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Optical sensors are a promising approach for assessing nitrogen (N) status of vegetable crops. However, their potential may be undermined if time of day influences measurements. This study evaluated the effects of time of day and N addition on measurements, made with two chlorophyll meters, SPAD-502 and MC-100, and two active canopy reflectance sensors, GreenSeeker handheld and Crop Circle ACS-470. Three treatments (N1, deficient, N2, conventional, and N3, excessive N application) of N concentration in the nutrient solution were applied by fertigation throughout a sweet pepper crop grown in soil in a greenhouse. Time of day of 12:00 and 15:00 h had an effect on measurements made with the SPAD-502, but only in the N1 treatment, suggesting that the effects of time of day were related to crop N status. This effect was slight, being 1.7 ± 0.02 SPAD units lower at 12:00 and 15:00 h compared to at 9:00 h (relative decrease of 3.6%). For the MC-100, a slight increase in Chlorophyll Content Index (CCI) values of 3.3 ± 0.1 units (relative increase of 6.3%) was observed at 15:00 and 18:00 h, relative to CCI values at 9:00 h, regardless of N treatment. The time of day effect on chlorophyll meters appears to be negligible in relation to the wide range of values measured in greenhouse-grown sweet pepper. Normalized Difference Vegetation Index, measured both with the GreenSeeker and Crop Circle, and Green Normalized Difference Vegetation Index, measured with the Crop Circle, were not affected by time of day in any of the N treatments, showing that these sensors and indices can be used with confidence at any time of the day.

A rapid analysis ion-selective electrode (ISE) system for measurement of [NO3−] in nutrient solution (NS), soil solution (SS) and petiole sap (PS) was evaluated. For each material, there were 797–2010 samples from 5 to 6 different crops, and from 2 to 4 different species. Accuracy was evaluated by linear regression (LR) with laboratory analysis (automated colorimetry, Cd reduction), and by relative error (RE), the average percentage deviation from laboratory analysis. For NS, the LR was y = 0.982x + 0.76, R² = 0.962 (n = 2010), for combined data from 5 crops (3 pepper, 2 cucumber). For SS, the LR was y = 0.975x + 1.13, R² = 0.965 (n = 797), for combined data from 5 crops (3 pepper, 2 cucumber). For undiluted PS, the LR relationship was y = 0.742x + 168.02, R² = 0.892 (n = 1425), for combined data from 6 crops (3 pepper, 2 cucumber, 1 melon). The underestimation was most pronounced at [NO3−] of &gt, 1500 mg NO3−–N L−1. For diluted petiole sap (dilution by 10 for pepper and melon, 5 for other species), the LR relationship was y = 1.010x + 99.26, R² = 0.927 (n = 1182), for combined data from 6 crops (2 pepper, 2 cucumber, 1 melon, 1 tomato). RE values for all measurements in composite datasets were 14%, 22%, 24% and 25% for NS, SS, undiluted PS and diluted PS respectively, and they were lower in concentrations most likely to be measured in practical on-farm work. The ISE system measured [NO3−] in NS, SS and diluted PS with sufficient accuracy to effectively guide on-farm decision making.

Excessive N application occurs in greenhouse vegetable production. Monitoring methods of immediately available soil N are required. [NO3&minus, ] in soil solution, sampled with ceramic cup samplers, and [NO3&minus, ] in the 1:2 soil to water (v/v) extract were evaluated. Five increasing [N], from very N deficient (N1) to very N excessive (N5) were applied throughout three fertigated pepper crops by combined fertigation/drip irrigation. The crops were grown in soil in a greenhouse. Soil solution [NO3&minus, ] was measured every 1‒2 weeks, and extract [NO3&minus, ] every 4 weeks. Generally, for treatments N1 and N2, both soil solution and extract [NO3&minus, ] were continually close to zero, and increased with applied [N] for treatments N3&ndash, 5. The relationships of both methods to the nitrogen nutrition index (NNI), an indicator of crop N status, were assessed. Segmented linear analysis gave R2 values of 0.68&ndash, 0.70 for combined data from entire crops, for both methods. NNI was strongly related to increasing [NO3&minus, ] up to 3.1 and 0.9 mmol L&minus, 1 in soil solution and extracts, respectively. Thereafter, NNI was constant at 1.04&ndash, 1.05, with increasing [NO3&minus, ]. Suggested sufficiency ranges were derived. Soil solution [NO3&minus, ] is effective to monitor immediately available soil N for sweet pepper crops in SE Spain. The extract method is promising.

