J. D. Oster

Phytoremediation of Sodic and Saline‐Sodic Soils

Sodicity-induced soil degradation is a major environmental constraint with severe negative impacts on agricultural productivity and sustainability in arid and semiarid regions. As an important category of salt-affected soils, sodic soils are characterized by excess levels of sodium ions (Naþ) in the soil solution phase as well as on the cation exchange complex, exhibiting unique structural problems as a result of certain physical processes (slaking, swelling, and dispersion of clay) and specific conditions (surface crusting and hardsetting). Saline-sodic soils, another category of salt-affected soils, are generally grouped with sodic soils because of several common properties and management approaches. Sodic and saline-sodic soils occur within the boundaries of at least 75 countries, and their extent has increased steadily in several major irrigation schemes throughout the world. The use of these soils for crop production is on the increase as they are a valuable resource that cannot be neglected, especially in areas where significant investments have already been made in irrigation infrastructure. It is imperative to find ways to improve sodic and saline-sodic soils to ensure that they are able to support highly productive land-use systems to meet the challenges of global food security.

Assessment and field-scale mapping of soil quality properties of a saline-sodic soil

Salt-affected soils could produce useful forages when irrigated with saline drainage water. To assess the productive potential and sustainability of using drainage water for forage production, a saline-sodic site (32.4 ha) in Californias San Joaquin Valley was characterized for soil quality. The objectives were (1) to spatially characterize initial soil physicochemical properties relevant to maintaining soil quality on an arid zone soil and (2) to characterize soil quality relationships and spatial variability. An initial mobile electromagnetic (EM) induction survey was conducted in 1999, with bulk soil electrical conductivity (ECa) readings taken at 384 geo-referenced locations, followed by an intensive mobile fixed-array survey with a total of 7288 geo-referenced ECa readings. Using the EM data and a spatial statistics program (ESAP v2.0), 40 sites were selected that reflected the spatial heterogeneity of the ECa measurements for the study area. At these sites, soil-core samples were taken at 0.3-m intervals to a depth of 1.2 m. Duplicate samples were taken at eight sites to study the local-scale variability of soil properties. Soil-core samples were analyzed for a variety of physical and chemical properties related to the soil quality of arid zone soils. Soils were found to be highly spatially heterogeneous. For composite soil-core samples taken to a depth of 1.2 m, ECe (electrical conductivity of the saturation extract) varied from 12.8 to 36.6 dS m−1, SAR from 28.8 to 88.8, and clay content from 2.5% to 48.3%. B and Mo concentrations varied from 11.5 to 32.2 mg l−1 and 476.8 to 1959.6 μg l−1, respectively. CaCO3, NO3− in the saturation extract, exchangeable Ca2+, Se, and As consistently had the highest coefficients of variation (CV) while pHe, ρb, and Ca2+ in the saturation extract consistently had the lowest CVs at all depths. A one-way analysis of variance (ANOVA) was used to spatially partition the local- and global-scale variability. Local-scale variability was greatest for pHe. Laboratory measurements of saturated hydraulic conductivity (Ks) were very low (0.0000846–0.0456 cm h−1), whereas field measurements were considerably higher (0.49–1.79 cm h−1). Based on the Cl− data, the leaching fraction (LF) for the entire study area was estimated to be 17%. Soil quality was reflected in yield and chemical analysis of forage. Forage Mo contents determined from newly established Bermuda grass varied from 1 to 5 mg kg−1 on a dry matter basis, and Cu/Mo ratios averaged 3.3, while forage yield in the establishment year declined with ECe, and failed to grow above ECe levels of approximately 22 dS m−1. The initial soil quality assessment of the research site indicated that the sustainability of drainage water reuse at this location would depend upon maintaining a sufficient LF with careful consideration and management of salinity, boron, molybdenum, and sodium levels.

