Soil with near-neutral pH, sometimes called “sweet soil,” is ideal for growing many vegetables and fruit. Listen to learn more about soil pH and the glories and pitfalls of lime.
This segment of The Old Farmer’s Almanac Garden Musings podcast series was written by George and Becky Lohmiller and is read by Heidi Stonehill, an Almanac editor.
The early settlers of North America encountered acidic soils in most areas east of the Mississippi, where normal rainfall leaches neutralizing elements like calcium, magnesium, and potassium from soil particles. Farmers used indicator plants to judge the soil: Clover, plantain, and wild carrots usually meant a near neutral pH (sweet) that was good for most fruits and vegetables; sheep sorrel, burdock, coltsfoot, and mullein indicated an acid (sour) soil. Sometimes farmers tasted a pinch of soil. If it tasted soapy, the soil was probably sweet enough for planting; if it was tart, it needed to be limed. Where limestone wasn’t available, wood ashes, bonemeal, and oyster shells were used.If your sod is, in fact, tender enough to till easily, you will get better results if you cover the area with black plastic for several weeks, which will kill the existing vegetation. This method, as well as the one below, should be preceded by cutting the grass as short as possible.
Most plants grow best when soil pH is between 6 and 6.8 (slightly acidic). It is in this range that most fertilizers will go to work. As early as medieval times, farmers realized that manure worked better on limed soil. Farmers today know the science behind this. If soil is too acidic, many fertilizer elements bond to metals in the soil and can not be taken up by plant roots. Fertilizing an acid soil is like throwing most of the fertilizer away.
In addition to its role as a neutralizer, lime acts as a conditioner. It bonds sandy soil particles together, increasing their water-holding capacity, and breaks up heavy clay soil, allowing air and water to penetrate. It also contributes calcium and magnesium, two nutrients that are vital for plant growth.
A sprinkling of lime in the planting hole when setting out cabbage, broccoli, and cauliflower helps control the fungus disease clubroot, and a light application watered into the tomato patch guards against blossom end rot, a result of calcium deficiency.
Some gardeners routinely spread a bag or two of lime onto their garden every year, figuring that it can’t hurt, but a pH that is too high (alkaline) can be just as damaging as one that is too low, especially for acid-loving plants like blueberries and potatoes. The only way to know the right amount of lime is by testing the soil with a soil test kit or pH meter.