How to Make Soil Acid

Any soils are naturally acid (sour, as some people say) so that no extra chemicals need be added to grow acid-soil plants. Other soils are neutral or alkaline (sweet), and these require special treatment to grow acid-soil plants satisfactorily.

The only positive way to tell the reaction of a soil, that is whether it is acid, neutral or alkaline, is by means of a soil test.

Plants requiring acid soils include rhododendron, azalea, mountain-laurel, blue-flowered hydrangea (otherwise it will be pink), heather, leucothoe, andromeda, trailing arbutus, partridge berry, wintergreen, and some wild flowers. Those that prefer but do not absolutely ‘require an acid soil include flowering dogwood, pin oak, and fringe-tree.

Soil reaction is expressed by a scale called pH. A reading of pH 7 is neutral with acidity increasing as numbers are lower. Acid-soil plants require a soil between pH 4.5 and pH 6. Below a pH of 4 the soil will be too acid for most plants, even acid-soil ones, to grow satisfactorily. Above 8.5 the soil will be too alkaline for many plants. A soil test will give the so-called pH reaction of the soil.

One recommendation for acidifying soil is a mixture of equal parts of:

• Powdered sulfur – need not be as fine as for dusting sulfur
• Iron sulfate (copperas)
• Aluminum sulfate
• Ammonium sulfate

One pound of this mixture scattered over 100 square feet will lower the pH point down to pH 6. Below this point it takes less to increase the acidity. The mixture is scattered over the surface of the ground. You may rake it in if you prefer, but it is not necessary. It may be applied in the fall or in the early spring, if you want to obtain full benefit. Otherwise it may be applied anytime the soil is found to be insufficiently acid.

A few hints on keeping soil acid are the following: Do not apply lime. Do not apply Bone meal or wood ashes; they will both sweeten the soil. Do not apply sand unless you are sure it contains no limestone. Do not use any water, in alkaline regions, other than rain water; otherwise the lime in it will counteract the acidity of the soil.

Maintain a 2- to 3-inch mulch of organic material over the entire bed. This may be peatmoss, fresh oak sawdust, well weathered sawdust, compost (if it has not had lime or wood ashes added to it), rotted straw, rotted manure, clover chaff, alfalfa chaff, chopped corn cobs or buckwheat hulls.

Related Articles Of Interest:

• Acid Soil
• Soil Testing Starts with pH
• Should You Test Soil Acidity and Alkalinity Before Landscaping?
• Why Are Some Soils Acid?