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.

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 cultivar

Two improved management packages of tillage, irrigation, and nitrogen (N) management were compared with conventional management (CM), in a sweet pepper crop, in a plastic greenhouse in southeastern Spain. Crops were grown in a layered “enarenado” soil. The two improved packages included prescriptive and corrective irrigation management using, respectively, the PrHo program and tensiometers, and prescriptive N management using the VegSyst-DSS program. In both improved packages, soil was tilled to 10 cm prior to transplanting after temporarily removing the sand mulch layer. In one of the improved packages, the “reduced input and tillage plus compost” (RIT + C), compost was incorporated during tillage. In the other improved package, the “reduced input and tillage” (RIT), no compost was added; otherwise, management was identical in both improved packages. Total volumes of applied irrigation and of drainage, and the total amounts of applied N were reduced in RIT and RIT + C compared to CM. However, the RIT and RIT + C packages were associated with slightly less fruit production than CM. This was attributed to higher root zone salinity and an apparent slight N deficiency. Biomass, fruit production and root growth were lowest in RIT + C, which were attributed to salts added in the compost. Relative to the untilled CM soil, soil tillage in the RIT package reduced soil bulk density and favoured deeper root growth. Compared to RIT, compost addition in RIT + C was associated with less root growth presumably because of higher salinity. To optimize N management, the use of N monitoring during the crop (i.e. corrective management of N) is required when prescriptive N management is being used. Long term practices of tillage and organic matter addition may be required to appreciably improve soil quality in this agricultural system. Care may have to be taken to ensure that excessive quantities of salts are not added when applying compost.

Chlorophyll meters are a promising approach for monitoring crop N status of intensively-produced vegetable crops. To do so effectively, it is fundamental that the nature and strength of the relationships between chlorophyll measurement and actual crop N status and yield, throughout the whole crop cycle, be evaluated. Another fundamental requirement for the practical use of chlorophyll meters for crop N monitoring is the availability of sufficiency values or ranges, for a given crop, that indicate N deficiency (below the value) or sufficiency (above the value). The SPAD-502 meter was evaluated as an estimator of crop N status and yield in cucumber, and sufficiency values of SPAD measurements were derived for maximum crop growth and for yield. Two crops were grown in soil in a greenhouse in Almeria (SE Spain), in different cropping cycles, in Autumn 2013 (from early September to late November) and in Spring 2014 (from early March to late May). Chlorophyll measurements were made on a weekly basis throughout the crop cycle and relationships were established between measurements and the Nitrogen Nutrition Index (NNI), i.e. the ratio between actual and critical crop N contents, and yield. Sufficiency values for maximum growth were based on the relationships with NNI and sufficiency values for maximum yield were based on linear-plateau relationships with yield. Relationships of SPAD measurements with crop NNI and yield had average coefficients of determination (R2) of 0.59 and 0.55, respectively, for most of the Autumn crop, and R2 values of 0.84 and 0.83, respectively, for most of the Spring crop. In both crops, relationships were weak in the initial vegetative phase and were stronger in the subsequent reproductive and harvest phases. Generally, there were small differences between the sufficiency values for maximum growth and those for yield, and between the three phenological phases. The average sufficiency value for all phenological phases for both maximum growth and maximum yield was 45.2 ± 0.7 SPAD units. This study showed that measurements of the SPAD-502 meter can be used as reliable indicators of crop N status and yield throughout most of the cycle of cucumber in Autumn and Spring growing seasons. The development and application of sufficiency values for maximum growth and maximum yield may improve N management of fertigated cucumber crops with frequent N application, by providing information for adjustments in N fertilization when SPAD measurements deviate from sufficiency values.

