An artificial capillary barrier to improve root zone conditions for horticultural crops: physical effects on water content

Capillary barriers (CBs) occur at the interface of two soil layers having distinct differences in textural and hydraulic characteristics. The objective of this study was to introduce an artificial CB, created by a layer of gravel below the root zone substrate, in order to optimize conditions for the cultivation of horticultural crops. Potential root zone formats were analyzed with and without the gravel CBs for variables including the following: depth of CB; barrier separating the root zone from the surrounding soil; and root zone soil texture. Field and simulated results revealed that artificial CBs increased root zone water content and changed water flow dynamics. Volumetric soil water content was increased by 20–70%, depending on the soil texture and depth of the barrier. Sandy soil texture and shallower placement resulted in relatively higher water content. For sandy soil without plants, a shallow (0.2 m depth) CB increased water content of the overlaying soil by 50% compared to the control. The introduction of a gravel CB below the root zone of pepper plants (Capsicum Annum L.) lead to 34% higher matric head, 50% lower diurnal fluctuations in matric head and 40% increase in pepper fruit yield. Increasing water content by way of artificial CBs appeared to improve the water use efficiency of pepper plants. Such an improvement could lead to reduced water and fertilizer application rates and subsequent reduction in contamination below the root zone. This is especially relevant for substrates of low water-holding capacity typically used in horticulture crop production.     

Whole-tree water balance and indicators for short-term drought stress in non-bearing ‘Barnea’ olives

Drainage-weighing lysimeters allowed monitoring of water balance components of non-bearing olive (Olea europaea cv Barnea) trees over a 3-month period including short-term events of controlled but severe water stress. The objective of the study was to evaluate a variety of soil and plant-based water status and drought stress monitoring methods on the basis of tree-scale evapotranspiration (ET). As the trees entered into and recovered from water stress, meteorological data, actual ET (ET_a), soil water content and changes in leaf turgor pressure were continuously monitored. Additionally, midday measurements of stem water potential, stomatal conductance, canopy temperature, and quantum yield of PSII photochemistry were conducted. Diurnal (dawn to dusk) measurements of all the above were made hourly on days of maximum stress. Shoot elongation rate was measured for periods of stress and recovery. Quantum yield of PSII photochemistry, stomatal conductance, and stem water potential all successfully indicated reductions in whole-tree water consumption beginning at moderate stress levels. These measured parameters fully recovered to the levels of non-stressed trees soon after water application was renewed. Shoot elongation was reduced 25-30% for the 10-day period during and following drought and recovered thereafter to levels of non-stressed trees. Whole-tree ET_a was reduced by as much as 20% even following full recovery of the leaf level parameters, suggesting reduced canopy size and growth due to the stress period. Non-destructive, continuous (turgor pressure) and remotely sensed (canopy temperature) methods showed promising potential for monitoring effects of water stress, in spite of technological and data interpretation challenges requiring further attention.     

Effect of irrigation water salinity on transpiration and on leaching requirements: A case study for bell peppers

Maximization of crop yields when the salinity of irrigation water is high depends on providing plant transpiration needs and evaporative losses, as well as on maintaining minimum soil solution salinity through leaching. The effect of the amount of applied irrigation water was studied regarding transpiration, yields, and leaching fractions as a function of irrigation water salinity. Bell pepper (Capsicum annum L. vars. Celica and 7187) in protected growing environments in the Arava Valley of Israel was used as a case study crop to analyze water quantity–salinity interactions in a series of lysimeter, field and model simulation experiments. Leaching fraction was found to be highly influenced by plant feedback, as transpiration depended on root zone salinity. Increased application of saline irrigation water led to increased transpiration and yields. The higher the salinity level, the greater the relative benefit from increased leaching. The extent of leaching needed to maximize yields when irrigating with saline water may make such practice highly unsustainable.     

Long-term growth, water consumption and yield of date palm as a function of salinity

Actual measurements of water uptake and use, and the effect of water quality considerations on evapotranspiration (ET), are indispensable for understanding root zone processes and for the development of predictive plant growth models. The driving hypothesis of this research was that root zone stress response mechanisms in perennial fruit tree crops is dynamic and dependent on tree maturity and reproductive capability. This was tested by investigating long-term ET, biomass production and fruit yield in date palms (Phoenix dactylifera L., cv. Medjool) under conditions of salinity. Elevated salinity levels in the soil solution were maintained for 6 years in large weighing-drainage lysimeters by irrigation with water having electrical conductivity (EC) of 1.8, 4, 8 and 12 dS m⁻¹. Salinity acted dynamically with a long-term consequence of increasing relative negative response to water consumption and plant growth that may be explained either as an accumulated effect or increasing sensitivity. Sensitivity to salinity stabilized at the highest measured levels after the trees matured and began producing fruit. Date palms were found to be much less tolerant to salinity than expected based on previous literature. Trees irrigated with low salinity (EC = 1.8 dS m⁻¹) water were almost twice the size (based on ET and growth rates) than trees irrigated with EC = 4 dS m⁻¹ water after 5 years. Fruit production of the larger trees was 35-50% greater than for the smaller, salt affected, trees. Long term irrigation with very high EC of irrigation water (8 and 12 dS m⁻¹) was found to be commercially impractical as growth and yield were severely reduced. The results raise questions regarding the nature of mechanisms for salinity tolerance in date palms, indicate incentives to irrigate dates with higher rather than lower quality water, and present a particular challenge for modelers to correctly choose salinity response functions for dates as well as other perennial crops.     

