David Studstill

Washing and/or Cutting Petioles Reduces Nitrate Nitrogen and Potassium Sap Concentrations in Vegetables

ABSTRACT Petiole sap testing using ion-specific electrodes is a simple method that can be used to guide in-season applications of nitrogen (N) and potassium (K) to vegetable crops. This method requires petiole sampling and sap extraction using a sap press. Because some vegetables are grown with foliar applications of N and/or K and because some crops have large petioles, petioles may need to be washed and/or cut before being pressed. Limited information is currently available on the effect of washing/cutting on sap-testing results. Hence, muskmelon (Cucumis melo L.), bell pepper (Capsicum annum L.) and tomato (Lycopersicon esculentum Mill) petioles were used to test whether washing/cutting reduced NO3 (nitrate)-N and K concentrations and changed the subsequent interpretation of plant nutritional status. Washing for 30, 60, or 120 sec in distilled water and cutting petioles before or after washing significantly reduced sap concentrations (p = 0.01 and p = 0.04 for NO3-N and K, respectively) in seven of 12 tests when compared with the control method (petioles cut and not washed). The average concentration reductions between the control and the lowest value among all the washing/cutting treatments were 30% for NO3-N and 19% for K. These losses due to washing/cutting are likely often to change the diagnosis of nutritional status from “sufficient” to “less than sufficient” and, therefore, may suggest the need for unnecessary fertilizer applications.

Depth and Width of the Wetted Zone in a Sandy Soil after Leaching Drip‐Irrigation Events and Implications for Nitrate‐Load Calculation

Abstract The quantitative assessment of nitrate‐nitrogen (NO3‐N) leaching below the root zone of vegetable crops grown with plasticulture (called load) may be done using 150‐cm‐deep soil samples divided into five 30‐cm‐long subsamples. The load is then calculated by multiplying the NO3‐N concentration in each subsample by the volume of soil (width×length×depth, W×L×D) wetted by the drip tape. Length (total L of mulched bed per unit surface) and D (length of the soil subsample) are well known, but W is not. To determine W at different depths, two dye tests were conducted on a 7‐m‐deep Lakeland fine sand using standard 71‐cm‐wide plasticulture beds. Dye tests consisted of irrigation lengths of up to 38 and 60 h, digging transverse sections of the raised beds at set times, and taking measurements of D and W in 30‐cm‐deep increments. Most dye patterns were elliptically elongated. Maximum average depths were similar (118 and 119 cm) for both tests despite differences in irrigation duration and physical proximity of both tests (100 m apart in the same field). Overall, D response (cm, both tests combined) to irrigation volume (V, L/100 m) was quadratic (Dcomb.avg=−2×10−7 V2+0.008 V+34), and W responses (using maximum and mean values at each 30‐cm increment depth, Wmax and Wmean, respectively) to D (cm) were linear (Wmax=−0.65D+114 and Wmean=−0.42D+79). Predicted Wmax were 104, 84, 64, 44, and 25 cm in 30‐cm depth increments. Load calculations using NO3‐N concentrations of 7.2, 5.0, 3.9, 3.0, and 2.9 µg/kg for the 15, 46, 77, 107, and 137 cm depths, respectively, were 21.2, 37.6, 28.2, and 39.1 kg/ha for W values of 40 cm, bed width (71 cm), Wmean, and Wmax, respectively. These load calculations ranged from simple to double based on the choice of W estimate used, which illustrates the importance of knowing W accurately when load is calculated from field measurements. These Wmax and Wmean values may be used for load calculations on sandy soils but are likely to overestimate load because they were determined without transpiring plants and may need to be adjusted for different soil types.

