Soil tests could be registering “false” pH

There is a real danger this wet fall and winter will cause many soil samples to register a false high pH. In fact, these false high pHs could be as off as much as 0.5 units. They may, for example, read 6.5 when they are really 6.0.

Two potential problems that may lead to false high pHs are not getting a good representative soil sample and if the soil lab is using the water method of measuring pH. Farmers may have a difficult time getting a good soil sample because really wet areas have to be avoided including spots where there is standing water. But soil labs using the water method is the biggest concern with false high pHs.

Why does wet weather cause pH values to be high? It has to do with salts (or lack of salts) in the soil and how it affects the reading when using a pH meter in the lab. Basically, a pH meter measures the “electrometric potential,” or charge, between a glass electrode and a reference electrode, that is directly related to hydrogen ions. This measurement, which is actually in millivolts, is then converted to a pH reading. A “wet” soil sample usually means salts have leached out and, if the sample is prepared with water, the readings are not as accurate.

The University of Georgia soil testing lab overcame the problem of false high pHs several years ago by preparing their soil samples with salt instead of water. Most private labs in Georgia and throughout the Southeastern U.S. are using water and not salt to prepare their samples. So a wise farmer should inquire if their soil samples are being prepared in water or salt. Southeast Farm Press recently published an article about the University of Kentucky soils lab switching from using the water method to the salt method.

So is the salt method better? Glen Harris, UGA Extension soil scientist, says as far as being a more accurate measurement of soil pH under conditions we are having right now with wet, leached out samples, the answer would be a definite “yes.” Harris says, “The only problem I have with the salt method is that up to this point everyone has always used pH water and no one has used the salt values. In order to reduce some confusion, the UGA soils lab actually estimates what the pH water value would be.”

Harris recommends that farmers delay soil sampling as much as possible until they can get a good representative sample. He advises growers who took soil samples during these wet conditions to go back and spot check soil samples later if the lab they are sending their soil samples to is using the pH water method (basically any other lab besides UGA or University of Kentucky).

If growers suspect they have a “false high” pH, then they should use the previous history of pH from each individual field. For example, if a grower did not lime last year and his pH goes from 6.0 to 6.5, then he could have a “false high.” False high pHs are more prevalent with higher pHs, so the situation where a grower thinks he has a 6.0 and he really has a 5.5 should be less likely.

TAGS: Management
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