Choosing the right site for a potato crop can have a big influence on crop health and profitability. Frontier Agriculture's Les Sykes addresses field selection.
Detail makes the difference – and for potato growers it all starts with field selection. Tough market conditions – resurgent prices for other commodity crops combined with supermarkets keen to keep a lid on potato prices – have left little room for error.
Planning starts now for the 2009 crop. Potential fields should be selected this spring and sampled after harvest.
The usual factors for site selection of soil type, depth, evenness, aspect and slope all affect final yields. Selecting a field where crops can be planted and emerge quickly is also important to make full use of all the available light, which directly relates to yield.
But growers also need to assess field shape. With land rents rising it has a significant impact on land use and irrigation efficiency, and marketable yields.
The example below shows a field mapped using GPS, part of Frontier’s Fieldselecta service, where only 1.31ha was lost from the field’s 19.23ha, helping to maximise total field efficiency. This approach also identified planted headlands (green) and appropriate irrigation runs (blue). The analysis improved the accuracy of input purchases and, in this instance, saved about £800.
Soil sampling
Soil sampling should deliver a useful understanding of pest, disease and nutrient status in a field. But there are several points to remember. For a start analysis can only give an accurate measurement of what is present in the sample. The greater the number of samples taken the greater the chance of accurately reflecting the field reality. How intensely you sample should reflect the risk of the problem and how much it costs to both sample and rectify any problems. But no sampling method can guarantee absence of a problem, so only use the results as an aid to management.
Nutrient sampling
Start by dividing blocks by soil type, either whole fields or into areas of obvious difference. Take about 40-60 cores per sample walking in a W pattern across each block. That should be enough to justify appropriate fertiliser applications, but also consider nutrient use in the whole rotation. Where precision agriculture is practised smaller blocks may be appropriate.
Pest and disease sampling
This should answer two key questions: Is there a problem in the field (and how big is it), and what is happening to that problem.The former can be tackled either by W-sampling blocks (40-60 cores per sample), which will help determine presence or absence, or by sampling each hectare from a recorded point and collecting 40-60 cores spiralling out to a radius of 10m. The latter method allows better quantification of a known infestation.To find out what is happening to a problem you need to take samples from the same small area for several years, preferably at a similar time of year. This allows population changes to be mapped, which, with clean land in short supply, will be increasingly important.How to use the results of sampling for various diseases and pests is summarised in the table.
Conclusions
Detailed planning before any cultivation is probably the most cost-effective spend on the crop. Prevention is better, and usually cheaper, than cure. Improved field selection encourages better cultivation and crop protection practices.
Sample Q&A
What am I sampling for?
Understand why you are sampling; eg, a) is there a problem in the field? b) How is that problem changing?
How intensively should I sample?
There is no single correct answer. For nutrient analysis one sample per field may be sufficient if the soil type and management are uniform.
Approaches for potato cyst nematode vary from a 100g sample from 4ha up to 1500g from 1ha (maximum in proposed EU Directive on PCN). We are moving towards a standard 200g sample/ha block, which is more repeatable and especially good at keeping a check on low populations.
Requirements may differ for specific tests; eg, rhizoctonia.
How should I sample?
Take small cores using a cheese or corkscrew corer. Samples must represent the area and the entire depth of the area tested.
What size sample should I take?
About three times the soil that will be analysed. Sub-sampling, however meticulous, introduces another chance for error.
How should I look after samples?
Keep cool and dispatch as soon as possible, particularly when analysing living organisms; for example, free-living nematodes.
What about interpretation?
Interpretation must reflect history, risk and cost of any action. Queries are usually about pest and disease, especially when results return as zero. This must not be interpreted as absence from the field, but only that none was present in the sample. Interpretation must consider whether a zero result is unexpected. If in doubt take another sample and take decisions based on all available information.