More Frequent Irrigations

Salt concentrations increase in the soil as water is extracted by the crop. Typically, salt concentrations are lowest following an irrigation and higher just before the next irrigation. Increasing irrigation frequency maintains a more constant moisture content in the soil. Thus, more of the salts are then kept in solution which aids the leaching process. Surge flow irrigation is often effective at reducing the minimum depth of irrigation that can be applied with furrow irrigation systems. Thus, a larger number of irrigations are possible using the same amount of water.

With proper placement, drip irrigation is very effective at flushing salts, and water can be applied almost continuously. Center pivots equipped with LEPA water applicators offer similar efficiencies and control as drip irrigation at less than half the cost. Both sprinkler and drip provide more control and flexibility in scheduling irrigation than furrow systems.

Preplant Irrigation

Salts often accumulate near the soil surface during fallow periods, particularly when water tables are high or when off-season rainfall is below normal. Under these conditions, seed germination and seedling growth can be seriously reduced unless the soil is leached before planting.

Changing Surface Irrigation Method

Surface irrigation methods, such as flood, basin, furrow and border are usually not sufficiently flexible to permit changes in frequency of irrigation or depth of water applied per irrigation. For example, with furrow irrigation it may not be possible to reduce the depth of water applied below 3-4 inches. As a result, irrigating more frequently might improve water availability to the crop but might also waste water. Converting to surge flow irrigation may be the solution for many furrow systems. Otherwise a sprinkler or drip irrigation system may be required.

Chemical Amendments

In sodic soils (or sodium affected soils), sodium ions have become attached to and adsorbed onto the soil particles. This causes a breakdown in soil structure and results in soil sealing or "cementing," making it difficult for water to infiltrate.

Chemical amendments are used in order to help facilitate the displacement of these sodium ions. Amendments are composed of sulphur in its elemental form or related compounds such as sulfuric acid and gypsum. Gypsum also contains calcium which is an important element in correcting these conditions. Some chemical amendments render the natural calcium in the soil more soluble. As a result, calcium replaces the adsorbed sodium which helps restore the infiltration capacity of the soil. Polymers are also beginning to be used for treating sodic soils.

It is important to note that use of amendments does not eliminate the need for leaching. Excess water must still be applied to leach out the displaced sodium. Chemical amendments are only effective on sodium-affected soils. Amendments are ineffective for saline soil conditions and often will increase the existing salinity problem. Table 15 lists the most common amendments. The irrigation books listed under the References section present equations that are used to determine the amount of amendments needed based on soil analysis results.

Pipe Water Delivery Systems Stabilize Salinity

As illustrated in Fig. 1, any open water is subject to evaporation which leads to higher salt concentrations in the water. Evaporation rates from water surfaces often exceed 0.25 inch a day during summer in Texas. Thus, the salinity content of irrigation water will increase during the entire time water is transported through irrigation canals or stored in reservoirs. Replacing irrigation ditches with pipe systems will help stabilize salinity levels. In addition, pipe systems, including gated pipe and lay-flat tubing, reduce water lost to canal seepage and increase the amount of water available for leaching.

Table 15. Various amendments for reclaiming sodic soil and amount equivalent to gypsum.

*Suitable for use as a water or soil amendment

^†Suitable only for soil application

References

Ayres, R.S. and D.W. Westcot. 1976. Water Quality for Agriculture. Irrigation and Drainage Paper No. 29. Food and Agriculture Organization of the United Nations. Rome.

Cuena, R.H. 1989. Irrigation System Design. Prentice Hall, Englewood Cliffs, NJ. 552pp.

Hoffman, G.S., R.S. Ayers, E.J. Doering and B.L. McNeal. 1980. Salinity in Irrigated Agriculture. In: Design and Operation of Farm Irrigation Systems. M.E. Jensen, Editor. ASAE Monograph No. 3. St. Joseph, MI. 829pp.

James, D.W., R.J. Hanks and J.H. Jurinak. 1982. Modern Irrigated Soils. John Wiley and Sons, NY.

Jensen, M.E. (Editor). 1980. Design and Operation of Farm Irrigation Systems. American Society of Agricultural Engineers, St. Joseph MI. 829pp.

Longenecker, D.E. and P.J. Lyerly. 1974. B-876 Control of Soluble Salts in Farming and Gardening. Texas Agricultural Experiment Station, Texas A&M University System, College Station. June. 36pp.

Pair, C.H. (editor). 1983. Irrigation. The Irrigation Assoc., Arlington, VA. 680pp.

Rowe, D.R. and I.M. Abdel-Magid. 1995. Handbook of Wastewater Reclamation and Reuse. CRC Press, Inc. 550pp.

Stewart, B.A. and D.R. Nielsen. 1990. Irrigation of Agricultural Crops. American Society of Agronomy. 1,218pp.

Tanji, K.K. 1990. Agricultural Salinity Assessment and Management. American Society of Civil Engineers. Manuals and Reports on Engineering Practice Number 71. 619pp.

van der Leeden, F., F.L. Troise and D.K. Todd. 1990. The Water Encyclopedia. Lewis Publishers. 808pp.