Locating the Variability of Soil Water Holding Capacity and Understanding Its Effects on Deficit Irrigation and Cotton Lint Yield

Precision irrigation equipment such as variable-rate center pivots is readily available to Tennessee growers and producers; however, little research exists describing its application to cotton grown in Tennessee. In order to optimize the use of variable-rate irrigation equipment and water resources, two experiments were performed to determine (1) whether or not ground-penetrating radar (GPR) and electrical conductivity (EC) measurements can be used to delineate variable-rate irrigation zones and (2) examine the response of cotton lint yield to varying rates and duration of irrigation.

GPR and EC measurements were recorded, validated using soil cores, and used to identify the subsurface variability of soil texture, depth to a sandy layer (DTS), and soil available water holding capacity (AWHC) on a research location in Jackson, Tennessee. A strong, linear relationship between DTS and AWHC (R²=0.92) indicated that a high resolution map of textural differences would provide a good approximation of AWHC variability. Both AWHC and DTS were related (R²>70%) to both GPR and EC data. Variable-rate irrigation zones representing textural and AWHC variability were successfully partitioned using a combination of GPR and EC data.

Past cotton research regarding lint yield response to irrigation suggests that cotton irrigated on soils with a high AWHC might be negatively affected by the high irrigation rates required to maximize the yield of cotton grown on low AWHC soils. Using three major soil blocks identified using GPR and EC data, varying levels of irrigation rate and duration were applied to cotton grown on soils with a low, moderate, and high AWHC. Statistical analysis shows that cotton grown on low AWHC soils responded differently (p=0.06) to irrigation treatments than cotton grown on moderate and high AWHC soils. In 2011, irrigating at 1.5 inches per week from first bloom until cracked boll resulted in approximately 1081 pounds of lint per acre increase in low AWHC soils but only 167 pounds of lint per acre increase in high AWHC soils. Also, this irrigation treatment resulted in a significant (alpha=0.10) lint yield decrease below maximum yield for high AWHC soils.