University of Maryland Extension

Reducing Nitrogen Applications in Watermelon While Maintaining or Increasing Yields

Author: 
Dr. Gerald Brust - IPM Vegetable Specialist

2008

Objective: With the emphasis on reducing excess nutrients applied to Maryland vegetable fields, which will reduce nutrient movement into the Chesapeake Bay, this research examined supplying nitrogen to watermelon plants only when needed as determined by the plant’s nitrate content.

Introduction: Nitrogen is one of the nutrients often supplied in excess to vegetable fields to ensure a good yield and quality of produce. Growers in the past have had to rely on the final yield to tell them if enough nitrogen was used on the field. This method told them nothing about if too much nitrogen was used in the production of their crop. One method to determine if the plant has enough nitrogen is to test for the nitrate content of the plant. One technique is to take tissue samples that are dried and sent off for analysis. This is time consuming and expensive and does not give growers time sensitive feedback. A real-time method used in the field is to test the plant petiole sap. The petiole concentrates nitrates and other nutrients that are then used in the leaf. Through several years of research on many vegetables a guide has been created that gives growers a range of values of nitrate concentration in the plant petiole sap for a given vegetable at a particular developmental point. If nitrate values fall within the range then plant growth and yield will be at their most efficient point. If the values are above the range there will be little increase in yield for the amount of nitrogen applied, i.e., the plant has too much nitrogen. If the values fall below the range the plant is short of nitrogen and will suffer yield and quality loss if nitrogen is not added.

Vegetable production fields are often irrigated using drip irrigation tubes. Not only can water be applied through the drip tubes, but so can nutrients and other chemicals. This would allow a grower to measure nitrate concentrations in the plant and apply nitrogen if needed. This would ensure that the plant has the nitrogen it needs at the time it needs it without having excess amounts in the soil. This experiment examined using a Cardy meter to measure the petiole nitrate concentration in watermelon to determine when nitrogen should be added through the drip irrigation.

Materials and Methods: The experiment took place at two sites: the Central Maryland Research and Education Center (CMREC) and The Lower Eastern Shore Research and Extension Center (LESREC) in 2008. Four levels of nitrogen concentrations were applied to the field in rows 4 ft wide by 100 ft long: Two rows of 80lbs (CMREC) and 100lbs (LESREC) of actual nitrogen applied (nitrogen source was a 45-0-0 calcium nitrate (one row received only 80 or 100lbs of N while the other row (80+ or 100+) received the 80 or 100 lbs plus more nitrogen fed through the drip when needed), one row of 120 lbs (CMREC), one row of 150 (CMREC and LESREC) and one row of 200lbs (LESREC). There were 4 replications. Watermelon transplants (Crimson Sweet) were set three ft apart within rows on June 8 (LESREC) and June 10 (CMREC). Petiole sap readings were taken weekly three weeks after transplanting by crushing 10 watermelon leaf petioles per row using a garlic press. Leaves selected for sampling were taken from the middle of a vine. The resulting sap was collected in vials and placed in a cooler until transported back to the lab where a few drops were placed on a Cardy meter for measurement. Once readings neared the bottom of the recommended range for the 80 and 100lbs of N per acre treatments an application of 20 lbs of urea was applied within 3 days only to the 80+ or 100+ lbs treatments. Yields (number and weight) were taken on August 3 (LESREC) and August 5 (CMREC).

Results and Discussion: Nitrate levels only neared the lower part of the recommended range once when watermelon fruit ranged in size from a golf ball to a softball in the 80 lbs or 100 lbs of nitrogen treatments. Twenty pounds of urea was then applied to the 80+ and the 100+ rows only. Weight of watermelon yield was significantly greater for the 80+ and the 100+ treatments compared with the 80 or 100 lbs treatments respectively (Fig 1, Columns with different letters are significantly different from one another at the P<0.05 level, using orthogonal contrasts). The 80+ or 100+ treatments were numerically the greatest yielding treatments compared with the 200, 150 or 120 lbs of N treatments. There were no differences in the numbers of watermelon fruit between treatments with an average of 17.7 (CMREC) and 16.4 fruit per treatment. This indicates that fruit set was the same for all nitrogen treatments. The Brix concentration (the measure of the soluble solids or sugar content of a watermelon fruit; a Brix concentration of 10o is considered a good value for watermelon sweetness by the USDA) showed that at CMREC the 150 and 80+ treatments were above 10o while the 120 and 80 lbs were just below 10o (Fig 2). However at LESREC all treatments were below 10o Brix, with the 100+ treatment having the greatest Brix concentration of 9o, which was significantly greater than the Brix reading in the 150 lbs of nitrogen treatment.

Fig 1a. Precision Nitrogen Application and Watermelon Yields

Yield of Watermelons (lbs/a) at LESREC

Fig 2a. Precision Nitrogen Applications and Brix Concentration

Fig 2b. Soluble solids in watermelon

To see how quickly plants take up an application of nitrogen I measured the nitrate concentration in the leaf petiole sap of watermelon in the rows that we applied urea to and those rows that we did not. Samples were only taken from the 80 lbs and 80+ lbs of nitrogen treatments for the first 48 hours (CMREC), and in all nitrogen treatments thereafter. At LESREC nitrate readings were taken from all treatments starting 24 hrs after urea application and for the next three weeks. Nitrate concentrations increased greatly after 24 hrs and significantly at 48 hrs in the rows in which nitrogen was applied compared to the rows it was not. Nitrate concentrations were significantly greater in the 80+ treatments than the 120, 150 or 200lbs of nitrogen treatments 4 days (CMREC) and 5 days (LESREC) after urea was applied. Ten days after application there was a rapid drop in all treatments. Dotted lines in figure 3 represent the lower and upper limits of recommended nitrate concentrations in petiole sap for watermelon in Maryland.

Figs 3a and b. Precision nitrogen application and nitrogen application

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