Science into practice - Watercress: tailored use of phosphate

Watercress requires supplementary phosphate fertilisation to produce marketable crops because the spring water used to grow it does not contain enough phosphate. Growers have long relied on phosphate fertiliser but there is no simple blueprint to follow.

HDC project FV338 (supported by the Environment Agency) was commissioned to study the effect of varying phosphate rates and application practices on the crop, and on phosphate discharges, which in excess may have environmental impacts.

Three phosphate application rates from 2,200kg/ha per year down to 900kg/ha per year were used in three planted and three regrowth crops. In both cases the lower phosphate regimes significantly slowed growth and reduced bed productivity.

Some low-phosphate treatments were associated with stem purpling, which would be unacceptable to retailers. Detailed advice on the levels of phosphate needed to reduce stem purpling are in the full HDC report.

Phosphate levels in the discharge water from the beds were related to fertiliser rates and formulations. Soluble fertilisers led to much higher levels in the discharge than when a less soluble source was used, so growers need to be particularly careful when using them. However, settlement systems, commonly used between the production beds and farm discharge points, significantly reduced phosphate discharge to watercourses.

An additional trial FV 338a, with recommendations due this year, tested the effectiveness and economics of using other technologies to remove phosphate from water, such as those used in sewage treatment plants. The work is vital to help the industry meet forthcoming EU water standards.

Horticultural Development Company

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