There are a number of factors to consider to determine the size of the area that a sample should cover. The time to process (extract, count and report on) a single sample ranges from a half hour to four hours or even longer and costs increase accordingly. Given the time involved in processing, one should consider that taking the time to obtain a "good" nematode sample, which often must be done when the weather is inclement or in a soil not conducive for digging, is worth the effort.
An informal survey of applied nematologists indicates that most feel comfortable advising growers based on samples taken on a 5 acre basis. However, based on processing costs involved, growers frequently opt for fewer samples taken from larger areas, typically arguing for 1 sample from each 20 acre block. If economics dictate sampling larger areas, then increasing the number of subsamples per sample can help to offset the loss of accuracy from sampling the larger area.
Prior to sampling, one should visually determine if the site has differences such as soil texture, soil moisture or drainage patterns, healthy vs unhealthy patches of plants, or cropping history that would warrant separate samples. Each subdivided area of the field is called a stratum. Each stratum should be represented by a separate sample to allow mapping of the nematode population and differential management as appropriate.
If plants are present in the area of interest, roots should be included in the sample. Small feeder roots are more likely to yield nematodes than larger structural roots. Roots should be placed in the same plastic bag as the soil and separated by the laboratory. Placing them in a separate bag, they are likely to dry out and be unuseable. Wrapping in wet paper towels encourages the growth of fungus prior to arrival at the laboratory.
If plants are showing damage symptoms, consider taking separate samples from healthy and unhealthy areas and include both roots (also tubers or bulbs) and above ground symptoms. It is often more desirable to sample at the outer edge of an unhealthy area rather than at the center, because nematodes tend to migrate to healthier areas as roots deteriorate.
Subsamples should be mixed well and approximately 1 quart or liter of soil placed in a plastic bag, labeled on the outside of the bag, sealed, and kept cool (12-15C) (not frozen) until arrival at the laboratory. Alternatively, the sample can be placed in a double plastic bag with a label written in pencil placed between the bags. Laboratories typically process about half a quart or half a liter of soil. Providing the extra soil will permit reprocessing if the sample is accidentally spilled or another calamity occurs.
There is no one correct number of subsamples to take. Generally, the potential for maximizing the detecion of the number of species present increases as the number of subsamples increases. For a diagnostic sample, a useful rule of thumb would be to collect as many subsamples as can be taken in a half hour. The number of subsamples that can be taken in this time will vary depending on a number of factors including the sampling device used, depth of roots, soil texture, and soil moisture level. Typically, a sample taken with an auger in this amount of time would comprise 3 to 5 cores, a shovel 5 to 12 areas, and an Oakfield or Viehmeyer tube 12 to 20 cores.
The Phytonematology Study Guide (DANR publication 4045) defines the basic minimum sample size as 15 to 20 cores, which should be used to represent 5 or less acres of uniform field history and soil condition. The basic sample size varies with the value and investment in the anticipated crop, the available management strategies, and supplementary information from the sampling (soil depth and hardpans, nutritional analyses, etc). In annual crops 15 to 20 cores should be taken to a depth of 12 to 18 inches. After prolonged fallow, deeper cores should be taken (to 36-inch depth). Deep rooted perennial crops should be sampled to a depth of 36 inches, or to the hardpan layer. Samples for cyst nematode can be easily taken from the soil surface.
In an orchard, samples are usually taken at the "dripline" of the tree. This is essentially at the outer edge of the canopy. Feeder roots should be included in the samples.
If an orchard or vineyard is drip irrigated, roots are likely to be most plentiful in the vicinity of the dripper and samples should be taken in this area. Vineyards should be sampled near the vine where roots and nematodes are least disturbed by tillage and cultural operations. Soil samples in established perennials should be taken to a depth of 36 inches.
It's important to let the laboratory know the crops of interest and cropping history because there are a number of extraction procedures available and some are more likely to detect one genus than another. Additional information would include the grower's name, location, soil type or texture, notable symptoms (yellowing, necrosis, root rotting, galling, wilting), last nematicide treatment and type, and any growth limiting factors other than nematodes that are known to be important in the field in question.