Soil sampling density in an ALC survey

Agricultural land classification sampling density is what decides whether an ALC survey can be relied on. The grading itself is a judgement built from soil and site observations, and the number and spacing of those observations determines how much confidence a local planning authority, or an inspector, will place in the result. A survey that grades 20 hectares from a handful of borings will be challenged, and on a site where best and most versatile land is in issue that challenge usually succeeds.

The grading matters because national policy asks decision makers to take account of the economic and other benefits of best and most versatile agricultural land, which is Grades 1, 2 and 3a. Land graded 3b carries far less protection.

What the survey has to establish

An ALC survey assigns each part of a site to a grade using the published classification guidelines, which work by identifying the most limiting factor at each location.

Three families of limitation are considered. Climate, derived from location and altitude, covering rainfall, accumulated temperature and moisture availability. Site, covering gradient, microrelief and flood risk. And soil, covering depth, texture, stoniness, wetness and, less often, chemical properties.

The interaction of those limitations determines the grade. A soil that would otherwise be Grade 2 falls to Grade 3 or lower if the wetness class or gradient says so, and climate acts as an overriding limitation in upland areas regardless of soil quality.

The survey output is a plan showing graded parcels with areas, supported by the field data that produced them.

Grid density and the size of the site

A detailed ALC survey works on a regular grid. The customary spacing for a detailed survey is 100 m, giving one observation point per hectare, with the grid tightened where soils are variable or the site is small, and loosened only for reconnaissance work that is not intended to support a grading conclusion.

On a small site that arithmetic gets uncomfortable. A 2 hectare field surveyed on a 100 m grid yields two boring positions, which is not enough to defend a subgrade boundary. The sensible response is to tighten the grid rather than to report a grading with an implied precision the data does not support.

State the grid used, the number of observation points, and their positions on a plan. A survey report without a boring location plan cannot be checked, and an officer who cannot check it will ask for the survey to be repeated.

Where the grid is irregular because of obstructions, buildings or standing crops, say so and show where.

Auger borings against soil pits

Most observations are auger borings, taken to a depth sufficient to characterise the profile relevant to the grading, conventionally around 120 cm where the soil allows.

Soil pits are dug at a lower density to examine structure, packing density and rooting conditions that an auger cannot show properly. The number depends on soil variability and on whether structural limitations are likely to be in issue, and it is worth agreeing that number with the local authority or its soil adviser before the survey rather than defending it afterwards.

Auger borings and pits record different things and the report should distinguish them clearly in the data tables. Conflating the two hides the fact that structural assessment rested on very few observations.

Recording depth, texture and stoniness

For each observation the field record should carry the position, the depth of each horizon, texture by hand assessment, stone content by size and percentage, colour and mottling indicating wetness, rooting depth and any impeding layer.

Texture assessment by hand is a skilled judgement and it drives the grade. Wetness class, derived from soil colour and mottling together with the profile and the climate data, is the other variable that most often decides whether land falls either side of the best and most versatile boundary.

Stoniness is frequently under-recorded. It affects both the soil depth available and the machinery workability, and a survey that notes stones without quantifying them has left out an input the grading needs.

Climatic and wetness limitations

Climate data is derived from the site location and altitude rather than measured on site, using the published datasets the guidelines rely on. The report should state the grid reference and altitude used, because a difference of 50 m in altitude across a sloping site can change the climatic interaction.

Wetness is the interaction of soil water regime with climate, expressed through the field capacity period and the number of days the land can be worked or grazed. This is where a survey carried out in unsuitable conditions goes wrong: soil examined when waterlogged gives misleading structural observations, and soil examined after a long dry spell can understate mottling.

Surveys can be carried out in most seasons, but not on frozen or saturated ground, and the report should record the survey dates and the antecedent weather.

Assigning subgrade 3a or 3b

This is the boundary that matters for planning, and it is the one most often contested.

The distinction between 3a and 3b turns on the severity of the limitations and the range of crops that can be grown consistently. Because it decides whether policy protection applies, the report must set out the reasoning for each parcel placed either side of the line, referring to the specific limitation and the observations supporting it.

A grading conclusion of 3a or 3b without stated reasoning is the single most common weakness in submitted ALC reports, and it is the first thing a county land agent or a rural planning consultant will attack.

Reporting so the grading can be checked

Build the report so somebody else could reach the same conclusion from the same data.

Include the survey method and the guidelines applied, the grid and observation density with a location plan, the full field data in an appendix, the climatic inputs with the grid reference and altitude used, the limitation assessment for each parcel, the graded plan with areas by grade, and the survey dates with ground conditions.

Where land is graded as best and most versatile, quantify it. A development that takes 1.8 hectares of Grade 3a needs that figure stated, because the planning balance depends on the amount as well as the grade.

An Agricultural Land Classification report of this kind is specialist work. The classification guidelines assume a surveyor experienced in soil description, and councils expect a suitably qualified author, with professional review strongly advised. Where ground investigation is also being carried out for a Geotechnical Assessment, coordinate the visits, because the trial pits can serve both purposes if positioned with that in mind.

The professional directory is the route to a reviewer who can check a draft before it is submitted. 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.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.