How to Read a Soil Analysis: pH, P, K and Mg Explained
A soil analysis is one of the cheapest ways to cut fertiliser bills and stay within phosphate limits — but only if you know what the numbers mean. Here is how to read the indices, the targets for Northern Ireland and the rest of the UK, and what to do about the results.
The Basics
Soil Indices and What They Mean
Most UK soil reports give each nutrient in milligrams per litre of soil (mg/l) and an index from 0 upwards. Index 0 means the soil is short and crops will respond strongly to fertiliser; higher indices mean the soil can supply more on its own. These are the bands from AHDB RB209 (2026):
| Index | P (Olsen, mg/l) | K (mg/l) | Mg (mg/l) |
|---|---|---|---|
| 0 | 0–9 | 0–60 | 0–25 |
| 1 | 10–15 | 61–120 | 26–50 |
| 2 | 16–25 | 121–180 (2-) / 181–240 (2+) | 51–100 |
| 3 | 26–45 | 241–400 | 101–175 |
| 4 | 46–70 | 401–600 | 176–250 |
| 5 | 71–100 | 601–900 | 251–350 |
Northern Ireland splits phosphorus index 2 into 2- (16–20 mg/l) and 2+ (21–25 mg/l) in the Nutrient Action Programme Regulations, because the phosphate limits for grassland depend on it.
Phosphorus
Phosphorus (P)
| Land | Target index | Source |
|---|---|---|
| Arable and grassland (UK) | 2 (16–25 mg/l) | AHDB RB209 |
| NI intensively managed grassland | 2+ (21–25 mg/l) | AFBI / DAERA |
| NI extensively managed grassland | 2- (16–20 mg/l) | DAERA NAP guidance |
| Vegetables | 3 (26–45 mg/l) | AHDB RB209 |
Above the target, the soil can supply what the crop needs and more phosphate simply raises the risk of losses to rivers and lakes — the main reason phosphorus is now the focus in Northern Ireland. In AFBI’s Soil Nutrient Health Scheme, which has sampled over 90% of NI farms, the average Olsen P across the first three zones was 26.7 mg/l — index 3, above the optimum. DAERA’s science group reports that around 40% of fields tested are at index 3 or higher.
Potassium
Potassium (K)
The target is index 2- (121–180 mg/l) for arable and grassland, in RB209 and in DAERA guidance. Silage fields lose a lot of potash in every cut, so K often falls fastest there. On sands and loamy sands the soil can’t hold much potash, so RB209 suggests a realistic upper level of about 100 mg/l (top of index 1). Slurry is a good source — most of its potash is available in the year it is spread; see the nutrient value calculator.
Magnesium
Magnesium (Mg)
RB209 lists index 2 as a target in its arable checklist, but also says index 1 is a satisfactory policy on light, stony or chalky soils in combinable rotations, where keeping index 2 is unnecessary and uneconomic. On grassland, magnesium matters most for stock: where there is a risk of grass tetany (hypomagnesaemia), RB209 says around 100 kg MgO/ha may be justified to keep soil magnesium at index 2. At index 0, RB209 suggests 50–100 kg MgO/ha every 3–4 years. If the field also needs lime, magnesian limestone supplies both.
pH and Lime
Soil pH and Lime
pH controls how well every other nutrient is used. RB209 gives an optimum pH, and suggests liming to a target 0.2 above it:
| Cropping | Mineral soil | Organic soil | Peaty soil |
|---|---|---|---|
| Continuous arable | 6.5 | 6.2 | 5.8 |
| Grass with occasional barley | 6.2 | 5.7 | 5.5 |
| Continuous grass | 6.0 (6.5 where clover matters) | 5.7 | 5.3 |
AFBI guidance for Northern Ireland is pH 6.0–6.5 for mineral grassland and 5.3–5.8 for peat. The Soil Nutrient Health Scheme found an average pH of 5.8 — below the range for mineral grassland — so a lot of NI grassland would benefit from lime.
Working out how much lime
RB209: lime needed (t/ha) = (target pH − current pH) × a factor for the soil type. For grassland (15 cm) the factors are 4 for sands, 5 for sandy and silt loams, 6 for clay loams and clays, 7.5 for organic soils and 12 for peaty soils (6, 7, 8, 10 and 16 for arable land to 20 cm).
The Rest of the Report
Organic Matter and Sulphur
Organic matter has no index — RB209 says it is difficult to define a critical level, and keeping or increasing it should be a priority. For nitrogen planning, RB209 treats under 10% as low to average in Northern Ireland and Scotland (under 4% in England and Wales).
Sulphur recommendations in RB209 aren’t based on soil tests. The risk is judged from soil type and winter rainfall, and confirmed with tissue tests. For grass, RB209 suggests 40 kg SO₃/ha before each silage cut where sulphur is needed.
Sampling
Taking a Good Sample
| Grassland | Arable | |
|---|---|---|
| Depth | 7.5 cm (15 cm for young leys or before reseeding) | 15 cm (about 23 cm if min-tilled) |
| How often | Every 3–5 years; every 3 if regularly cut | Every 3–5 years |
| When | September to February for long-term grass | Ideally straight after harvest |
| Cores | At least 25, in a W pattern | At least 25, in a W pattern |
| Area per sample | No more than 4 ha | No more than 4 ha |
Avoid gateways, troughs, dung and urine patches and feeding areas, and wait at least 12 weeks after phosphate, potash or manure and 6 months after lime. The Northern Ireland regulations set the same basic method for phosphorus testing — 75 mm for grassland, 150 mm for arable, no more than 4 ha per sample and at least three months after any phosphate.
FAQs
Frequently Asked Questions
Q.What is a good soil P index?
AHDB RB209 sets index 2 (16–25 mg P/l Olsen) as the target for arable and grassland. In Northern Ireland, intensively managed grassland is managed to index 2+ (21–25 mg/l), and extensively managed grassland to index 2- (16–20 mg/l). Above the target, the soil can supply the crop and extra phosphate mostly adds to the risk of losses to water.
Q.What is a good soil K index?
Index 2- (121–180 mg K/l) for arable and grassland, in both RB209 and DAERA guidance. Sands and loamy sands often can’t hold that much potash, so a realistic target there is the top of index 1 (around 100 mg/l).
Q.What pH should grassland be?
RB209 gives an optimum of pH 6.0 for continuous grass on mineral soils (6.2 with occasional barley), 5.7 on organic soils and 5.3 on peaty soils — and suggests liming to 0.2 above the optimum. AFBI guidance for Northern Ireland is pH 6.0–6.5 for mineral grassland and 5.3–5.8 for peat. Clover swards benefit from the upper end.
Q.How often should I take soil samples?
Every 3–5 years under RB209 (every 3 years for regularly cut grass). The Northern Ireland regulations treat sampling every fourth year as the basis for phosphate recommendations. Results can be used for up to four years.
Q.How deep should soil samples be taken?
7.5 cm for long-term grassland and 15 cm for arable land (and for young leys or grass about to be reseeded). Take at least 25 cores in a W pattern across an area of no more than 4 ha, avoiding gateways, dung pats and feeding areas.
Q.Should I test for sulphur?
RB209 doesn’t base sulphur recommendations on soil tests. It uses soil type and winter rainfall to judge the risk, backed up by tissue testing in spring — a grass sample below 0.25% sulphur, or with a nitrogen to sulphur ratio above 13:1, points to deficiency.
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