It's That Time...Start Thinking About a Fall Soil Test
Posted: September 24, 2015
Take an hour now, as the season winds down to take your test. Once the ground is frozen it is awfully hard (and cold) to be soil sampling. A good soil sample has to be 6” deep, or to the plow depth. If you only take the top 2” because it is frozen underneath, results will come back off the charts. The lab is assuming you sampled 6”, not the top 2” where the nutrients are highly concentrated.
Fertile soil is the basis for healthy crops, don’t leave it as an after-thought.
The first step to taking a good soil test is deciding what area to sample. You could take a sample for every two acres or so. But if you have fields with different histories – manure applied, cover crops etc, it is best to do separate samples.
Now that you have decided what area to sample, take multiple samples randomly throughout the field, placing all 15-20 sub samples together in a CLEAN bucket. To make sure the samples are random, walk the field in an X or Z pattern. You will want each sample to be 6” deep. Scrape off debri or plant material before you take the sample. If you don’t have a soil probe you can use a shovel or a trowel. I like to use a trowel in shaly or rocky ground where the probe does not work well. For each trowel-full of soil I use a butter knife to cut off and discard the sides leaving an even two inch wide strip down the center of the trowel. This way I don’t get more of the top layer than the bottom. Since the top often has more nutrients, more top layer would throw off my results.
Next I mix all my small sub samples together in the clean bucket and take out one cup and put it in a plastic bag to send to the lab. When I am doing multiple samples I mark the bag of course.
It is tempting to just take a few shovel-fulls on the edge of the field and call it good. Keep in mind that the top 6” of an acre of soil weighs 2 million pounds. You are sending the lab one cup of soil. They will only use one teaspoon. So make sure you send them a sample that represents your field, all 2 million pounds. That means a lot of sub-samples (somewhere around 10-15 is usually a good number). Then mix them well before you take out a cup to send to the lab.
There are many labs you can send your soil test to. Make sure you pick a reputable lab that is familiar with soils from your area. In Pennsylvania I like to use Penn State Ag Analytical Lab because I know that PA soil types are in their database, and they will give me recommendations that make sense for this area. Whichever lab you choose, stick with it. That way you will see how your soils change over time in response to management.
If this seems complicated, don’t worry. There are directions for how to take the sample right in your soil test kit. Pick one up from your county Extension office. For soils information check out our factsheets in the soils section.