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Arnold W. Schumann

Fertilizer spreaders capable of variable rate application are increasingly important for enhancing nutrient management in horticultural crops because they improve placement and increase nutrient uptake efficiency. Matching applied fertilizer to fertilizer requirements represents a significant input cost saving for the grower and a reduction in potential pollutant loading to ground and surface water. Variable rate fertilization (VRF) is a precision agriculture technology made possible by embedded high-speed computers, accurate Global Positioning System (GPS) receivers, Geographic Information Systems (GIS), remote sensing, yield or soil maps, actuators, and electronic sensors capable of measuring and even forecasting crop properties in real time. For tree crops like Florida citrus (Citrus spp.), the most important function of the VRF spreader is to detect and avoid fertilizing spaces of the orchard not occupied by trees. Treeless spaces are becoming more common in Florida as diseases such as citrus greening (Candidatus Liberibacter asiaticus) and canker (Xanthomonas axonopodis) cause the removal of thousands of trees every year. VRF works best under those conditions. Because VRF exploits crop and soil variability, it has no value in a perfectly uniform field. VRF enables smaller trees including resets to be fertilized at lower, most appropriate rates, thus minimizing any excess application. This article examines the existing knowledge on using precision agriculture and variable rate technology to keep water and nutrients in the root zone of horticultural crops, thus facilitating maximum uptake efficiency.

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Juan Carlos Melgar, Arnold W. Schumann and James P. Syvertsen

We determined if frequency of application of irrigation water plus fertilizer in solution (fertigation) could modify root and shoot growth along with growth per unit nitrogen (N) and water uptake of seedlings of the citrus rootstock Swingle citrumelo growing in a greenhouse. In the first experiment, all plants received the same amount of water with sufficient fertilizer N but in three irrigation frequencies applied in 10 1.5-mL pulses per day, one 15-mL application per day, or 45 mL applied every 3 days. Plants irrigated at the highest frequency grew the least total dry weight and had the highest specific root length. Plants with lowest irrigation frequency grew the most and used the least water so had the highest water use efficiency. There were no irrigation frequency effects on relative growth allocation between shoot and roots, net gas exchange of leaves, or on leaf N. A second experiment used identical biweekly irrigation volumes and fertilizer rates, but water and fertilizer were applied using four frequency combinations: 1) daily fertigation; 2) daily irrigation with fertilizer solution applied every 15 days; 3) fertigation every 3 days; or 4) irrigation every 3 days and fertilizer solution applied every 14 days. Total plant growth was unaffected by treatments, but the highest frequency using the lowest fertilizer concentration grew the greatest root dry weight in the uppermost soil depths. Roots grew less and leaf N was highest when N was applied every 15 days, implying that root N uptake efficiency was increased when fertigated with the highest fertilizer concentration. All plants had similar water use efficiencies. A third experiment was conducted with irrigation every 3 days and with four different N application frequencies: every 3, 6, 12, or 24 days using four fertilizer concentrations but resulting in similar total N amounts every 24 days. There were no differences in growth, gas exchange, or water use efficiency. Given the fact that all treatments received adequate and equal amounts of water and fertilizer, fertigation frequency had only small effects on plant growth, although very high frequency fertigation decreased N uptake efficiency.

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Davie M. Kadyampakeni, Kelly T. Morgan, Arnold W. Schumann and Peter Nkedi-Kizza

Citrus (Citrus sp.) root length density (RLD) can help in understanding and predicting nutrient and water uptake dynamics. A study was conducted at two sites in Florida to investigate root and water distribution patterns among different irrigation and fertigation systems. The results over the 2 years showed that RLD was highest in the 0- to 15-cm soil depth and decreased with depth for all treatments at both sites. About 64% to 82% of the fibrous roots (<1 mm diameter) were concentrated in the irrigated zones of drip- and microsprinkler-irrigated trees and 18% to 36% were found in the nonirrigated zones at the Spodosol site (SS). At the Entisol site (ES), the RLD (<0.5 mm diameter) in the 0- to 15-cm depth soil for intensive microsprinkler or drip irrigation was 3- to 4-fold (nonirrigated zone) and 4- to 7-fold (irrigated zone) greater at the 0- to 15-cm soil depth than that for conventional irrigation system. The trees at SS were symptomatic for Huanglongbing (HLB; Candidatus Liberibacter asiaticus) in the second year, while those at ES were asymptomatic throughout the study. This might have limited the density and extent of root distribution at SS. The water contents remained either close to or slightly above the field capacity. The results showed higher RLD for intensive irrigation and fertigation practices in irrigated zones compared with conventional grower applications suggesting greater water and nutrient uptake potential for the former.

