For our purposes here, there are three main points to cover about soil testing: how to do it, why to do it, and how to use the test results? We’ll tackle how first.
How do I soil test?
You may have seen home soil testing kits in your local garden center. The quality, reliability, and breadth of these kits can vary widely; so to have confidence in the accuracy of your results, why not let a professional testing lab handle the test for the same money? A web search or call to your local extension office will turn up several lab possibilities. But be sure to compare the specifics of each option. Fees vary, as do actual services performed. For example the labs recommended in my area charge anywhere from $5 to $18 and the specific nutrients tested for vary, but not necessarily in correlation to price! Your extension office may have a handout detailing the specifics of labs in your area.
In order to have your soil tested by a lab, you need to send them a sample. There is a “right way” to do this. When you’ve settled on which lab you’ll use, download any forms you need from the lab website and check to see if the lab or your extension office provides a sample bag. If not, you’ll need a zip lock bag.
To collect the soil sample, follow the instructions your lab provides. But the basic process looks something like this… Dig a slice of soil (plunge the shovel 6-8” into the soil, lift out, set the shovel close behind the cut and plunge in again) from several spots in the area you intend to plant. Mix the soil from all the slices together in a clean bucket, leaving out rocks, plant material, and other debris. Mix it really well so the sample you send the lab really represents the whole area you plan to use. Once the soil is well mixed, measure out about a cup and a half of soil. It should not be wet! If the soil feels like it’s got a lot of moisture in it, spread your sample on a plate to air-dry a bit before packaging it (don’t bake it). If the sample is dry, go ahead and pour it into either the sample bag you were provided or your zip lock bag. Seal the bag and follow the packaging and mailing directions on the form provided by the lab, but be sure to indicate on the lab form that the area you are testing will be a food garden. The recommendations you receive back could be very different if the lab thinks you are testing a lawn.
A short while after sending off your soil sample, you will receive back a report from the lab. Some lab reports are gussied up with colorful charts, but most look like something out of, well, a…laboratory (ahem). To the uninitiated, soil reports can seem as if they require a Chemistry degree to decipher; but honestly, they’re not that complicated.
Different labs may convey the information slightly differently. But all the reports will tell you about the levels of plant-available nutrients in your soils, provide amendment suggestions, and indicate your soil pH. Many will also tell you the level of organic matter in the soil sample, but some laboratories charge extra for this, so make sure you check this before sending in your sample. There may be other information in the report; but for beginners, the nutrient levels, pH, and organic matter readings are probably enough to tackle. Don’t be shocked if your report indicates that your soil sample contained only about 5% organic matter- this is about average. If your sample tests much lower than that, you’ll know you need to beef it up a bit.
Why should I soil test?
Your soil test report provides crucial information about the ability of your soil to support healthy plant growth. As mentioned in the first installment of this series, most foods humans eat can be traced back to the soil one way or another. Healthy soil produces healthy plants. The benefits continue passing along up the food chain. As a gardener growing some of your own food, you have a prime opportunity to make sure that your food comes from the healthiest soil possible. Your soil report provides lots of information to help you in that quest.
How do I use the test results?
The amendment recommendations for your soil that you receive in your soil report are based upon the sample you sent in and the assumption that you will be growing a food garden. The recommendations will be presented as a number of pounds per thousand square feet. In order to correctly amend your soil, you will need to determine how many square feet of growing area you will be amending. If your plot is roughly square or rectangular, you can multiply the length measurement by the width measurement and be done with it. Other shapes will require a little more work to calculate.
As an example, let’s assume your garden plot is 20 feet by 20 feet. That’s a total of 400 square feet. That’s only 40% of the 1000 square feet your amendment recommendations are based upon. So, to figure how much you should apply to your plot, multiply the amount recommended in your report by 40%. To illustrate, if your report recommends 6.0 pounds of potash, that’s really 6 pounds per thousand square feet. For your 20x20 plot, you’ll only need 2.4 pounds. But remember, your results are based on the size of your plot. If your plot is 840 square feet, that’s 84% of the 1000 square feet; if it’s 620 square feet, that’s 62%.
Once you know how much of a recommended amendment to use on your garden space, you need to decide how you want to apply it. Commercial fertilizers, synthetic or organic, are widely available but are not the only options. A little independent research is in order here to ensure the product you choose is in keeping with your ethical framework.
It is important to understand that pH is a logarithmic and not linear measurement scale. In order to understand it, however, we can compare it to a standard number line of equally spaced marks with the center mark numbered zero, marks to the right of zero numbered in positive increments of one, and marks to the left of zero numbered in negative increments of one ( …,-2, -1, 0, 1, 2…). On our pH number line, we’ll use the same equally spaced marks; however, our center mark is numbered 7 and represents a neutral condition. The numbers to the left and right of 7 decrease or increase by one as on a standard number line, but they represent exponential changes in acidity or alkalinity. Numbers lower than 7 represent relative acidity, while number higher than 7 represent relative alkalinity. To illustrate the degrees of change, we can consider some common materials. If we compare baking soda solution, which has a pH of 8.4, with milk of magnesia, which has a pH of 10.5, what we need to understand is that we’re not really talking about a (more or less) “two point change.” The difference in alkalinity between the two is far more significant. Milk of magnesia is roughly one hundred times more alkaline than is the baking soda solution. If we then compare our baking soda solution (pH 8.4) with common vinegar (pH 2.4), the “six point” difference actually indicates that vinegar is one million times more acidic than the baking soda solution.
Different plants have different preferences with regard to pH; however, most garden crops are happy in the 6.0-7.0 range. Some notable exceptions are found among berries: blueberries prefer 4.0-5.0, blackberries 5.0-6.0, and black raspberries 5.0-6.5. So if you plan to grow berries, consider setting aside space for them outside your regular garden plot.
Soil pH has a lot to do with where you live. For example, much of the Mid-Atlantic has acidic soil, while areas such as
Arizona and tend to be alkaline. Reducing acidity is a fairly simple matter of applying agricultural lime. Your soil report will indicate an appropriate amount of lime to use, but remember that it will be expressed as pounds per thousand square feet and do the conversion to determine the actual amount for your plot. Alkaline soil is a little trickier to amend. Alkaline soils often have some limiting factor that needs to be addressed with more care than can be addressed here. If you find yourself with alkaline soil, consult the local extension office or a reliable local nursery for advice. Utah
The importance of pH to gardeners is really twofold. The plants’ preferences are important. But pH also affects nutrient availability. As luck would have it, the same 6.0-7.0 range in which most vegetable patch plants thrive happens to also be the range at which most soil nutrients are most available. Pretty smart plants, huh?
We discussed the types of matter that make up organic matter in the first article of this series, which you can find here. We will discuss the form that is perhaps most familiar to gardeners, compost, in more depth in the next article in this series- so we won’t go into it now.
Stay tuned… Next up will be compost and composting…