29 November 1996

Too much water as bad as too little – and waste of cash

In the first of a series of articles looking at ways to cope with scarce water resources, Edward Long finds out one potato farmers precision solution

POTATO growers who irrigate crops probably do not appreciate the damage being done by overwatering. Not only does it waste a scarce resource, it also compromises yield and quality.

So says Suffolk potato grower Anthony Mayhew. He is convinced a more precise approach is needed.

"In the past we have applied too much water from rainguns, particularly on windy days and this was economically damaging," says Mr Mayhew, who farms 160ha (400 acres) of blowing sand at Sheepdrift Farm, Brightwell near Ipswich, plus rented land in the windy Suffolk coastal strip.

"Wind alters the distribution pattern and the downwind rows get far more water than those on the upwind side. Too much water early in the season stops the potato crop in its tracks, as root development is halted. In excessively wet conditions young roots can even retract, setting the crop back," he says.

That hits both yield and quality. In a worst-case scenario the yield of an excessively watered crop could be halved, he believes. To avoid that Mr Mayhew uses a sophisticated approach to irrigation scheduling.

Water comes from a borehole, lake, and gravel pit, with licences allowing 5.5m gallons/year from the borehole and a total of 20m gallons from all three sources.

"We could always do with more, but there is no prospect of us getting any extra as the Environment Agency has clamped down on all applications. So we must make the best possible use of what we have. Booms are a lot better at getting it to where it is needed than rainguns. But for the future I am keeping all options open. I am looking very seriously at trickle systems."

Maximum benefit

But it is no use applying water more accurately unless you know when to put it on for maximum benefit, he stresses.

For the past three years his application rates and timings for potatoes have been made according to the Cambridge University Growers Research Associations system.

CUPGRAs Mark Stalham visits each field after planting to check soil type, stone content, and soil penetrability. Each week information about the crops canopy development is faxed to Cambridge and Mr Stalham obtains further data from local hydrological stations.

Together with farm data this allows the evapo-transpiration rate to be calculated. Details of the amount of water needed, and when it should be applied, are then faxed to the farm office.

The little-and-often application policy, which is geared to the needs of the crops during the critical early development phase and to scab control, involves a maximum of 8mm of water every four days if needed.

Five weeks after tuber initiation the policy changes to an SMD-linked system.

The CUPGRA approach has been used since Mr Mayhew lost faith in neutron probes, which he says did not seem sufficiently accurate at the time. They did not provide a clear indication of what was happening in the surface layer of soil where the quality of an early baker crop can be made or lost.

More precise irrigation is vital, says Suffolk grower Anthony Mayhew. Boom irrigation helps in pre-pack turnip and potato crops.

Getting the most out of potato crops demands a detailed knowledge of the crops water status, argues Mr Mayhew.


&#8226 Cropping:ha(acres)potatoes60(150)sugar beet24(60)dwarf beans24(60)p-pack turnips 100(250)flax60(150)

&#8226 Water: 20m gallons from borehole, lake, gravel pit.

&#8226 Irrigators: 2 Briggs booms; 2 300m centre pivots; 2 Briggs reel/rainguns for use after canopy closes.

&#8226 CUPGRA forecasting early in season, 8mm max application.

&#8226 SMD forecast later.