Project Summaries

10-647TN  Project Manager: E. M. Barnes


Brian Leib, University of Tennessee

In west Tennessee, the soil under most center pivot systems is mixed. In the more upland areas, the hill tops have deep well drained soil, the side slopes are shallow from erosion, and the low areas have deep poorly drained soil. Along the Mississippi bottom, the soil can vary from very sandy to heavy clay over very short distances. It is possible that each center pivot will require a customized irrigation schedule to optimize yield depending on the mix of soil in that field. Should the irrigation schedule follow the lowest water holding soil, the predominant soil, or a rate somewhere in between the two soil optimums? Will a variable rate pivot be a profitable approach to cotton irrigation requirements in mixed soils? To start to answer some of these questions a new experiment was initiated in 2010 to 2012. 

At the West Tennessee Research and Education Center in Jackson, Tennessee, a field was located that was known to contain variable soil due to the depth of silt loam over course sand. In order to utilize this field for a plot experiment, the soils needed to be differentiated by location. A combination of Ground Penetrating Radar (better for deeper sand layers), Electrical Conductivity (better for shallower sand layers) and verification with soil cores was used to delineate zones. Three soil zones were identified with an average depth to sand of 20, 30 and 50 inches with corresponding average water holding capacity of 0.7, 1.1 and 1.9 in./ft. It is interesting to note that the shallow depth to sand areas can be easily identified by an aerial photograph of the cotton plots.

The optimum irrigated and rain fed yields by individual year and as an average over three years, provides some valuable perspective. The average rain fed was 437, 807, 1,236 lbs./acre in order of increasing water holding capacity. This result verifies the division of soil types but it remains unexplained why individual year yields within a soil type are so similar. It would seem that the different rainfall years would have produced different yields within the same soil type of the rain fed treatments. The greatest yield increase from irrigation was obtained in the sandiest soil while the highest irrigated yield per acre was obtained in deep silt loam soil. Interestingly, the greatest yield response per unit of irrigation applied occurred in the middle water holding soil.

After combining the three years of results, it was determined that a field with approximately 26% sand and 74% silt areas was the breakeven condition. At this mix of soils in a field, it did not matter which soil was irrigated for optimum yield, the average yield would be the same. If the field had less sandy areas than 26% it was better to irrigate to optimize the silt soil, i.e., the predominant soil. If the field had more than 26% sandy areas, it would be better to irrigate to optimize the sandy area, i.e., the lowest water holding soil. If the field could be irrigated to optimize yield in both soil types via variable rate irrigation, a yield increase of over 50 lbs./acre could be obtained in a range of sandy areas from 10% to 60%. The optimum yield increases from variable rate irrigation occurred at the breakeven point resulting in 120 lbs./acre increase in lint yield.


Project Year: 2012

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