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Soil Testing 101

The following 101, on soil testing, comes from Rachel, a HOMEGROWN Life blogger and a woman who knows a thing or two about growing. You can read even more about Rachel's urban farming endeavors in her personal blog, Dog Island Farm. Thank you, Rachel, and please keep sifting for good ideas!

SOIL TESTING: WHY IT MATTERS

Soil testing is incredibly important when you’re planning to produce food from your yard. It tells you what you’ve got and what you need. Plants won’t give you their best if they don’t have the proper nutrients or the proper pH levels. The three most common nutrients that plants need, referred to as macronutrients, are nitrogen, phosphorus, and potassium—collectively known as NPK. But NPK isn’t the only level you need to know when it comes to your soil. Other important nutrients, or micronutrients, include magnesium, calcium (especially important for tomatoes, peppers, and eggplants), manganese, and iron (especially key for citrus).

It’s also important to know the structure of your soil and how much organic matter is in it. This affects how much you should water, as well as your plants’ ability to take up that water. High clay soils are more prone to runoff and take longer for water to infiltrate, while sandy soils drain very fast and don’t hold adequate amounts of water long enough for plants to access it.

IDENTIFYING KEY NUTRIENTS

How do you find out what you need to know? Well, you could buy one of those at-home soil tests, but I wouldn’t recommend it. I've found those kits to be dreadfully inaccurate, and they don’t give you all of the information you need to determine what's in your soil.

Instead, I recommend finding a soil lab that can do the testing for you. I use A&L Western Agricultural Labs. You simply print out their form, fill a quart-sized Ziploc bag with soil, and ship them those things, plus a check, and they'll email you the results. I chose their SC3 Soils Analysis with recommendations. Pictured at right is their soil report, recommendations included. You can also find a soil laboratory through your county extension or local university. (Visit Find Good Food: State Resources for help near you.)

DETERMINING SOIL STRUCTURE

As for determining your soil's structure, a home test works just fine. All you need is water and a jar with a lid. Simply add to the jar 1 part soil to 2 parts water. Close the lid and shake for one minute. Allow this mixture to settle.

Settling can take anywhere from a couple of hours to a couple of days, depending on what you've got in your soil. The first particles to settle will be the sand, since they're larger and heavier. Silt is second, followed by clay—the smallest and lightest soil particles.

From my own test, pictured above left, it appears that our soil is equal parts of clay, silt, and sand, but what does that mean? Well, take a look at the handy diagram below to determine what soil structure that trio represents.

As you can see, the 30 percent lines of each side of the triangle (clay, silt, sand) meet to form a trapezoid that we're in the dead center of: the Clay Loam category. Loam is considered the best balanced soil structure because it offers both optimum water retention and water drainage. Clay loam doesn't drain as quickly, but it retains water well.

The most common—and concerning—soil structure is clay soil. Clay soil can be very difficult on plants, as it doesn’t drain well and makes it difficult for water to percolate in. It also contains fewer air pockets, which roots need. If you have clay soil, your best option is to dig in a lot of organic matter. An easier option, however, is to forgo using this soil and just build raised beds. The good news is, HOMEGROWN can you help there. Check out the Composting 101 for a how-to on creating your own organic soil additive and read through the Raised Beds 101 for a step-by-step tutorial on gardening above ground.

SPEAK UP!

Got a question for Rachel? Or a tip to add about testing? Or a recommendation for a good soil lab? Post it below and keep the conversation rolling. You might also be interested in joining the Urban Gardeners group, and you can always find more things to plant, grow, cook, preserve, make, and craft in the HOMEGROWN 101 library.

ALL IMAGES: COURTESY OF RACHEL