Nutrient management plan · Soil test
How to read a soil test report (soil analysis), line by line
The short answer
Before you look at a number, look at which method measured it and which unit it comes in. Then read it against the scale of that same method, and if you can, of the same laboratory. A phosphorus result can be low with one method and fine with another: Olsen, Bray-1 and Mehlich-3 do not give the same figure on the same soil.
First things first
The method and the unit, before the number
Each laboratory extracts the nutrients from the soil with a different solution (the extractant), and each extractant pulls out a different amount. So an interpretation table only works for the method it was built with.
On your report, look next to each line for the name of the method (Olsen, Bray-1, Mehlich-3, ammonium nitrate, ammonium acetate…) and for the unit. The equivalences people mix up most:
- ppm = mg/kg. Same thing, two names.
- mg/l is not mg/kg. Laboratories in the UK give results in mg/l. Do not compare them with ppm without checking.
- P or P2O5, K or K2O. P × 2.29 = P2O5, and K × 1.2 = K2O. Fertiliser (fertilizer) recommendations are often given as P2O5 and K2O (lb per acre in the US), while soil results are given as P and K. Check before you compare.
- lb/acre and ppm. In the US, labs report in ppm or lb/acre. For a 6-inch (15 cm) layer, lb/acre ≈ ppm × 2.
UK reports
Indices 0 to 6 and mg/l
In the UK, laboratories measure phosphorus with the Olsen method, and potassium and magnesium with ammonium nitrate. The result is given in mg/l, and it is then turned into an index from 0 to 6 (the scale used in RB209, the UK nutrient management guide). Soil pH is measured in water, at a ratio of 2.5 parts of water to 1 of soil.
The index is a band, not a measurement: it comes out of the mg/l figure. Ask your lab for the mg/l as well as the index. Two fields with the same index can sit at different ends of the band.
US reports
ppm, lb/acre and the state's own scale
In the US, phosphorus is usually measured with Bray P1 or Mehlich-3. Mehlich-3 gives about 35% more than Bray. Bray P1 is not recommended when the pH is 7.3 or above: there, Olsen is used. Potassium is measured with ammonium acetate or with Mehlich-3.
The categories (very low, low, optimum, high) and the recommendations are calibrated by each state and by each laboratory. There is no single national table. That is why the lab and the state matter as much as the number.
pH
Acidity or alkalinity
Compare the pH only if it was measured the same way (in water or in a salt solution, and with the same amount of water). A different method gives a different pH.
Very acid soil and very alkaline soil both limit what the plant will be able to take up. The pH you want is not the same for every crop: read it for the crop you are going to grow.
Organic matter
Organic matter and the C:N ratio
Organic matter is calculated from organic carbon. Check whether your report gives it in % or in g/kg (10 g/kg = 1%), and which method was used.
Its reading depends on texture: a clay soil holds more than a sandy one. Compare it only with soils of the same type. The C:N ratio tells you how easily that organic matter breaks down and releases its nitrogen. It is a guide, not a measurement of the nitrogen available at the right moment.
Texture and CEC
What the soil can hold
Texture is the split between sand, silt and clay. The more clay, the more nutrients the soil can hold.
CEC (cation exchange capacity) is the soil’s pantry: how much calcium, magnesium, potassium and sodium it can hold. It is given in meq/100 g or cmol(+)/kg, which are the same unit.
A trap in soils with free lime: when the lab measures calcium, its extractant can dissolve part of the lime. Calcium then comes out higher than it is, and the sum of the cations can go above the CEC. That excess calcium is not real; it is an effect of the method.
Phosphorus
Olsen, Bray-1 and Mehlich-3 cannot be compared
Phosphorus is the line most often misread. Olsen is the usual method in the UK, and in the US for alkaline soils. Bray-1 and Mehlich-3 are the common ones in the US. Mehlich-3 pulls out phosphorus, potassium, calcium, magnesium and trace elements in one go.
On the same soil the methods can give different numbers, and the gap changes from one soil to another. A single factor or equation would often give wrong estimates, so we do not convert between methods: each number is read against the scale of its own method.
Tell us which method your lab uses. We work with Mehlich-3, Olsen and Bray-1 results; if your lab uses another, we tell you before you pay.
Potassium and magnesium
Potassium and magnesium
Potassium is measured with ammonium acetate or with Mehlich-3 (US), or with ammonium nitrate (UK, together with magnesium). Check the unit before you compare two reports: an error of a factor of 10 or 1,000 does happen.
Its reading depends on texture and CEC: a clay soil holds more potassium than a sandy one, so the same figure does not mean the same thing. Together with the crop and the history of the field, the result tells you whether to replace what the crop removes, to build up a poor soil, or to apply nothing because the soil already has enough.
Mistakes
The most common mistakes
These are the mistakes we see most when people read a soil test:
- Comparing this year’s test with one from five years ago, done by another lab with another method.
- Reading a Mehlich-3 result with the Olsen or Bray table, or the other way round.
- Mixing up P and P2O5, or K and K2O.
- Mixing up mg/l and mg/kg (ppm).
- Using the table of one crop for another: vegetables and vines do not have the same needs as wheat.
- Sampling badly: too few cores, mixing different zones, or sampling just after spreading fertiliser.
Sampling
How to take the sample
A test is only as good as the sample. The common-sense rules are the same everywhere:
- One sample per uniform zone. If a field has two soils, take two samples.
- Many cores in a zigzag across each zone, mixed in a clean bucket.
- At the same depth every time, in the cultivated layer.
- After harvest and before spreading fertiliser. If you repeat the test, do it at the same time of year and after the same crop.
- In a clean bag, labelled with a permanent marker.
What farmers ask us
Common questions about soil tests
Which values are normal?
There is no single table: it depends on the method, the crop, the soil type and who built the scale. In the US it also depends on your state. The first question is always which method was used.
Is there a formula to go from Mehlich-3 to Olsen or Bray?
Equations have been published, but they depend on the soil and keep a margin of error. The safe way is to read each method against its own scale.
How often should I test?
In England, the Farming Rules for Water ask for soil test results no more than five years old when you plan an application. We explain it in the nutrient planning rules. For a nutrient balance, think in terms of the rotation, not a single year.
How many kilos per hectare is 1 ppm?
It depends on depth and soil weight. For 30 cm and a soil of 1,400 kg/m³, 1 ppm is about 4.2 kg per hectare (3.7 lb/acre). In the US, for a 6-inch layer, lb/acre ≈ ppm × 2.
Does my soil have a lot of free lime?
If the soil is calcareous and your report does not give it, ask the lab for active lime, not only total lime.
Can you interpret my soil test?
Yes, and remotely. Send us the full report and the history of the field, and we tell you what is in excess and what your crop is missing. The first one is free, from 25 ha (62 acres): see the terms, section 7. See also the nutrient management plan and pricing.
Sources: UK laboratory methods, mg/l and indices 0 to 6: PAAG report 2012-13; Defra index scale datasheet. US units and methods: Iowa State University Extension; University of Arkansas Extension FSA-2118; Ohio State University Extension; University of Delaware. Agronomic information, not legal or regulatory advice.
Know someone this would help?
Send them this page. It will save them some searching.