Large nitrogen (N) fertilizer applications are a feature of intensive vegetable production systems, and optimal N management is required to maximize N use efficiency and minimize N losses. Vegetation indices (VIs) of canopy reflectance, measured with proximal sensors, are generally strongly related to crop N status. For practical application, sufficiency values that distinguish between N deficiency and sufficiency are required. In this work, sufficiency values of VIs for maximum crop growth and for yield were determined for two cucumber crops grown in contrasting seasons (Autumn and Spring). Canopy reflectance was measured with a Crop Circle ACS-470 sensor. Sufficiency values for maximum growth were based on the relationships of VIs with the Nitrogen Nutrition Index (NNI), i.e. the ratio between actual and critical crop N contents. Sufficiency values for maximum yield were based on linear-plateau relationships of yield with VIs. Strong relationships were obtained between all VIs and both NNI and yield for most of the weekly measurements during both crops. For NNI, best-fit relationships were linear, quadratic, power or exponential, and had coefficients of determination (R2) of 0.61–0.98. For yield, most linear-plateau relationships between yield and VIs had R2 values of 0.47–0.89. VIs based on reflectance in green and red edge had slightly better relationships with NNI and yield than VIs in the red, with the Green Normalized Difference Vegetation Index (GNDVI) and the Green Ratio Vegetation Index (GRVI) being the most sensitive and consistent indices for estimating both crop NNI and yield. Relationships between VIs and NNI and yield for all weekly measurements of each crop, and for the two crops combined, were also analyzed to provide unique sufficiency values for maximum growth and yield that applied to the entire crop cycle of each crop and of both crops considered together. Overall, there were slight differences between sufficiency values for maximum growth and for maximum yield and the unique sufficiency values were generally intermediate to the weekly sufficiency values. This study demonstrated the potential for using VIs for monitoring crop N nutrition and yield in cucumber. The calculated sufficiency values of VIs may facilitate the use of proximal optical sensors in farming practice for optimal N management through periodic monitoring for deviation from sufficiency values.

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 Almer&iacute, a, 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.

Aim of study: Soil compaction causes an increase in bulk density, resistance to penetration, low diffusion of oxygen and water in the soil. Tillage is one of the techniques to alleviate compaction. The objective of this work was to evaluate the effects of tillage on sweet pepper grown in greenhouse soil.Area of study: The experimental work was conducted in a plastic greenhouse at the Experimental Station of the University of Almería (SE Spain).Material and methods: The soil was ploughed with a single pass with ripper to 15 cm depth and with rotavator to 10 cm depth. The control treatment was soil untilled. Crop dry matter production and root length growth and density of sweet pepper were evaluated, in addition to soil characteristics such as bulk density, resistance to penetration and soil matric potential.Main results: Tillage reduced soil bulk density from 1.70 to 1.60 kg L-1 in the 10-40 cm of soil depth. There was a notable reduction in irrigation (12%), total N applied (13%), drainage (91%) and N leaching (95%) in the tillage treatment. However, tillage did not improve significantly crop dry matter production and yield. The absence of tillage effect is possible due to a slight reduction in the bulk density of the soil.Research highlights: The tillage treatment produced a notable reduction in irrigation, total N applied, drainage and N leaching when compared to the control.