Gypsum usage in irrigated agriculture: A review

Gypsum is the source of calcium most commonly used to reclaim sodic soils and to improve soil water infiltration when it has been decreased by low electrolyte concentration. The electrolyte concentration of the soil water and exchangeable sodium fraction,ENa, are the two predominant chemical factors that influence soil hydraulic properties. In sodic soil reclamation with gypsum, the maximum electrolyte concentration increases with increasingENa: for soil solutions in equilibrium with both gypsum and anENa of 0 and 0.4, the concentrations are 15 and 133 mol m−3. These concentrations are generally adequate to maintain the existing hydraulic conductivity. Higher concentrations can increase the soil hydraulic conductivity and the rate of reclamation. Thus, combining calcium chloride or sulphuric acid with gypsum, can reduce both the time and amount of water required to achieve reclamation.Infiltration is especially sensitive to low electrolyte concentration. Thus, surface application of gypsum or its direct addition to irrigation water, can potentially increase water infiltration rates of soils susceptible to aggregate dispersion and crusting. This is particularly true in areas where rainfall is interspersed with the application of irrigation water — as in the Midwest and Plains States of the US — or where mediterranean or monsoon climates prevail and wet and dry (irrigation) seasons alternate. As supplemental irrigation and the use of poor quality irrigation waters (sewage effluents, brackish well waters, irrigation drainage waters, etc.) intensify the need for gypsum will increase and phosphogypsum may help meet this need.Recent data indicate phosphogypsum dissolves faster than mined gypsum, and hence it develops a higher electrolyte concentration during an infiltration event. This attribute of phosphogypsum, in conjunction with its acid content, would increase its effectiveness as compared to mined gypsum for both sodic soil reclamation and maintenance of infiltration rates. The acid content of phosphogypsum is of direct benefit for reducing aggregate dispersion through promotion of soil particle flocculation and bonding by calcium, iron and aluminium released by dissolution of calcite and other soil minerals. It would decrease soil pH, thereby increasing the availability of trace metal nutrients and of phosphate which are typically deficient in sodic soils (ENa > 0.15) because of high pH. Also, the phosphate content of phosphogypsum has value as a phosphate fertilizer.Significant expansion of the agricultural use of gypsum would depend on its application in both irrigated and dryland agriculture to increase water infiltration. An annual production rate of phosphogypsum of 30 × 106 Mg (1 Mg = 1 tonne) is sufficient to treat 73 000 km2 (29 000 mi2) at a rate of 4 Mg per ha, or nearly half the total area irrigated in the USA. Although extensive farmland areas with limited rainfall exist along the Gulf of Mexico within the North American continent, market development within this area would require extensive field evaluation by local agricultural research personnel in cooperation with the phosphate fertilizer industry to determine if the economic benefits exceed the cost of phosphogypsum.

Drainage water reuse

Saline-sodic (4 < EC[dS/m] < 30; 10 <SAR < 40) drainage water can be used toirrigate crops that are moderatelysensitive, moderately tolerant and tolerantto salinity. However, in order to besustainable, particular attention isrequired towards crop selection, control ofsoil salination, and crop and soilmanagement to maintain soil permeability towater and air. Potential negative impactsof B, Mo and Se on crop yields, foragequality and wildlife also must be takeninto account and, if necessary, mitigated.The focus of this paper is the Californiaexperience, along the Westside San JoaquinValley (WSJV), related to these concerns.

Economic and agronomic strategies to achieve sustainable irrigation

The achievement of sustainable irrigation in arid regions requires greater attention to waterlogging, salinization, and degradation of ground and surface waters, which are among the problems that continue to threaten productivity and degrade environmental quality. We consider sustainability to be achieved when irrigation and drainage are conducted on-farm, and within irrigation districts, in a manner that does not degrade the quality of land, water, and other natural resources, either on-farm or throughout an irrigated region. Sustainability may also be described as maintaining the productive resources required for irrigation, so that future generations may have the same opportunity to use those resources as we do. Given the increasing importance of irrigated land for food production, the time has come when it is vital to intercept, reuse, and isolate drainage waters within the regions in which they are generated. Adoption of this strategy can be enhanced by policies that require farmers, and irrigation districts, to consider the off-farm impacts of irrigation and drainage. Such policies include linking water rights with salt rights to require the monitoring and management of both irrigation water and the salt loads in drainage waters. We review the knowledge gained since the early 1970s regarding the economic and agronomic aspects of irrigation and drainage, with a focus on drainage water reduction and sequential reuse of drainage water on salt-tolerant crops. Economic incentives that motivate farm-level and district-level improvements in water management are also reviewed. We conclude that adequate knowledge exists for implementing strategies that focus on water use and salt disposal within irrigated regions, and we recommend policies that will motivate improvements in productivity and enhance the likelihood of achieving sustainability.

