Interpreting Soil Test Results: A Practical Guide

A soil test only helps if you can interpret what it tells you. Many farmers receive test results and find themselves uncertain about what the numbers mean or how to act on them. This guide walks through how to read a soil test report, understand the key measurements, and translate results into practical decisions for your farm.

Soil testing provides a snapshot of your soil’s chemical condition - nutrient levels, pH status, and organic matter content. Interpreting these results correctly is the essential link between testing and improved soil management. Getting this right saves money on unnecessary inputs and identifies genuine deficiencies before they limit production.

Whether you test regularly or are starting for the first time, understanding your results puts you in control of your soil management rather than guessing or following generic recommendations.

Who this guide is for

This is for Irish farmers who have received soil test results and want to understand what they mean. It’s also useful for those considering testing who want to know what information they will receive. The focus is on standard agricultural soil testing as practised in Ireland, covering the main parameters reported by Irish laboratories.

What a soil test report contains

Standard Irish soil test reports include several key measurements. Understanding each one helps build a complete picture of soil condition.

pH

Soil pH measures acidity or alkalinity on a scale from 0 to 14, with 7 being neutral. Most Irish agricultural soils fall between pH 5.0 and 7.5.

What it means:

• Below 6.0: Acidic. Nutrient availability is reduced, particularly phosphorus and molybdenum. Aluminium may become toxic to roots. Lime is likely needed.
• 6.0–6.5: Slightly acidic. Suitable for most grassland. Some tillage crops prefer slightly higher pH.
• 6.5–7.0: Optimal range for most crops and nutrient availability.
• Above 7.0: Alkaline. Trace element availability may be reduced, particularly manganese, copper, and zinc. Common on limestone-derived soils.

pH directly affects how available other nutrients are to plants. A soil can have adequate phosphorus but if pH is too low, that phosphorus becomes locked up and unavailable. Correcting pH is often the most cost-effective first step in improving fertility.

Phosphorus (P) index

Phosphorus is reported as an index from 1 to 4, based on Morgan’s extractable phosphorus levels.

Index 1: Deficient. Response to phosphorus fertiliser is likely and often dramatic. Building soil P is a priority.

Index 2: Low. Response to fertiliser is probable. Maintenance plus build-up applications are appropriate.

Index 3: Target. Adequate for crop needs. Apply maintenance amounts to replace what crops remove.

Index 4: Excess. No response to fertiliser expected. Reduce or eliminate P applications until levels draw down.

The index system simplifies interpretation - you do not need to know the exact mg/L figure, though it is usually reported alongside the index. What matters is whether you are below, at, or above target.

Potassium (K) index

Potassium uses the same 1–4 index system as phosphorus.

Index 1: Deficient. Grass and crop response to potassium likely. Silage fields and light soils are particularly prone to deficiency.

Index 2: Low. Response probable. Build-up applications appropriate alongside maintenance.

Index 3: Target. Adequate supply. Maintain by replacing offtake.

Index 4: Excess. No fertiliser response expected. Reduce applications. Very high K can interfere with magnesium uptake in grazing animals.

Potassium behaves differently from phosphorus in soil - it is more mobile and more readily leached, particularly from light soils. Silage systems remove large quantities in each cut, making potassium status a common issue on intensive grass farms.

Organic matter

Organic matter is typically reported as a percentage of soil weight. Irish agricultural soils commonly range from 3% to over 20%, depending on soil type and history.

What levels mean:

• Below 4%: Low for mineral soils. May indicate degraded structure, limited water-holding capacity, and reduced biological activity.
• 4–8%: Moderate. Common range for well-managed mineral soils.
• 8–15%: High. Often found in soils with history of grass or organic inputs.
• Above 15%: Very high or peaty. Different management considerations apply.

Organic matter influences almost everything else - structure, water retention, nutrient cycling, and biological activity. Trends over multiple tests matter more than single readings.

Additional parameters

Some laboratories report additional measurements:

Magnesium (Mg): Important for animal health on grazing land. Deficiency causes grass tetany risk. Reported as index or mg/L.

Sulphur (S): Increasingly tested as atmospheric deposition has declined. Deficiency now common, particularly in high-yielding crops.

Trace elements: Copper, zinc, manganese, boron, and others may be tested where deficiency is suspected. Not routine in standard testing.

Lime requirement: Many labs calculate and report the tonnes of ground limestone needed to reach target pH, based on current pH and soil buffering capacity.

Understanding the index system

The index system used in Ireland simplifies nutrient management by grouping continuous measurements into practical categories.

How indices work

Rather than telling you “your soil has 5.2 mg/L phosphorus,” the index system tells you “your soil is at Index 2 for phosphorus.” This immediately indicates the appropriate management response:

• Index 1–2: Build up soil levels while meeting crop needs
• Index 3: Maintain current levels by replacing what crops remove
• Index 4: Reduce inputs and draw down excess

The index boundaries are set based on research correlating soil test levels with crop response. Fields at Index 1 or 2 typically show clear yield responses to fertiliser; fields at Index 3 or 4 typically show little or no response.

Index 3 as target

For most farming systems, Index 3 is the target for both phosphorus and potassium. At this level:

• Crops have adequate nutrient supply
• No yield penalty from deficiency
• Fertiliser use is maintenance only - economically optimal
• Environmental risk from excess is minimised

Getting all fields to Index 3 is a reasonable long-term goal, though it may take years of strategic application to build up depleted fields.

The problem with averages

Many farms have fields ranging from Index 1 to Index 4. Applying the same fertiliser rate across all fields wastes money on high-index fields while under-supplying low-index fields.

Individual field testing and targeted application based on results is far more efficient than blanket approaches based on farm averages.

Common patterns in Irish soils

Certain patterns appear frequently in Irish soil test results.

