pH: What’s the Big Deal?

Potential Hydrogen is Important When You Want to Maximize Your Crop Yields

pH: What’s the Big Deal?

By Nancy P. Farris, South Carolina

Potential Hydrogen (PH) is measured as a scale of 0 to 14, with 7 being neutral. At pH 7, soil contains the same rate of hydrogen ions (H+) and hydroxide ions (H-). At pH 6.0, soil contains 1 part (by weight) of hydrogen ions per million parts. Since the scale is based on logarithms, dropping 1 point on the scale, bringing it to 5.0, translates to 1 part hydrogen ions per hundred thousand parts. That makes a big difference is the soil’s capacity to absorb more hydrogen.

For optimal growth and production, plants need to obtain elements, each of which has either a negative or a positive charge. To persuade soil to give up the element, the plant must offer a medium of exchange: hydrogen. During respiration, plants break down water (H20) and use the oxygen. Hydrogen is left as a byproduct. If the soil can absorb hydrogen (positive charge) it will give up an element with a negative charge, such as calcium.

When tomatoes don’t get enough calcium, the plant presents fruit marred by ugly blossom end rot. You would think, “I’ll just add calcium to the soil.” But if the soil already contains enough hydrogen, there can be no deal, even if the soil has a surplus of calcium.

Will the Potential Hydrogen Situation Affect Yields?

According to one USDA report, corn grown at a pH of 6.8 will yield up to 100 bushels per acre. If too much lime is spread over a field, bringing pH to 7.5, the yield will be reduced to about 85 bushels per acre.

The chemistry may seem confusing and trying to convert per acre yields to my small plot might give me blurred vision, but I know bad things result if I ignore the pH rating of my soil. Blossom end rot is not pretty, club root in cabbage does not appeal to me, and I want that extra 15 percent yield from my corn.

Most garden crops grow best in soil with pH of 6 to 6.8. A few things, like potatoes, watermelon, strawberries and blueberries, like an acid soil: pH 5.5 to 6.0. Squash, beans, tomatoes and corn prefer soil closer to neutral: pH 6.0 to 6.8.

How Can You Know the PH of Your Soil?

Various kits for testing soil are available from garden supply companies. A pH meter is easy to obtain. County agents will accept a soil sample and send it to the lab of the agricultural college of your state.

What can you do to adjust the pH of your soil?

If soil pH is too acidic—below 6.0—you can add lime or wood ash. Spread these materials on the soil in fall in order for soil to absorb them. Ten pounds of dolomitic lime per hundred square feet should raise pH about one point.

Wood ash has a potent alkalizing effect. About one pound per hundred

square feet can raise the pH by a point. About two cups of wood ash is usually enough for a 50-foot row.

Lowering pH in alkaline soil (above 7.0) is more difficult and takes longer. Products containing sulfur are offered in garden supply stores for use in lowering the soil pH. Adding organic matter such as aged manure, greensand or peat may help lower pH. A mulch of pine straw or oak leaves (or both) slowly lowers pH. Green manure crops during the off season will help add more neutralizing organic matter to soil. It will take at least a season to get the benefits of the organic matter.

Soil Tester
If soil pH is too acidic—below 6.0—you can add lime or wood ash. Spread these materials on the soil in fall in order for soil to absorb them. Ten pounds of dolomitic lime per hundred square feet should raise pH about one point.

What Could You Do in the Short Term?

First, contact your county agent, who might know of varieties which will grow in soil of high pH.

Try planting in containers or raised beds filled with soil of the proper pH. I have grown a cucumber in a rusted out washtub, a bell pepper in a large flower pot, and a tomato plant in a five gallon bucket. I create a drainage hole in the bottom of the container and provide adequate water throughout the growing season. Containers dry out relatively fast.

Purchasing soil can get expensive, unless you make part of it yourself.

I make compost from pine straw that has served as goat bedding or chicken litter. When we clean out the barn in fall, the straw has been broken up and permeated with

manure. We pile it on a concrete slab to slow insect invasion. Throughout the year, we toss on fall leaves, weeds from the fall/winter garden, and occasionally a shovelful of dirt.

By spring, the pile has shrunk and decomposed to something like coarse dirt. The pH usually registers about 6.0. I mix that at a rate of one part with two parts of good soil. To loosen it and improve drainage, I add a shovelful of perlite to a wheelbarrow full of the compost/dirt mixture. Since my soil tends to heavy clay, I add a shovelful of builder’s sand to the wheelbarrow.

To fill a container, I start with about an inch of gravel to add weight in case of high winds. I add an inch of builders sand to improve drainage. Then fill with the compost/soil mixture. I set one plant in each container and sometimes add an herb like anise hyssop to attract pollinators, or a marigold to repel tomato worms.

In a raised bed, I would spread rough compost as the bottom layer; it will continue to decompose, providing bottom heat for early plantings. Add a good soil mix, and then plant three or four rows of vegetables. Make the bed just wide enough so the gardener can reach from each side to the middle. The soil does not become compacted by foot traffic during the growing season.

Time and effort spent in getting the soil pH at the best level should help ensure a good crop of tasty, nutritious vegetables.

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