In the Garden – November


The garden column on the role of soil texture in growing healthy plants generated some interest, which leads me to a good holiday gift suggestion for gardeners. In fact, it’s a gift that can be used right away, without waiting for spring – a soil test kit.

While I am an advocate of using professional soil tests for crops, a handy little home kit provides good information about your soil, teaches you some basics of soil chemistry, is much more accessible than the pages of chemical analysis yielded by a professional test, and is a great way to interest older children in chemistry or soil science. It’s sort of a friendly ‘better get to know your soil’ kit.

I’m thinking of the Rapitest Soil Test Kit, produced by Luster Leaf Products. It’s available at most hardware stores and garden centers, as well as online. To use the kit to test four key chemical properties of soil, you place a specified small amount of soil and water (or soil solution) into each of four vials, add a chemical that reacts with a color change, and compare results to a color chart. Replacement capsules of chemicals can be direct-ordered from Luster Leaf by downloading an order form and mailing it in. That’s right – the good old USPS snail mail way.

The first property the kit assesses is pH, an inverse measure of the concentration of free hydrogen ions in the soil. Why does that matter? Hydrogen ions combine with other elements to form acids. Plant roots cannot absorb minerals and nutriments unless they are dissolved, and acids dissolve these and make them available for absorption.

Of course, too much acid, and the roots could be exposed to toxic levels. If there’s too alkalinity, then the plant will be malnourished. So the most important key for soil, after soil texture, is pH because everything else – including the absorption of the other nutriments tested – will depend on these two soil characteristics.

For the pH part of the Rapitest, a few minutes after you add the chemical reagent to the appropriate amount of soil and water, the water will turn some shade between yellow (acid) and green (alkaline). The color is compared to a chart that yields the measured pH of the soil. A pH of 7 is considered neutral, lower numbers are acidic (sour, as in vinegar), and higher numbers are alkaline (base or sweet as in baking soda, not candy).

Generally, rainy climes have acidic soil because rain is naturally slightly acidic and also washes away some elements like calcium that make a soil alkaline, leaving a lot of free hydrogen ions knocking around. This is the case in Point Roberts, where most surface soils have a pH between 6 and 6.5 (acid to slightly acid). Fortunately, this is an ideal range for most plants, allowing them to absorb nutriments in the soil. But not all plants evolved in slightly acid soil like ours, and you might need to research the needs of plants you want to grow.

We have no native lawn-type grasses like creeping red fescue because most evolved in grassland prairie soils that are more neutral. The same is true of members of the onion family. Our slightly acid topsoil is going to be mildly toxic to such plants. In order to have a lush lawn and great onion crops, then, you should move the pH of your soil toward a more neutral 7. The fastest way to do that is to add lime, which is why you often see packages of lime alongside grass seed in garden centers. Make sure that you follow directions and don’t overdo it. I once spilled baking soda on my lawn. It instantly killed the acid-loving moss in the lawn, but it also killed the grass. It took a lot of rain to return the soil to the neutral pH that grass loves. A slower but less finicky way is to add wood ashes to the compost you use for neutral-loving plants.

Conversely, some ornamental shrubs like rhododendrons, fruit shrubs like blueberries, and several common crops grow best in a moderately acidic soil because they have specific needs for dissolved minerals that are more available in acidic soil.

For these plants, you might need to acidify your soil. If you need immediate results, like we did for an ailing cucumber bed at the Benson Road Garden (aka co-op) this summer, Nielson’s Building Center can order you aluminum sulfate to add to the soil. A better but slower solution is to prepare your beds with generous amounts of coarse compost, which releases carbon dioxide as it decomposes, in turn forming a weak acid.

Remember that healthy soil will have both the appropriate texture and pH before you even consider what fertilizers are needed … more on that some other time.


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