| MESH/INCH | SCREEN OPENING (MICRONS) |
| 325 | 45 |
| 400 | 38 |
| 500 | 25 |
Some laboratories have conducted extensive tests to determine extraction efficiency for the methods they utilize and nematodes typically recovered. If so, this will usually be indicated on the laboratory report. Extraction efficiency indicates the percentage of nematodes recovered from a sample to which a known number of nematodes has been added. A useful procedure for determining extraction efficiency is to add a specific number of nematodes at various life stages, including eggs or vermiform juveniles of different sizes, to several soils of varying textures. Then, by using standard extraction procedures, extract nematodes from the soil to determine the percentage extracted.
Compare the screen size openings of the smallest mesh sieves typically utilized for nematode extraction with the body diameters of common plant parasitic nematodes. It should be evident that only nematodes landing on their sides will be retained on a sieve while those hitting on their heads or tails will easily pass through.
| If nematodes are reported/ 1000 cc soil. | |
| Example - | |
| Correction Factor | |
| run 350 cc soil on elutriator | X 3 |
| elutriator collects 1/5 of sample | X 5 |
| count 1/2 of dish | X 2 |
| =X30 | |
| 1 nematode counted = 30 reported |
Several factors not often evident from the laboratory report are common to most processing labs. Laboratories may report the numbers of nematodes found based on a unit that is different from that actually processed.
An example is illustrated by the table above. Samples may be sumitted with different volumes of soil or different extraction methods may utilize different volumes. For consistency and ease of interpretation, labs will utilize a correction factor to be able to report on a consistent basis.
Some methods such as elutriation will allow a portion of the sample to run down the drain particularly if soils are heavy or contain large amounts of organic matter which will clog screens prior to the end of the processing time. A correction factor is again utilized to return to a standard quantity to report.
Finally, only a portion of the sample dish may be counted when viewed under a microscope. This may be because nematodes laying close to the edge of the dish may not be viewed and identified accurately, so that a portion of the dish is marked off with a grid as the counting area. When samples include large numbers of nematodes and it would take a long time to count each one individually, only a portion of the marked grid is counted and multipled by a factor which would indicate the entire dish had been counted. Sometimes these counting multiplication factors are recorded and other times the multiplication is done mentally as the sample is counted and not written down.
In any event, the utilization of multiplication factors is common practice in nematology laboratories. One should be aware that a laboratory report may indicate the presence of a certain number of nematodes in a certain volume or weight of soil or plant material when fewer were actually counted from a smaller (or possibly larger) volume or weight of sample. The following approximation from the Phytonematology Study Guide (DANR publication 4045) is useful for converting nematodes per soil volume to nematodes per soil mass. Sandy soils usually have a bulk density (B.D.) of 1.4 to 1.6 and clay soils range from 1.1 to 1.3. For example, in sandy soil, 200 root knot/250 cc at B.D. 1.6 = 200 root knot/400 g. Therefore, 1 kg soil contains 500 nematodes. This assumes a moist soil in which nematodes can remain viable.