Mark Plunkett: Efficient fertiliser application delivers a double dividend for the farm and the environment

The '4 Rs': Preparing a fertiliser plan and consulting it on a regular basis provides the basis for using applied nutrients in the right place with the right product, at the right rate and at the right time. PHOTO: ROGER JONES

Mark Plunkett

March 18 2021 07:17 AM

Over the coming weeks and months, cereal and other crops will receive the majority of their N, P, K & S requirements to build and develop grain yield potential.

This is World Water Quality week and there will be a number of online events to guide the efficient use of applied nutrients to reduce losses to the environment.

Using applied nutrients as efficiently as possible delivers a double dividend in protecting the environment and increasing overall farm profitability.

Targeting applied nutrients based on soil fertility status and crop nutrient requirements is the starting point.

Correct soil pH

Preparing a fertiliser plan and consulting it on a regular basis provides the basis for using applied nutrients in the right place with the right product at the right rate and at the right time ( the 4 Rs).

Lime is a key nutrient in correcting soil pH to increase nutrient efficiency (esp. N & P).

Maintaining optimum soil pH increases the availability of soil nutrients and also increases plant use efficiency of applied nutrients in manures & fertilisers.

Aim to maintain soil pH 6.5 to 6.8 depending on the crop rotation. For spring crops apply lime as recommended on the soil test report and work into the seedbed prior to sowing.

P & K Index 3

Building soil P and K levels to agronomic optimum P (> 6.1mg/l) & K (>101 mg/l) Index 3 will further increase N efficiency and build grain yield potential.

For spring barley select a suitable fertiliser blend and adjust application rates based on soil P & K Indexes. Supplying a good balance of N, P & K at establishment will be critical to getting the crop off to a good start.

The first three to six weeks are critical for rapid root and tiller development — the foundation for building yield potential. It will also ensure the crop has the ability to utilise applied nutrients (N & S) thereafter.

Aim to apply all P & K at sowing time and supply 25-30pc of the crop’s total N requirements at this stage.

Calculate crop N

To determine the crops N requirements take the average grain yield for the last 3 years.

For every 1 t/ha grain yield above the base yield of 6.5t/ha increase or decrease by 20kgN/ha.

For example, where continuous feeding spring barley (Soil N Index 1) has yielded an average of 7.5t/ha (3t/ac), this crop will require 155kgN/ha (135 +20). Adjust crop total N for seedbed N, therefore top dress the remaining at 110 to 115kg N/ha (85 to 90 units/ac) from early to mid-tillering stage.

This can be applied in a single application or alternatively a split application (70:30).

Splitting the application increases the N use efficiency while reducing the risk of N losses through either leaching or volatilization.

Where the above crop is grown after beans (Soil N Index 2) the total crop N requirements can be reduced by 35kgN/ha (28 units/ac) to take account of the N fixed by the previous spring bean crop.

This crop will require a total N of 120kgN/ha (96 units/ac) after beans.

Organic fertilisers

Where organic manures such as cattle or pig slurry have been applied adjust crop nutrient advice to take account of N, P & K in manures.

The table shows available N, P & K values per 1,000gals. For example, take an application of 2,000gals/ac of pig slurry: this will supply 38 units N/ac, 14 units P/ac & 40 units K/ac.

This can reduce a spring barley crop’s N requirement by 30pc and supply a proportion of the P & K requirements. Where possible have organic manures tested to determine actual nutrient values.

To maximise the use of organic manures ensure manures are well agitated and applied evenly.

For maximum N recovery apply with low emission techniques such as the trailing shoe and plough in within three to six hours.

Organic manures bring the advantage of adding soil organic matter which will help feed soil biology, help build soil organic matter & improve soil health.

Other nutrients

Cereals have a sulphur (S) requirement of 15kg S/ha. Sulphur increases plant N efficiency, so aim to apply S with main N top dressings. Cereals have a low requirement for magnesium (Mg) and in general Irish soils have a good Mg supply.

Trials would indicate that it is very difficult to find a yield response to Mg in cereal crops.

Cereals also have a requirement for trace elements such as copper, manganese and zinc.

Where soil test results show low levels (Index 1 or 2) or where there is a history of a deficiency, ensure good seedbed preparation and consolidation to improve root to contact and nutrient availability.

Apply trace elements at the two to four leaf stage and repeat where necessary.

Mark Plunkett is a Teagasc soil and plant nutrition specialist