How to Read a Soil Test Report: Annotated Fields and Blend Decisions (2026)

Learning how to read a soil test report is the fastest way to stop guessing about fertiliser. A soil analysis report looks like a page of numbers, but each field answers a practical question: is the soil acidic, which nutrients are low, and can the soil hold what you apply? This guide walks through the common fields in the order most laboratories print them, explains what each one means, flags where methods and units differ between countries, and then shows how the results lead to a blend decision. It is written for growers, resellers and agronomy teams who buy fertiliser in volume, but the interpretation steps apply equally to a small farm or a market garden.

Key takeaways

• Check the method and units before the number: Colwell, Olsen, Bray and Mehlich-3 phosphorus values are not interchangeable.
• Read pH, salinity and organic carbon first, because they decide whether nutrient additions will pay.
• Phosphorus needs two numbers to interpret: the extractable P and a buffering measure such as PBI.
• Soil test results are a snapshot of one sample; use paddock history, crop removal and a yield target to turn them into rates.
• The blend decision comes last: fix limiting factors, then match nutrient ratios to the gap between the test and the target.

What a soil analysis report actually contains

Most reports have the same skeleton: sample details, a table of measured values, a rating or interpretation column, and sometimes a recommendation. Laboratories in different countries use different extraction methods, so the first job is to know what you are looking at.

Sample details, depth and paddock identity

Confirm the paddock name, sampling date, GPS or zone reference and the depth. Topsoil is usually 0-10 cm (0-4 in) or 0-15 cm (0-6 in), and some nutrients, notably sulphur and nitrate nitrogen, are often sampled deeper, for example 0-60 cm (0-24 in). A result is only meaningful against a critical value for the same depth and method. If two reports for the same paddock disagree sharply, a difference in depth or sampling pattern is the first thing to check.

Laboratory methods and units

Concentrations may appear as mg/kg, ppm (the same thing for practical purposes), or as lb/acre in some regions. Cation exchange capacity appears as cmol(+)/kg in Australia and much of the world, and as meq/100 g in older or North American reports; the numbers are equivalent. Percent values such as organic carbon are by weight. Always read the footnotes, as these state the extraction used for each field.

Ratings versus raw numbers

Ratings such as low, marginal, adequate or high are based on critical values, the soil test level at which 90-95 percent of maximum yield is reached in trials. Those values depend on crop, soil type, region and method. A rating printed by a laboratory built for one region can mislead if your crop and soils sit elsewhere, so treat it as a starting point and discuss it with a local agronomist.

Reading pH, salinity and organic carbon first

These three fields set the conditions in which every nutrient behaves. Fertiliser applied to a soil with a serious pH or salinity problem often returns less than the same product on a sound soil.

pH in water versus calcium chloride

Soil pH is reported either in water (pH w) or in 0.01 M calcium chloride (pH CaCl2). The CaCl2 value is typically about 0.5 to 1.0 unit lower than the water value, because the salt solution mimics soil water more closely and is less affected by seasonal salt changes. A pH CaCl2 of 5.0 is not the same as a water pH of 5.0. Many crops begin to see aluminium toxicity risk when pH CaCl2 falls below roughly 4.8-5.0, but sensitivity varies by species and soil. Alkaline soils, above about pH CaCl2 8, can reduce the availability of zinc, manganese and iron, and phosphorus can be fixed with calcium.

Electrical conductivity and salinity

Electrical conductivity (EC) estimates soluble salts, in dS/m or mS/cm. Check whether the laboratory used a 1:5 soil-to-water extract or a saturated paste, since the two scales differ and results need conversion by soil texture. High EC reduces water uptake and can burn seedlings near band-applied fertiliser, so it influences both the product choice and placement.

Organic carbon and organic matter

Organic carbon (OC) is reported as a percentage. Organic matter is often estimated as OC multiplied by about 1.72, although factors vary between laboratories. Soils with low OC, such as sandy topsoils under 1 percent, generally hold fewer nutrients, mineralise less nitrogen and buffer less against acidity. Higher OC supports nutrient cycling, but it does not remove the need to replace what the crop takes off.

Soil nutrient levels: phosphorus, potassium and sulphur

These are the three macronutrients most often decided directly by soil tests, alongside nitrogen, which is handled differently as explained in the next section.

Phosphorus: extraction method and buffering

The phosphorus figure depends on the method. Colwell P (bicarbonate extraction) is standard in Australia, Olsen P is widespread in Europe, New Zealand, China and parts of North America, and Bray-1 and Mehlich-3 are used in the United States and elsewhere. A value of 25 mg/kg can be adequate under one method and low under another. In Australia, Colwell P should be read together with the phosphorus buffering index (PBI), which measures how strongly the soil binds applied P. In soils with a high PBI, the same Colwell value corresponds to a lower plant-available supply, so the critical value is higher. If your report has a P number but no buffering information, ask for the missing measure before settling on a rate.

