Acid Soil: How To Make Soil More Acidic

Do you know that?… Acid soil determined with a soil test, exhibit differences in texture (or physical condition) and in fertility than neutral or alkaline soil.
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 A combination may be neither acid nor alkaline, but neutral.

Acid Soil Pin

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.

Related: Learn about Lowering Soil pH with Vinegar

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

Where underlying rocks are:

  • Quartz
  • Granite
  • Sandstone
  • Shale
  • Gneiss
  • Mica schist
  • 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 like types of hydrangeas or azaleas.


How To Test Soil pH – Try A Simple Litmus Test

The soil condition may be roughly guessed at from the kind of plants growth coming 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 structure 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 garden centers and Home Improvement stores, which by means of a color chart, show the degree of acidity.


What Does The pH Scale Indicate?

Soil acidity is measured by what scientists refer to as its hydrogen ion concentration and designated for convenience as pH levels. 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.

How To Change Soil pH

Here’s how to make soil acidic and how to make soil alkaline:

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 organic materials as the following worked into the soil and also as organic mulches:

  • Peat (peat moss)
  • Oak or pine leafmold
  • Well-rotted hardwood sawdust
  • Pomace (dried pulp from cider mills)
  • Fresh coffee grounds (fresh ones are acidic while used coffee grounds are neutral)
  • Wood chips

… all applied to make soil acidic as necessary.

It is not known for certain whether the demand of acid soil plants is for acidity or for other pH 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 acidify soil rather than chemical acids.

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

Improving Acid Soil And Making Garden Soil Better

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 potting soil or potting mix for acid-loving plants.

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,.

They should be 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 you have a raised garden bed built on a concrete slab, 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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