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Kirandeep K. Mann, Arnold W. Schumann, Thomas A. Obreza and Jerry B. Sartain

Citrus production in Florida is commonly affected by a high degree of spatial variability of soils. Therefore, this study developed rapid indicator crop bioassays to evaluate the relationships between indicator crops and citrus production at various soil depths. A citrus grove was divided into five productivity zones based on existing tree canopy volume using GIS software (“very poor,” “poor,” “medium,” “good,” and “very good”). Visual ratings of percentage cover were collected from each zone using a 1-m2 quadrant. Six random soil samples were collected between the tree rows from each productivity zone at four depths (0 to 15, 15 to 30, 30 to 45, and 45 to 60 cm). Greenhouse bioassay experiments used sorghum and radish crops grown in soil sampled from four depths. Overhead photographs of potted radish plants were captured periodically with a SLR digital camera to calculate leaf area by image processing. Shoot weights, shoot length, root weights, and leaf nutrient concentrations were measured at harvest (56 and 21 days after germination for sorghum and radish, respectively). Germination, shoot length, and shoot weight of sorghum and radish were significantly affected by the productivity zone. Sorghum (0 to 30 cm), radish (0 to 45 and 0 to 60 cm) and weed cover were strongly correlated (r ≈0.50 to 0.60***) with citrus yield and canopy volume at the lower two depths. The strong relationships (r > 0.50***) of sorghum and radish shoot weights and weed cover with soil properties at greater depths demonstrated the important role of cumulative root zone depth of 60 cm in differentiating citrus productivity. These results revealed that citrus production in poor areas of the grove was limited by the shallow depth of productive soil, and citrus productivity could be successfully mapped using indicator crop bioassays with soil samples taken at multiple depths.

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Rhuanito S. Ferrarezi, Alan L. Wright, Brian J. Boman, Arnold W. Schumann, Fred G. Gmitter and Jude W. Grosser

Completely enclosed screen houses can physically exclude contact between the asian citrus psyllid [ACP (Diaphorina citri)] and young, healthy citrus (Citrus sp.) trees and prevent huanglongbing (HLB) disease development. The current study investigated the use of antipsyllid screen houses on plant growth and physiological parameters of young ‘Ray Ruby’ grapefruit (Citrus ×paradisi) trees. We tested two coverings [enclosed screen house and open-air (control)] and two planting systems (in-ground and container-grown), with four replications arranged in a split-plot experimental design. Trees grown inside screen houses developed larger canopy surface area, canopy surface area water use efficiency (CWUE), leaf area index (LAI) and LAI water use efficiency (LAIWUE) relative to trees grown in open-air plots (P < 0.01). Leaf water transpiration increased and leaf vapor pressure deficit (VPD) decreased in trees grown inside screen houses compared with trees grown in the open-air plots. CWUE was negatively related to leaf VPD (P < 0.01). Monthly leaf nitrogen concentration was consistently greater in container-grown trees in the open-air compared with trees grown in-ground and inside the screen houses. However, trees grown in-ground and inside the screen houses did not experience any severe leaf N deficiencies and were the largest trees, presenting the highest canopy surface area and LAI at the end of the study. The screen houses described here provided a better growing environment for in-ground grapefruit because the protective structures accelerated young tree growth compared with open-air plantings while protecting trees from HLB infection.

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Rhuanito S. Ferrarezi, Alan L. Wright, Brian J. Boman, Arnold W. Schumann, Fred G. Gmitter and Jude W. Grosser

Huanglongbing (HLB) disease is a threat to most citrus (Citrus sp.) producing areas and is associated with the bacterium Candidatus Liberibacter asiaticus. The disease is transmitted by the vector asian citrus psyllid [ACP (Diaphoria citri)]. Antipsyllid screen houses can potentially reduce and eliminate HLB development in young citrus plantings by excluding the insect vector. These structures are also anticipated to represent a new environmental platform to cultivate high-valued fresh citrus. The purpose of this investigation was to evaluate the effect of screen houses on excluding infective ACP from inoculating grapefruit (Citrus ×paradisi) trees and determine changes on environmental conditions caused by the screen cloth. We tested two coverings [enclosed screen house and open-air (control)] and two planting systems (in-ground and container-grown), with four replications arranged in a split-plot experimental design. Psyllid counting and HLB diagnosis were performed monthly, and the antipsyllid screen excluded the HLB vector from the houses. ACP and HLB-positive trees were found only at the open-air plots. Weather monitoring was performed every 30 minutes from 22 Feb. to 31 July 2014. Solar radiation accumulation averaged 6.7 W·m−2·minute−1 inside the screen houses and 8.6 W·m−2·minute−1 in the open-air. Air temperature was greater inside the screen houses whereas wind gusts were higher in the open-air. Reference evapotranspiration accumulation averaged 3.2 mm·day−1 inside the screen houses and 4.2 mm·day−1 in the open-air. There was no difference in cumulative rainfall between screen houses and open-air. The antipsyllid screen houses reduced solar radiation, maximum wind gust, and reference evapotranspiration (ETo). The environmental conditions inside the protective screen houses are suitable for grapefruit production.