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United States Department of Agriculture

Agricultural Research Service

Research Accomplishments
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Since its inception, the U.S. Salinity Laboratory has been the world's premier research institution in the field of saline and sodic soils and irrigation water quality.
Initial research efforts focused on the diagnosis of saline and sodic soils and the development of reclamation techniques, with subsequent research emphasizing the management of saline and sodic soils for crop production. More recently the research focus has shifted to preservation of water quality of both surface and ground waters. Current research includes ecological concerns such as toxic element and pesticide pollution. Throughout its history, the Salinity Lab's research has always addressed problems pertinent to irrigation agriculture.
Criteria for Diagnosing Saline and Sodic Soils
Laboratory personnel established the criteria for diagnosing saline and sodic soils. Electrical conductivity (EC) of the soil saturation extract was introduced as a practical index of soil salinity. The threshold ECe value of 4 dS/m is still being used world wide to diagnose and classify saline soils. A threshold of 15 for the exchangeable sodium percentage (ESP), indicating soil sodicity and permeability and structural problems, also remains the standard diagnostic criterion used throughout the world.
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Suitability of Water for Irrigation
Key practical diagnostic criteria used to evaluate a water's suitability for irrigation and its potential for degrading soils were developed at the Salinity Laboratory. These include electrical conductivity (EC), sodium adsorption ratio (SAR), adjusted SAR, and boron (B) hazard. Electrical conductivity is the universal standard measure of a water's salinity hazard used world wide. Sodium adsorption ratio is also a universal standard and indicates a water's potential to cause sodic conditions and poor soil structure. Both of these indicators are critical for management decisions and together constitute the basis of a classification system for waters with respect to their salinity and sodicity hazard, developed by the Laboratory. Adjusted SAR was developed to correct the measure of sodium hazard for the tendency of calcium carbonate to precipitate from irrigation waters and to improve the appraisal of water quality, prediction of potential infiltration problems, and subsequent management decisions. The above concepts, pioneered by Laboratory personnel, are advocated in an FAO paper that provides water quality guidelines used around the world, especially in developing countries. Laboratory researchers developed the carmine colorimetric method for B analysis in irrigation waters. This standard procedure for measuring this element often present in toxic concentrations in irrigated agriculture which has been used for many years in laboratories throughout the world. A series of papers by Laboratory personnel established the relationship between soil ESP and SAR. This research provided an easier, more practical way to diagnose sodic soils and to establish their management requirements.
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Understanding Plant Responses to Salinity
In a series of papers Plant Science group researchers established the theory of osmotic adjustment which forms the basis for understanding plant response to salinity. This research established that the adverse effect of salts on plants is a general stress and not usually due to a specific ion toxicity. The Laboratory has been at the forefront of determining the boron and salt tolerance of enumerable plant species. A landmark Laboratory study quantified all available salt tolerance data by presenting threshold salinity values for yield decrease and linear yield decrease per unit of salinity. Thus a given crop's response to salinity can be describe using only two variables, thereby simplifying the selection of an appropriate crop for waters and soils of a given salinity. Salt tolerance tables, thresholds, and yield responses are provided in all manuals and handbooks dealing with crop production on saline soils and/or with saline waters and are used world-wide.
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Salt Balance and Leaching Requirements
Laboratory personnel instituted the concepts of salt balance and leaching requirements. The salt balance is the difference between the salt input and the salt output for a given irrigation project, and is used to evaluate the adequacy of drainage facilities, leaching programs, and water requirements for removing salts, and sustaining irrigation in general. This method is still used in monitoring programs by many irrigation projects. The leaching requirement establishes the fraction of irrigation water that must be leached through the root zone to maintain an acceptable level of salinity for cropping purposes. Comprehensive leaching requirement studies carried out by Laboratory personnel found that past recommendations for leaching requirements were excessive and that plants grew well under much lower leaching fractions. These findings evolved into the minimum leaching concept, an important breakthrough that allows reductions in salt loading without yield decrement, thus preserving scarce water resources. A series of innovations in high frequency irrigation advocated by Laboratory personnel allowed additional reductions in leaching fraction. Minimized leaching concepts developed by the Laboratory were at the core of the water quality control measures adopted for implementation to control salinity of the Colorado river. Improved water management practices pioneered by the Laboratory allowed a reduction in size of the world's largest desalting plant in Yuma, Arizona by a factor of two, resulting in tremendous cost savings.
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Reclamation of Saline and Sodic Soils
Laboratory scientists have been at the forefront in developing reclamation procedures and guidelines for saline and sodic soils. To reclaim saline soils, leaching strategies especially continuous ponding and intermittent ponding were developed by Laboratory scientists and are universally used. To reclaim sodic soils, the Laboratory pioneered the use of the soil amendments: gypsum, sulfuric acid, sulfur, and calcium chloride to replace exchangeable sodium along with leaching. The gypsum requirement, the amount of amendment required to affect reclamation of a given amount of exchangeable sodium, was developed at the Salinity Laboratory and is the universally-used reclamation standard. To reclaim sodic soils of very low permeability, a high salt-water-dilution method was developed at the Laboratory. This method drastically shortened the reclamation time which was expected to take years to a few days and was accomplished without addition of an amendment. Laboratory personnel established that use of sulfuric acid in combination with gypsum or calcium chloride could reduce the time and leaching needed to achieve reclamation and reduce amendment cost as compared to the use of gypsum alone. Use of sulfuric acid could also accelerate the reclamation process of high boron soils.
