Deficiencies in phosphorus and potassium draws concern for pasture growers, tour shows

By MANDY JOHN | Central Texas Edition

Dr. Tony Provin, Extension soil chemist for Texas A&M, shows participants at DeLeon tour how to use a probe to get an accurate soil nutrient reading. -Staff photo by John

October 3, 2002 -- Cattle ranchers were able to view and discuss important forage issues during the Forage Tour that took place on Sept. 26 in DeLeon. One of the important issues discussed was on phosphorus and potassium deficiencies in irrigated Coastal bermudagrass fields.

"Three years ago a few colleagues and I had a meeting where we discovered that potassium was too low in certain farming areas in Texas. So, we decided to create a study to find out how to solve this problem," stated Dr. Tony Provin, Extension soil chemist and director for the Texas A&M University soil lab. "The same study that is occuring here (DeLeon) is also occuring in Liberty, Van Zandt, and Nacadoches counties," he said.

"By doing this study we hope to find out how nutrients in the soil are doing based on the timing and the rate that potassium is added to an area. Basically, what we are trying to find out is if there is a different outcome when potassium is added all at once at the beginning of the year versus adding potassium heavily during the beginning and end of the year and then continually adding a little at a time throughout the year. Essentially, potassium is the anti-freeze to a plant and so it's very important to have it around, especially during the colder months," Provin said.

"The plot used in the DeLeon study was 12 feet by 18 feet and we came out to find out what the deal was with the low nutrient content in the soil. Twenty-six inches of this soil was sandy soil. These 26 inches are special because this site is a clay area. When it rains, you can actually squeeze water out of the clay. We tried to bring a truck to break the clay ground, but have had no luck in it. We also tried to probe the ground with a drill, but instead it just shook the ground.

Billy Mac Barnes' farm, where the plot was located, was said to have limited water irrigation supply. "There would be more of a response to phosphorous if it would rain more or if there was more water available," Provin said.

Billy Mac Barnes, left, and Charley Boswell check out the test area that was used in the nutrient study. -Staff photo by John

He said there was a greater need for phosphorus than all the cows combined. The goal, however, was not to create another environmental problem.

Provin said that at the Mac Barnes' farm there was limited opportunity for run-off in both compost and manure. Both the compost and manure was putting out more carbon than nitrate presenting a problem of nitrate deficiency in the ground. A larger amount of compost or manure than crops was a problem because the primary job of the microbes in those items was to reproduce. They do this by using nitrogen. If there is not enough nitrogen available to them, they are going to take it away from the soil.

Another field tested was at the Boswell Farm, also in Comanche County. The test field was sampled in March 2001. The results of this test indicated a pH level of 4.1, which was said to be a very low result. In April 2001, two tons of ag-lime was applied to raise the pH levels. Due to drought conditions however the field laid fallow throughout the summer and fall.

Charlie Boswell, owner of the Boswell Farm, said that in February 2002 an application of ag-lime was added to 25 tons per acre of wet manure with a follow-up of close shredding to better spread the manure and to control some of the late season weeds.

No herbicides were used on the field.

One application of commercial nitrogen fertilizer was applied, Boswell said, between the first and second cuttings.

Since the three cuttings the test field has yielded 87 tons of hay with an average protein value of 11.08 percent crude protein. The trace mineral levels also showed notable increases.

"Using manure to keep phosphorus, potassium and trace minerals at optimum levels along with conventional inputs of nitrogen as needed should result in substantial increases in yield and quality, along with decreased water usage and lower fertilizer costs," Boswell said.

"The soil phosphorus level from the soil sample we took after the first cutting went up to 50 pounds per acre," added John Ed Brothers, of Agricultural Consulting Service, speaking about the Boswell sample. "There was also a light increase of potassium and the nitrogen levels fluctuated."

"Sometimes it helps for us to compare our materials to the commercial supplies. It's hard to compare, but it can gear us to what is available and will be available in the future," Brothers said.