The Agricultural Land Classification is a widely used classification systems that “provides a framework for classifying land according to the extent to which its physical or chemical characteristics impose long-term limitations on agricultural use.” With its diverse landscape the Chew Valley has some very good agricultural land that is used for arable crops and some less good land that is used for pasture.
Principle Physical Factors and Land Grades
Agricultural land is graded according to a set of principle physical factors, namely:
- Climate – temperature and rainfall
- Site – gradient, microrelief, flood risk
- Soil – texture, structure, depth, stoniness
The following Grades of land are identified by the classification. A principle point of note is that the digital map data for England does not include Grade 3b. This issue is addressed by this study.
- Grade 1 – excellent quality agricultural land
- Land with no or very minor limitations to agricultural use. A ver wide range of agricultural and horticultural crops can be grown and commonly includes top fruit, soft fruit, salad crops and winter harvested vegetables. Yields are high and less variable than on land of lower quality.
- Grade 2 – very good quality agricultural land
- Land with minor limitations which affect crop yield, cultivations or harvesting. A wide range of agricultural and horticultural crops can usually be grown but on some land in the grade there may be reduced flexibility due to difficulties with the production of the more demanding crops such as winter harvested vegetables and arable root crops. The level of yield is generally high bet may be lower or more variable than Grade 1.
- Grade 3 – good to moderate quality agricultural land
- Land with moderate limitation which affect the choice of crops, timing and type of cultivation harvesting or the level of yield. Where more demanding crops are grown yields are generally lower or more variable than on land in Grades 1 and 2.
- Subgrade 3a – good quality agricultural land
- Land capable of consistently producing moderate to high yields of a narrow range of arable crops, especially cereals, or moderate yields of a wide range of crops including cereals, grass, oilseed rape, potatoes, sugar beet and the less demanding horticultural crops.
- Subgrade 3b – moderate quality agricultural land
- Land capable of producing moderate yields of a narrow range of crops, principally cereals and grass or lower yields of a wider range of crops or high yields of grass which can be grazed or harvested over most of the year.
- Subgrade 3a – good quality agricultural land
- Land with moderate limitation which affect the choice of crops, timing and type of cultivation harvesting or the level of yield. Where more demanding crops are grown yields are generally lower or more variable than on land in Grades 1 and 2.
- Grade 4 – poor quality agricultural land
- Land with severe limitations which significantly restrict the range of crops and/or level of yields. It is mainly suited to grass with occasional arable crops (e.g. cereals and forage crops) the yields of which are variable. In moist climates, yields of grass may be moderate to high but there may be difficulties in utilisation. The grade also includes very droughty arable land.
- Grade 5 – very poor quality agricultural land
- Land with very severe limitation which restrict use to permanent pasture or rough grazing, except for occasional pioneer forage crops.
- Non Agricultural Land – ‘Soft Uses’
- ‘Soft’ uses where most of the land could be returned relatively easily to agriculture, including: golf courses, private parkland, public open spaces, sports fields, allotments and soft-surfaced areas on airports / airfields. Also active mineral workings and refuse tips where restoration conditions to ‘soft’ after-uses may apply.
- Urban Land – ‘Hard Uses’
- Built up or ‘hard’ uses with relatively little potential fro a return to agriculture.
In our scheme the criteria for woodland creation are that only poor quality (Grade-4) and marginal (Grade-3b) land should be considered for conversion to woodland, so Grade-1, Grade-2, Grade-3a and Grade-5 land should be excluded from consideration.
ALC Codes and Areas, pre-adjustment
| Grade | Code | Area England (Ha) | Area Wales (Ha) |
| Grade 1 | 1 | 354,694 | 5297 |
| Grade 2 | 2 | 1,849,234 | 135203 |
| Grade 3 | 3 | 6,291,737 | 622695 |
| Grade 3b | 31 | ||
| Grade 4 | 4 | 1,840,345 | |
| Grade 5 | 5 | 1,100,761 | 465484 |
| Non Agricultural Land | 6 | 657,283 | |
| Urban | 7 | 952,221 |
ALC Grades, numeric codes and areas in hectares
Data Processing to Include Grade 3b in England
The Agricultural Land Classification (ALC) maps were generated in 1988 by the Ministry of Agriculture Fisheries and Food using rather general information. In England the maps do not include the Gradient Criteria for site limitations, presumably because no detailed digital terrain model for England was not yet generally available for England at that time. We have augmented the ALC map data using digital terrain data from the Ordnance Survey using the MAFF grades below.
The gradient limits for the ‘b’ grade is shown below as deep red / burgundy. This actually applied to all grades of land, not just 3b.
The grading of the land into ALC categories is complex and gradient is just one consideration. As the gradient (slope) becomes steeper the land-grades are pushed into the higher grades. Grade-1 land on a very steep gradient doesn’t make sense as excellent quality agricultural land because it may not be possible to safely cultivate it, even though it may have good quality soil and climate. Land on steep slopes on any grade has therefore been pushed into Grade 3b, making space for woodland.
The gradient (slope) is therefore a determining factor. Table 1 above is interpreted as follows:
- Land on slopes between zero and 7 degrees (the limit) will be either Grade-1, Grade-2 or Grade-3a (if it complies with other criteria)
- Land on slopes between 7 and 11 degrees will be Grade-3b
- Land on slopes 11 and 18 degrees will be Grade-4
- Land on slopes greater than 18 degrees will be Grade-5
For all intents and purposes this means that slopes greater than 7 degrees can be assigned to a marginal class (whether it be 3, 4 or 5) on which it is dangerous to operate machinery and where there “is often an attendant risk of soil erosion” (MAFF 1988).
Adding this slope information adds a level of detail that was not previous available in the ALC England data. It also adds to the area that may be considered for woodland creation, providing additional area and/or choice. Limiting woodland opportunity to both Grade-4 and Grade-3b land should provide adequate opportunity for woodland creation to double tree cover.
This section is based on the the following report and associated data that are available under the OGL: Ministry of Agriculture, Fisheries and Food, 1988, Agricultural Land Classification of England and Wales, Revised guidelines and criteria for grading the quality of agricultural land.