Effect of Cultivar on Chlorophyll Meter and Canopy Reflectance Measurements in Cucumber

Optical sensors can be used to assess crop N status to assist with N fertilizer management. Differences between cultivars may affect optical sensor measurement. Cultivar effects on measurements made with the SPAD-502 (Soil Plant Analysis Development) meter and the MC-100 (Chlorophyll Concentration Meter), and of several vegetation indices measured with the Crop Circle ACS470 canopy reflectance sensor, were assessed. A cucumber (Cucumis sativus L.) crop was grown in a greenhouse, with three cultivars. Each cultivar received three N treatments, of increasing N concentration, being deficient (N1), sufficient (N2) and excessive (N3). There were significant differences between cultivars in the measurements made with both chlorophyll meters, particularly when N supply was sufficient and excessive (N2 and N3 treatments, respectively). There were no consistent differences between cultivars in vegetation indices. Optical sensor measurements were strongly linearly related to leaf N content in each of the three cultivars. The lack of a consistent effect of cultivar on the relationship with leaf N content suggests that a unique equation to estimate leaf N content from vegetation indices can be applied to all three cultivars. Results of chlorophyll meter measurements suggest that care should be taken when using sufficiency values, determined for a particular cultiva

The Use of Chlorophyll Meters to Assess Crop N Status and Derivation of Sufficiency Values for Sweet Pepper

Chlorophyll meters are promising tools for improving the nitrogen (N) management of vegetable crops. To facilitate on-farm use of these meters, sufficiency values that identify deficient and sufficient crop N status are required. This work evaluated the ability of three chlorophyll meters (SPAD-502, atLEAF+, and MC-100) to assess crop N status in sweet pepper. It also determined sufficiency values for optimal N nutrition for each meter for pepper. The experimental work was conducted in a greenhouse, in Almería, Spain, very similar to those used for commercial production, in three different crops grown with fertigation. In each crop, there were five treatments of different N concentration in the nutrient solution, applied in each irrigation, ranging from a very deficient to very excessive N supply. In general, chlorophyll meter measurements were strongly related to crop N status in all phenological stages of the three crops, indicating that these measurements are good indicators of the crop N status of pepper. Sufficiency values determined for each meter for the four major phenological stages were consistent between the three crops. This demonstrated the potential for using these meters with sufficiency values to improve the N management of commercial sweet pepper crops

Recovery of 15N Labeled Nitrogen Fertilizer by Fertigated and Drip Irrigated Greenhouse Vegetable Crops

The stable isotope 15N was used to assess the recovery of mineral N fertilizer applied to fertigated and drip-irrigated spring muskmelon and autumn-winter sweet pepper crops grown in greenhouse soil plots. They received 92–96% of mineral N fertilizer as NO3−. 15N-labeled Ca (NO3)2 fertilizer was applied to crops during vegetative growth and fruit production phases. Crops were grown with either conventional management or combined improved N and irrigation management. Improved management for both irrigation and N was based on the combined use of models, to estimate crop requirements, and of monitoring of soil parameters. In sweet pepper, from conventional management, 15N recoveries from the 15N applications made during vegetative growth and fruit production were 66% and 58%, respectively. With improved management in sweet pepper, the corresponding 15N recoveries were 82% and 77%. In muskmelon, 15N recoveries from conventional management from the 15N applications made during vegetative growth and fruit production were 71% and 42%, respectively. With improved management, the corresponding 15N recoveries were 68% and 44%, respectively. The results demonstrated that combined drip irrigation and fertigation systems with frequent irrigation and N fertilizer application can have very high recovery of applied N fertilizer, of 77–82%

Assessing Performance of Vegetation Indices to Estimate Nitrogen Nutrition Index in Pepper

Vegetation indices (VIs) can be useful tools to evaluate crop nitrogen (N) status. To be effective, VIs measurements must be related to crop N status. The nitrogen nutrition index (NNI) is a widely accepted parameter of crop N status. The present work evaluates the performance of several VIs to estimate NNI in sweet pepper (Capsicum annuum). The performance of VIs to estimate NNI was evaluated using parameters of linear regression analysis conducted for calibration and validation. Three different sweet pepper crops were grown with combined irrigation and fertigation, in Almería, Spain. In each crop, five different N concentrations in the nutrient solution were frequently applied by drip irrigation. Proximal crop reflectance was measured with Crop Circle ACS470 and GreenSeeker handheld sensors, approximately every ten days, throughout the crops. The relative performance of VIs differed between phenological stages. Relationships of VIs with NNI were strongest in the early fruit growth and flowering stages, and less strong in the vegetative and harvest stages. The green band-based VIs, GNDVI, and GVI, provided the best results for estimating crop NNI in sweet pepper, for individual phenological stages. GNDVI had the best performance in the vegetative, flowering, and harvest stages, and GVI had the best performance in the early fruit growth stage. Some of the VIs evaluated are promising tools to estimate crop N status in sweet pepper and have the potential to contribute to improving crop N management of sweet pepper crops

