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A Short Essay on Soil pH By Hugh Wiberg

Orginally published in the March, 1997 N.E.P.G.A. Newsletter

As I converse with pumpkin growers from all over New England, I get the feeling that a surprisingly high percentage of our members have not tested their pumpkin patch's pH for more than two years, and in many cases, even longer.

If your soil's pH is well below 6.5, this just might explain why you have yet to grow a pumpkin weighing over 500 pounds.

First, let's define soil pH. pH, by definition, refers to the degree of acidity or alkalinity of a given soil. The extent of one or the other is essentially predetermined by the chemical nature of the rock from which the soil has partially resulted. Most of our New England soils are slightly acid, in the range of 5.5 to 6.5 on the chemist's pH scale, which starts at 0 (ultra acid) and goes to 14 (ultra alkaline). Acid rain, resulting from industrial pollutants drifting into our area from the mid west, have tended to reduce our soil's pH over the last 60 or 50 years in an ongoing way. Therefore, it is very necessary for all giant pumpkin growers to check to be sure that our soils have not dropped below a pH of 6.5.

All vegetables, including pumpkins, do best on soils with a pH of 6.6 to 7.2. The microscopic organisms in the soil which transform decomposing plant and animal substances into usable plant nutrients thrive at a pH at or just under neutral (7.0).

It is relatively simple to check your soil's pH. Most garden supply outlets and nurseries sell inexpensive soil test kits, which come with easy to follow directions.

Many of the larger nurseries are now equipped to test your soil's acidity, either free or for a small charge. Another approach is to write to your state university, care of their agricultural extension, to inquire about their procedure for mailing or delivering soil samples for testing, at a nominal fee. The advantage of going this latter route is that, in addition to a pH report, you will also find out if your soil is deficient in nitrogen, phosphorous, potassium, and the important trace elements.

If it is shown that your soil is acid, the solution~is to add crushed limestone which, fortunately, is relatively inexpensive. The accepted rate of application to raise the pH one point (example, from 5.8 to 6.8) is 20 pounds per 450 square feet, or 50 pounds per 1100 sq. ft., which is roughly a plot measuring 30 by 35 feet. Slight errors either way will do no harm. The crushed limestone can be applied either very late in the fall, or very early in the spring. Limestone acts (and moves through the soil) very slowly. Mix it into the sub soil when you spread your manure and other organic materials late in March or early in April. By June and July it will be "working" for you, and this application should cover your needs this year and next year.

All of the successful growers check their soil pH annually. Don't procrastinate this spring!

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