Scheduling Drip Irrigation for Bell Pepper Grown with Plasticulture

ABSTRACT Producing economical yields of bell pepper (Capsicum annuum L.) while conserving water and nutrients requires an integrated approach to fertilization and irrigation. Detailed fertilization recommendations exist for bell peppers grown in Florida, but current irrigation recommendations are based on historical weather data and year-to-year adjustments. The objective of this study was to develop and test a crop factor (CF) for bell peppers grown with plasticulture and irrigated daily. Crop water use (ETc) was calculated daily by multiplying CF with Class A pan evaporation (Ep). Values for the proposed CF were 0.20, 0.40, 0.80, 1.00, and 0.80 for periods 1–2, 3–4, 5–11, 12, and 13 weeks after transplanting, respectively. Daily Ep values were converted, to irrigation volumes using 10 mm Ep = 835 L/100 m of bed length. ‘Brigadier’ bell peppers were established in a factorial combination of 75%, 100%, and 125% of the recommended 224 kg N/ha rate and 33%, 66%, 100%, and 133% of the reference irrigation rate (I3) based on the proposed CF. Soil water tension (SWT) was monitored twice weekly in all the plots receiving the 100% nitrogen (N) rate. The numbers of days SWT remained below the recommended 15 kPa increased quadratically in both years as irrigation rate increased. Only the 100% and 133% I3 irrigation rates maintained SWT within the 0 to 15 kPa range at 15 and 30 cm depths on most days in 2001 and 2002. In 2001, bell pepper yields tended to increase as N rate increased. For 125% N rate, total, marketable, and fancy bell pepper yields responded quadratically to irrigation rates. Highest yields occurred between 115% and 124% of I3. In 2002, bell pepper yield did not respond to irrigation rate, and responded quadratically and linearly for N rates of 75%, 100%, and 125%, respectively. Highest bell pepper yields occurred with 125% N rate and 133% I3 irrigation rate. These results suggest that highest yields of bell pepper grown in the spring with plasticulture may be achieved with a combination of 125% of the University of Florida recommended N rate and irrigation scheduled in real time using 1.25 × CF test values of 0.25, 0.50, 1.25, 1.00, and 1.00 for the periods 1–2, 3–4, 5–11, 12, and 13 weeks after transplanting, respectively, with 10 mm Ep = 835 L/100 m of bed length.

Petiole Sap Testing Sampling Procedures For Monitoring Pumpkin Nutritional Status

With increasing fertilizer costs and the development of nutrient Best Management Practices (BMPs), pumpkin (Cucurbita spp.) growers need a way to monitor nutrient concentration during crop production. Ion specific electrodes are available to determine in the field, nitrate nitrogen (NO3-N) and potassium (K) concentrations in the plant, but currently, no recommendations exist for sampling procedures for pumpkin. Pumpkins were grown at two locations to determine if the number of petioles sampled, the part of the petiole tested, or if the presence of developing fruit on a runner, affected NO3-N and K concentrations in the sap. Nitrate and K concentrations were determined using Cardy meters on the proximal, middle, and distal sections of petioles and on increasing number (from 1 to 60) of middle petiole sections. Nitrate (p=0.17) and K (p=0.84) concentrations were not significantly influenced by petiole section. The presence of a developing fruit on the sampled runner did not significantly influenced NO3-N (963 and 913 mg/L without and with fruit, respectively; p=0.51) and K (3188 and 3000 mg/L, respectively; p=0.14) concentrations. The coefficients of variation for NO3-N and K remained below 10% for sample sizes greater than 10 petioles. Therefore, growers who want to determine NO3-N and K in pumpkin petioles should use the middle section of at least 20 petioles from the most recently, fully mature leaves from representative plants. #This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series No. R-09111.

Development of a Nitrogen Fertigation Program for Grape Tomato

ABSTRACT Grape tomatoes (Solanum lycopersicon L. var. cerasiform) have recently gained in popularity among consumers because they can be eaten without being cut, they are deep red in color, and their flavor is intense and pleasant. Current nitrogen (N) fertilization recommendations were developed for determinate tomato varieties that have a 3-month long growing season, whereas that of the indeterminate grape cultivars may be up to six months. ‘Tami G’ grape tomatoes were grown on a Lakeland fine sand at the North Florida Research and Education Center—Suwannee Valley, near Live Oak, FL in Springs 2005 and 2006 using standard plasticulture practices under 0%, 33%, 66%, 100%, 133%, and 166% of the current recommended N rate for round tomato (224 kg/ha). Due to a longer growing season in 2006, plants received an additional three weekly injections of 22 kg/ha of N each in the 100% rate, that were also proportionally applied to the other treatments. Tomatoes were transplanted March 24, 2005 and April 4, 2006 and harvested, weighed and graded five (2005) and seven (2006) times. Season marketable yield (SMY) responses to N rates were quadratic (both years P < 0.01) and highest SMY (40,340 and 36,873 kg/ha) occurred with 314 and 280 kg/ha of N in 2005 and 2006, respectively. Fruit soluble solids concentrations ranged from 6.25 to 7.5, and 7.0 to 8.3° Brix in 2005 and 2006, respectively, and were not significantly affected by N rate. These results suggest that N fertilization for grape tomato grown in Spring with plasticulture could be done by incorporating 56 kg/ha of N in the bed, followed by daily rates ranging from 0.5 to 3.5 kg/ha/day. Because the length of the growing season for grape tomato may vary, emphasis should be placed on daily N rates and irrigation management, rather than on seasonal N rate.