Effect of cultivar on measurements of nitrate concentration in petiole sap and leaf N content in greenhouse soil-grown cucumber, melon, and sweet pepper crops

Excessive N fertilizer applications in intensive vegetable production in soil is commonly associated with appreciable N losses causing negative environmental impact. Measuring petiole sap [NO3−‒N] and leaf N content (%) are simple and practical monitoring methods to assess crop N status for improving N fertilizer management. The effect of cultivar on petiole sap [NO3−‒N] and leaf N content was evaluated. One cucumber, two melon, and two sweet pepper crops were grown in different cropping periods, with three cultivars in each crop. Three N treatments, deficient (N1), sufficient (N2) and excessive (N3) N supply, were applied by combined fertigation with drip irrigation. For a given N supply, there were often significant differences between cultivars in petiole sap [NO3−‒N] and leaf N content in cucumber, the two melon crops and one pepper crop. This was, particularly so with the sufficient (N2) and excessive (N3) N supply. In the cucumber and two melon crops, there were consistent differences in petiole sap [NO3−‒N] between cultivars in two or three of the different N treatments. In some crops, very little petiole sap [NO3−‒N] was measured with deficient (N1) N supply. In the two pepper crops, the differences between cultivars were less clear than with cucumber and melon. In general, for the three species examined, petiole sap [NO3−‒N] was subject to more consistent and larger effects between cultivars, than was leaf N. Average differences between cultivars in petiole sap [NO3−‒N] of 200‒450 mg NO3−‒N L−1 were observed during periods of 4‒6 weeks in cucumber and melon. The differences between different cultivars of the same species in petiole sap [NO3−‒N] and leaf N content, when receiving the same N supply, has implications for the practical applications of these methods for monitoring crop N status

Crop response of greenhouse soil-grown cucumber to total available N in a Nitrate Vulnerable Zone

Intensive vegetable production in soil is commonly associated with low N use efficiency (NUE) and consequently appreciable N losses that have negative environmental impacts. Improved N management practices for intensive vegetable crops require detailed knowledge of crop response to N supply. This study evaluated the effects of increasing total available N (TAN, i.e. the sum of soil mineral N at planting, N mineralized from organic matter, and mineral fertilizer N applied by fertigation) on cucumber grown in soil in a greenhouse. Parameters assessed were: yield, dry matter production (DMP), crop N uptake, nitrogen use efficiency (NUE) and potential NO3− leaching loss. The study was conducted in three growing seasons, in autumn, spring and late spring. Three commercial cultivars were examined, in the Late Spring crop, to assess possible cultivar differences. Five N treatments were applied, in the Autumn and Spring crops, as different N concentrations in nutrient solution that were applied in all irrigations throughout the crops. The applied N concentrations were N1: 0.7–1.0 mmol L-1, N2: 4.7–5.7 mmol L-1, N3: 12.1–13.8 mmol L-1, N4: 16.3–17.6 mmol L-1 and N5: 19.7–21.1 mmol L-1. The cultivar ´Strategos´ was used in both crops. Three N treatments (N1: 2.4 mmol L-1; N2: 8.5 mmol L-1and N3: 14.8 mmol L-1) were continuously applied throughout the Late Spring crop to three different cultivars (´Strategos´, ´Padrera´, and ´Mitre´). Total and marketable yield, relative to maximum value, and DMP were strongly related to TAN in linear-plateau relationships for the three growing seasons and three cultivars. Using relationships that include data from the three cropping seasons and the three cultivars, TAN values for maximum DMP, total yield, and marketable yield were 222 ± 15 kg ha−1, 221 ± 14 kg ha−1 and 228 ± 15 kg ha−1, respectively, for the Autumn, Spring and Late Spring crops. The relationships of crop N uptake to TAN, and DMP to crop N uptake, were described by a logarithmic equation. The relationship of N uptake efficiency to TAN (i.e. N uptake/TAN) was described by an exponential decay equation. Considering all crops and cultivars, these relationships were described by individual equations with R2 values of 0.75-0.96. The consistency of these relationships indicate that there are general responses of greenhouse-grown cucumber to N, which is not affected by growing season or cultivar. Measured NO3− leaching losses were low because of good irrigation management. Residual mineral N was considered to be indicative of the potential NO3− leaching loss; residual soil mineral N increased exponentially with TAN, being 196 and 330 kg N ha−1 for the highest N treatments in the Autumn and Spring crops, respectively. The information provided by this study will enable the total N supply (TAN) to be matched to cucumber crop N requirements thereby reducing excessive N supply and consequent negative environmental impacts

Sample Temperature Affects Measurement of Nitrate with a Rapid Analysis Ion Selective Electrode System Used for N Management of Vegetable Crops