Behavior of 42 crop species grown in saline soils with high boron concentrations

Abstract The coastal region of northern Chile is a desert and the salinity and boron levels in the soils can be high. The irrigation water is also saline (3–9 dS/m), with high concentrations of sodium, chloride, and boron. Despite these conditions, the irrigation of alfalfa, winter grains, and vegetables has been practiced on the alluvial soils near the rivers, since before the arrival of the Spanish in the 16th century. A field experiment was conducted in 1989 and 1990 to document the effects of irrigation on the growth and yield of 42 crop species near the city of Calama. The EC of the Loa river water used in the study was 8.2 dS/m and the boron content was 17 mg/l. This EC level exceeds the threshold salinity of most crops, and the boron level exceeds the threshold level for all crops. The crops were planted in December of 1989 and harvested the following May. Drip irrigation was used. The plant growth and crop yields of artichoke, asparagus, broad bean, red and sugar beets, Swiss chard, carrot, celery, a local variety of sweet corn, potato, prickly pear cactus, onion, shallot, spinach, were greater than expected based on published information. If separate effects of salinity and boron were additive, little or no growth would be expected for all 12 of these crops. Interactions likely occur which increase the individual tolerance coefficients for boron and salinity when a crop is exposed to both sources of stress at the same time. Foliar levels of boron may be reduced because high soil salinity levels reduce plant water uptake. The milder climate in Chile compared to that in Riverside, CA, where much of the salt and boron tolerance data has been obtained, could be partially responsible for the better crop response to salinity and boron than expected. Finally, the productivity of the local variety of sweet corn suggests that it is a more salt-tolerant variety, which has arisen as a consequence of seed selection practiced since the time irrigation began in the region which predates the 16th century.

Soil responses to sodicity and salinity: challenges and opportunities

Exchangeable sodium and low salinity deteriorate the permeability of soils to air and water. The susceptibility of soils to sodicity and low salinity depend on both the inherent properties of the soils (e.g. texture, mineralogy, pH, CaCO3, sesquioxides, and organic matter content) and extrinsic, time-dependent properties (e.g. cultivation, irrigation method and wetting rate, antecedent water content, and the time since cultivation). Whereas the effect of inherent soil properties on the soil response to sodicity has been studied and modelled, especially under laboratory conditions, the effect of soil management on the physical response of soils to sodicity has been studied very little. Consequently our ability to predict the changes in soil permeability under field conditions is limited. Including the effect of management on the physical response of soils to sodicity and low salinity is the main challenge facing researchers, consultants, and farmers.

An economic analysis of irrigation systems.

SummaryAn array of irrigation systems are available which can be broadly classified as being gravity flow or pressurized. Pressurized irrigation systems provide better control on the amount of applied water and, in most cases, better irrigation uniformity than gravity flow systems. They also have a higher initial capital cost than gravity flow systems and an analysis is required to determine whether the improved performance of pressurized systems justifies the additional costs. An economic analysis was done on several irrigation systems which included consideration of farm management costs associated with a given irrigation system, shifts in crop yield and drainage volumes associated with the optimal management of each irrigation system, and costs associated with disposal of drainage waters. Cotton was selected as the crop for analysis. Irrigation uniformity is a significant determinant to the results. Although irrigation uniformities can be highly variable based on design, maintenance and management, a typical uniformity for each irrigation system was selected. For the conditions of the analysis, gravity flow systems were calculated to be more profitable than pressurized systems if there was no constraint on the amount of drainage water generated or cost for its disposal. Imposition of costs for drainage water disposal induced a shift whereby pressurized systems became more profitable than gravity flow systems.

Comparison of Gypsum and Sulfuric Acid for Sodic Soil Reclamation

In some field and laboratory studies , H2SO4 has shown better reclamation efficiency than gypsum, but the explanation for this has been debated. We tested the hypothesis that significant amounts of HCO might be formed during the reaction of HSO3 with CaCO3 leading to additional Ca2+ in solution and enhanced displacement of exchangeble Na +. To determine if H2SO4 is more efficient than gypsum , we compared four equivalent treatments of H2SO4 and gypsum on three soils and two clay minerals. Gypsum and H2SO4 were reacted with calcareous sodic soils and clay minerals in closed-system batch studies, with CO2 pressures typical of soil root zones. Column studies comparing two equivalent treatments of H2SO4 and gypsum were conducted on one of the soils and produced results comparable to the batch studies. Soluble ions, electrical conductivities (EC), and mineral saturation indices were determined and compared between treatments . Clay dispersion and average dispersed particle size were compared between amendments...

