Daniel Hudson, UVM Extension Agronomist
Soil sampling can feel like a lowly task, especially when you have other urgent things to do, but your crop yields and forage quality depend on you having good information. Getting good information depends on a careful sampling process that acknowledges the fact that you are trying to get a 1 cup soil sample to accurately represent up to 40 million pounds of soil (the approximate weight of the top 6.7 inches of soil in a 20 acre field).
- Farmers should own the basic tools of the trade: get a good soil probe or auger. Other tools include plastic buckets, zip lock bags, a thin spatula, and something to break soil cores apart in the bucket. Yes, you now have another excuse to buy an ATV.
- Divide your crop land into ‘sampling zones.’ A sampling zone should be 20 acres or less in size, contain fairly similar soil types, and have a uniform management history. Aside from soil types and management history, if crops in certain parts of a field tend to grow differently from the rest, that is a good indicator that you should create more than one zone in the field. If any of the aforementioned criteria does not fit,
break the field up into more than one zone. In some cases, it is appropriate to combine small contiguous (or extremely close) fields into one sampling zone if soil type, slopes, management history are basically the same. In this case, total size of the zone should still not exceed 20 acres.
- Do not take samples from places within a management zone that do not conform to the zone overall. If most of the field has good surface drainage, do not take cores from the tiny wet area (etc.). If the odd section of the zone is big enough to justify it, you can take a separate sample for that area. Otherwise just avoid it.
- Establish a system for keeping track of which samples came from which zone/field.
- Using a zig-zag or grid pattern, collect 15-20 soil cores from within each sampling zone. For standard soil testing of corn and forage fields, cores should be taken from the top 6 to 8 inches of the soil. Place the cores in the plastic bucket as they are collected.
- If you are sampling a corn field where fertilizer was band-applied and/or fields where manure was injected, follow the guidance given here.
- If manure was spread or deposited recently, do not probe through the manure. Move the probe over to a nearby place that is not covered with manure. [Ideally, soil sampling will be done prior to late-summer or fall application.]
- If there is a lot of living or dead plant material on the surface, gently scuff it aside with your boot or the tip of the probe, taking care not to displace actual soil.
- If green grass, rock, or thatch is obvious in the sample, remove it prior to sub-sampling. Don’t obsess about pebbles.
- It is probably better to rely on the soil laboratory to screen out plant roots in the early stages of sample preparation.
- After 15-20 cores from a zone have been collected in a bucket, the cores
should be broken up (using a trowel, spatula, etc). Mix as completely as possible.
- Wet soils or soils with layers of clay will often be more challenging and may need to be air dried and crushed before thorough mixing is possible. To air dry samples, spread them out in a thin layer in trays lined with paper. Applying a fan will hasten the process, but do not apply heat.
- Obtain a representative sub-sample from the sample collected. Depending on the diameter of your soil probe and the depth of your sample, you could have between 2 and 4 cups of soil from a management zone from which you need to take a 1 cup sub-sample to submit to the lab. Develop a procedure to get a truly representative sub-sample from the entire sample from that zone.
- The process I often use to get a representative sub-sample is as follows:
- LABEL THE BAGS you are using to submit the samples: date, your name, field name. Imagine getting all the work done and realizing that some or all of the bags are not labeled! The Field Name should be a short but recognizable version of what you call the field on an every day basis. Labeling the sample “Field 1” just because you sampled it first is not a good idea...
- Dump the thoroughly mixed/crushed soil in a pile on a flat, clean, and washable (or disposable) surface.
- Divide the pile of soil into 4 – 8 pie-shaped wedges.
- Take two opposite wedges – entirely, and put them in the appropriate labeled bag.
- A thin, unslotted kitchen-type spatula is a good tool for dividing and collecting the wedges. Attempting to use just your hands can result in a skewed result as finer soil particles will stay on the surface or fall between your fingers.
- If you need a little more soil to attain the volume you need to send to the lab, take some soil from the other wedges using a similar approach.
- Put the sub-sample in the appropriately labeled plastic bag.
- Fill out the paperwork, and submit the samples to the lab. Don’t forget to include the “crop code.” If you are a Vermont farmer who hopes to get a state-approved nutrient management plan, BE SURE the lab you choose uses the “Modified Morgan extraction procedure.”
- Send the bagged samples/sub-samples to the soil analysis laboratory as quickly as possible. For a complete list of certified soil labs, click here. If there is going to be ANY delay in sending it to the lab, refrigerate (but do not freeze) the sample. Soil is ‘alive’ and chemically reacting, so the longer you delay submitting your soil sample, the lower your data quality from the sample will be.
- Air-drying the soil samples is an extra step, but it is an even better alternative to refrigeration if your goal is to slow the deterioration of soil sample quality. Air drying should be done at room-temperature (no heat). Spreading the samples out on clean paper and applying a fan will hasten the drying process. In this case, ‘air drying’ means that it needs to get ‘dry-as-dust,’ with no sign of moisture.
- Interpret and follow soil test report recommendations. Most land-grant universities have publications designed to help with this. Here in Vermont, we use Nutrient Recommendations for Field Crops in Vermont. You should print a copy of this type of document and familiarize yourself with:
- Ideal soil status for your crop with respect to: pH, phosphate, potash, magnesium, sulfur, boron, and other micro-nutrients.
- Nutrient removal rates and management guidelines for various crop.
- Nutrient sources and management guidelines for those practices.
- Other crop-specific information for geographic area.