Express News Weekly Article
Saturday, January 15, 2005
By Calvin Finch, Conservation Director, SAWS, and Horticulturist

Soil Tests Are Not Useful

            It is often claimed that a healthy soil is the key to good gardening and an attractive landscape.  I agree that a gardener that understands his or her soil is more likely to be successful with plants but the success usually comes from utilizing the soil we have rather than any treatment that results in changes that make the soil “healthier.”  Over the next few weeks I will explore (with breaks for time sensitive topics) ideas that will make you more knowledgeable about soil and should help you be more successful in your gardening.

            Among the topics that will be covered are:

o       Is a soil test useful for landscape care in San Antonio?

o       Is a balanced fertilizer that best for gardening in San Antonio?  Can we enhance certain plant activities, such as blooms, by increasing the level of a particular nutrient in a fertilizer mix?

o       What is the best nutrient mix for a lawn fertilizer in San Antonio?  Why would we select a fertilizer that does not meet the best mix for our soil conditions?

o       Can we permanently alter the alkalinity of our San Antonio soils to make it better suited for plant growth?

o       What is the difference between organic and manufactured fertilizers?  Why would one or the other be more effective for growing plants in the San Antonio area?

o       How can organic material increase both drainage and water retention characteristics of our soil?  Is there any way to increase organic material in an established lawn or garden?

o       Why do we often have to add iron to our lawns and gardens when our soils have large amounts?  How is iron availability affected by alkalinity, soil moisture, soil temperatures and plant type?

The first topic I want to cover is soil testing.  It is my contention that they are not particularly useful to a gardener in San Antonio trying to have a productive garden or attractive landscape.  This is not a popular conclusion.  A large number of respected area horticulturists recommend soil tests for good gardening results.  The reasons for my conclusion that the tests are not useful, include the fact that soil tests are not very accurate, as many as 95% of all tests for San Antonio soils turn out the same, and homeowners and hobbyists do not have access to fertilizers that allow us to address variations in the test results if any exist.

Soil tests are approximate measures of soil nutrient levels.  The testing laboratories do not claim very high accuracy, certainly less than 75% for any particular nutrient on any single test.  Here in San Antonio with our alkaline soil, many procedures may even be less accurate depending if the test was developed for acidic or alkaline soils.  The soil tests usually do not determine available nutrients.  If you check a Texas A&M soil test for our typical heavy soil, it will usually say iron levels are high.  As we all know, however, high levels of iron in our alkaline soil do not translate into readily available iron.

Advocates for the value of soil tests will say that the recommendation from the soil scientist is at least as important as the nutrient level numbers.  In the case of the iron, a good test will advise the recipient that the availability of the iron in an alkaline soil is not the same as a high level in the soil.  So it is, for 95% of the tests for soil in San Antonio.  The soil tests will determine that our soils have low nitrogen, high phosphorous and high potassium.  They will determine that we have high iron.  They will go on however and say that since the soil is highly alkaline, the iron is not available and we may have to provide iron to St. Augustine grass and other plants that grow better in acidic soils.  The analysis will also recommend that you add nitrogen.  There!  Those are the results provided by a soil test for most home landscapes in San Antonio. 

Further, it really does not matter what the test says, we have a limited choice of fertilizers from which to choose to address the soil test results.  If you want to protect the aquifer and area watersheds you must select a 3-1-2 slow release manufactured fertilizer or an organic fertilizer.  If you are willing to take a chance that you can apply the fertilizer efficiently and avoid burning your lawn, ammonium sulfate 21-0-0 could be used.  In many cases, however, 21-0-0 fertilizer is a threat to the environment because it is volatile, most of it is lost to the air or leached before the lawn can use it. 

The bottom line is that even if soil tests were very accurate and if there was variation in test results from landscape to landscape, there are not enough choices of fertilizer blends to address the exact nutrient recommendation provided by the soil test.  Why waste the time and expense for a soil test?