Every season, with every plant you grow, nutrients are used up or leached out by water. Doing a soil test when preparing for the next season is the only way to know what nutrients need replenishing.
Sampling Your Soil
Whether using a home test kit or sending your soil to a lab, your soil test results will only be as good as the sample you take. Sampling errors result in incorrect fertilization or amendment recommendations. Here’s LaMotte’s recommended soil sampling procedure:
- Choose the areas to be tested. It is ideal to test beds where different crops have grown each in separate tests.
- Take 5 or 6 samples: the idea is to take a little bit of soil from 5 or 6 spots in the bed to be tested. To do this, scrape away the top debris or mulch, then dig a hole 6″ deep, with a slight angle with a small shovel. Set this soil aside, then take a thin slice from the angled side of the hole, trying to take a slice from the hole depth. Put this slice into a clean bucket. I use a plastic shopping bag to line the bucket, since I never have a clean bucket. The bag is also a good idea since a few of the detergents that would be used to clean the bucket will leave residual elements that could skew the soil test results. The bag should not have been used to carry fertilizer or similar products, though. Do not use paper or cloth bags or packaging to collect, store, or ship the sample. Repeat 5 or 6 times to get a good sampling of all of the areas of the garden bed.
- If you are dealing with a large garden bed, it’s a good idea to draw a map, marking your sample points.
- Dry out sample by spreading it out on a clean tray as soon after collection as possible, and let it sit at room temperature in a well-ventilated area. Any sample allowed to sit in a plastic bag or container at room temperature or higher, will undergo changes due to the soil organisms present. Most often this will result in higher nitrate and sulfate readings. Moist samples kept in tightly closed containers can also lose nitrates, resulting lower readings. Drying out the sample before packaging it or analyzing it will stop biological activity before significant increases in readings. Note, however, that potassium readings may be more accurate if the soil is not dried, due to clay particles locking up the potassium as the sample is drying; drying at high temperatures can increase this problem. (Note: if you are sending your soil sample to a lab, defer to their instructions regarding drying or not drying the soil)
- Thoroughly mix the dried sample to redistribute the nutrients throughout the sample.
- After the soil sample has been dried enough to sift easily through the fingers, sift the soil using mesh the size of window screen. I use regular window screen stretched across an embroidery hoop. (Note: if you are sending the sample to a lab, they do not request, and would probably rather you didn’t sift the soil)
- At this stage, the sample can be stored in a container and kept in a warm (room temp) dry place, shipped to the lab, or do your own test using a home soil test kit
Recommended Soil Testing Labs
- Timberleaf Soil Testing
- Peaceful Valley Farm Supply: they don’t actually do the test, but send the soil to a lab, which I can’t remember the name of
Recommended Soil Test Kits
Most soil fertility reference books seem to think the home test kits are of little use. The main problem with these, in my opinion, is that you only get N, P, K, readings and not Magnesium or Iron or other elements, which are needed in order to determine how much N, P, or K, are needed. There are kits that can be purchased with a full panel of tests, but they are quite expensive, ranging from $600 to $1000, and up for more sophisticated equipment. I made the plunge and purchased one of these, but still rely on labs to guide me in learning to interpret the data. In a separate article, I will outline the steps of test kit, along with the benefits and problems with it.
That being said, LaMotte seems to be the most highly regarded manufacturer of soil test kits, so here’s their lineup (including the big daddy test kit that I own):
- LaMotte Soil Test Kit (Item #TM001): pH, N, P, K
- LaMotte Advanced Soil Test Kit (AST-5): pH, nitrate, phosphorous, potassium, humus
- LaMotte Advanced Soil Test Kit (AST-15): same as above, plus: calcium, magnesium, ammonia, manganese, aluminum, nitrite, sulfur, chloride, iron and copper
Make sure that the reagents in the kit have a good bit of time before they expire, before making a purchase.