By STACY CAMPBELL
Cottonwood Extension District
Since I have had some questions recently on soil sampling, I thought it might be worth re-visiting. Not always a fun activity but the return on investment (ROI) is guaranteed!
First and foremost what are you sampling for? This will determine how deep of a sample you will need to get. For instance, if you are sampling for phosphorus, potassium, soil pH and even micronutrients such as zinc, and iron, then a 0 to 6 inch sample is necessary; a piece of cake.
However, if you also need to sample for nitrogen (N), and possibly even chloride and sulfur then it gets tougher. Nitrogen, sulfur (S), and chloride (Cl) easily move with water and therefore considered mobile in the soil, as opposed to phosphorus that only moves a little in the soil. To sample properly for N, S, and Cl, K-State recommendations are to sample to a depth of 24 inches.
I will be the first to admit that sampling to a depth of two feet can be a bit tiring with a hand probe, even when soil moisture is good. But if you really want a more accurate N, S & Cl soil test, you should sample to 24 inches. If the soil is dry, sample as deep as you can and record what the average sampling depth was on the form that goes to the lab.
For example if you probe to an average depth of 16 inches, make sure it is recorded on the form. Be sure you completely fill out the entire form. It will ask what the intended yield goal is and crop to be grown, what the previous crop was, dryland or irrigated, and if any manure has been applied; make note of that too. The more information the lab has the better the recommendations will be.
Some folks are using cordless drills and an auger type of bit to pull the 24 inch samples. We have the auger bit and bucket for check-out, as well as hand probes for check-out at our offices.
If you are using a hand probe take along a can of WD-40 and oil up the probe tube before using it. The soil cores come out so much easier, and, no, the WD-40 will not affect the soil in any way to foul up the results.
Other items needed are two clean plastic buckets to put the 0 to 6 and 0 to 24 inch samples in, the soil sample form from the lab you use, and bags to put the soil into to mail to the lab—about a pint of soil is needed.
K-State recommends taking at least 20 to 30 cores per field, which is a bitter pill to swallow that most don’t. The more cores you take the better representation of the field you will get—to a point. I would recommend taking at least 12 to 15 cores per field, mix thoroughly in a clean bucket, and then fill your soil container from this mixture.
It is actually recommended to divide your fields into uniform areas. For example, if the field has differences in topography--bottom ground, hill sides or sloping, soil texture, etc.
On fields that have been reduced-tilled or no-tilled for several years, a split sample from the top 6 inches (i.e. 0 to 3 inches and 3 to 6 inches) is encouraged to assess pH to determine if there is any nutrient stratification—decrease in soil pH near the surface.
Avoid sampling in old fence rows, dead furrows, low spots, feeding areas, and other areas that might give unusual results. If information is desired on these unusual areas, obtain a separate sample from that area.
County extension offices have the soil sample forms and bags, and most if not all will have a soil probe you can check out and use.
I better quit while I still might be ahead. If I keep pointing out all of the fine details that land grant universities and soil scientists recommend in order to take soil samples, I will probably scare off all that read this.
The key is to just get started and at least try sampling a few fields, and with time and some experience it will get easier and you will probably learn some tricks to make the experience less time consuming and labor intensive.
The other alternative is to have someone who does this service come out and pull the samples.
Either way it will still pay dividends and you will know how much fertilizer to apply, rather than guessing.
Stacy Campbell is a Crop Production Extension Agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact him by e-mail at [email protected] or by calling 785-628-9430.