Tests suggest magnetic scan is handy technique
GPS soil mapping creates
pretty charts for the office
wall. But does it produce
valuable management data?
Andrew Swallow reports
ELECTROMAGNETIC scanning seems set to refine soil mapping and could help growers spot structural problems and rationalise cultivation decisions, says three leading soil scientists.
The conclusion comes after a meeting with Solutechs Justin Smith – the man behind the MagnaScan mapping system (Arable, Aug 27, 1999) – to test the technique on a field in Beds.
MagnaScan maps of the field indicate at least three distinct soil types (see map, areas 1, 2 and 4), plus a seam of apparently lighter material on top of a field ridge (area 3).
Soil pits dug across the field, according to the map areas, confirmed huge variations in soil texture and type.
"It is one of the most variable fields I have ever seen," says independent soils and sugar beet adviser Philip Draycott.
The yellow area on the map indicates a light soil texture at one low-lying end of the field, giving way to much heavier soil on a slope. Digging reveals Bearsted series sandy loam soil and Oxford clay, respectively.
Both areas would be found by traditional soil mapping methods, says Cranfield University land resource and global information systems specialist Ian Bradley.
But the third pit at the top of the slope (area 3), reveals a clay-capped glacial gravel. "There is no way a traditional soil map would have picked up that third pit in the gravel," says Dr Bradley.
Such detailed knowledge of the field would be important for its management because the gravel seam, unlike the clay on the slopes or Hanslope series soil (medium brown on map) on the top, would not hold a mole drain channel, he says.
"It is not a problem in terms of drainage itself, but because of where it sits in the field."
The fact the gravel and Bearsted soils appear the same on the map, but are actually very different, highlights the need for mapping to be backed up with spadework, says Mr Smith.
"The map only tells us the conductivity is low for a reason, but does not give the reason itself. It tells us to go and look." Dr Draycott agrees. "You have got to dig pits, but with the map and pits you are going to have more understanding of what is going on."
Growers will know how much of a field is suitable for certain cultivations, allowing them to calculate the cost benefit of any operation, says Mr Smith.
Dr Draycott agrees the maps could help growers target suitable techniques by distinguishing soil types more precisely.
"The morning has shown how valuable MagnaScan could be to a soil surveyor. But we have not been able to test the instruments ability to pick up compaction as we have not found any such areas in this field."
Given a fairly uniform field, Mr Smith says that the MagnaScan data can be analysed to pinpoint slight changes in conductivity that could be due to structural problems.
"We have done lots of fields where we have gone out with a penetrometer and found compacted areas that we have identified with the MagnaScan. They tend to be easier to find in the clayier soils." *
• Four soil types identified.
• Advance on soil survey maps.
• Helps target cultivations.
• Spadework still essential.
• Compaction identification claim.
So what does Magnascan technique tell us? Soil scientists Ian Bradley (left), Dick Godwin and Philip Draycott got to grips with four soil types revealed by the gps-guided scanner in a Beds field recently.
Grower Mark Brinkley, who contract farms the field where the MagnaScan maps were tested, believes that with local field knowledge such data is not needed. But for growers taking on new land, or with very large areas to manage, it could be a useful tool. "I knew where to go and look for the different soil types, and Justin Smith has proved he could have taken me there with these maps. But the maps are useless unless they are interpreted. It is the management that can come from them that is of value." Dr Draycotts advice on the field tested is to plough this autumn, because only the Hanslope series, and possibly the Bearsted soil, would be suitable for discing. No compaction or plough pans means no subsoiling is needed either, he adds. "That is very handy because ploughing is exactly what we will be doing," says Mr Brinkley.
More accurate targets for tillage needed
Techniques such as Electro-Magnetic Inductance (EMI) scanning of soils could help growers target cultivations better, says Cranfield Universitys Dick Godwin. In the field tested, only the Bearsted soil at the foot of the hill is ever likely to need sub-soiling, he says. But that soil would be marginal for minimum tillage.
"There are very few occasions when busting is needed on clay soils," he stresses. Tramlines, gateways and turning areas may be exceptions, but they make up less than 20% of a typical field.
MagnaScan maps could help growers tell where to dig pits to check soil structure and define areas for remedial action, he suggests.
If sub-soiling is needed, it should be done when the soil is dry and the land left as long as possible before any further passes are made.
"The soil is at its most vulnerable immediately after sub-soiling. You have to tread very lightly once it is done and it should almost be the last operation. If you are very bold you could sub-soil after sowing."
Spring sown crops, on sandy or sandy-loam soils, in drought seasons are most likely to show a yield response, he adds.