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March 14, 2013

Sodden Land Solutions Pinpointed

Too little attention to the soil is partly to blame for sodden land across much of the country, delegates to a recent Lackham College workshop run by the Agrii-sponsored Soil & Water Management Centre heard from leading soil scientist, Professor Dick Godwin.

However, while near record rainfall was clearly a factor, his message was that there were plenty of steps growers could take in the short, medium and long term to ease future field operations.

Prof Godwin outlined trials conducted at Letcombe Laboratory in the 1970s which found wheat yield losses caused by waterlogging were less than expected – at worst 20%.

“The good news is that a crop will take 21 days of continuous winter waterlogging before it runs into a problem,” he said. “And it’s more sensitive to waterlogging later in the season.”

However, good drainage clearly boosted output. Studies on heavy land between 1970 and 1974, for instance, showed a combination of drains and moling added 1t/ha to wheat giving only 4.5t/ha when undrained.

Drains plus subsoiling, instead of moling, added 0.6t/ha – which could have been higher if the potential benefit of earlier drilling could have been included in the experiment.

“I keep hearing that fields are saturated. But underneath they’re probably not – unless they’re in high ground-water areas,” he pointed out. “Everywhere else it’s mostly a problem is caused by a nearly impermeable layer of soil near the surface with the water sitting above – a perched water table.”

“In many cases the surface may be flooded but the soil is only at field capacity underneath.”

How quickly surface water naturally filters down depends on several factors, the crucial one being the condition of the soil near the surface, he explained.

“A fundamental problem is that in many cases the soil pores in the top six inches are too small because the ground has been tilled and/or run over too much.
Old permanent pasture could readily absorb a typical 4-5mm/hr of rain, he noted. “But clean, tilled bare ground is barely able to cope with peak rainfall rates.

“The effect of compaction caused by wheels is particularly noticeable. In a recent trial where water filtered through unmarked soil at 20mm/hr, a single pass reduced that to under 6mm/hr, and after two passes it was down to less than 2mm/hr.”

Offering short term advice, Prof Godwin showed a picture of a sandy loam field with surface ponding only a few yards from a ditch. “It’s because the ditch is blocked,” he explained. “Very rarely do you see anyone cleaning ditches nowadays.

“We need to study old drainage plans, walk ditches, clear blockages and make sure the drain outfalls are fully functioning.”

‘Fire-brigade’ options to get rid of surface ponding included mini-moling.

“Work as shallow as possible, say only 12-15in,” he advised. “It doesn’t need high power; and don’t use an expander.  As an alternative dig a short ditch or plough a single furrow to create a groove to take the water away.”

In extreme cases it might be worth drilling holes to help surface water penetrate or even digging soak-aways, Prof Godwin suggested. “By sacrificing a small area you could be doing yourself a long term favour.”

Broadcasting seed via quad-bike, or if necessary by hand, could also help by establishing a covering of plants to “pump out water” once the surface layers have dried. “This will enable the soil profile to dry out at depth – otherwise it will remain at or close to field capacity for most of the season.”

In the medium term, growers with problems on clay soils should plan to mole drain them to connect to the backfill above field drains in May/June – if necessary through standing crops, he advised. “The mole’s leg helps get surface water away faster – and think about subsoiling damaged areas after harvest.”

Apart from improving overall field drainage systems, long term options to avoid waterlogged fields included installing grass waterways, grading low spots on flat land, and raising soil organic matter levels, which he admitted was a particularly long term goal.

More immediately, wider use of low ground pressure tyres which add only 50p/ha and £1/ha respectively to the costs of running a combine and tractor over their lifetimes should be considered.

By Kind permission of Farmers Weekly