Fertiliser use on Irish farms: crop nutrition and planning

Fertilising wheat: nitrogen allowance by soil index

The Teagasc allowances are agronomic optimum rates tied to a reference yield.

Before you read the table, write down the crop, variety, preceding crop, intended market and expected harvest for each field, and use the same field name on soil results, manure records and work instructions. If you expect more than the reference yield, note what that expectation is based on, since last year's output may not describe every field.

Where proof of higher yields is available, an additional 20 kg N/ha may be applied for each tonne above the reference yield. A 10 t/ha winter wheat crop on an index 1 site therefore has an allowance of 230 kg N/ha instead of 210 kg N/ha.

Agronomic optimum and economic optimum

The agronomic optimum is the nitrogen rate at which yield is maximised; nitrogen beyond it does not generate additional yield. The economic optimum is the rate at which profit is maximised once the price of nitrogen is set against the value of the grain; fertiliser beyond it might add yield but at a financial loss.

When fertiliser prices rise and the grain price does not match the rise, the economic optimum falls before the agronomic optimum, and the nitrogen applied should be adjusted accordingly.

Bring the actual fertiliser quotation and your grain price assumption to the adviser, and keep both with the recommendation. Then a later price change can be checked against the same basis.

Break-even ratio: adjusting the rate to fertiliser and grain prices

The relationship between fertiliser cost and grain price is the Break-Even Ratio (BER). The economic optimum occurs where the marginal yield response equals the BER.

For winter wheat with CAN (27% N) as the nitrogen source, the Table 1 rate is adjusted for grain price. At €200/t the adjustment is:

With CAN at €550/t and winter wheat at €200/t, the Table 1 rate is reduced by 27 kg N/ha, giving 183 kg N/ha for a 9 t/ha crop of winter wheat on a continuous cereal site.

The table is worked out for CAN. If a merchant offers another nitrogen product, ask which nutrient basis its price and calculation use before comparing offers. If a price or a crop expectation changes, note which one changed, keep the old plan, and leave applications already made as they were.

Lime, phosphorus and potassium come before nitrogen

The plan starts from up-to-date soil samples in a farm Nutrient Management Plan. Lime is the cheapest fertiliser and makes other nutrients more available. Phosphorus and potassium are matched to crop offtakes at soil indices 1 to 3. At soil index 4, where pH is lower than 7, there is no need to apply P and K fertiliser.

Straight fertilisers suit fields where only one nutrient is required. At soil index 1 or 2, drilling the compound fertiliser at sowing time is strongly worth considering.

Give the adviser the laboratory report and the sampling references, and mark on a map the area each sample represents. Label samples from different parts of a field separately. If the crop behaves unexpectedly, keep the full report and ask whether more soil or plant testing is needed before you assume nitrogen is the limit.

When comparing fertiliser products, ask the merchant for the declared nutrient content and how the grade is expressed. Keep product weight and nutrient weight in separate columns, with kilograms, tonnes and hectares marked, so a colleague can repeat the arithmetic if a consignment is substituted or the field area changes:

Leafwrit

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Organic manures and straw reduce the fertiliser needed

In fields where organic manures are applied, added fertiliser is reduced. Where straw is incorporated, potassium fertiliser is reduced accordingly.

Record each manure application with the material, source, date, quantity and field, and pass on any analysis you have. For straw, note whether it was removed or incorporated, and mark any area handled differently.

Sulphur for wheat: soils, symptoms and timing

Atmospheric deposition of sulphur in Ireland has fallen to about 1–2 kg/ha a year, so sulphur is now more likely to limit crop performance in intensive tillage and grassland systems.

Up to 95% of soil sulphur is held in organic forms that are not immediately available to plants. Mineralisation releases plant-available sulphate, driven by soil temperature, moisture and biological activity, so supply is often lowest in early spring when soils are cold.

Sulphate is not strongly held by soil particles and behaves like nitrate nitrogen, so it is readily lost through leaching, particularly in free-draining soils and during periods of high rainfall.

Light, free-draining soils such as Brown Earths and Brown Podzolics are more prone to sulphur deficiency and respond highly to sulphur fertilisation. Gley soils and peats supply more sulphur through mineralisation and are generally less responsive. Long-term tillage systems make deficiency more likely.

Sulphur is a component of the amino acids cysteine and methionine, and adequate sulphur supply is critical for efficient nitrogen use. Where nitrogen is applied at high rates, sulphur demand increases and deficiency is more likely if sulphur is not supplied alongside it.

If a crop looks pale, tell the adviser the soil texture, drainage and cropping history of the field. Describe which leaves are affected, how the symptom is spread across the field and when it first appeared, and photograph an affected patch next to a healthy one. Ask whether plant analysis is needed, and keep the sample date, leaf position and recent applications with the result.

Sulphur deficiency can resemble nitrogen deficiency, with pale or yellowing leaves. It appears first in younger leaves, because sulphur has limited mobility within the plant.

There is no reliable soil test to predict sulphur deficiency. Assessment is based on crop response, plant analysis and soil type. Adequate sulphur in plant tissue is above 0.2% of dry matter, with a nitrogen-to-sulphur ratio of less than 15:1; where these thresholds are not met, a response to sulphur is likely.

Sulphur is commonly applied with nitrogen fertilisers, as ammonium sulphate or blends containing sulphur. Because it leaches, applications are most effective when timed to coincide with active crop growth.

Tell the adviser which product and which pass you plan, and ask how the sulphur fits the existing nutrient plan. Do not assume every compound fertiliser supplies the sulphur the crop needs, and recheck the composition if the product is substituted.

Before the spreader or sprayer goes out, work out the nutrient rate first and calibrate the machine for the actual product with the manufacturer's settings. If an application is postponed, note the reason and do not record the planned date as work done.

Fertilizer for farming: how farmers use fertilizer on wheat

Farmers use fertilizer to supply the nutrients a crop takes up. Fertilizing wheat and fertilizing winter wheat start with the nitrogen allowance by soil index, after lime, phosphorus and potassium have been checked. Record every fertiliser you spread in the nutrient records.

Next step

Check the liquid-fertiliser nozzle guide if the fertiliser goes on as a liquid, read the winter wheat guide for variety and establishment, and keep the fertiliser records.