Blog Posts‎ > ‎Bob's Notes‎ > ‎

Soil Test Results Reveal a Lot

posted Sep 16, 2010, 7:33 PM by Bob Whitney   [ updated Sep 16, 2010, 7:34 PM ]

From 2008-09 there were 573 soil samples submitted to the Texas AgriLife Extension Service Soils Lab in College Station. Dr. Tony Provin is the Extension Soil Chemist and Soils Lab Director and he sends me the combined results of those soil samples every year so that I can see any trends. I love to use these for folks so that they have a general idea of what to expect and also they can compare their individual results with the county averages. Dr. Provin sends me results on all the nutrients but I am only discussing the ones that make the most difference.

First on the list is pH which is a measure of the alkalinity or acidity of a soil. This affects how plants grow because it affects nutrient uptake. We can’t grow some plants here simply because our soil pH is too high. Of 573 samples, 445 samples ranged from 7.1 - 8.2 pH. 56 were below 7.0 or slightly acidic and 72 samples were above 8.2 or moderately alkaline. At 8.2 you can really see iron chlorosis problems show up in plants.

Nitrogen is used in large quantities by plants and it what really makes plants grow and look green. So we need lots of nitrogen and most soils in our area are low or should be. Some samples are high which means that growers are probably over-fertilizing and not soil testing before applying fertilizer. Of all the samples 312 were below 10 ppm, which means 20 lbs per acre of nitrogen. 121 samples were 11-20 ppm and 140 samples were above 20 ppm which is very high. The highest sample was 1047 ppm which is unbelievable.

Phosphorus results were a lot more scattered in results. 50 ppm is considered the critical value above which you would not need to apply any phosphorus. Values less than 50 would mean an application but still you wouldn’t need to add a lot. 173 samples had less than 10 ppm, 101 samples were between 11 ppm and 20 ppm, 134 between 21 and 50, 165 greater than 50 ppm. In our high pH soils when you start to get higher phosphorus levels you can really see problems with iron chlorosis in plants. Fortunately fertilizer companies are starting to see the problem in Texas and selling fertilizers with little or no phosphorus for homeowners.

Basically of the 573 samples we don’t have a potassium shortage! 466 samples had more than 175 ppm which is the critical value for potassium. This means that no potassium is needed and probably won’t be needed for a long time.

Lastly I have included calcium. Calcium is one of those nutrients we have an abundance of in our soils. That is not hard to understand when you think about all the limestone (calcium carbonate) that is in and under our soils. One of the largest limestone mines is in our county so finding calcium is not too hard! Of all 573 samples565 had more than 3000 ppm. So don’t add any limestone or gypsum to these soils, you are only wasting your money.