Potassium: ppm values and the CEC context

Potassium is reported as exchangeable K, in mg/kg or cmol(+)/kg. To convert mg/kg to cmol(+)/kg, divide by about 391. Sandy soils with low CEC can show low K and respond quickly, whereas clay soils may hold large reserves in non-exchangeable forms that a standard test does not show. Look at K as a percentage of the CEC and at the crop’s removal rate, not at the single number. For a comparison of potassium sources, see our guide on SOP vs MOP.

Sulphur: why the sampling depth matters

Sulphur is commonly measured as KCl-40 sulphur or as sulphate-S, in mg/kg. It is mobile and can leach below the topsoil, so shallow samples may under-read the supply available to deeper roots. Crops with a high sulphur demand, such as canola and legumes, are particularly responsive on low-OC soils. Treat a low sulphur reading together with the crop type and rainfall pattern rather than in isolation.

Nitrogen, CEC and exchangeable cations

These fields influence how much you apply and which products suit the soil, but they rarely produce a single fixed rate on their own.

Mineral nitrogen and why it is a snapshot

Nitrate and ammonium nitrogen show what is in the soil on the sampling day. They change with rainfall, temperature and microbial activity, so a test taken months before sowing may not describe the real supply. Deep nitrogen tests, in regions where they are used, can help set rates for cereals, but most nitrogen programs are set from yield target, crop response, previous crop and in-season rainfall. Choosing between nitrogen sources is a separate question; our comparison of ammonium sulphate and urea covers it.

Cation exchange capacity

CEC measures the soil’s ability to hold positively charged nutrients such as calcium, magnesium, potassium and sodium. Typical ranges are below 5 cmol(+)/kg for coarse sands, 10-25 for loams and above 30 for heavy clays, though local ranges vary. Low-CEC soils need split or smaller, more frequent applications of mobile nutrients; high-CEC soils tolerate larger applications but may need more to shift the balance.

Calcium, magnesium, sodium and soil structure

Reports often list exchangeable Ca, Mg, Na and K, with ratios such as Ca:Mg. Evidence for a universally ideal ratio is weak; the more reliable signals are absolute levels, exchangeable sodium percentage (ESP) and aluminium. An ESP above about 6 percent suggests sodicity, which can cause dispersion, crusting and poor infiltration, and usually calls for gypsum and management changes rather than NPK.

Trace elements on a soil analysis report

Zinc, copper, manganese, iron, boron and molybdenum are needed in small amounts, and tests for them are less standardised. Zinc and copper are commonly extracted using DTPA, manganese using DTPA or other extractants, and boron using hot calcium chloride or similar. Critical values depend heavily on the method and crop. The table below summarises the main behaviours worth knowing.

Element	Typical risk situation	What to check on the report
Zinc	High pH, high P, sandy or calcareous soils	DTPA-Zn against the crop’s critical value
Copper	Sandy, leached or high-OC soils	DTPA-Cu, particularly for cereals
Manganese	Deficiency in high pH; toxicity in acid soils	pH and Mn together
Boron	Light soils, high rainfall; toxicity in some subsoils	Boron level and depth
Molybdenum	Acid soils, legume crops	Often inferred from pH rather than measured

Trace element decisions should also use tissue testing during the season. A borderline soil test combined with a clean tissue result may not justify an addition, while a low tissue test confirms a gap.

From soil test results to fertiliser recommendations: a step-by-step method

Reports that include soil test fertiliser recommendations are useful, but they are generic. Use the following sequence to check or build your own recommendation.

1. Set the yield target for the crop and season, using local history and rainfall outlook.
2. Estimate nutrient removal at that yield, such as kilograms of P, K and S per tonne of grain, hay or fruit.
3. Compare each soil result with the critical value for the same method, crop and soil type.
4. Decide whether each nutrient needs build-up, maintenance or nothing this season.
5. Address non-nutrient limits (pH, sodicity, salinity, compaction) before or alongside fertiliser.
6. Convert the nutrient plan into products, then into a blend and a rate per hectare (or per acre).

Set a yield target and crop removal

Crop removal is the baseline for maintenance. If a soil is at or above the critical value, replacing removal keeps it there. If it is below, add removal plus a build-up allowance, spread over seasons if cash flow or response data suggest this.

Fix limits before adding nutrients

Applying more phosphorus to a strongly acidic soil with aluminium toxicity will not give the response a soil at pH CaCl2 5.5 would give. Lime, gypsum or drainage work belongs first in the plan. Our article on how to read a soil test and choose a fertiliser gives a shorter version of this sequence.

Worked example: an annotated report turned into a blend decision

The following is an illustrative case for a cereal-canola rotation on a light-to-medium loam. The numbers are invented for explanation and are not a recommendation for any real paddock.