Sustainable N management of intensive vegetable crops requires accurate and timely on-farm assessment of crop N status. Proximal fluorescence-based sensors are promising tools for monitoring crop N status, by providing non-destructive optical measurements of N-sensitive indicator compounds such as chlorophyll and flavonols. The ability of the Multiplex ® fluorescence sensor to determine crop N status was evaluated in two indeterminate cucumber crops grown in contrasting seasons (autumn and spring). Three fluorescence indices, leaf chlorophyll (SFR) and flavonols (FLAV) contents, and their ratio (Nitrogen Balance Index, NBI) were evaluated, and their consistency between the two crops compared. Actual crop N status was assessed by the Nitrogen Nutrition Index (NNI), calculated as the ratio between the actual and the critical crop N contents (i.e., the minimum N content for maximum growth). There were strong relationships between each of SFR, FLAV and NBI with crop NNI, for most weekly measurements made throughout the two crops. For the three indices, coefficients of determination ( R 2 ) were mostly 0.65–0.91 in the autumn crop, and 0.71–0.99 in the spring crop. SFR values were generally comparable between the two crops, which enabled the derivation of common relationships with NNI for individual phenological phases that applied to both cropping seasons. FLAV and NBI values were not comparable between the two crops; FLAV values were appreciably higher throughout the spring crop, which was attributed to the higher solar radiation. Consequently, phenological relationships of FLAV and NBI with NNI were established for each individual cropping season. Our findings suggested that SFR was the most consistent index between cropping seasons, and that NBI was the most sensitive index within each season. Regardless of the index and crops, all fluorescence indices were weakly related to crop NNI during the short vegetative phase while stronger relationships were found in the reproductive and harvest phases. This study also showed that the three fluorescence indices were sensitive to and able to distinguish deficient from optimal crop N status, but that they were insensitive to discriminate optimal from slightly excessive N status. Overall, this study demonstrated that fluorescence indices of chlorophyll content (SFR), flavonols content (FLAV) and nitrogen sufficiency (NBI) can be used as reliable indicators of crop N status in cucumber crops; however, there was variability in FLAV and NBI values between cropping seasons and a lack of sensitivity in the range of optimal to slightly excessive crop N status.

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 % N c = 4.71 × D M P - 0.22 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.

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.

Intensive vegetable production is commonly associated with excessive nitrogen (N) fertilization and associated environmental problems. Monitoring of crop N status can enhance crop N management. Chlorophyll meters (CMs) could be used to monitor crop N status because leaf chlorophyll (chl) content is strongly related to crop N status. To monitor crop N status, relationships between CM measurements and leaf chl content require evaluation, particularly when excessive N is supplied. The SPAD-502 meter, atLEAF+ sensor, MC-100 Chlorophyll Concentration Meter, and Multiplex sensor were evaluated in sweet pepper with different N supply, throughout the crop, ranging from very deficient to very excessive. CM measurements of all sensors and indices were strongly and positively related to leaf chlorophyll a + b content with curvilinear relationships over the entire range of chl measured (approximately 0–80 μg cm-2). Measurements with the SPAD-502, and atLEAF+, and of the Multiplex’s Simple Fluorescence Ratio index (SFR) had asymptotic responses to increasing leaf chl. In contrast, the MC-100’s Chlorophyll Content Index (CCI) had a progressively increasing response. At higher chlorophyll a + b contents (e.g. >40 μg cm-2), SPAD-502, atLEAF+ and SFR measurements tended to saturate, which did not occur with CCI. Leaf chl content was most accurately estimated by CCI (R2=0.87), followed by the SPAD-502 meter (R2=0.85). The atLEAF+ sensor was the least accurate (R2=0.76). For leaf chl estimation, CCI measured with the MC-100 meter was the most effective of the four sensors examined because it: (1) most accurately estimated leaf chl content, and (2) had no saturation response at higher leaf chl content. For non-saturating leaf chl content (approximately 0–40 μg cm-2), all indices were sensitive indicators. As excessive applications of N are frequent in intensive vegetable crop production, the capacity of measuring high leaf chl contents without a saturation response is an important consideration for the practical use of chlorophyll meters.

Optimal nitrogen (N) management is essential for profitable vegetable crop production and to minimize N losses to the environment that are a consequence of an excessive N supply. Proximal optical sensors placed in contact with or close to the crop can provide a rapid assessment of a crop N status. Three types of proximal optical sensors (chlorophyll meters, canopy reflectance sensors, and fluorescence-based flavonols meters) for monitoring the crop N status of vegetable crops are reviewed, addressing practical caveats and sampling considerations and evaluating the practical use of these sensors for crop N management. Research over recent decades has shown strong relationships between optical sensor measurements, and different measures of crop N status and of yield of vegetable species. However, the availability of both: (a) Sufficiency values to assess crop N status and (b) algorithms to translate sensor measurements into N fertilizer recommendations are limited for vegetable crops. Optical sensors have potential for N management of vegetable crops. However, research should go beyond merely diagnosing crop N status. Research should now focus on the determination of practical fertilization recommendations. It is envisaged that the increasing environmental and societal pressure on sustainable crop N management will stimulate progress in this area.