Soil fertility first: making every kilo of fertiliser count

With fertiliser prices still volatile in recent years and tighter environmental rules in place, the best return on any farm this spring will come from soil fertility done right, Padraig Glynn, Teagasc Advisor, tells us more.

Teagasc research is clear: fields at the correct pH with Index 3 phosphorus (P) and potassium (K) use nitrogen (N) far more efficiently, grow more grass or crops, and waste less nutrient to the environment.

Start with soil samples

You can’t manage what you don’t measure. Whole-farm soil sampling every 3–4 years underpins a compliant Nutrient Management Plan (NMP) and the Department’s National Fertiliser Database requirements. One sample for a maximum of 5 ha per sample (ideally <4 ha), taken with a corer to 10 cm depth. At a small cost per hectare, samples pay for themselves by directing lime, slurry and fertiliser to where they’re needed most and reducing or omitting them where they’re not. For arable land, soil sampling is mandatory, and without a valid test, P Index 4 is assumed—meaning no chemical P allowance and no organic P imports. Farms with a grassland stocking rate over 130 kg N/ha must also have valid samples to justify chemical P.

Lift pH with lime firstly as lime is the cheapest fertiliser you can buy. On mineral soils, aim for pH 6.3 for grassland and 6.5–6.8 for tillage. Correcting acidity unlocks N already in the soil, improves P availability (often lifting soils by up to one P Index over time), and boosts response to applied fertiliser. Teagasc shows N use efficiency can rise from about 35% on low-fertility soils to over 50% by fixing pH, and to around 63% where pH, P and K are all at target. Spread lime as per the soil report, prioritising the most acidic fields, and plan applications over 1–3 years if large tonnages are required.

Target Index 3 for P and K Index 3 is the “optimum” zone that balances yield, nutrient efficiency and cost. Index 1 and 2 fields are short and give the biggest response to applied P and K—these need a build-up strategy. Index 3 fields need maintenance rates to replace offtakes. Index 4 presents a saving opportunity: you can harvest the soil reserve for a period (often 2–3 years, depending on the test value) and omit P or K, then retest and revert to maintenance advice.

Make best use of slurry and FYM organic manures as these are your lowest-cost P and K. Typical cattle slurry contains about 9 units N, 5 units P and 32 units K per 1,000 gallons (guideline—test if possible). Use low-emission spreading (LESS) where applicable to capture more available N and place spring applications on high-demand fields. Target silage ground first (highest offtake), then low index grazing fields. Top up with fertiliser to reach your P and K targets.

Choose balanced fertiliser products. After lime and slurry, select compounds that match your soil indexes, stocking rate and enterprise:

• Grazing platforms (at Index 3) typically need modest P and K to replace yearly offtakes; products like 18-6-12 from March to August can maintain indexes where required, with protected urea used to supply N in line with measured grass growth. Where clover is present and fixed N rises, N rates can be reduced accordingly.
• Silage fields remove large amounts of K and significant P—prioritise slurry plus top-ups (e.g., protected urea and muriate of potash or N+K blends) to achieve target total N and restore K.
• Tillage crops require pH 6.5–6.8 and a balanced P-K-S programme, with placement P especially valuable at sowing on Index 1–2 soils.

Key compliance points to note

• Soil samples required to justify chemical P on arable and on grassland farms >130 kg N/ha; no samples mean P Index 4 is assumed (no chemical P).
• Sample: whole-farm soil analysis every 3–4 years, mapped by field.
• Lime: correct pH to target for enterprise; schedule deliveries and spreading windows.
• Index 3: build P and K where low; maintain where optimal; harvest reserves (temporarily) where high—then retest.
• Slurry: apply early, target high-demand/low-index fields, and use LESS to capture N.
• Balance: choose fertilisers that complement organic manures and include sulphur where warranted; remember one missing nutrient limits the response to others.