high, is not to apply that nutrient, but to utilise the soil reserves until a pasture response to applying that nutrient is more likely.

If soil tests show extreme acidity or salinity, then responses to fertiliser will be limited until soil is limed or the salinity level decreased.
Fertiliser test strips are a useful tool to help identify whether it’s worthwhile applying a particular nutrient over a whole paddock.

2. Stocking rate
Fertilisers are a cheap way to provide extra feed when an increase in stocking rate is planned. Phosphorus in particular has been shown to increase pasture production if other factors are not limiting pasture growth. Where a decision has been made to increase stocking rates well above the typical stocking rate for a given district, then increased fertiliser use above the typically recommended rates is recommended. Increasing stocking rates is not an option for some farmers as it exposes them to extra risk and may involve changes in management.

3. Pasture utilisation
- Stocking rate should be matched as closely as possible to the amount of feed produced. Pasture that is grown and not eaten or cut for hay or silage is wasted pasture. It is therefore pointless and costly to increase fertiliser use to increase pasture production if existing pasture is already poorly utilised.

Pasture utilisation can be estimated by assessing the quantity of pasture residues from the previous spring that are still present in the following late
winter/early spring period.

Improved pasture species generally have higher protein content and digestibility than unimproved species and therefore should be encouraged to increase pasture utilisation.

4. Pasture composition - Often, native grasses, introduced annual grasses, weedy perennial grasses and weeds do not respond well to fertiliser. Improved pasture species such as white clover, sub clover, Phalaris, perennial and annual ryegrass do respond well to higher fertility. Therefore, fertiliser priority should be given to newly sown pastures or pasture with a reasonable content of and improved pasture grasses.

Because of the time and money invested in sowing new pastures it is crucial that they receive adequate fertiliser.

Pasture composition can often improve markedly with increased fertiliser and/or grazing pressure. Some paddocks, however, will require renovation with improved pasture species before a worthwhile response to fertiliser can be achieved.

5. Growing extra feed - If hay or silage is to be cut or extra feed is required at certain times of the year, extra fertiliser may need to be considered. It is generally more profitable to utilise extra feed by grazing than by cutting hay or silage.
When favourable seasonal conditions result in an early autumn break, the early production of feed may influence some farmers to reduce fertiliser rate for that year. However, long-term soil fertility needs to be considered. Late breaks may mean a shortage of winter feed and encourage the use of fertilisers to promote pasture growth. Where the growth period over summer has been extended by regular rains and cool weather,
extra fertiliser could also be considered.

6. Economic considerations - A common response by farmers is to only apply fertiliser when they can afford it. However, other things also need to be considered.

• Optimum economic rate - If a particular soil nutrient is limiting pasture growth, then each extra kilogram of that nutrient applied will increase pasture production until a point is reached where at a particular rate of that nutrient no extra pasture production is gained. This rate is the point of maximum production, but it is not an economic fertiliser rate to aim for. The economic optimum rate of fertiliser is below the point of maximum production and occurs when the cost of the extra fertiliser used starts to be higher than the value of the extra pasture grown. Some value has to be placed on the pasture produced and the fertiliser used, to establish this optimal fertiliser rate.

• Cost of fertiliser - The cost of fertiliser is the cost ex. works plus transport and spreading costs. Before spending money on fertiliser, consideration should be given to alternative investments and returns.

• Value of pasture grown - It is difficult to put a dollar value on extra pasture produced. Extra feed is likely to be worth more on a dairy farm than on a sheep farm. As the aim is to convert pasture grown into a saleable commodity such as wool, milk meat or hay, commodity prices will have a big impact on the value given to extra pasture grown.

Image: www.magic1278.com.au 

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