Soil Water Movement in Vegetables Grown with Plasticulture

View the project final report

Commodities

• Vegetables: tomatoes

Practices

• Crop Production: fertigation, nutrient cycling, application rate management
• Education and Training: demonstration, on-farm/ranch research
• Production Systems: general crop production

The long-term sustainability of commercial vegetable production requires increased fertilizer and irrigation efficiency. Three vegetables growers recognized as leaders in fertilizer and irrigation management in North Florida were selected to demonstrate how irrigation and fertilizer management are linked together and how management may prevent water movement below the root zone of two muskmelon and one watermelon field, all grown with plasticulture. The approach was to create irrigation rates by using drip tapes with different flow rates, inject colored dye in the irrigation water three times during the growing season, and digging the dye to determine its position. Similar results were found at all three locations: water movement was greater early in the season (1 to 5 weeks after establishment) and moved below the root zone (20 to 30 inches deep). Vertical movement was greater on a loamy soil than on the two sandy soils. The uniformity of water movement decreased as depth increased. Overall these results show that some leaching is likely to occur on light-textured soils, even when recommended practices are followed. Educational efforts should focus on fertigation management during the first weeks after crop establishment. Based on these observations, cooperators are considering improving their fertigation practices by using two drip tapes, reducing preplant fertilizer, using a 100% injected N/K program, and/or adding organic matter to the soil. The efficiency of these additional practices on further reducing water movement and fertilizer leaching will be studied next with these farmers and with organic growers who use different nutrient sources. This project shows that vegetables growers are more likely to try and adopt sustainable practices when they actively participate in the process than when these changes are mandated through legislation.

Introduction

Irrigation management is directly linked not only to yield and economical value of vegetable crops, but also to long-term sustainability and environmental impact of vegetable production. Precise knowledge of where irrigation water goes has direct implications not only on irrigation management, but also on fumigant application (Hochmuth et al., 2002; Santos et al., 2003) and fertilizer leaching (Simonne et al., 2002). The recommendations of UF/IFAS for irrigation management for vegetable crops include using a combination of target irrigation volume, a measure of soil moisture to adjust this volume based on crop age and weather conditions, a knowledge of how much water the root zone can hold, and an assessment of how rainfall contributes to replenishing soil moisture (Simonne et al., 2003). Improving irrigation management in vegetable crops has been limited by the fact that water movement in soil is a process that cannot be easily seen because it occurs under ground.

A direct knowledge of how much water can be stored in the root zone can be gained by visualizing water movement in the soil using soluble dye (German-Heins and Flury. 2000). A blue dye and controlled irrigation conditions were used to visualize the wetting pattern of drip irrigation using different drip tapes on sandy soils representative of vegetable producing areas of Florida (Santos et al., 2003; Simonne et al., 2003, 2004). As research tools, these dye tests were used to describe the shape of the wetted zone for several water volumes applied by drip irrigation, determine height, width and depth of the wetted zone, and determine if soluble fertilizer and the water front represented by the dye move together in the soil (Simonne et al., 2004). As educational tools, these dye tests have been used to show growers how deep water moves into several soils and how drip tape flow rate and emitter spacing affects wetted zones. While novel in their approach, these dye tests have used single irrigation events and were done without an actively transpiring vegetable crop.

Past educational efforts and fertilization recommendations generally attempted to reduce environmental impact by reducing fertilizer application rates. While this approach is theoretically valid, it is not practical since fertilizer costs only represent 10% to 15% of the overall pre-harvest production costs. Fertilizer are often applied at rates above the crop nutritional requirement as a means to decrease the risk of reduced yields due to shortage of fertilizer, especially close to harvest. We believe that it is possible to follow a different approach to improving fertilizer management. As water is the vehicle to soluble nutrient movement in the root zone and below, it is possible, in theory, to improve nutrient management by improving irrigation management. If irrigation water stays in the root zone, smaller amount of fertilizer are likely to be leached. If growers are shown how their current irrigation schedule affects water movement in their fields, they are more likely to understand how water and nutrients are linked. With this integrated approach, sustainability becomes more compatible with economical profitability. Therefore, the goals of this project were to demonstrate to cooperating growers how irrigation and fertilizer management are linked together and how management may prevent water movement below the root zone.

More specifically, the objectives of this project were to (1) establish a partnership with three key growers and discuss fertilizer and irrigation management, (2) determine the position of the water front throughout the growing season, (3) diagnose crop nutritional status, and (4) determine nitrate distribution in the soil profile at the end of the season. From a producer’s stand point, this information will be used to increase sustainability by reducing water used and environmental impact of vegetable production. From a regulatory stand point, this information will contribute to demonstrate the efficacy of possible nutrient/water Best Management Practices and set practical management expectations.