How can the variability in my soil sample results be explained?


(from Dr. Terry Niblack , University of Illinois at Urbana-Champaign & Dr. Pat Donald, University of Missouri)

Soybean cyst nematodes (SCN) are not evenly distributed in a field. For example, in a single field with an average number of eggs of 50,000, the actual number of eggs in any one section of that field might be anywhere from 0 to 100,000. Due to this natural variability in the distribution of SCN, it is very important to take a sample. Some of the questions we've received about this variability in SCN egg counts are given below, along with the answers.

If I take a sample in May and get an egg count of 25,000 (high), then take a sample in June from the same field and get a count of 500 (low), what happened?

One of two things: 1) you took "bad" samples (see below), or 2) most of the eggs hatched by June because of the rise in soil temperatures and the presence of soybeans. This can happen especially if the field was rotated to a nonhost for one or two of the previous years. The nematodes that succeed in infecting won't produce new eggs for about 35-40 days, so as you can see, sampling after planting for about 35-40 days will give you a wrong idea about how many SCN you have. The best times to sample are just before planting or just after harvest.

If you take a sample from a field, split it in two, and get back an egg count of 10,000 on one and 30,000 on the other one, what happened?

This may be a little hard to believe, but once your egg counts get above a certain level (say, 5,000), differences of 25%, 50%, or even 100% can occur in two tests run on the same sample, depending on how high the average number is. SCN cannot reproduce in the absence of a host root, and because it tends to cluster in certain areas on the roots, cysts tend to be clustered together in a sample. The eggs we count are released from the cysts, which can contain anywhere from 50 to 500 eggs apiece. Combining these factors (clustering plus variation in the number of eggs in a cyst), it's easy to see how chance can alter the results. The real question is - is this important? The answer to that one is no. The important result you obtained is that you have high levels of SCN in that field, and need to take steps to reduce the potential for damage.

Yes, but I want to know exactly how many cyst nematodes I have in that field.

O.K., we can try. What you need to do is take one sample (1-inch diameter by 6 inches deep) every 15 feet in the north-south direction of your field, and do the same thing in the east-west direction. This will be about 194 samples per acre (or $1,940 at $10 per sample). According to research done in North Carolina, we'll expect to get about 46% zeros (meaning, 46% of the egg counts will be 0), and our estimate of the average number of eggs will not be that much better than if we took a single 'good' sample (see below) from the field.

But if I can't know the exact number of SCN eggs there are, how will I know when the number is low enough for me to grow soybeans without yield loss?

Take a GOOD sample! At planting in the spring, or just after harvest in the fall, walk your field in a zig-zag pattern. Each time you zig or zag - in other words, each time you change direction - take a small sample to a depth of 6 inches with a soil probe, garden trowel, or spade. Put the sample into a bucket and keep walking. After you've taken 30 or so small samples, each one at a different site in the field, mix up what you have in the bucket. Mix it well, until the soil is of uniform texture (no lumps, rocks, stems, etc.). Then take a sample from the bucket, about a pint of soil, and put it in a plastic bag - no cardboard boxes, please! Keep the bag out of direct sunlight until you can take it to your Extension office or send it to a nematology lab for analysis. If you let the bag cook in the sun or dry out, you should collect a fresh sample.

So, what you're telling me is that I can never know exactly how many SCN eggs are in my field. I can only get an estimate of the average depending on how "good" the sample is. If that's so, then why do you send me a number of eggs and not just a high, medium, or low rating?

Because, if the sample is 'good', the estimate of the average number of eggs is a pretty good indicator of the number of eggs actually there. If your field is not infested, you'll never get a number higher than 0. If your field is infested, whether it's high, medium, or low, you still need to start considering the presence of SCN when you choose soybean varieties or plan your rotations. If the egg count is high, you can expect some yield loss on a susceptible variety. If the egg count is low or medium, you may not see yield loss this year or next, but you will eventually if you continue to grow susceptible soybeans.

Some labs count cysts, not eggs. Is one better than the other?

That depends on what you want to know. If you want to know whether you have SCN and don't care about the numbers, then a cyst count is fine. But if you are monitoring a rotation (sampling every couple of years to see what the SCN numbers are doing), then you really need an egg count. This is because resistant varieties are not completely resistant that is, they allow some reproduction. But cysts produced on some resistant varieties have fewer eggs than those produced on susceptible ones, so that a cyst count of 20 could mean an egg count of 1,000 (medium level) or it could mean an egg count of 10,000 (high level).

Some labs count "viable eggs". Is this better than just an egg count?

We don't know the answer to this one. It's unlikely that a "viable egg" count is really valuable, because of the method used to determine viability, so that's a decision for you to make. But, wherever your SCN tests are done, be sure you provide the one thing only you can provide: a GOOD sample.