A MODEL OF CLAY SWELLING AND TACTOID FORMATION

An electrostatic model for the stability of clay tactoids (stacks of parallel clay platelets at ~10 Å separation) in an aqueous solution has been developed. The counter ions located in the interstitial water layers are assumed to be in equilibrium with the bulk solution. Generally, the counter-ion charge density is slightly different in magnitude from the platelet charge density. Approximating the discrete charges by homogeneously charged planes, a one-dimensional potential distribution can be calculated. From this the Gibbs energy of electrostatic interaction (using single platelets as a reference) can be computed. The model predicts that clay minerals with high (vermiculite, mica) and low (pyrophyllite, talc) degrees of cationic substitution form stable tactoids. For smectites, charge density, electrolyte concentration, and counterion species determine the swelling characteristics. At a particular charge density, lower valences of the counter ions and lower electrolyte concentrations lead to increased swelling. If tactoids are formed, the number of platelets is governed by a dynamic equilibrium between electrostatic forces, van der Waals forces, and external forces, such as shear forces due to hydrodynamic flow.РезюмеРезюме—Была развита электростатическая модель стабильности глинистых тактоидов (групп параллельных глинистых пластинок, отделенных 10 Å) в водном растворе. Предполагается, что противоионы, расположенные в промежуточных водных слоях, находятся в равновесии со всем раствором. Обычно, плотность заряда противоионов отличается немножко по величине от плот-ности заряда пластинок. Одноразмерное распределение потенциала может быть вычислено, заменяя дискретные заряды равномерно заряженными площадями. Из этого можно вычислить энергию Гиббса электростатического взаимодействия (используя отдельные пластинки как базу). Модель предсказывает, что глинистые минералы с высокой (вермикулит и слюда) и низкой (пиро-филлит, тальк) степенью катионной подстановки образовывают стабильные тактоиды. Плотность заряда, концентрация электролита и противоионы определяют характеристики набухания смекти-тов. При определенной плотности заряда низшие валенции противоионов и низшие концентрации электролита приводят к увеличению набухания. Если формируются тактоиды, число пластинок определяется динамическим равновесием между силами: электростатическими, Ван дер Ваальса и внешними, такими как срезывающие силы, возникающие при гидродинамическом течении. [E.C.]ResümeeEs wurde ein elektrostatisches Modell für die Stabilität von Ton-Taktoiden (Pakete von parallelen Tonplättchen mit ~10 Å Abstand) in einer wässrigen Lösung entwickelt. Dabei wird angenommen, daß die Gegenionen im Zwischenschichtwasser im Gleichgewicht mit der Gesamtlösung sind. Im allgemeinen ist die Größe der Ladungsdichte der Gegenionen etwas verschieden von der der Lagerungsdichte der Tonschichten. Betrachtet man die einzelnen Ladungen annähernd als homogen geladene Ebenen, so kann eine eindimensionale Potential-Verteilung berechnet werden. Daraus kann die Gibbs’sche Energie der elektrostatischen Wechselwirkung berechnet werden, wobei einzelne Tonschichten als Bezug verwendet werden. Das Modell sagt voraus, daß Tonminerale mit einem hohen (Vermiculit, Glimmer) und einem niedrigen (Pyrophyllit, Talk) Kationenaustausch stabile Ton-Taktoide bilden. Bei Smektiten bestimmen Ladungsdichte, Elektrolytkonzentration und Art des Gegenions die Art der Quellung. Bei einer bestimmten Ladungsdichte führen niedrige Wertigkeiten der Gegenionen und niedrige Elektrolytkonzentrationen zu einer Zunahme der Quellbarkeit. Wenn Ton-Taktoide gebildet werden, wird die Zahl der Tonschichten durch ein dynamisches Gleichgewicht Zwischen elektrostatischen Kräften, Van der Waals’schen Kräften und äußeren Kräften, wie z.B. Scherkräften, aufgrund hydrodynamischen Fließens, bestimmt. [U.W.]RésuméOn a dévelopé un modèle électrostatique pour la stabilité de tactoïdes argile (empilements de plaquettes d’argile parallèles à ~10 Å de séparation) dans une solution aqueuse. Les contre-ions situés dans l’eau interstitiale sont supposés être en équilibre avec la solution en masse. Généralement la densité de charge du contre-ion diffère légèrement en magnitude de la densité de charge de la plaquette. On peut calculer une distribution potentielle à une dimension en approximant les charges discrètes par des plans chargés de manière homogène. A partir de ceci, l’énergie d’interaction électrostatique Gibbs (utilisant des plaquettes simples comme référence) peut être calculée. Le modèle prédit que des minéraux argile ayant de hauts (vermiculite, micas) et bas (pyrophillite, talc) degrés de substitution cationique forment des tactoïdes simples. Pour les smectites, la densité de charge, la concentration d’electrolyte, et l’espèce de contreion déterminent les caractéristiques d’enflement. A une densité de charge particulière, des valences de contre-ions plus basses, et de plus basses concentrations d’electrolyte, mènent à un accroissement d’enflement. Si des tactoïdes sont formés, le nombre de plaquettes est gouverné par un équilibre dynamique entre les forces électrostatiques, les forces van der Waals, et des forces externes telles des forces dues au flot hydrodynamique. [D.J.]