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Weed learning module
How we estimate the density of weed seeds in the soil.
The number and size of weeds present in a field changes rapidly over the course of a growing season, depending on how long since the field was cultivated, how well it was cultivated, the type of crop and other factors. The density of weed seeds in the soil seed bank changes more slowly. For the many weed species that have long lived seeds, the density of the seed bank provides a long term average of the effectiveness of management practices. It is thus a good way to evaluate differences in weed control between treatments over the long run.
Weed seeds are a very small component of the soil. Finding a few weed seeds in a large volume of soil requires special equipment and a lot of work.
First of all, seed density varies enormously over short distances. A one inch diameter soil core may contain no seeds of, say, common lambsquarters whereas another core a few inches away may contain dozens of seeds if part of a fruiting stalk happened to get buried there. Thus, to get a good estimate of seed density we have to take many large soil cores. In these experiments, we take 12 2.75-inch diameter cores to a depth of 8 inches in each plot. That is a lot of soil! To reduce the volume to an amount we can handle in the lab, we screen the soil to remove rocks, and weigh the soil that is left. We then thoroughly mix the soil by rolling it around on a tarp, and take a 5 lb sub-sample for analysis.
Next we run the 5 lb subsample through an elutriator. This is a device that separates the coarse soil organic matter, including weed seeds, from the mineral portion of the soil. We pour the sample into one of the large tubes. Four water jets pummel the soil and break it up, and the force of the rising water carries silt, clay and organic matter up and over the elbow at the top. A sieve catches the coarse organic matter, but lets the silt, clay and humus pass through into the large tub. We make a lot of mud with this machine! The sand and gravel remain at the bottom of the tube and are cleaned out at the end of each run. We then dry out the organic matter and our hard working undergraduate assistants pick out, identify, and count the seeds.
How many samples do you guess a skilled operator can elutriate in an hour? How long do you think it takes to sort out and count the seeds from one sample? Make your guess, and then find the answer here.
Commercial elutriators that can separate organic matter from both coarse sand and gravel and fine silt and clay only handle a few ounces of soil at a time. Our elutriator is home-made and designed to handle large samples. It has four tubes so that two samples can be washing while the operator cleans out the sieves and bottom gravel on the other two tubes.
Thought question:
How do you calculate the number of seeds per square foot of soil from the number of seeds found in the elutriated subsample? When you think you have it figured out, find the answer here.
Return to weed learning module main page.
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