The technical support staff at AGVISE has been answering questions this fall regarding phosphorus (P) soil test levels being higher this fall compared to previous years. A few clients have even asked if the average P soil test level is drastically higher this fall. When we summarized all of the data, we found the 2003 phosphorus test levels are slightly higher than previous years, but the change is between 1.5 and 2 ppm higher compared to last year (see figure). It is obvious from the figure, that the average P soil test has been increasing the last two years. It is also obvious that during the extended period of wet years (1992-2000), the average P soil test level has been dropping.

We have spent some time trying to determine the reasons for the upward trend in P test levels the last two years. In the dry years of 1988-1990, P levels were higher as well. 2002 was a very dry year in portions of our trade area and 2003 has been very dry since about mid July over the entire trade area of the Northwood laboratory. I am confident that the dry conditions the past two years has some role in the slightly higher P soil test levels. A Canadian researcher from the University of Saskatchewan, Dr. Les Henry, has documented similar increases in P soil test levels during drought conditions. Dr. Henry feels the increase in P soil test level is the result of some soil chemistry changes brought on by the dry conditions.

Another factor that may affect P soil test levels is the quality of the soil sample obtained under dry conditions. If a topsoil sample is collected from a soil that has not been tilled, you do not get the same sample as when the soil has been tilled. Soil samples from tilled fields generally have a lower P test level due to problems with lumps of topsoil that push around the soil probe and are not included in the soil sample. Plugged soil probes can also be more common in tilled soil. In the past two years, due to dry conditions and an early harvest, many more fields have been tested before they were tilled. Samples collected before tillage are always the best quality samples, and usually have a slightly higher P test level. I was out in western North Dakota during harvest and virtually no fall tillage had taken place. In 2002, we documented un-worked fields can have a significantly higher P test than tilled fields. A customer zone sampled a field before tillage and marked the points with GPS. They resampled the same points a month later after a primary tillage and the P test levels decreased 15-30% after tillage.

In summary, it appears that testing for phosphorus during a prolonged dry spell can and does affect P soil test levels (due to several reasons). If soil moisture conditions are better next fall, I would expect P test levels to drop slightly (based on past experience). If you have any questions, please give our technical support staff a call.

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