Your Garden’s Soil pH Matters

Learn what causes acidic soil and alkaline soil, plus how to apply the results of a soil pH test in your organic garden.


| April/May 2014



Soil pH

Blueberries and potatoes prefer acidic soil (low pH). Asparagus likes alkaline conditions (high pH).


Illustration by Elayne Sears

To ensure that your garden crops make the most of the rich, organic soil you create, you need to understand your soil’s pH. The pH describes the relative acidity or alkalinity of your soil’s makeup, and it has important implications for plant health and growth. Soil pH impacts beneficial fungi and bacteria in the soil and influences whether essential minerals are available for uptake by plant roots.

What Is Soil pH?

A solution’s pH is a numerical rating of its acidity or alkalinity. All pH is measured on a logarithmic scale from zero (most acidic) to 14 (most alkaline, or basic); 7.0 is neutral. The pH scale is used by chemists to measure the concentration of reactive hydrogen ions (H+) in a solution.

Most food crops prefer a pH of 6.0 to 6.5, but you can have a productive food garden as long as your pH is about 5.5 to 7.5 (see chart in slideshow). A difference of just 0.5 may not seem like much, but the pH scale is logarithmic, which means, for example, a pH of 7.0 is actually 10 times less acidic than a pH of 6.0. Potatoes and most berries, which grow best in more acidic soil, are the main exceptions to the average preferred pH range.

A soil’s pH results from interactions among native rocks, plants and weather conditions over many years, and it varies with climate and physical surroundings. In moist climates that support dense forests, such as those east of the Mississippi River and along the Pacific Coast, soil tends to be acidic, with pH ratings usually between 4.0 and 5.5. The grasslands of the comparatively dry Midwest often have slightly acidic soil (6.0 to 6.5), while most arid regions, such as the Rocky Mountains, are dominated by alkaline soil (7.0 to 7.8). Local differences in rock can cause huge variations within these general patterns, however — for example, when weathered limestone creates alkaline patches in otherwise acidic landscapes, or when elevation leads to more or less rainfall. Plus, soil is often severely disturbed during construction, and sometimes native topsoil is completely lost.

Some synthetic chemical fertilizers — mainly those high in ammonium or sulfur — can make soil more acidic, as can tillage methods that reduce soil’s levels of organic matter. Acid rain caused by air pollution from coal combustion began to acidify streams and soil during the late 1800s, and continues to push soil in some regions into the acidic range every time it rains. In addition to outside influences, some types of organic matter, such as peat moss and pine needles, acidify naturally during decomposition.

Alkaline soil occurs naturally in places where soil is formed from limestone or other calcium-rich minerals, and high water-evaporation rates common in arid climates aggravate the problem by loading the topsoil with accumulated salts. Many garden plants can still thrive when grown in alkaline soil that has been generously enriched with organic matter, which also improves the soil’s ability to retain water. Mulches also will slow the buildup of salts in plants’ root zones by reducing the amount of surface evaporation.

daisy
1/2/2015 2:29:49 PM

If you just add compost ( leaves, grass, hay, and cow or horse, chicken or rabbit manuer (sp) to your garden every planting, the soil will naturally balance itself and the texture improves itself, as well. Sometimes technicalities just complicate and confuse the issue.






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