A Lesson In Soil Acidification

Cultivated soil in humid regions will become increasingly acid if steps are not taken to reverse the process. That's because soil water will dissolve the more alkaline substances like calcium, sodium, magnesium, and potassium faster than acidic materials like carbon. In soil management language, the alkalis "leach out" sooner than the acids.

Why plants won't tolerate highly acid conditions is not completely understood. Slowdown of beneficial microorganism action is part of the reason; increased toxicity from certain trace elements like aluminum is another. Deficiency of calcium and magnesium is a third possibility. The best explanation may be that in acid soils, chemical reaction can lock up major nutrients,
especially phosphorus, making them unavailable to plants.

Heavy use of inorganic, high-analysis fertilizers is known to cause soil to become more acidic. Organic gardeners don't have to worry about that, but the same result can stem from using organic fertilizers that have an acidifying effect. Cottonseed meal is a prime example. Mixing a little bone meal with it helps head off the problem.

The surest way to determine the pH of your soil is to have a good soil test done. A simple test for pH you can do yourself is with blue litmus paper, available from drug stores. Blue litmus turns pink when brought into contact with an acid (even a weak acid like vinegar) and turns back to blue if dipped in lime water.

Soil Test Instructions: Get some soil from the garden. Try to take soil under the surface rather than off the top. Get samples from two or three different locations. Mix up all the soil you've collected in a clean bucket, then pour clean rain water over it.

Place several pieces of litmus paper into the mud you've made in the bucket, being careful that your hands are clean of any acid substance before you handle the paper. Wait ten seconds or so and withdraw one piece of the paper. Rinse it off with clean rainwater. If pinkness shows already, the soil is quite acid. The intensity of the pink color is another indication of degree of acidity.

Pull another piece of the paper out in about five minutes. If pink, the soil needs lime, but not as much as when the color changes right away. If after fifteen minutes the blue paper shows little or no change to pink, your soil probably doesn't need lime. This method is none too exact. But it will give you an idea of where your soil stands.

Curing Acid Soil: Lime is the cheapest and easiest way to cure acid soil disorders. Freshly burned lime is called quicklime; hydrated lime has been slaked. Don't use quicklime, because it can destroy soil humus. Hydrated lime might too, but it or ground limestone are the preferred materials. Hydrated lime is more potent than ground limestone and acts quicker. Where you would apply fifty pounds of ground limestone to a 1,000-square-foot plot, thirty-five pounds of hydrated lime would be sufficient.

Agricultural-ground limestone is the commonest and safest liming material in use. There are two kinds, generally: calcic limestone and dolomitic limestone. Many gardeners prefer the latter because it contains magnesium in addition to calcium and so fertilizes a little better while it neutralizes the soil. A general rule of thumb in applying limestone is this: to increase pH by one unit, spread on every 1,000 square feet thirty pounds of limestone, if the soil is very sandy; if a sandy loam, spread fifty pounds; on a loam, seventy pounds; and on a heavy clay, eighty pounds.

Spread the lime on top of the soil in the fall after you have plowed or spaded deeply. Lime should not be plowed under, because it leaches down into the soil too fast anyway. On lawns and pastures, spread in late summer if possible, though any time will do. It's best not to apply lime with other fertilizers. And don't use hydrated lime where plants are already growing in the garden. Hydrated lime can injure plant roots. Furthermore, don't lime areas around your acid-loving plants, nor any area where run-off water might carry the lime downhill to such plants. Lime is poison to blueberries, azaleas, and the like.

The best way to lime soil is with non-leached hardwood ashes. You can't get them in quantity or if you could, they'd be too expensive, but use them whenever you can. If you have a fireplace or are heating and cooking with a wood stove, save the ashes as if they were gold. If you can't put them directly on the soil, store in a dry place, since rain quickly leaches out the lime and potash in them.

The hardwood ashes gives a better liming effect than lime. Part of the better response that you get from wood ashes on sweet corn is due to the potash in the ashes, but the lime must also be in a more available form, too, than in limestone. Coal ashes are of little or no value.

When using ground limestone, don't expect a tremendous response the first year you apply it. The year after will be better. About every four years liming will usually need to be repeated. However, where it is really needed, liming gives fantastic results. Because of that, the temptation is to over-lime, a mistake easy to make on a small garden plot. Over-liming is as bad as not liming at all. A pH of 7.5 is a signal you've overdone it.