pH-The Heart and Soul of Soil

You would never plop a baby in a bathtub without first testing the temperature of the water, so why would you plant a tree or shrub without first testing the soil for its pH level? The baby will wail if the water is too hot or cold, but the plant, lacking vocal cords, will simply languish, maybe even die if the soil is too acidic or too alkaline. Death comes from starvation; the plant can’t absorb the nutrients crucial to its health. Acidic soil causes nutrients to leach out rapidly before the roots can grab them whereas alkaline soil ties up the nutrients, binding them so tightly that they are unavailable to the roots. Once diagnosed either of these conditions is easily corrected.

Kits to test the soil’s pH are available at most garden centers or over the Internet. Simply follow the instructions, taking care to draw and test samples from several areas in the garden. Once the readings are taken, interpreting them is easy: a pH reading of 7.0 is neutral, below 7.0 means that the soil is acidic or sour, and higher than 7.0 the soil is sweet or alkaline.

Even if you have no pH kit you can test the acidity or alkalinity of your soil with a simple DIY experiment. Put 2 teaspoons of soil into each of 2 separate cups. Add ½ cup of vinegar to one, and if it fizzes the soil is alkaline, with a pH between 7.0 and 8.0. To the second soil sample, mix in enough distilled water to make a slurry and add ½ cup of baking soda. It the slurry fizzes your soil sample is acidic with a pH between 5.0 and 6.0. If there’s no reaction to either experiment, you’re in luck – your soil is neutral.

Once you have tested the soil and figured out whether it is sweet or sour you’re ready to plant, but before you plunk that pricey specimen in the ground, check the plant’s pH requirements and amend the soil accordingly. Some plants prefer one end of the range or the other; broad leaf evergreens like rhododendrons and azaleas want a mildly acidic soil, but clematis and geraniums hate acidity and will thrive only in a slightly alkaline soil. As a rough rule of thumb, to raise or neutralize the pH of an acidic soil and slow the leaching of nutrients, work dolomitic limestone into the garden, and to lower the pH of an alkaline soil and free up nutrients, apply aluminum sulfate – always according to the directions on the package.

Even without a pH test sometimes the tree or shrub will tell the gardener what elements are missing from its diet, deficiencies that can be corrected once their symptoms are recognized. For instance, if the plant suffers from a shortage of nitrogen, the primary nutrient responsible for green leaves and robust growth, its mature leaves will turn yellow, and the stems tend to be short and reddish in color. To remedy this problem, apply a fertilizer that has a high first number listed on the bag, such as 30-10-10. This means that 30% of the active ingredients in the fertilizer are nitrogen – the other two numbers refer to the percentage of phosphorus and potassium, respectively. If the foliage looks bronze or very dark green and the plant appears stunted, your soil probably lacks phosphorus, the basic nutrient essential for healthy roots and big gorgeous flowers. To correct this condition you need a fertilizer with a high second number, such as 10-30-10, or work a healthy dose of bone meal into your garden. The final basic element is potassium, and a deficiency is marked by curling leaves often mottled with brown spots, symptoms that show up when the pH is less than 6.0 or greater than 7.0. This problem can be alleviated by the application of a fertilizer with a high third number or a compost made primarily from food products – think, banana peels. A balance of these three basic nutrients, nitrogen, phosphorus and potassium, determines the general health of plants, but secondary nutrients, sometimes known as trace elements, are almost as important.

When the soil is acidic three secondary elements, calcium, magnesium and sulfur, flow out of the porous soil too quickly for the plant to latch on to them. To correct the problem, apply dolomitic lime at regular intervals or work crushed egg shells into the soil. If calcium is lacking, the new growth of the plant emerges distorted, but in the case of tomatoes the new growth may look healthy, but the fruit will be rotten on the bottom. The gardener can spot a magnesium deficiency if the mature leaves are yellow and they drop early. A temporary fix is to spray the foliage with a solution of Epsom salts, one cup per gallon of water, but to cure the problem for good give the soil a shot of dolomitic lime. A lack of sulfur shows up as universal chlorosis, a yellowing of all leaves, both new and old, a symptom that can be alleviated with a dose of superphosphate.

The two micronutrients most affected when the soil is too alkaline, or when the pH reads above 7.5, are zinc and iron. A zinc deficiency, known as the “little leaf” symptom, is characterized by increasingly smaller leaves, and to correct for a shortage of zinc, shove several glazier points in the soil around the plant, an old but highly effective cure. If the leaves of your plant are yellow, but the veins are still green then the plant is suffering from a lack of iron that can be corrected by dousing the plant, leaves as well as roots, with chelated iron.

Relying on the plant to tell you what’s missing from its diet can be risky; the same symptoms may signal several different deficiencies. For a more accurate diagnosis test your soil’s pH and avoid all of these disfiguring, life-threatening maladies.