The practical value of portable hand-held ion selective electrode sensors (ISE) for on-farm [NO3−] measurement to assist with crop N management of vegetable crops has been demonstrated in numerous previous studies. They provide rapid, in-situ measurement of the nitrate concentration ([NO3−]) in nutrient and soil solutions, and in petiole sap. Sample temperatures, for on-farm measurements, vary appreciably. This study evaluated the effects of sample temperature on [NO3−] measurement using two different models of a commonly used, commercially available, portable ISE meter. The temperatures (5, 10, 15, 20, and 25 °C) examined were in the range likely to be encountered in practical on-farm work. Aqueous solutions of 6, 12, and 18 mmol NO3− L−1 were prepared from KNO3, Ca(NO3)2 and NaNO3. [NO3−] was measured in three replicate samples of each of the three concentrations, made from each NO3− compound, at each temperature. The results consistently and clearly demonstrated a strong negative linear relationship between temperature-induced errors and sample temperatures. The temperature-induced error was considerable for cooled samples, being +50% at 5 °C and +31% at 10 °C. At sample temperatures of 17–20 °C, the temperature effects were minimal. Above this range, the temperature effect caused underestimation. At 25 °C, the temperature-induced error was −24%. These results show that care must be taken to ensure that sample temperatures do not erroneously affect the measurement of [NO3−] with ISE meters. Particular care needs to be taken with both refrigerated and warmer samples

Sweet pepper and nitrogen supply in greenhouse production: Critical nitrogen curve, agronomic responses and risk of nitrogen loss

Intensive vegetable production in soil is often associated with large N losses to the environment. To contribute to improved N management of sweet pepper, this work developed a critical nitrogen curve (CNC). It also developed N recommendations and examined N use efficiency (NUE) and potential NO3− leaching loss in relation to increasing total available nitrogen (TAN). TAN is the sum of the soil mineral N at planting, N mineralized from soil organic material, and mineral N fertilizer. Three sweet pepper crops were grown in soil with autumn-winter cropping cycles in greenhouse conditions. Five different N concentrations in the nutrient solution were applied throughout the crop cycle: very N deficient (N1), N deficient (N2), conventional N management (N3), excessive N (N4) and very excessive N (N5). A critical N curve of was determined for sweet pepper. Relative yield of the three crops had a strong linear-plateau relationship (R2 = 0.66) with integrated nitrogen nutrition index (NNIi). Maximum yield was associated with an NNIi of 0.86. In the three crops, total yield, dry matter production (DMP) and crop N uptake were generally strongly related to increasing TAN. An optimal TAN value (minimum TAN for maximum yield) of 425 kg N ha−1 was determined using a linear-plateau regression model. N uptake efficiency (NuptE) decreased exponentially with increasing TAN, from almost 0.90 kg kg−1 in the N1 treatment to 0.30 kg kg−1 in the N5 treatment. The sum of residual mineral N and leached NO3−–N was considered to be potential NO3− leaching loss. Potential NO3− leaching loss increased exponentially, with increasing TAN, to 686–1034 kg N ha−1 in the highest N treatments. For the optimal TAN value, NuptE was 0.63 kg kg−1 and the potential NO3− leaching was 125 kg N ha−1. The CNC and derived NNI values provide valuable information for N management of pepper. Consideration of TAN as the crop N supply enables maximum yield with less fertilizer N and less risk of N loss

Use of fluorescence indices as predictors of crop N status and yield for greenhouse sweet pepper crops

To increase nitrogen (N) use efficiency and reduce water pollution from vegetable production, it is necessary to optimize N management. Fluorescence-based optical sensors are devices that can improve N fertilization through non-destructive field monitoring of crop variables. The aim of this work was to compare the performance of five fluorescence indices (SFR-R, SFR-G, FLAV, NBI-R, and NBI-G) to predict crop variables, as dry matter production, crop N content, crop N uptake, Nitrogen Nutrition Index (NNI), absolute and relative yield, in sweet pepper (Capsicum annuum) crops grown in greenhouse. Fluorescence measurements were periodically made with the Multiplex® 3.6 sensor throughout three cropping cycles subjected to five N application treatments. The performance of fluorescence indices to predict crop variables considered calibration and validation analyses. In general, the five fluorescence indices were strongly related with NNI, crop N content and relative yield. The best performing indices to predict crop N content and NNI at the early stages of the crops (i.e., vegetative and flowering phenological stages) were the SFR indices, both under red (SFR-R) and green (SFR-G) excitation. However, in the final stage of the crop (i.e., harvest stage), the best performing indices were NBI, both under red (NBI-R) and green (NBI-G) excitation, and FLAV. The two SFR indices best predicted relative yield of sweet pepper at early growth stages. Overall, the fluorescence sensor and the fluorescence indices evaluated were able to predict crop variables related to N status in sweet pepper. They have the capacity to be incorporated into best N management practices