A bunch of numbers. That’s all you have, and you have no idea what they mean. At least you can read a soil test and figure out how much P and K you need.

But what you got in the mail today were the results of the soil sample you sent to a laboratory several weeks ago to determine the seriousness of your problem with—cue the scary music—soybean cyst nematode.

scn is so insidious that it doesn’t even deserved to be capitalized. But scn is nearly everywhere in the Cornbelt. In the November 22 edition of the farm gate you learned that 62% of Cornbelt farmers don’t think they have a problem with scn. But 61% of Cornbelt farmers have a problem with scn. Let’s imagine that many of those took the statistics to heart and shipped off a soil sample and just received the results in the mail.

Greg Tylka, a plant pathologist at Iowa State University, walks us through the interpretation of the results. He says you need to know what was counted by the lab, whether it was the eggs in a soil sample, or whether it was the number of cysts (dead females that contain eggs) and the juveniles. If you have a number representing cysts, multiply that by 100, because there are about 100 eggs per cyst. The juveniles don’t last very long in the soil, and their numbers are somewhat of a ho-hum.

The scn number you have is based on so many cysts (or eggs) per volume of soil. Your lab results may be for 100 cubic centimeters (cc) or 250 cc of soil. Your treatment program will be based on either 100 or 250 and if you have the wrong one, all you have to do is convert to the other. If you have X number of scn eggs per 100 cc, and you need it in terms of 250 cc, just multiply your egg number by 2.5. To go the other way, divide by 2.5.

The number of scn eggs will vary widely from state to state, from county to county, from field to field, and from plot to plot within a field. scn may move an inch in its lifetime, so it is not like GMO pollen. The eggs will be moved by tillage equipment, and soil left on the shovels will move scn from field to field. But from plot to plot within a field, the egg count might be well into the 5-digit range, and 50 feet away it might be zero. Tylka says, “Even though variability cannot be eliminated, increasing the number of soil cores and decreasing the area from which a sample is collected make the SCN soil sample results more meaningful.”

Tylka says scn management issues kick in at the threshold of 12,000 eggs per 100 cc of soil, and that begins with the growing of a non-host crop such as corn. And if you are increasing your corn planting this year, the fields with the higher scn populations may be the ones you begin with.

Iowa State recommends a six year rotation if your scn counts are more than 12,000 eggs following a year of corn, or more than 16,000 eggs following a year of soybeans:

• Year 1: Plant a resistant variety from breeding line PI88788
• Year 2: Plant a non-host crop, such a corn, oats, or alfalfa.
• Year 3: Plant a resistant variety, other than what you grew in year 1
• Year 4: Plant a non-host crop, such a corn, oats, or alfalfa.
• Year 5: Plant a susceptible variety
• Year 6: Plant a non-host crop, such a corn, oats, or alfalfa.

Summary:

Soybean cyst nematode can eliminate any profit in your soybean crop, by cutting yields without you knowing it. However, by evaluating the results of an scn soil test, and knowing whether you have a count of cysts or eggs, you can begin a management program in 2007.

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