How to Take a Soil Test

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Soil Sampling and Testing

Robin Sweetser

Do you test your soil? It's a good time to evaluate your soil's health so that any fall amendments can break down over the winter in time for spring planting. Here's why it's worth taking the time to lay the groundwork for a better garden next year.

You don't have to dig too deeply to discover the secret of great gardening; it is your soil. Without healthy soil it is very difficult to have a successful garden and fall is the best time to evaluate your soil's health.

Before you start dumping on the lime and fertilizers, your first step should be taking a soil sample to send off for testing. A good soil test will evaluate the basic texture of your soil—and, silt, or clay—and determine its acidity—the pH level. The available amounts of nutrients including magnesium, calcium, phosphorus, and potassium will be calculated and recommendations will be made for raising each to the correct levels for optimum plant growth. Armed with this knowledge, you can make the proper improvements. Too much of a good thing can be just as harmful as too little so let your soil test be your guide.

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Where to get a Soil Test

Home test kits are available but they are not as accurate or thorough as professional testing.  Contact your county extension office for soil testing information and instructions. Most have websites; fees for soil testing, along with the proper forms, can be found there. Here's a list of cooperative extension services by state.

How To Take a Soil Test

  • To take a representative sample, scrape away any surface litter, plant residues, leaves, etc.
  • Avoid sampling in a spot where ashes have been dumped, manure or compost stored, or brush burned.
  • Cut straight into the soil with a shovel or trowel 6 to 8 inches deep making a V-shaped hole.
  • Cut a 1 inch wide slice of soil the length of the hole from one side. Take a 1 inch strip from the center of this slice to use in your sample.

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  • Repeat sampling randomly around the garden and mix the samples together in a clean glass jar or bucket. Since we use raised beds we take a slice from each bed and mix them all together.
  • Measure out a cupful of soil, dry it indoors for a few days, and seal in a plastic bag with your information on it.
  • Send it in with the proper forms and fees.

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When you get your test results the fertilizer recommendations will be quite specific, and speak to three elements:  nitrogen, phosphorus, and potassium.

Nitrogen

N stands for nitrogen which helps plants make leafy growth. For nitrogen they will tell you how much manure to use.

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If you are using fresh manure, spread it on in the fall so it can break down over the winter and be safe for spring planting. If you prefer, dried blood, alfalfa, soybean, or cottonseed meals can be substituted. Nitrogen is released quickly from them so it is best to wait until spring to add it to your soil. 10 pounds of blood meal supplies the same amount of nitrogen as 10-20 bushels of manure but without the added benefit of all the organic matter that manure contains.

Phosphorus

P stands for phosphorus which is necessary for germination, strong root growth, flowers, and fruit. It helps plants absorb minerals, grow strong stems, and withstand disease. Rock phosphate provides phosphorus, magnesium, and trace minerals. Rock powders are wonderful soil enhancements. They are slow acting but long lasting so they need to be applied only every 3-4 years. Bone meal and bone char are more readily available sources of phosphorus.

Potassium

K stands for potassium or potash. It regulates the flow of water in plant cells and is necessary for flowering, fruiting, and disease resistance. A lack of potassium will cause plants to have weak stems and stunted growth. For added potassium you can use granite dust or greensand which is made from glauconite, an ocean mineral high in potassium and iron. Wood ashes are also high in potash.

Most gardens have room for improvement. Take the time now for a better garden next year!

See more about NPK Ratios And What This Means.

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Comments

Scott Davis (not verified)

1 year 5 months ago

Great article. We had our soils tested by UMass, they were by far the least expensive of all the testing sources we found and are much more accurate than the DIY kits.
We live in Vacaville, Ca. where the soils are heavy clay, our area is a former Pecan/Walnut orchard. Temps get to 100 in the day and drop to 65 at night through June so some plants need extra help.

Rononpi (not verified)

3 years ago

Thanks so much for your instructions regarding the proper method of collecting soil samples for testing. I am Master Gardener and one of my volunteer activities is performing the soil tests on samples that people bring into our local Extension Service office. All too often, I receive a sample that is mostly mulch or grass. You really need to collect your sample from the root zone which is about 6 to 8 inched below the surface for most garden plants.

Susan Snow (not verified)

4 years 1 month ago

When I first started gardening in 1970, I amended my soil with blood meal, alfalfa, soybean, or cottonseed meal. But, now, they all contain GMOs thanks to Monsanto. I now use kelp meal, worm castings from red wigglers fed organic produce wastes, and composted leaves, unsprayed grass clippings, and a variety of animal manures from livestock not grown with GMO grains or grasses.

What goes into the soil comes out in food, so I am extremely careful with my supplements.
Can glacial till be used to add minerals OR will it put unwanted heavy metals like chromium 6, arsenic, lead, mercury, and other poisons?

From peer reviewed studies around the world, I'm learning that GMOs include high levels of herbicide and antibiotic resistant bacteria such as
Clostridium botulinum. C. botulism if found at poultry houses, it is also found if dairy livestock in Germany an the source IS animal feed sprayed with Glyphosate resistant herbicides and the pathogenic bacteria to which the feed is resistant. See M.Krueger et al, 2012, 2013, 2014, 2016. This highly resistant bacteria is killing dairy cows, is was found at farms, in farm house dust, and in the feces of the animals, humans, and human children with dire consequences!

Suggestions are wanted on other uncontaminated sources of soil amendments to add beneficial minerals to the soil.

Thanks!

Susan Snow

Glacial till can be naturally high in some types of heavy metals, depending on its source. How do you feel about using other kinds of rock powders such as basalt or granite dust? They are good at re-mineralizing soil. The best thing you can do is to be aware of your sources. I always get my soil amendments from a trusted organic company. Check out the blog post on soil amendments for more information.

Bill Marrs (not verified)

4 years 1 month ago

Your plants Need 8 Macro-Nutrients and 12-19 Micro-Nutrients (Trace Minerals). The Macro-Ns are - from the air - Carbon (from CO2) and Oxygen (for the roots); The Macro-Ns from the Soil are Nitrogen, Phosphorous, Potassium, Calcium, Magnesium, and Sulphur. Important Micro-Ns are Iron, Manganese, NH3 compounds (Ammonia+), Sodium, Silicon, Zinc, Copper, Boron, and Molybdenum. The ONLY Fertilizers I use are Organic Bone Meal (for Calcium and Phosphorous), 1 year old horse manure, and 2 year old poultry manure. Potting, Planting, and Growing Soil is Always - 35% Silica Sand, 30-50% Humus (from COLD Compost), and 15 to 35% Screened Clay (no rocks) - for the Abundant Trace minerals. Soil pH is Maintained at 6.0 to 6.5 using Only Ferrous Sulfate (which works over 6-12 months). Remember: Hot Composting Produces Fine Mulch - and NO Humus. Bill - Master Gardener - 25 years.