How to Read a Soil Analysis Report

1. Soil pH

Soil pH is the measure of acidity or alkalinity — the single most important figure on your soil report. It directly controls how available nutrients are to plants and how effectively fertilisers and organic manures work. Even if you apply the correct amount of phosphate fertiliser, a soil at pH 5.5 will make less than half of it available to the crop.

UK agricultural soils typically range from pH 4.5 (very acid, upland peat) to pH 8.0 (naturally calcareous chalk soils). The optimum range for most agricultural crops is pH 6.0–7.0.

If your pH is below the target for your intended crop, lime is required. The amount depends on your soil type — clay and peaty soils resist pH change much more than sandy soils, so they need significantly more lime per unit of pH rise. Use the Lime Application Rate Calculator to calculate the correct rate for your soil.

2. Phosphorus (P) — Olsen P and P Index

Phosphorus on your soil report is usually shown as the Olsen P value (mg/litre) and a corresponding P Index from 0 to 4+. The Olsen P method is the standard extraction used by most UK laboratories and reflects the plant-available phosphorus in the soil.

Phosphorus is essential for root development, establishment and energy transfer in plants. It is relatively immobile in soil — it does not leach readily but also does not move far from where it is placed, which is why even distribution across the field matters.

Most UK arable fields should be maintained at P Index 2 — the target for the majority of arable crops and grassland. At Index 2, apply phosphate to replace what the crop removes (maintenance application). At Index 0 or 1, apply additional P above maintenance to build the index over several seasons. Never apply P fertiliser to an Index 3 or 4 field — you are adding cost with no agronomic benefit.

3. Potassium (K) — K Index

Potassium (K) is the nutrient that drives yield, water use efficiency and disease resistance. It is particularly important for grassland, root crops and potatoes. Unlike phosphorus, potassium can leach from light sandy soils — on these soils, annual top-up applications are essential even at Index 2.

K Index is split at Index 2 into 2L (lower) and 2U (upper) to help distinguish between the arable target (2L) and the grassland target (2U).

High K can suppress magnesium uptake by plants, particularly on grassland — if your K index is 3 or 4, check your Mg index carefully. In ruminants, high K grass is a risk factor for grass staggers (hypomagnesaemia) in spring.

4. Magnesium (Mg) — Mg Index

Magnesium is essential for chlorophyll production and is the central atom in every chlorophyll molecule — low Mg causes yellowing between leaf veins (interveinal chlorosis). In livestock systems, Mg deficiency in grass is directly linked to grass staggers in cattle and sheep.

Mg is measured in mg/litre and converted to an index. Sandy soils are most prone to Mg deficiency; heavy clay soils generally have adequate natural Mg.

If your Mg index is 0 or 1 and you are also liming, consider using magnesian limestone rather than standard ground limestone — it supplies both calcium and magnesium in a single application. If you are not liming, apply kieserite (sulphate of magnesium) or Epsom salt for faster availability.

5. Soil Organic Matter (SOM%)

Soil organic matter is reported as a percentage of the soil sample. SOM underpins soil health — it drives biological activity, water holding capacity, structural stability and the slow release of nitrogen, phosphorus and sulphur throughout the season.

High SOM increases the soil's buffering capacity — meaning it resists changes in pH and releases nutrients slowly. This is important when interpreting your lime requirement: a field with 5% SOM will need significantly more lime to raise pH by one unit than a field with 2% SOM of the same mineral soil type.

6. Sulphur (S) — an increasingly important nutrient

Sulphur is not always included in basic soil analysis but has become increasingly important since the UK reduced sulphur dioxide emissions from industry. Atmospheric sulphur deposition has declined by over 90% since the 1970s and soils can no longer rely on it as a free input.

Sulphur deficiency shows as yellowing of the youngest leaves first (unlike nitrogen deficiency which starts with the oldest leaves). It is most common in oilseed rape, cereals on light sandy soils, and grassland.

If sulphur is included in your analysis, a result below 10 mg/kg (or a soil sulphate-S below 10 mg/litre) indicates potential deficiency and a response to sulphur application is likely. AHDB recommend 25–50 kg SO₃/ha for most arable crops where deficiency is indicated.

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