Landscape
And
Nursery
Dialog

Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State University

February, 1996

Changes in the OSU Horticulture Soil Test

Keywords: soil/media testing, soils, landscape, nursery

I n January of 1996, important changes were made in the OSU Horticulture Soil Test offered through the Research Analytical Extension Laboratory (R.E.A.L. for short). This test is used primarily for field soil testing as well as growing media that contain > 20% soil by volume. The soil test laboratory procedures have not changed, but the units of reporting some of the elements have. All macro- and micronutrients that were previously reported in pounds per
acre (lb/A) will now be reported in parts per million (PPM). Soil laboratories across the country are making this change in order to standardize reporting.

Table One demonstrates how the same soil sample would appear under the old and new reporting methods. To convert from lb/A to PPM, divide by two. The REAL lab will be including a notice with all Horticulture Soil Test reports to notify growers of this change. Growers who fail to make note of the change are likely to worry that their soil test values look rather low!

While standardization of testing and reporting procedures across the country will ultimately be a positive move, one concern I have is that the change to PPM may facilitate using the wrong set of guidelines for interpreting mineral soil tests. For example, when potassium was reported in pounds per acre, a grower would not mistakenly use soilless medium testing guidelines which are reported in PPM. Now that both tests are reported in PPM,
confusion may arise.

Not only are the procedures for soil and soilless media testing very distinct, the units of reporting, parts per million, do not mean the same thing in the two tests! Theoretically, soil test values are based on the weight of the soil. For example, 39 PPM potassium in the soil test translates to 39 parts potassium per million parts dry soil, by weight.

In contrast, PPM in the soilless tests are concentration units. Nutrients are extracted from soilless media, and their concentration is measured in the solution. The amount of nutrients are not related back to the weight of the medium. For example, 39 PPM potassium in a soilless medium test would mean 39 milligrams potassium per liter of extracting solution (A concentration of 1 PPM of any substance = 1 milligram per liter).

In next month's L.A.N.D. column I will review the guidelines I use for interpreting the Horticulture Soil Test.

Table One. The Horticultural Soil Test, before and after January 1, 1996.

	pH	P lb/A	K lb/A	Ca lb/A	Mg lb/A	CEC meq/100g	% Ca	% Mg	% K
old test	6.4	85	298	3330	120	15	57	12	6.1
		PPM	PPM	PPM	PPM
new test	6.4	42.5	149	1665	60	15	57	12	6.1

Return to Landscape and Nursery Dialogue Archives