When designing a sampling plan for nematodes, there are practical and theoretical considerations to be made, which are balanced with the amount of time and money available for the study.
• Goal of the study and required accuracy: The goal of the study will greatly determine the degree of accuracy required. For qualitative studies such as diagnosis of a plant disease or taxonomic work, the requirement is relatively low. In an ecological study of nematode communities more accuracy is needed, because rare species should be included in the sampling. Even more effort is needed in sampling for disease control, for example, to certify that a certain field is disease-free. In general, a higher accuracy costs more time and money.
• Variation in space (horizontal and vertical distribution): One of the biggest problems in sampling is that nematodes have a patchy distribution, that is, they are not randomly distributed in the soil. This has biological reasons (concentration around roots or in islands of organic debris), agronomic reasons (cropping history; planting diseased material creating "hot spots" of disease), as well as physical reasons (e.g., texture gradients). If any of these factors are known, sampling variability can be decreased by sampling and analyzing such areas separately (called stratified sampling). In terms of vertical variation, most nematodes (and other soil organisms) can be found in the topsoil (0-10 cm). In agricultural fields, when sampling roots, sample depth is often as deep as the plow depth (typically 15 cm) or deeper. Again, it depends on the purpose of the study.
• Variation in time: Seasonal cycles and life cycles will influence the results of the sampling. Seasonal fluctuations in moisture, temperature, and food availability will influence abundances of different species in different ways. For sampling plant parasitic nematodes, knowledge of the life cycle is most relevant. For population studies of an endoparasite, sampling soil when the crop is on the land does not make sense, because most nematodes will be inside the roots.
• Statistical considerations: The purpose of sampling is to estimate, as accurately as possible, nematode abundance in a site. The "sampling error" (expressed as variance or standard deviation) is the sum of systematic and random errors, introduced at any stage in the sampling process (taking cores, mixing soil, extracting soil, counting subsamples, etc.). Systematic errors can be reduced by improving methods and working as carefully as possible. Random errors can be reduced by taking more and bigger samples, as much as time and money allows. A rule of thumb is that variability within a sample (or plot) is usually smaller than between samples (or plots). Thus, it makes more sense to take many samples than to analyze many sub-samples (likewise, it is more efficient to sample many different plots than to take many samples within the same plot). Random sampling is best but not always most practical. Systematic sampling (e.g., taking samples along a transect, every few meters) is more often used. In that case, the starting point should be randomly chosen.
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