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Mitigation of phosphate and sediment

2 August 2011

Cultivated stubble and cover cropping can reduce surface runoff and phosphorus loss from land the winter before potatoes by up to 60 per cent, according to Dr Martyn Silgram of ADAS.

Furthermore, when a mustard cover crop was grown, 15-20 kg N/ha that would otherwise have been lost via leaching, was conserved in the soil.  Results also show that typically 5% of rainfall and irrigation applied to potato crops are lost as runoff, the majority down stone rows.

Potatoes are often considered a ‘high risk’ crop due to the intensive tillage processes prior to planting (e.g. bedforming, destoning), raised bed systems which channel water in furrows, the frequent traffic for spraying and irrigation, and typically late harvests. 

Project Leader Dr Martyn Silgram notes that soil compaction associated with these processes can limit rooting depth, reduce water and nutrient availability, reduce tuber yield and quality, increase the risk of waterlogging and tuber rot diseases, and lead to inefficient use of irrigation and fertilisers.

In conjunction with the Potato Council, the Defra-funded ‘MOPS2’ project led by ADAS is evaluating sustainable management options for the potato rotation to minimise the risk of soil compaction, improve surface infiltration, and minimise surface runoff and losses of soil (sediment), phosphorus, nitrogen and surface applied products (e.g. herbicides) to water courses.

Before potatoes

Runoff water was collected and analysed over the winter period from replicated 60-100 m long field sections with 4° slopes on a silty clay loam in Herefordshire and a loamy sand in Shropshire. 

Results from the winter before planting potatoes show the majority of surface runoff, sediment and phosphorus loss, occurs down compacted and unvegetated tramline wheelings used for spraying operations in the previous cereal. These losses can be substantially reduced via light autumn cultivation (e.g. power harrow) or the use of a cover crop (Figures 1 & 2). 

This is consistent with a separate ADAS-led HGCA/Defra LINK project that is exploring methods of minimising soil compaction in winter cereals.  A mustard cover crop was used before planting potatoes as it grows rapidly (locking up N that would otherwise be leached over-winter), has biofumigant properties, is not frost hardy, and decomposes rapidly.

Figure 1.  Four treatments the winter before potatoes

Over-winter runoff and diffuse pollution losses were greater from the stubble tramline area compared with the stubble area.  Mustard cover crops proved effective, establishing rapidly and taking up c.15-20 kg N/ha which would otherwise have been leached over-winter.  

The mustard died off following frosts, with residues breaking down rapidly and disappearing well before bedforming in March.  Shallow tillage produced a rough aerated surface which helped infiltration but also broke apart clods, leaving soil more vulnerable to erosion from intense rainfall events.

Figure 2.  Over-winter surface run-off

 

During potatoes

Using the same sites as before, treatments characterised the importance of different pathways of loss in potato systems, to enable effective targeting of management options in future work.  Losses from no stone rows, stone rows without traffic, and stone rows receiving spray and irrigation passes were measured (Figure 3).  An additional treatment explored whether the careful use of a winged tine could break the compaction just beneath the stone layer in a no-traffic row

Figure 3. Runoff down stone row receiving traffic during 25 mm irrigation event, and lack of runoff from adjacent non-traffic rows

Stone rows had much greater losses than rows without stones, with the greatest losses from stone rows receiving repeated traffic (P<0.05; Figure 4).  Losses from stone rows with traffic were 15-18% of water applied in rain and irrigation (Figure 5).  The tine in a stone row proved effective at reducing losses (P<0.05) with no discernible effect on yield.

Figure 4.  Runoff and sediment results during the potato season

Figure 5  Runoff by treatment, expressed as a % of rainfall and irrigation

However, if we upscale these results to reflect field-scale practice, then there is a ratio of 12:12:2 between no stone, stone, and stone traffic rows in each 24 m width.  At whole field scale, overall runoff losses represented 4.2 - 5.0% of the rainfall plus irrigation water applied during the season – a significant waste of water and fuel, with implications for irrigation scheduling and yield quantity and quality.  At whole field scale, it was the stone rows without traffic which were the most important location for runoff because they were so numerous, with the majority of field scale losses originating from this location.

Summary

Results show tramline wheelings are important pathways for runoff and diffuse pollution when potatoes follow cereal harvests, but with careful timing, cover crops and shallow cultivation are practical methods which can substantially mitigate such over-winter losses. 

In the potato season, stone rows are the main loss pathway for runoff and associated pollutants (sediment, phosphorus, nitrogen, surface applied products).  Work is now investigating a range of practical options for managing the stone rows between potato beds.

For more details contact Martyn Silgram: martyn.silgram@adas.co.uk.

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