A comparison between spatial clustering models for determining N-fertilization management zones in orchards

Precision Agriculture - Site-specific agricultural management (SSM) relies on identifying within-field spatial variability and is used for variable rate input of resources. Precision agricultural...     

An artificial capillary barrier to improve root-zone conditions for horticultural crops: response of pepper, lettuce, melon, and tomato

Capillary barriers (CBs) occur at the interface between two soil layers having distinct differences in hydraulic characteristics. In preliminary work without growing crops, it was demonstrated that CBs implemented in sandy soils increased hydrostatic volumetric water content by 20–70%, depending on soil texture and depth of barrier insertion. We hypothesized that the introduction of an artificial CB at the lower root-zone boundary of horticultural crops can increase yields as a result of increased water content and uptake efficiency. The effects of introduced CBs on soil water content, plant growth, and yields of bell peppers (Capsicum annum L), lettuce (Lactuca sativa L), tomatoes (Lycopersicon esculantum L.), and melons (Cucumis melo L.) were studied in a desert environment in southern Israel. Inclusion of a CB increased soil water content by 60% and biomass and fruit yields by 25% for pepper, and increased matric head and biomass yield by 80 and 36%, respectively, for lettuce. Neither tomatoes nor melons reacted significantly to the presence of CBs, in spite of increased soil moisture. Daily soil matric head amplitude was reduced fivefold when lettuce was grown with a CB. Spatial variability was highly reduced when a CB was present. When peppers were grown with a CB, the standard deviations of water content and biomass yield were reduced by 40% relative to control.     

The Contribution of Foliar Exposure to Boron Toxicity

Boron toxicity is commonly considered in terms of plant B uptake and accumulation. This work tested the hypothesis that foliar exposure of water with high B content leads to more severe toxicity reactions in plants as compared to exposure to high B simply through the soil solution. Growth and B uptake were studied in corn, tomatoes, onions, celery and radish where B laden water was applied with and without a component of foliar contact. Increased visual symptoms of B toxicity and decreased yields were found in plants with foliar applied B for all of the crops, while associated increased B tissue concentrations were not measured. The results imply that the relative toxicity of B entering through the leaves is greater than that of B entering via roots. Biomass reduction due to B was found to be a function of neither absolute B accumulation nor relative B mobility in the plant.     

The influence of bearing cycles on olive oil production response to irrigation

Water requirements for olive oil production and the effects of deficit irrigation were determined while considering the relative fruit loads on trees occurring as a result of biennial bearing cycles. Two Israeli olive (Olea europaea) varieties (Barnea and Souri) were evaluated for growth and yield parameters in a 4-year field study where five relative irrigation rates were applied. Increasing irrigation increased stem water potential, vegetative growth, and olive fruit yield with the increases tapering off at application rates reaching 75–100% of potential crop evapotranspiration. Tree water status, growth, and fruit characteristic parameters were highly affected by both fruit load and by irrigation level. Oil yield increases as a function of increased irrigation were initiated for each cultivar only following an ‘off’ season when the treatments lead to higher vegetative growth. The increased oil yields as a function of increased irrigation were primarily explained by higher tree-scale capacity for carrying fruit, especially as irrigation alleviated measureable water stress. For the Barnea cultivar in ‘on’ years, a secondary effect due to increased oil per fruit as irrigation increased was evident, particularly at the higher application rates.     

The effects of crop load and irrigation rate in the oil accumulation stage on oil yield and water relations of ‘Koroneiki’ olives

The interactions between irrigation rates applied during the oil accumulation stage and crop load were studied in a six-year-old very-high-density Koroneiki (Olea europaea L.) orchard. Five irrigation rates, determined as thresholds of midday stem water potential, were applied from July 1st until harvest in 2008 and 2009 and from July 1st to the end of September in 2010. Oil yield increased with increasing crop load in all the irrigation treatments. Oil yield did not respond to increasing irrigation at very low crop load and the higher the crop load the higher the response to irrigation. There was no response to irrigation at the lowest crop loads, but the higher the irrigation rate the higher the oil yield at high crop loads. The predicted commercial oil yield at common fruit counts increased from 1.99 t/ha at the lowest irrigation rate to 3.06 t/ha at the highest irrigation rate. Stomatal conductance decreased with decreasing stem water potential but leveled off at 30–60 mmol m^?2 s^?1 at stem water potential values lower than ?4.0 MPa. High crop load increased stomatal conductance and decreased stem water potential relative to low crop load at low and medium irrigation rates. The effect of crop load on water relations became evident by the end of August and was well pronounced at the beginning of October. Physiological and irrigation water management implications related to the interactions between tree water status and crop load are discussed.