How to sample

Taking Samples

Be consistent in the way you take samples. Consider the variation or it will make your efforts meaningless.

1. Take enough cores to make a good composite sample. Pull five to ten cores from the same "performance area", that is, where the soil conditions and plant performance are the same. For example, do not blend sample cores from a loamy area with cores from heavy clay, or a strong area with a weak area within the same field.
2. Organisms are different between plant species. Bare soil has different organisms than areas with litter. Soil under old logs will have different organisms than soil under new logs. Samples taken without regard to this variability can mask the effects of management practices from background variation.
3. The numbers of organisms in litter layers are always higher than in the true soil. The litter layer (O horizon) can be defined as the material that can be recognized as plant material. Don't take litter from one place and compare with soil from another place - you are comparing apples and oranges. But maybe you want to compare apples and oranges.
4. Sub-samples should be taken from similar areas within one area, e.g., all taken 3 inches from the stem of plants, or from under a log, or at the same soil depth, to reduce variation and increase detection of management effects.
5. Sample so you measure what you want to know.
1. For example, if you want to know if your tomato patch has a typical food web, randomly sample throughout the tomato area. On the other hand, if you want to know whether soil next to tomatoes contains normal numbers of organisms, sample an inch or two from the tomato stems, but not in the walkways between rows of tomato plants. Often sampling near sick plants to compare the organisms in soil from around healthy plants allows determination of pathogen or pest problems.
2. Another example: Agricultural systems are more uniform because of plowing. Tree distribution in forests is not uniform, and understory plants impose variation. Different amounts of litter and woody debris cause variability. Reduce variability by collecting from similar areas within the forest.
3. If samples are collected from within three feet of the dominant tree, or only from bare patches, or only from places where the soil is covered by litter, or only under N-fixing shrubs, or only within an inch of thistle plants, then variability will be lower than if samples were taken from all areas.

In each case,

1. Define the area you want to sample from, for example where all the plants are equally poorly growing, or equally as good.
2. Once the area is defined, draw a map and section off the area. Number each box.
3. Randomly select numbers. The numbered slips of paper in the hat trick works fine.
4. How many places to sample from? Typically, a minimum of 3 MUST be taken. Five is better. 10 even better, you get the idea. More is better, but how many to be really representative? People who send in rude little notes saying all our data is meaningless because we only combined 5 samples and their statistical procedures show that 20 is the absolute minimum just prove that they have no idea what they are talking about. Highly uniform areas only need 3 samples to be representative, while 20 samples may not be enough if the area is really heterogenous – which means it is very variable. 200 samples may not be enough. But practicality has to come into play here. If by removing 200 samples from your field you have no soil left, it doesn’t do you much good to have done the sampling. So, typically, someplace between 5 and 10 samples. Use your judgement.
5. Can you "stratify" your sampling to reduce variation? Sure. You should. But how?
6. What is it you are really asking questions about? Sample from just those areas. Don’t sample from the weed root systems if you want to know about your crop plant. As you go out from a stem of a plant, the ratio of fungi to bacteria, the root-pests, the organisms attracted to the roots of your plant change in density. So, sample half-way between the stem and the drip-line of the plant.
7. What’s a drip-line? When it rains, where does the rain fall un-impeded by the aboveground part of the plant? Where does the plants’ biomass prevent rain from falling un-impeded to the ground – that edge is the dripline. On a wheat plant, this will be right up against the stem. For most cactus, sample place, but how about a potato? Or an apple tree. Some distance out, and you help reduce variability by sampling the same distance, half way between the stem and the dripline. This takes care of big tree versus little trees. But avoid the patch of poison ivy, or hemlock, or other variation-causing plants in the area under a big plant. In a crop field, what direction out from the plant do you go? Along the plant row, not towards the wheel row.
8. What about a tree with grass right up to the trunk of the tree? Well, what do you want to know about? The grass or the tree? Or the ecosystem? If you want to know about what the roots of the trees are "seeing", you may need to dig down to find the TREE roots. The grass is the focus? Sample the grass roots. If you want to sample the ecosystem in general, you would need to take multiple samples from all the types of major plant species in a representative way.
9. In general though, we sample the 0 to 3 inch depth of soil (0 to 5 cm depth). We have done depth sampling many times, and generally, the soil food web remains remarkably consistent through the lateral and fine-root sections of a root system. So, sampling at the soil surface down 3 inches is usually representative of the root system.
10. But the roots of trees may be deeper than that, and so in that case, tell us that you sampled deeper in the soil profile, and how deep you took the sample – and roots.
1. Numbers of organisms are usually greatest near the surface of the soil and decrease with depth.
2. But how rapidly do numbers of organisms decrease with depth? It depends on the plant species and on organic matter in the soil profile. Because roots feed bacteria and fungi, the numbers of active bacteria and fungi are usually greatest where fine roots are most dense. Soil organic matter also is food for bacteria and fungi, so numbers are also high in places with high organic matter. Sometimes the distribution of these two food resources is correlated, sometimes not. Thus, sometimes the highest densities of organisms occur at 5 feet deep in the soil. But in general, numbers are highest near the surface.
3. How far down does soil go? First, we have to define what we mean by soil. Soil is the combination of the mineral parent materials (sand, silt and clay), the life in that soil (bacteria, fungi, protozoa, nematodes, microarthropods, worms, insects), and the organic matter made as the result of these organisms consuming and processing dead plant and animal material. So, how far down into the soil surface do these things go? We're at 12 miles and still going. Numbers certainly fall off after you get past the root zone, but there still are active, living organisms quite deep into the soil surface (check some of D. Balkwill's papers!)
4. How far down does the root zone go? Depends on the plant. Some weeds have very shallow roots, but consider Douglas fir with root systems down deeper than 40 feet. Walk down into a cave, and look up at the ceiling of the cave. There are many instances where caverns at 200 feet depth have tree roots growing into the cavern. And there are active, living soil foodwebs around those roots.
11. What if there is a litter layer? It must be brushed aside until you hit material that has sand/silt/clay in it. In forest soils, that can be difficult to assess, however, so brush aside litter that you can recognize as being plant material. You may end up with some OH horizon material in the sample, but that is ok.
12. Once the place to be sampled is determined, a soil corer, a bulb planter, a hand trowel or an apple corer can be used to obtain the soil sample. Remove all the roots from the cored area.
13. Remove the soil from the collector, and place in a clean container.
14. Collect the other cores from the pre-determined points and add to the container.
15. Mix the soil samples together. Break up lumps, chunks and clods, and remove rocks.
16. Place enough soil in a SANDWICH size sealable plastic baggie to fill it half full (about 500 grams).
17. Make sure all the roots that can be easily found are placed in the plastic baggie as well. A small fistful of roots are needed for mycorrhizal colonization assessment.
18. For microarthropods, a sample 5 inch by 5 inch, by 2 inch depth is needed. Take a coal or fire place scoop and scoop out a single sample from each area. Place in a plastic baggie (quart size generally works). You might want to do multiple samples per area, since you cannot composite microarthropod samples to reduce variability the way the smaller organism sample can be treated.
19. Pull leaf samples (see section on leaf sampling) and place in a separate baggie.
20. Send to SFI by overnight mail if activities are desired. By second day mail if activities are not required.