Recently, I received a letter from a gardener interested in finding a way to make his alkaline soil permanently acid. This is known as lowering the pH.
I think his questions and the answers will interest other readers. His letter follows:
“I would like information on the various materials which he can use to change soil with a pH of about 7.0-7A to about 6.0 for roses and 5.0 for blueberries.”
A bed was dug to a depth of 15′ feet, and a considerable amount of imported peat moss was mixed with the soil, which had a pH of about 7.2.
In addition, elemental sulfur, of about 325 mesh fineness, and aluminum sulfate were mixed with the soil.
The pH was about 5.0 a few weeks later, but now, two years hence, the pH is back up to about 7.2 “I have read articles advising the use of peat moss, sulfur, aluminum sulfate, and commercial tannic acid for lowering the pH of soils.
I know the aluminum sulfate will leach out over time, but I thought the sulfur would give a long-lasting effect.
My experience does not indicate that the sulfur did any good. We have not used any tannic acid but understand it has lasting effects.
I would appreciate any information which will help me establish and maintain the soil at less than 7.2 pH in our yard.
Questions to Ask in Lowering Soil PH
In addition to general information, I would appreciate the answers to these questions:
1. “Is tannic acid a practical agent to use for producing and maintaining a pH of 5.0-6.5 in the soil where pH is normally about 7.2? If so, where can it be obtained?
2. “Does elemental (dusting) sulfur have any effect on the pH of the soil?’’
3. “How strong a solution of aluminum sulfate is safe to apply to the ground around roses, blueberries, etc.?’’
My answers follow:
“It is a mistake to assume that once the soil has been acidified, the job is done once and for all.
The soil in your area is alkaline because alkalinizing agencies exist in the soil to keep it that way.
Earthworms, working from below the acidified area, keep bringing lime to the surface in their castings. Water from domestic supply systems probably comes through limestone.
Also, drainage water from higher ground brings in water of a higher pH than that of the soil. Acidifying materials must be applied constantly to maintain the pH level desired.
In theory, humic acid would be the ideal acidifying agent since it is found in soil. However, tannic acid is the closest thing to it.
If one could buy tannic acid, it would be an ideal material, but tanners no longer use this substance.”
Recommendation From Florists’ Greenhouse Practices
“Sulfur is the third most logical material. As for aluminum sulfate, I would not use it under any circumstances. It is recommended only by those who know nothing about the subject.
They pick up the recommendation from florists’ greenhouse practices, an entirely different field.
In the garden, constant use of aluminum sulfate will soon destroy roots, and the plants will die. I see no reason for trying to modify the pH of your soil for roses if it is now 7.2.
At this reading, the plants might have slight iron chlorosis if your soil is low in iron, but that can be adjusted with applications of chelated iron.”
PH Readings
“It is true that between pH 6.0 and 6.9, all the elements needed by roses are available, so there might be some point in bringing the reading to below 7.0. But to go as low as 6.0 wastes money and energy.
Blueberries grow best at 5.5; the reason is not that the plants need this low reading, but the mycorrhiza (highly specialized root fungi), which serve the roots of blueberries instead of root hairs, will not survive at higher pH readings.
However, if the soil is high in organic matter (as much as one-third of the total bulk of the soil), then they can tolerate a reading of up to pH 6.0.”
44659 by Na