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Friday, December 3, 2010

Vegetable Gardening: How to test soil pH By Mac Pike

What is a PH level , how do we test it, and what are the implications for a productive garden?
Plants do not draw their food directly from garden soil, but rather use the water and air present in the soil and the energy derived from sunlight to manufacture their food from the nutrients the soil contains. This process is known as photosynthesis. Major nutrients include potassium, nitrogen and phosphorus, but there are many other nutrients like calcium and magnesium that plants require for optimum growth.

Plants need these nutrients to be present, but even if the nutrients are present plants can only utilize them if they can free the nutrients from the soil matrix. This is where the PH scale enters into the process.
The PH scale is used to measure the acid or alkaline properties of a soil sample. It is a 14 point logarithmic scale where 7 is the neutral point. Numbers below 7 indicate acidity; numbers over 7 indicate an alkaline soil. As the scale is logarithmic, each number indicates a ten fold change from the next. A rating of PH 6.0 is 10 times more acetic than 7.0, 5.0 PH is 100 times more acetic and so on.
Of the plants most commonly grown in the home garden, most do well in a slightly acetic soil; tomatoes, cabbage, peppers, lima beans, cucumbers all thrive in a soil with a PH level between 6.0 and 7.0. This is the range when they can most readily synthesize soil nutrients into food, mainly carbohydrates, through the photosynthetic process.  As the PH varies from this optimal range less and less of the vital nutrients like calcium, potassium and phosphorus are available to the plants. Depending on how far from optimal PH a given soil may be, plants may produce poorly or not at all, at the extreme ranges they will not grow.

Fortunately the problem of toxic PH levels can be addressed, but to address the problem one must recognize the problem. PH levels cannot be accurately determined by visual inspection of the soil or plants, although the condition of the plants may give a skilled observer some clues. To get an accurate picture of soil PH, a soil PH test is necessary.

Soil testing kits:
Commercial testing kits are readily available online, from garden supply houses and from gardening centers. They are relatively inexpensive, reasonably simple to use and generally have enough supplies to run several tests, which is always a good idea. A typical kit

is available from the Accugrow Company and a number of retail outlets. It provides simple reagent strips adequate for 10 separate PH tests.

To use the strips the gardener prepares slurry from the soil being tested, using distilled (PH neutral) water. A reagent strip is inserted into the slurry. The strip changes color in response to the PH factor and after a specified period of minutes this color is compared to a color chart included in the kit. When a match is found the number on the chart is the PH level.

Another PH testing using reagents is offered by Manutec through a number of outlets. To use this test a sample of soil is mixed on a sterile plate with 3 to 5 drops of reagent. The resulting mix is then dusted with a powder which changes color in repose to the PH value of the soil. After one minute this color is compared to a color coded chart; as before, the matching color will yield a number which is the PH level for the sample.

The kit does not test for nutrients but provides supplies for multiple PH tests. The price is again in the $25.00 range.

PH meters:
Electronic PH measuring devices called PH meters are widely available. These devices are simple to use and will last a long time if properly cared for, but depending on their degree of accuracy may be a bit more expensive than reagent kits.

Instructions for testing vary with design and manufacturer. In some cases soil samples are taken, returned to your home or greenhouse, mixed with distilled water and then tested by inserting the probe or probes into the mixture. A digital read out or gauge and needle will indicate the PH number.
dual probe meter is a testing device that requires you to dig a small hole in the garden soil, pour in distilled water, stir to slurry, and insert the probe into the resulting muck. The reading appears on a readout or gauge. Probes must be thoroughly cleaned with distilled water prior to the next test.
Testing devices of the single probe variety, simply press the probe into the soil, and read the PH level directly without prior digging or mixing of slurry. This is a simple to use and increasingly popular home testing device.

Many of these devices test soil moisture content and overall more.

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