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Five simple steps to boost and manage soil fertility

You don't feed your animals without looking at their condition -- so why do it to your soils? Very often, the soils of a farm are something that can be taken for granted.

Trends are now emerging which show that soil fertility levels are declining across all farm types as a result of the reduction in fertiliser usage in recent years. The proportion of soils being analysed with low fertility levels has increased by more than 30pc in the past four years (see figure 1, right).

Soils with low fertility have lower productivity and result in higher fertiliser input costs. For example, research work has shown that low soil phosphorus levels can cost the farm an average of around 1.5t of grass dry matter production per hectare per year. At current feed prices, this is potentially worth up to €400/ha.

Managing soil fertility in five steps

1) Soil samples: Have soil samples taken for the whole farm. Unless you know what is in the soil, it is impossible to know how much extra it needs. Repeating soil analysis over time is also critical to monitor the effectiveness of the fertiliser strategy.

2) Lime: Soil pH should be the first thing to get right if soil test results show a lime requirement. Lime should be applied to neutralise acidity and raise the pH.

For mineral soils, a pH of 6.3 is recommended for grassland and 6.5 for cereals. Acidic soils will result in reduced nutrient release from soil and poorer response to fertilisers.

3) Index 3: At optimum fertility levels, nutrients being removed in products need to be replaced. High fertility soils are a resource and should be exploited. Low fertility soils need to be nurtured. For simplicity, soil test results for each nutrient are divided into one of four categories called an index. The index of the soil is usually shown on the soil test report. The aim is to have soils in Index 3. For soils in Index 3, the fertiliser programme should be designed to replace the nutrients being removed, thus maintaining the soil fertility level. Indexes 1 and 2 are low, and require additional nutrients to increase the fertility levels. Index 4 is high, and presents an opportunity to save money on fertiliser inputs and exploit the soil resources.

4) Slurry and organic fertilisers: While organic fertilisers can be more difficult to manage than chemical fertiliser, they usually represent good value, particularly where high nutrient inputs are required to increase fertility levels.

Begin by using organic fertilisers as efficiently as possible, then top up with fertiliser as required. Target organic fertiliser applications at fields that have high phosphorus (P) and potassium (K) requirements. Apply in cool and moist weather conditions to promote high nitrogen (N) recovery.

5) Balanced nutrient supply: If one nutrient is deficient, no amount of another nutrient will overcome this.

For example, if a field is deficient in K, then excess N application will not be fully used. Make sure the fertiliser compound is supplying nutrients in the correct balance for the crop and soil, and to complement other fertilisers.

Implementing these simple steps will go a long way to ensuring that the production potential of the farm is being realised and that fertiliser inputs are being used as efficiently as possible.

Stan Lalor and Mark Plunkett are Teagasc researchers at Johnstown Castle, Wexford

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