Soil resource and grading in an agricultural land assessment

An Agricultural Land Classification survey grades land from 1 to 5 according to the extent to which physical characteristics limit its agricultural use. Grades 1, 2 and 3a together make up best and most versatile land, which national policy protects, and grade 3b does not. That single division decides the outcome of a great many rural applications, and it cannot be established from the published provisional maps, because those show grade 3 undivided.

So the practical purpose of most ALC work on a planning application is narrow: to establish, by field survey, whether the site is 3a or 3b.

What the classification measures

The method in use is the revised guidelines published in 1988, which assess the long-term physical limitations on agricultural use rather than current cropping, management or profitability. A well farmed field and a neglected one with the same soil and climate receive the same grade.

Limitations fall into groups. Climate is assessed first and acts as an overriding constraint, using altitude, rainfall, accumulated temperature and moisture measures for the location. Site factors follow: gradient, microrelief and flood risk. Then soil factors: texture, depth, stoniness, structure and chemical properties. Finally the interactive limitations, chiefly wetness and workability on the one hand and droughtiness on the other, which arise from the combination of soil and climate rather than from either alone.

The grade is set by the most limiting factor, not by an average. A deep, fertile, level soil in a high rainfall location with a severe wetness limitation will not be graded 1 however good everything else is.

That is why a desk assessment of soil association data gives an indication and not an answer. Two fields on the same mapped soil association can carry different wetness classes depending on drainage history and position in the landform.

Field survey method and density

A detailed survey is carried out on a regular grid across the site. The usual density is one auger boring per hectare, which is a 100 metre grid, with additional borings where the pattern changes and soil pits at intervals for full profile description.

Each boring records depth, texture by horizon, stoniness, colour and mottling, structure where a pit allows it, and the depth to any impeding layer or to gleying. Those observations feed the wetness class and the available water capacity calculations that drive the interactive limitations.

Grid density matters because the 3a and 3b boundary often runs through a site rather than around it. A survey at a coarser density can miss a change in soil depth or texture that moves a hectare across the line, and an officer with a neighbouring survey at proper density will notice.

Plot the boring positions on the report drawing with their reference numbers. A survey whose observation points are not shown is difficult to test and correspondingly easy to discount.

Where grade 3a and 3b divide

This is the working heart of most reports, and it usually turns on two things.

Wetness and workability: the wetness class derived from the depth to gleying and the soil texture, combined with the field capacity period for the location, determines how many days the land can be worked. Where that falls below the threshold for the reference crops, the grade drops.

Droughtiness: the available water capacity of the profile, calculated for wheat and for potatoes and compared with the moisture deficit for the location. Shallow soils, stony soils and sands hit droughtiness limitations quickly, and the two reference crops can give different answers, with the more limiting one governing.

Gradient and stoniness can also be the limiting factor on their own, and on some sites depth to rock decides it without any calculation being needed.

State which limitation determined the grade for each mapped unit. A report that gives a grade without naming the governing limitation invites a challenge, and where Natural England or a county adviser reviews it, that is the first question asked.

Best and most versatile land in policy

National planning policy asks decision makers, where development of agricultural land has been shown to be necessary, to prefer areas of poorer quality land over higher quality land. The wording and the location of that policy have changed between editions of the National Planning Policy Framework, most recently with the edition published on 17 August 2026, so cite the current edition and verify the reference at the time of writing.

Two procedural points follow from the grading.

Where a proposal is not in accordance with the development plan and involves the loss of a substantial area of best and most versatile land, the local planning authority is required to consult Natural England under the development management procedure rules. The threshold is set in those rules and it is worth checking against your site area before assuming the consultation will not be triggered.

And on larger schemes, soil and agricultural land is a topic that can be scoped into an Environmental Impact Assessment, where the assessment has to go beyond grading to consider effects on the agricultural holding as a business.

An illustrative example. A solar proposal on 32 hectares is assessed as predominantly 3b with a band of 3a along one edge, the limitation being droughtiness over a shallow gravelly soil across most of the site. The application reduces the array footprint to avoid the 3a band, keeps sheep grazing across the whole site, and commits to a soil management plan. The land quality argument is largely removed from the debate, which is what a good survey buys.

Soil resource plans and reuse on site

Grading answers whether the land is protected. It says nothing about what happens to the soil when the site is built.

A soil resource survey records the depth and quality of topsoil and subsoil across the site, and a soil resource plan sets out how much of each will be stripped, where it will be stored and what it will be used for. On a development with landscaping, that plan is what stops good topsoil being sold off site and replaced with imported material of unknown provenance.

The handling principles are set out in the published code of practice on the sustainable use of soils on construction sites. The points that matter on site are stripping only when the soil is dry enough to move without smearing, keeping topsoil and subsoil separate, limiting stockpile heights, avoiding trafficking over stockpiles, and reinstating in the correct order with relief of compaction between layers.

Where land will be returned to agriculture after a temporary use, that reinstatement specification carries the whole restoration case, and authorities increasingly condition it. Tie the soil resource plan to the Landscape Strategy so the planting specification and the soil available to it agree.

Timing the survey

Auger survey needs soil that can be sampled and described, which rules out frozen ground and prolonged waterlogging. It also needs access, which cropping patterns constrain on an arable holding.

In practice the workable periods are wider than people assume, and the constraint is usually access rather than season. Speak to the occupier early, agree access windows around drilling and harvest, and allow for a return visit where the first pass shows a boundary needing definition.

What you cannot do is survey in standing water and describe a wetness class from it. If the season defeats the programme, say so and reschedule rather than submitting a survey with a caveat that undermines its own conclusion.

Evidencing the result

The report should let a reviewer reproduce the grading. Include the survey date and weather conditions, the climate data source and the interpolated values used, the observation point plan with references, representative profile descriptions, the calculations behind the wetness and droughtiness assessments, a grade map, and an area statement in hectares by grade with the best and most versatile total stated separately.

Authorship matters here. An Agricultural Land Classification survey is expected to be carried out by a soil scientist with relevant field experience, and councils usually expect a suitably qualified author, so professional review is strongly advised. Where you need a reviewer, PlanScope's professional directory is the route to review and sign-off.

Send the draft to the case officer before the application goes in where the 3a and 3b split is finely balanced. It is much easier to agree a boundary at that stage than to defend it at committee.

This is general guidance, not legal advice. Check the position with your local planning authority, Natural England and a qualified soil scientist before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.