Agricultural land quality varies significantly across the UK with two grading systems in place. In Scotland the Land Capability Classification is used to divide all land into seven grades. In England and Wales the Agricultural Land Classification (ALC) uses a combination of climatic, site, soil and interactive limitations to classify land into five grades (1-5) in order to enable sustainable choices to be made about the sustainable release of greenfield land to meet, housing, commercial and infrastructure demands.
The earliest ALC maps, published in the 1970s were not based on detailed survey and provided generalised information only. The current version of the classification is based on more quantitative criteria with Grade 3 subdivided into sub-grades 3a and 3b and with Grades 1, 2 and 3a subsequently characterised as representing the 'best and most versatile' agricultural land - that which is most flexible, productive and efficient in response to inputs and which can best deliver future crops for food and non-food uses (such as biomass, fibres and pharmaceuticals).
Advice on ALC surveys of land under consideration for development is given in Natural England’s Technical Information Note TIN049 which points out that the series of maps published in the 1970s are not sufficiently accurate for use in assessment of individual fields or development sites, and should not be used other than as general guidance. TIN049 states that for a detailed assessment ‘ALC surveys are undertaken, according to the published Guidelines, by field surveyors using handheld augers to examine soils to a depth of 1.2 metres, at a frequency of one boring per hectare’ ‘supplemented by digging occasional small pits (usually by hand) to inspect the soil profile’. The ALC module of the Working with Soil training programme states minimum competencies for persons undertaking detailed agricultural land classification surveys.
If you need a soil scientist Land Research Associates can almost certainly help.