What a Soil Assessment Measures, and What It Misses

A standard soil test measures chemistry: pH, available phosphorus, exchangeable potassium, calcium, magnesium, and often organic matter and cation exchange capacity. It does not measure structure, biology, or drainage, and those are the properties that decide whether a field is actually improving. A grower who wants to know if a field is getting better needs both kinds of information, and needs to know which numbers answer the question. A field reference for this kind of field observation, the sort of material that treats soil pits, drainage, and site reading as a documented practice, is the territory covered by Northern New England field reference work on ecological assessment.

What does a standard soil test report?

A routine laboratory test, the kind ordered through a state extension service or a commercial lab, returns a small set of extractable nutrients and a pH reading. The usual panel includes pH, buffer pH on acid soils, organic matter by loss on ignition, available phosphorus, exchangeable potassium, calcium, magnesium, and sometimes sulphur, zinc, manganese, boron, and copper. Cation exchange capacity is calculated from the base cations. Results arrive as parts per million or pounds per acre, with a recommendation for lime and fertilizer.

The method matters. Phosphorus extracted with Mehlich 3 is not the same number as phosphorus extracted with Bray P1, and the two are not interchangeable across labs. Organic matter by loss on ignition is not the same as organic carbon by combustion. A grower comparing a 2019 report from one lab with a 2024 report from another is often comparing methods, not fields. The first rule of tracking change is to keep the same lab, the same extraction, and the same depth of sampling.

What the report describes is a chemical snapshot of the plough layer, usually the top 15 to 20 centimetres. It is a genuinely useful snapshot. It catches acidity before it limits yield, it flags phosphorus loading that threatens water quality, and it gives a defensible basis for nutrient budgeting. It is also narrow. A field can test optimum on every line and still shed water, crust after rain, or fail to hold a seedbed.

Which measures show structure rather than chemistry?

Structure is physical, and chemistry tests do not see it. The measurements that do see it are mostly made in the field or in a pit, not in a lab.

• Bulk density. A known volume of soil is dried and weighed. Values above roughly 1.6 grams per cubic centimetre in a silt loam suggest compaction that roots will struggle to penetrate. The measurement is simple and repeatable, and it tracks change over years.
• Infiltration rate. A ring or a small ponded plot, timed as water enters. Slow infiltration after a rain is the clearest field evidence of surface sealing or a plough pan.
• Aggregate stability. How well soil crumbs hold together when wetted. Slaking and dispersion in a jar of water indicate weak aggregation, which is a structural problem even when fertility is adequate.
• Penetrometer resistance. A cone pushed into the soil at consistent moisture reads a resistance profile. A sharp increase at 15 to 25 centimetres is a compaction layer, not a nutrient shortage.
• Root observation. Dig a spade of soil and look. Roots that turn sideways at a boundary are describing the soil more honestly than any laboratory number.
• Earthworm counts and residue cover. Both are crude, both are cheap, and both respond to management within a season or two.

The point is not that one method replaces another. It is that structure and chemistry fail in different ways, and a grower watching only one of them will miss half the story.

What does a soil assessment not tell you?

A soil assessment does not tell you what the soil will do next year. It reports a condition at a moment, under the moisture and temperature of that moment, and it cannot forecast a season.

It also does not tell you about the things that most often limit a field:

• Drainage and water movement. A test does not know where water sits in March. That requires a pit, a topographic map, and observation after rain.
• Depth to a restricting layer. A plough pan, a fragipan, or bedrock at 40 centimetres changes everything about how a field should be managed, and no routine test reports it.
• Biological activity. Microbial biomass, mycorrhizal colonisation, and the active fraction of organic matter are measurable, but they are not part of a standard panel and they are expensive to track.
• Weed, pest, and disease history. These live in field records, not in a lab report.
• Slope, aspect, and erosion risk. A soil sample from a knoll and a sample from a swale can return the same numbers and behave completely differently.
• The margin of error. Laboratory results carry analytical uncertainty, and sampling variability within a field is usually larger than the analytical error. A change of a few parts per million between years may be noise.

A grower who wants to know whether a field is improving should treat the laboratory report as one instrument among several, and should record the field observations that the report cannot contain.

How often should the same field be sampled?

Every two to three years is the common recommendation for routine fertility monitoring on cropland and pasture. Annual sampling is rarely justified for chemistry alone, because the year to year variation from moisture and sampling position can exceed the real trend. On sandy soils, on fields receiving heavy manure or compost, and on ground where phosphorus loading is a concern, a shorter interval of one to two years is defensible.

Structural and biological indicators follow a different clock. Bulk density, infiltration, and aggregate stability change slowly, and a three to five year interval is enough to see a direction. Earthworm counts and residue cover can be checked every year at no cost.

The discipline that matters more than frequency is consistency. Sample the same depth, in the same season, along the same transect, at the same points, and send the samples to the same laboratory. A field sampled at 15 centimetres in October one year and at 20 centimetres in April the next has not been monitored. It has been sampled twice.

Reading the two records together

The useful habit is to keep two records side by side. One is the laboratory series: pH, phosphorus, potassium, organic matter, and the methods used. The other is the field series: infiltration after a soaking rain, penetration resistance at a marked spot, residue cover, earthworm counts, and a photograph from the same corner of the field each year.

When the two records disagree, the disagreement is the finding. A field with optimum fertility and slow infiltration is not a fertile field with a minor problem. It is a compacted field that will lose the fertility it has. A field with modest fertility and good structure is often the better long term asset, because structure is slower to build and slower to lose than a nutrient level.

What to ask a laboratory or a consultant

Before ordering a test, ask which extraction methods are used, what the detection limits are, and whether the lab reports organic carbon separately from organic matter. Ask whether the laboratory participates in a proficiency testing program. Ask what depth the recommendation assumes.

Before commissioning a field assessment, ask whether the work includes a soil pit, a drainage observation, and a written description of the limiting layer. An assessment that returns only a nutrient panel is a laboratory service, not an ecological one, and it should be priced and used as such.

The question a grower is really asking, whether a field is improving, is answered by the trend in both records, not by any single number. Chemistry tells you what is in the soil. Structure tells you what the soil can do with it.