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Plant Growth Stages and Salinity Tolerance
Laboratory research established that plants exhibit differences in salinity tolerance at various growth stages. The information was exploited by the chemistry group to devise a cyclical strategy where good quality water was used for growth of sensitive crops and during sensitive growth stages, while saline drainage water was used for the growth of tolerant crops or during tolerant growth stages. A large-scale field experiment demonstrated that crop yield and quality could be sustained while conserving water, thereby minimizing the off-site pollution potential of drainage water disposal, and maintaining soil tilth and permeability. Reuse strategies advocated by the Salinity Laboratory have served as models for numerous reuse experiments in other locations around the world. The U. S. Bureau of Reclamation and the California Resources Agency have adopted minimized leaching and drainage water reuse concepts pioneered at the Laboratory, to conserve water, minimize drainage volumes, and protect water quality as the heart of the San Joaquin Valley Drainage Program.
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Measuring Bulk Soil Electrical Conductivity
Laboratory research has resulted in development of unique instruments and associated technology for sensing bulk soil electrical conductivity. A porous matrix salinity sensor was developed which imbibes soil water and measures its electrical conductivity directly. A commercially available salinity sensor is based on the design developed at the Laboratory. A four-probe electrode system and an inductive electromagnetic methodology were developed at the Laboratory to allow soil salinity measurements in situ, thereby eliminating the need for taking soil samples in the field and determining their salinity in the Laboratory. Both of these units are now commercially produced and are in use throughout the world. An added advantage of the electromagnetic induction instrument is that the measurements can be made without soil-instrument contact, making access holes for electrodes unnecessary. Tractor-mounted versions of the four-probe electrode system and self-propelled automated electromagnetic induction systems have recently been developed at the Laboratory; this equipment allows automated on-the-go salinity measurements and data logging with spatial referencing. This breakthrough technology has exponentially increased the amount of salinity measurements that can be taken and drastically reduced the manpower involved. It also allows salinity to be determined at a level of detail never before possible by soil sampling. With these methodologies researchers can measure, monitor, and map soil salinity, detect the presence of shallow water tables, assess the adequacy of leaching and drainage practices for soil salinity control, and identify areal diffuse sources of salt loading from irrigation. Changes in salinity can be determined readily so that the impacts of management practices can be evaluated and the sources of salt loading and pollution pinpointed. Scientists studying volatile pesticides have developed a wide variety of sampling equipment which can be used to determine the concentration or volatilization rate of volatile compounds in the near-surface atmosphere, and include atmospheric sampling masts, flux chambers and soil gas sampling equipment.
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Development of Methods for Measuring Soil Properties
Historically, an early strength of the Laboratory was in the development of experimental methods for measuring soil properties. An apparatus was developed to measure modulus of rupture as an index of soil crusting. Since soil crusting critically influences seedling emergence, the measurement method is important for evaluating soil treatments, amendments, and management practices. Laboratory personnel also developed the pressure plate apparatus still used to measure water contents at very low soil water potentials, thermocouple psychrometry methods for making soil water potential measurements, and heat dissipation sensors for measuring matric potential in situ. All of these measures of soil water content are still in use today and provide important knowledge of the amount of water available for plant growth. The design for the heat dissipation sensor has been commercialized. Laboratory researchers similarly pioneered experimental methods for measuring hydraulic conductivity of soils such as the one-step outflow and constant flux methods, among other approaches. Hydraulic conductivity of a soil is a measure of its ability to transmit water and is a critical parameter necessary to describe water entry into soil, movement of water to plant roots, water flow to drains, and evaporation of water from the soil surface. A recent experimental advancement at the Laboratory was the development of a time-domain reflectometry (TDR) method for the simultaneous determination of both soil water content and soil salinity. Time-domain reflectometry can be used to make rapid measurements in the field without the uncertainties resulting from two separate measurements and is an effective tool for research. A recent innovation in this area is the surface TDR probe, which allows the water content at the soil surface to be determined, with important implications on measuring rates of volatilization of pesticides from soils.
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Describing the Movement of Water, Salts, and other Chemicals with Models
Many pioneering advances in mathematical analyses of water flow were carried out at the Laboratory. The so-called Richard's equation was developed by L.A. Richard of the Salinity Laboratory. This mass balance equation describing the dynamics of water flow in saturated and unsaturated soils is an integral part of all transport models in use to this day. Additional work by Laboratory personnel provided simplified mathematical analyses of water flow in unsaturated soils. Recent theoretical advances occurred in the development of analytical and numerical models for describing the movement of water, salts, and other chemicals in soils and groundwaters. Hydraulic properties of unsaturated soils were described and predicted by new theoretical equations developed at the Laboratory. This information is necessary to accurately describe water flow and transport in numerical models.
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Last Modified: 12/13/2006
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