Low pH

Acidic soils are common in Ireland due to high rainfall, acidic parent materials, and nitrogen fertiliser use. Many fields test below pH 6.0 and would benefit from lime.

Low pH is the single most common limiting factor in Irish soils. Correcting it is often the highest-return investment available - lime is inexpensive relative to its impact on nutrient availability and grass production.

Variable phosphorus

Within-field phosphorus variation is often extreme. Areas receiving concentrated inputs (around feeders, near gates, in old farmyards) may be Index 4 while less-frequented areas of the same field are Index 1.

This variation means that single samples per field may miss important patterns. Sampling in multiple zones or using GPS-guided sampling reveals the true picture.

Potassium depletion on silage ground

Fields cut repeatedly for silage often show falling potassium levels. Each silage cut removes substantial potassium - typically 30–40 kg K₂O per tonne of dry matter. Without adequate replacement through fertiliser or slurry, indices decline.

Testing silage fields separately from grazed areas helps identify this pattern.

High potassium on livestock farms

Conversely, fields receiving regular slurry or farmyard manure often show elevated potassium. Slurry is high in potassium relative to other nutrients, and repeated application accumulates K in soil.

High-K soils can suppress magnesium uptake in grazing animals, increasing grass tetany risk. This is particularly relevant for spring-calving herds.

From results to decisions

Interpreting results is only useful if it leads to action. Here is how to translate test data into practical decisions.

Prioritise pH

If pH is below target, address it first. Nutrient applications to acidic soil are less effective because availability is reduced. Lime before fertiliser.

Calculate lime requirement based on current pH, target pH, and soil type. Light soils need less lime than heavy soils to achieve the same pH change. Many labs report lime requirement directly.

Address deficiencies

Index 1 and 2 fields should receive build-up applications alongside maintenance. The goal is to reach Index 3, where maintenance-only applications sustain adequate levels.

Build-up takes time - typically several years of above-maintenance application. Patience is required; soil nutrient levels change slowly.

Maintain adequate fields

Fields at Index 3 need only maintenance applications - enough to replace what crops remove. This is the economically optimal zone: adequate nutrients without waste.

Maintenance rates depend on crop type and yield. Silage removes more than grazing; cereals have different requirements than grass.

Reduce inputs on high-index fields

Index 4 fields do not need phosphorus or potassium fertiliser. Applying more wastes money and increases environmental risk.

Allow crop offtake to draw down excess levels. Monitor through regular testing to see when levels approach Index 3.

Consider organic manures

Slurry and farmyard manure supply nutrients that count toward requirements. Account for their contribution when planning fertiliser applications.

Targeting manure to lower-index fields and away from high-index fields helps balance nutrients across the farm.

Common interpretation mistakes

Several errors are common when interpreting soil tests.

Ignoring pH

Focusing on nutrient indices while ignoring low pH is counterproductive. The lime requirement may be the most important result on the report.

Single-sample assumptions

One sample point represents only a small area. Within-field variation means single samples may not capture the full picture, especially in fields with variable history.

Testing too infrequently

Soil conditions change over time. Annual cropping removes nutrients; lime effects wear off; organic matter trends develop over years. Testing every 3–5 years tracks these changes.

Not retesting after changes

If you have made significant changes - heavy lime application, nutrient build-up, different cropping - retest to confirm the expected effect.

Comparing different labs

Different laboratories may use different extraction methods or report results in different formats. Comparing results between labs can be misleading. Where possible, use the same laboratory consistently for comparable results over time.

Building a soil testing programme

Single tests are useful; systematic testing over time is powerful.

Baseline testing. Test all fields to establish current status. This reveals the starting point and identifies priority areas.

Regular monitoring. Retest every 3–5 years under consistent sampling protocols. This tracks trends and confirms whether management is achieving intended effects.

Consistent sampling. Use the same sampling depth, timing, and method each time. Variation in technique adds noise to results.

Record keeping. Maintain records of results, inputs, and management. This allows meaningful interpretation of changes over time.

For more guidance on sampling, contact Teagasc or your local accredited soil laboratory.

Frequently asked questions

Why do my results vary between samples in the same field?

Within-field variation is real. Nutrient distribution depends on history - where animals gathered, where inputs were applied, where water flows. This variation is information, not error. Consider zone sampling to capture it.

How accurate are soil test results?

Laboratory analysis is precise, but results depend on sampling quality. A carefully collected composite sample from a uniform area gives reliable results. A single core from a non-representative point does not.

Should I test every field?

Fields with different histories, soil types, or management should be tested separately. Very uniform areas under identical management might be grouped. In general, more resolution is better if budget allows.

How quickly should I expect changes?

pH changes slowly - lime takes 2–3 years to reach full effect. Nutrient indices change over years with consistent management. Expect gradual trends, not dramatic shifts.

What if I cannot afford recommended applications?

Prioritise based on likely return. Lime on acidic soils typically gives the best return. Target fertiliser to the most responsive fields (Index 1–2) rather than spreading thinly everywhere.

Do I need to test for trace elements?

Routine trace element testing is usually unnecessary unless deficiency symptoms appear or specific problems are suspected. Standard testing covers the main limiting factors for most farms.

Where to go from here

Soil test results are the foundation of informed soil management. Understanding what they tell you enables decisions based on evidence rather than guesswork.

If you have not yet tested, getting a baseline for your fields is the first step toward strategic management. If you have tested, acting on the results - addressing pH, building deficient nutrients, maintaining adequate levels - translates information into productivity.

Soil testing connects to the broader goal of soil health. Chemical fertility is one component alongside structure, biology, and organic matter. Understanding your test results helps integrate nutrient management into a complete approach to building productive, resilient soil.

For guidance on collecting samples, contact Teagasc or your local accredited soil laboratory.