Sampling soils for clubroot
Time of sampling
Fields in which brassicas are to be sown or planted may be sampled at any time of the year, but samples are most conveniently taken when the soil is soft and any growing crop is small. As the test takes 6 weeks after setting up, fields must be sampled well in advance of sowing.
Five hectares agricultural brassicas (e.g. oilseed rape); 2 hectares or individual breaks for horticultural brassicas (e.g. calabrese).
Draw a map of the field and, if necessary, split it into 5 hectare blocks using features such as gates and trees as reference points. Sample each 5 hectare block separately. Gateways and wet hollows should be sampled separately as these areas are most likely to be heavily contaminated and could give a false clubroot rating to the main sampling area.
Sample size and pattern
Fifty cores should be taken from each sampling area to give a soil sample of 4.5 lb (2 Kg). The cores should be collected at regular intervals as the sampling areas is traversed in a “W” or multi “W” pattern.
Sampling tools and sampling depth
A narrow bladed fern trowel or auger should be used and the cores should be taken to a depth of 6-8” (15-20 cm). An auger will be essential where the soil is compacted. Remove excess soil from the sampling tools between different sampling sites.
All samples should be collected in heavy gauge polythene bags. Each bag should be tied as soon a it has been filled to prevent contamination. Label each bag separately giving the name of the farm and field or part-field.
Remember the validity of the clubroot test is determined primarily by the sampling procedure in the field.