Field	Result	Reading	Blend implication
pH (CaCl2)	4.9	Acidic, aluminium risk in subsurface	Plan lime; fertiliser alone will not fix this
Colwell P / PBI	22 mg/kg / 90	Likely below target at this buffering	Build-up plus maintenance P from MAP or DAP
Colwell K	85 mg/kg	Low to marginal	Add potassium, using SOP if sulphur is also needed
Sulphur (KCl-40)	5 mg/kg	Low for canola	Include sulphate-S in the blend
DTPA zinc	0.4 mg/kg	Borderline	Add zinc to the blend or plan a foliar option
Organic carbon	1.1%	Low	Expect modest mineralisation; budget nitrogen in-season

The decision here is a layered one. Lime is scheduled first. Phosphorus and sulphur are handled with a phosphate source plus a sulphate source; potassium sulphate covers both K and S in one granule, which reduces the number of components. Nitrogen is then adjusted as a separate component, with urea or a nitrogen-rich blend, according to the yield target and rainfall. Where the sources have different nutrient ratios, as with MAP and DAP, see MAP vs DAP: nutrients, pH and crop fit for the trade-offs.

Common mistakes when you interpret a soil test

• Comparing across methods. Using an Olsen critical value on a Colwell result gives a false rating.
• Ignoring buffering and texture. A phosphorus number that looks adequate may be low for a high-PBI soil.
• Treating one sample as a whole farm. Variable paddocks need zone sampling, or the average hides both the deficient and the oversupplied areas.
• Chasing ratios. Perfect Ca:Mg ratios are rarely a yield driver; absolute levels, ESP and pH usually matter more.
• Skipping removal. A good test today does not mean the nutrient will remain adequate after several high-yielding crops.
• Testing at the wrong time. Nitrate and sulphur vary seasonally; take samples in a consistent window each year for comparable trends.

Keeping reports year to year, with the same method and sampling pattern, shows whether the program is building, holding or mining the soil.

Turning your report into an order

Once you have decided nutrient amounts per hectare, the next step is to convert them into a product mix and a tonnage. Options include a granular compound where each granule carries the same analysis, or a blend where separate granules such as urea, MAP, DAP and potassium sulphate (SOP) are combined to a target analysis. Our guide to compound versus blended fertiliser explains when each is preferable.

At Natures Fertilizer Pty Ltd we blend to the customer’s soil test, quote each order individually in writing, and supply Australia-wide or for export, typically in volumes of 10-25 MT or more. Send us your soil analysis report with the crop, target yield and delivery location, and we can discuss the blend options. For a full worked order, see custom fertiliser blending from soil test to order. Outside Australia, local advisers should confirm critical values and regulations for your region before a blend is finalised.

Conclusion

A soil analysis report becomes useful once you know the method behind each number, read pH, salinity and organic carbon before the nutrients, and interpret phosphorus, potassium and sulphur against crop and soil type. Add crop removal and a yield target, resolve non-nutrient limits, and the report points to a blend: which nutrients to build, which to maintain, and which to leave alone. Keeping consistent testing records makes each following season’s decision easier.

Frequently asked questions

How do I read a soil test report if I am new to it?

Start with the sample details and methods, then read pH, salinity and organic carbon, then the nutrient values against their critical levels for your crop. Finally, compare nutrient gaps with crop removal and decide on a blend. An agronomist can help confirm the critical values for your region.

What is a good soil pH for most crops?

Many field crops perform well at a pH of around 5.5 to 7.0 in water, which corresponds to roughly 4.8 to 6.5 in calcium chloride. Optimal ranges vary by crop and soil, so always check which pH method the report uses before comparing.

Why are my phosphorus results different between laboratories?

Laboratories use different extraction methods, such as Colwell, Olsen, Bray or Mehlich-3, which remove different amounts of phosphorus. The numbers are not directly interchangeable, so compare each result only with critical values for the same method.

What is CEC on a soil analysis report?

CEC is cation exchange capacity, the soil’s ability to hold nutrients such as calcium, magnesium, potassium and sodium. Sandy soils have low CEC and hold fewer nutrients, while clay and high-organic soils hold more. It is reported in cmol(+)/kg or meq/100 g, which are equal.

Can a soil test tell me how much nitrogen to apply?

Not on its own. Nitrogen in the soil changes quickly, so a test is a snapshot. Rates are usually set from the yield target, crop type, previous crop, organic carbon and seasonal rainfall, sometimes supported by deep nitrogen testing.

How often should I test my soil?

Many growers test each paddock every one to three years, in the same season and at the same depth, to track trends. Sulphur and nitrate vary more within a year, so testing before key decisions may be useful for those nutrients.

Do I need a trace element test as well?

It depends on soil type, pH and crop. Zinc, copper, manganese and boron are the usual ones to consider, and tissue testing during the season helps confirm whether a borderline soil result is a real limitation.

Can you blend fertiliser to match my soil test?

Yes. Natures Fertilizer Pty Ltd blends to the customer’s soil test and quotes each order individually in writing, with delivery across Australia or export. Contact us with your report, crop and location to discuss options.