58, Winter 2005
Soil management for drylands
by Jeffrey C. Silvertooth
"The primary goal in a plant production system is to grow good healthy plants, and soil and water management (quantity and quality of water) are critical parts of such systems."
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There are numerous examples of poor management of arid and salt-affected soils from around the world. Soil salinity has plagued mankind for thousands of years in various locations on virtually every continent. For example, the Hohokam were an agricultural people whose civilization flourished in what is now central Arizona from about 300 BCE to roughly 1200 CE; around that time, however, many of their settlements were abandoned. One of the proposed causes for this decline and disappearance is soil salinization resulting from poor management of the soil/irrigation systems the Hohokam had developed in the Gila River Valley. And such problems still occur with regularity: around the world, it is estimated that over 6 million ha (~14 million acres) of land are lost each year due to drainage and salinization problems.
Soil salinity and sodicity are natural components of desert agricultural systems and have plagued many attempts at developing and maintaining crop production systems in arid regions. The first step in developing appropriate management schemes for dealing with saline and/or sodic soil conditions involves proper identification (Table 1).
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By definition, a saline soil is a nonsodic soil containing sufficient soluble salt to adversely affect the growth of most crop plants, with a lower limit of electrical conductivity of the saturated extract (ECe) being 4 deciSiemens per meter (dS m-1), which is equivalent to a value of 4 millimhos per centimeter (mmhos cm-1). It is important to note that this is a lower limit; soil salinity conditions are actually dependent upon the crop in question. Plants (crop plants or any others) are not equal in terms of their sensitivity or tolerance to salinity. For example, cotton is considered to be relatively salt tolerant (Table 2) compared to many other common crop plants. In contrast, lettuce, like many other vegetable crops, is rather sensitive to salinity. Therefore, salinity management for these two crops, even when grown on similar soils, will result in different water and management requirements.
To manage soil salinity effectively one must calculate and use a leaching fraction (LF) for any given crop. The LF is essentially the amount of irrigation water needed above and beyond the actual crop consumptive use requirements to remove soluble salts from the soil profile through the process of percolation under saturated soil conditions. The LF can be calculated for a specific crop and irrigation water situation as follows:
ECe = equals the tolerance level for the specific crop or plant species in question expressed as the EC (dS m-1) of the saturated paste extract of the soil, and
ECw = EC of the irrigation water being used.
Crop seedlings are particularly sensitive to soil salinity, and early stages of growth are critical with respect to salinity management. Published levels of crop/plant salinity tolerance are usually those developed for established plants. As a "rule of thumb" for most crops, salinity tolerance levels for germination and plant establishment are usually about 1/2 of the published levels for that particular crop.
Because management of soil salinity requires leaching of the soil profile, it is an issue of irrigation management. Saline soils do not require the application of soil amendments. However, since leaching requires additional irrigation water above the basic amounts needed to support crop consumptive use (CU) needs, it must be taken into account when planning for water management and allocation. For example, within a give plant production system (farm, nursery, garden, golf course, park, etc.) if only the CU needs are met, there will not be enough water for adequate leaching; this will limit both the short- and long-term sustainability of the system in question. This is true at the regional level as well: If for example a regional water district allocates only the amount of water needed to support local plant CU needs without accounting for leaching requirements, this will limit the long-term sustainability of all plant production systems within that region.
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where concentrations denoted by brackets are expressed in mmoles per liter.
Sodium is an important element in soils because it develops a large hydrated sphere and when adsorbed onto soil colloids (very small soil particles) it tends to separate the individual soil particles, resulting in their dispersion. Dispersed soil particles are a real problem because this condition leads to soil crusting, reduced water infiltration, and reduced soil aeration. Collectively, this creates very poor soil physical conditions and poor conditions for plant growth. Water management on soils that have significant exchangeable sodium levels (high SAR values) is also very difficult; very poor irrigation water efficiencies are a common result.
Management for sodic soils generally requires two steps:
Sodic soils may also require the application of soil amendments, such as gypsum (CaSO4) as a Ca source. Management of either saline or sodic condition requires proper identification of soil conditions in addition to good management of the irrigation water applied to the field in question.
The irrigation water being used is, itself, often a primary source of the soluble salts being applied to the field. Irrigation waters with ECw values of 0.7 to 3.0 dS m-1 may pose slight restrictions in use, with severe limitations being associated with waters having ECw values > 3.0 dS m-1. Therefore, not only is the quantity of available irrigation water important in a plant production system; the quality of the water is crucial, as well.
The primary goal in a plant production system is to grow good healthy plants, and soil and water management (quantity and quality of water) are critical parts of such systems. In arid regions, these factors (plants, soils, and water) and their management are especially critical to the development and management of sustainable systems over time.
Bernstein, L. 1964. Salt tolerance of plants. USDA Bulletin 283.
Maas, E.V. 1984. Salt tolerance of plants. In Handbook of plant science in agriculture, ed. B.R. Christie. Boca Raton, FL: CRC Press.
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Dr. Jeffrey Silvertooth (firstname.lastname@example.org) is Professor and Head of the Soil, Water and Environmental Science Department at the University of Arizona.
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