Acid Soil: Making Soil Acidic

garden soil being worked

Soils may be either acid, neutral or alkaline – determined with a soil pH test – as well as exhibit differences in texture (or physical condition) and in fertility. The degree of acidity or alkalinity has an important bearing on the health of many plants.

Acids and alkalis are opposite and if mixed together tend to neutralize each other, so that a combination may be neither acid nor alkaline, but neutral. A well-known acid (acetic) is that which gives sourness to vinegar; lime in its various forms (garden lime) is a familiar, mild alkali; caustic soda is a more powerful sort.

Compounds containing both lime and the acetic acid of vinegar occur in soils, but acid soils contain more of carbonic, humic, tannic and other acids, many of which are complex and not yet well understood.

Where underlying rocks are quartz, granite, sandstone, shale, gneiss, mica schist or slate, the soil above or derived from them is usually acid, though sometimes shales and sandstones are neutral or moderately alkaline instead.

The soil above limestone, marble or serpentine, on the contrary, is nearly always alkaline. Where drinking-water from shallow wells is “hard” expect the soil nearby to be alkaline. If this “hard” water is used in sprinkling, it may injure acid-soil plants.

roto tilling the soil

Guessing Soil Acidity By Plants Grown?

The soil condition may be roughly guessed at from the kind of plants which grow naturally upon it; where acid soil plants predominate, look for acid soil, while alkaline plants indicate soil-alkalinity.

It is often said that where you find sheep-sorrel the soil is acid, but this weed is not a reliable indicator. Swamps are usually acid unless surrounded by limestone land. Poor sandy soils are often acid, also peat or muck soils.

To quickly determine whether a soil is acid, neutral, or alkaline, a soil pH test is made from samples tested by the old litmus method, slips of red or blue litmus paper being inserted in a slit made in a cupful of moistened soil.

If the blue paper turns red, the soil is acid; if the red paper turns blue, the soil is alkaline ; while if either paper turns purple it indicates a neutral condition. Much more satisfactory are compact testing sets sold at seed stores, which by means of a color chart, show the degree of acidity.

What pH Means

Soil acidity is measured by what scientists refer to as its hydrogen ion concentration and designated for convenience as pH. The scale reads from 1 to 14; 7 is neutral, and readings above that point show alkalinity, below it, acidity.

Tests even more accurate than those of the testing sets may be had by sending soil samples to the State experiment station or agricultural college.

Changing Soil pH

To make an alkaline or neutral soil acid, or to increase acidity in one already on the acid side, chemicals may be added, commonly aluminum sulphate, sulphur, or tannic acid.

Aluminum sulphate should be used cautiously, not more than 5 lbs. per 100 sq. ft. at one application, spread evenly and watered in. Sulphur, also dangerous in excess, should be scattered at the rate of not more than 3 lbs. per 100 sq. ft. Tannic acid may be used rather liberally – 1 part to 50 of water.

For ordinary gardening it is better and perfectly safe to use such acid materials as the following spaded into the soil and also as a mulch: Peat (peat moss), oak or pine leafmold, well-rotted hardwood sawdust, or pomace (dried pulp from cider mills), all applied as necessary.

It is not known for certain whether the demand of acid soil plants is for acidity or for other soil conditions which go with acidity; until this important question has been decided, natural acid materials such as those just specified should be supplied to their soil rather than chemical acids.

The type of soil desired by such plants is loose, light, moist and largely organic, and rich in thoroughly decayed leaves, wood and other vegetable substances.

Improving Acid Soil

The acid-forming materials should be worked into the soil in quantities depending on its original nature.

Poor ground where the topsoil has been removed requires at least an equal bulk of peat or leafmold to give it a proper consistency or friability, while a rich, light topsoil needs only enough added to create distinct acidity. One part each of peat, leafmold and sand mixed make an excellent soil for acid-soil plants in pots or as a bed.

When making an acid-soil bed on strongly alkaline soil (that testing pH 8 or more), the peat and other materials should not be dug in or placed in a hole, but incorporated in a raised bed on top of the natural soil; more acid material should frequently be added as a mulch to maintain the acidity.

Unless the bed is kept raised, the natural “sweetness” of the surrounding soil will encroach upon it. If the local water is alkaline, such beds should be sprinkled only with collected rain water, which is always acid.


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