18 August 1995

PUT SOIL ON THE MAP…

Soil mapping can help plan cultivation strategies and much more besides. Charles Abel examines the value of such a map

SOIL mapping offers a better introduction to precision farming than satellite assisted yield mapping, reckons Cultivations 95 host farm manager, Bob Green.

Soil type has a profound effect on crop yields, but wont vary from year to year, he explains. "It provides something consistent on which to base management decisions, including liming, base nutrients, cultivations and crop choice."

Indeed, he is so convinced of the value of the soil map produced for Bedfordia Farmings land around Milton Ernest, Bedford, in 1989 that several new blocks have been mapped recently and a further 240ha (600 acre) bought recently will "definitely" be mapped this winter.

"Compared with the cost of the land, £4/acre or so is nothing, but provides lots of helpful information," he comments.

ADAS adviser, Selwyn Richardson, agrees. "Soil maps can help most farmers, but will be particularly useful to those taking on new land, whether buying, contract farming or working on a share farm arrangement. They save years of building up knowledge of the local soils."

At Bedfordia, 28 soil types are scattered across the 2000ha (4940 acre) unit, a legacy of the Great Ouse river cutting through the local geology. The lightest sand is very light Newport series as at ADAS Gleadthorpe, Notts, with Denchworth heavy Oxford clay providing the other extreme.

The accompanying report explains many of the implications of the soil types identified. At Bedfordia it brought savings. "Weve saved a fortune on targeting our liming." Earlier advice had urged the treatment of ground the soil map shows is not prone to acidity.

Testing is now targeted to certain soils, allowing split treatment of fields. Some areas, including Newport and Wick series light land, are tested annually. Others are tested every two or three years, and a lot, including Hanslope series clays, never.

Use of P and K has also been refined, according to the release of nutrient from different soils and yield potential which influences offtake. "Natural release rates from the boulder clays are around 50kgK/ha, so we can modify applications accordingly," says Mr Green. Some fields at index 4+ have not been treated for six years. But others, rated 4 for phosphate and 1 for potash clearly need a specific treatment.

Again split field treatment is considered, particularly on fields over 15ha (37 acres). But Mr Green is cautious about varying rates too much.

Mr Richardson agrees. "Variable rate fertilising is best suited to large fields with very variable soil types. Chalk downland is ideal, where silts in the valley move through to heavy clay caps on hilltops."

Compaction checks can also be targeted. "We now know weak structured sandy soils can be just as prone to compaction, but deeper than usual, at 18in. That means sub-soiling. The Hanslope clays usually show any compaction at 14in, which we can cure with a cheaper flat-lift ," says Mr Green.

Like an encyclopedia, the soil maps reveal more information every time they are consulted. They are a vital aid to planning operations, prompting detailed discussions about the merits of alternative approaches. Mr Green looks forward to getting the maps on screen, so other data can be compared on the farm computer.

How mapping is done

Mapping starts with a look at geological maps to identify underlying influences and maps from the Soil Survey and Land Research Centre which show detail on a four-miles-to-the-inch scale.

"Those show us what to expect and help us target the sampling. Well usually do a core every five acres to start with, dropping to one every half acre or more often where soil types are changing a lot," comments Mr Richardson.

That provides data from which a soil map can be produced, indicating the areas for each type using the SSLRC naming system. A typical farm of around 240ha (600 acres) includes about six soil types. "Differences in crop productivity can be quite marked-thats what makes it worth investigating."

Winter is the best time for sampling-wet soils make for easier sampling and visible changes such as cloddiness provide extra information, comments Mr Richardson.