Friday, March 28, 2008

Testing your soil pH

Soil pH, what is it and why does it matter?

A few years ago I was told a story about an old farmer that used to taste his soil. He would stand out in the field and pick up a handful of dirt and actually taste it. He could tell if the pH was acidic by just a quick nibble! Well, I am not sure I want to do that but every year we check the pH of our soil with a small testing kit or we have an evaluation of our soil done by our agricultural extension service.

The small test kit is fairly accurate but of course the extension service can be more exact as well as tell you how to correct the pH to the appropriate level for vegetables. The soil pH value, which is really the Potential Hydrogen of a liquid mixed with your soil, is a measure of soil acidity or alkalinity. This pH value directly affects the nutrient availability to plants so that even if you have lots of great nitrogen, phosphorus and potassium etc. in your soil, the roots of your vegetable plants may not be able to take them up because of the chemical actions that must take place.

The pH scale ranges from 0 to 14, with 7 as neutral. Numbers less than 7 indicate acidity while numbers greater than 7 indicate alkalinity. Soil humus, the dark black stuff in your soil, contains the highest CEC or cation exchange capacity which means that plants are most able to transfer nutrients from it and compost runs a close second. If the plant cannot break down the nutrients properly they don't get fed. This can lead directly to Blossom End Rot which is a condition where the plant cannot take up calcium. Using some types of fertilizers which leave salts behind such as ammonium or urea, which is in most grass/turf fertilizers, can make soil more acidic.

In areas with plentiful rainfall it is almost always necessary to add lime, which is ground limestone, to your garden every year to correct pH to the neutral level and in areas with very little rainfall over the year, it is likely that your soil is alkaline or may have a buildup of salts. Rainfall passing through the soil leaches out basic nutrients such as calcium and magnesium from the soil. They are replaced by acidic elements such as aluminum and iron. For this reason, soils under high rainfall conditions are more acidic than those which were formed under dry conditions. Sulfur can be added to alkaline soils to correct pH or gypsum can be added to flush away salts in alkaline conditions which can correct pH levels slighly.

What is neutral? How do I correct pH? Vegetable plants prefer to have the soil pH between 5.5 and 6.5 and above or below that range must be limed or adjusted with sulfur to bring soil back into the neutral range. For small adjustments, 1 lb of lime per 100 sq. ft is enough but if your garden is new, it may require more than that to bring it up to "normal" such as 2 lbs. per square yard. Liming is basically adding natural limestone to the soil which over a period of time will change the pH value. Wood ashes can also be used so those with fireplaces can add them every winter to the garden. Two materials commonly used for lowering soil pH are aluminum sulfate and sulfur. These can be found at most garden supply centers. Aluminum sulfate will change the soil pH instantly because the aluminum produces the acidity as soon as it dissolves in the soil. Sulfur, however, requires some time for the conversion to sulfuric acid with the aid of soil bacteria. The conversion rate of the sulfur is dependent on the fineness of the sulfur, the amount of soil moisture, soil temperature and the presence of the bacteria. Sulfur can be very slow and take several months to correct pH. So most people use the aluminum sulfate. Both of these should be worked into the soil after applying to be most effective. If these materials are in contact with plant leaves as when applied to a lawn, they should be washed off the leaves immediately after application or it can burn leaves. Take extreme care not to over-apply the aluminum sulfate or the sulfur.

Purchase a soil test kit or garden lime from our catalog.

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