by Amber

Is Garden Soil Testing Important?

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Why worry about garden soil testing when slow release fertilizers never harm your plants? Maybe not, but you’re overlooking what soil is, and what it does. Soil testing tells you what deficiencies exist in garden soil so you can correct what is lacking before planting. If you’ve got one nutrient in high concentration, it can still cause plant health issues – regardless of how safe and easy that fertilizer label professes to be.

Efficient gardening is about building soil, not about fertilizing plants. Soil fertility comes from the soil. Inputs, whether organic amendment or chemical, should only restore what is missing, but most gardeners today rely on the convenience of polymer-coated fertilizers that you apply once to feed the plants all season. What if your plants don’t need anymore of something that’s in those pellets?

Too much of anything is never a good thing – be you plant, animal, or human. Over fertilizing your garden plants causes growing problems, just as deficiencies do. Excess ruins the balance that should exist in your soil too, and for the best plant health and harvest quality, you want your crops to have balanced nutrition. This is why fertilizer has an analysis. It tells you how potent each component is, which important to getting desired results from plants at different stages of growth. If your soil is already high in one of the macro nutrients – you certainly shouldn’t add more of it. By the same token, if your soil is lacking in an important micronutrient, a soil test is the only way to find out you will have problems if you don’t correct the deficiency. Of course, you could wait until the plants are looking sickly, or the fruits are messed up to worry about it. At which point, part of your potential harvest is lost, or worse, the problem might be irreversible.

Soil Test Samples

Remember that your plants pull nutrients in through their roots, and those roots spread out into the depth of garden soil. A proper sample for soil testing is taken from beneath the surface. You only want up to a cup of soil per sample, and getting this with a shovel is pretty near impossible if you want good test results. You need to pull ‘core samples’ – about 1′ across and 6-8″ deep. They make tools for the task, but they’re an expense that most home gardeners won’t be able to justify. Spending $40-$90 USD on a garden tool like this isn’t necessary though. You can pull samples with items found at the closest hardware store and a small sledge hammer.

With compacted heavy clay soils you might have problems getting the core sample out of the pipe if the soil is dry. You can deep moisten the ground where you want to pull the sample, or wet it in the pipe to force it out of your homemade tool. It’s perhaps best to moisten the ground first. The soil should be dry before sending the sample to the lab.

Don’t pull just 1 core sample from the garden! Nutrient content and organic matter will vary from spot to spot. You want an overall reading, so pull 5-6 core samples from random spots. Dump them all into a plastic bucket… NEVER into something metal! It could cause bad test results. Remove all the stones. Mix it well, and put about 1 cup of the blended soil into the sample container to send to the lab. These directions for home gardens. For acreage, you’ll want to get a different test package designed for agriculture that involves more samples sent in.

Soil Test Results

Where you send your soil sample will have some bearing on how good the results are. Not that the lab does a poor job, but you can only correct things you know are out of whack, and like anything else, here too, a complete picture costs more than a partial one.

For $15-$25 you can get a simple basic analysis that will tell you your soil pH, and whether you have the right amount of nitrogen, potassium, and phosphorous to grow the crops you indicated on the form submitted with the samples. These come from your state university soil testing lab, and the kit will also be available through your county Extension Service office. The results come with recommended rates of application to correct anything out of balance, and are received within 1-3 weeks – depending on how backed up the lab is.

But these tests were designed many years ago based on the needs of commercial farmers. Not that they shouldn’t be worried about their soil being micronutrient deficient, but that’s of little importance when the chemical company would love to sell them more stuff to correct the problems the imbalance causes. And then there’s the fact that not just your plants need those micronutrients – people need them too. Isn’t more nutrient-dense fruits and vegetables part of the reason you are growing your own? If your soil doesn’t put these minerals into your harvest, you will be shorted important nutrition too.

Note that some state university labs do not test for nitrogen. My state does, but it appears that others may assume that farmers already know to at least put down nitrogen with every new crop. This would be a big problem for many plants the home gardener would want to grow, especially lettuces and greens. Do some fact finding before blindly going for the cheapest option – inquire first.

Independent soil labs will give you a more complete soil analysis under the basic test package, including nitrogen. Sometimes these commercial labs ‘basic package for home gardens’ also includes secondary nutrients: sulphur, calcium, and magnesium. If not, it will be in the add-on testing packages, along with the important micronutrients: iron, manganese, copper, zinc, boron, molybdenum, and aluminum. Since these labs aren’t the recipient of land grant university subsidies, the test will cost more, but the turn around time tends to be faster, and $50-$65 or so for a complete soil analysis and input recommendations is definitely reasonable. It’s certainly a lot less frustrating than guessing! And much more reliable than those cheap little home test kits.

To find your state lab – Google “(insert state name) state university soil test lab”.  They will sell you the test kit, and mail it to you, if you don’t want to make the trip to the Extension Office and pick one up. Independant labs will all be about the same price. Check out: Control LabsMidwest Laboratories, or Woods.


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The garden played a starring role from spring through fall in the house Amber was raised in. She has decades of experience growing plants from seeds and cuttings in the plot and pots.