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Similar to chapter 2.pptx lecturenote on irrigation (20)
chapter 2.pptx lecturenote on irrigation
- 1. CHAPTERTWO: IRRIGTION WATER QUALITY • Irrigation water quality refers to the kind and amount of salts present in the water and their effects on crop growth and development. • Salts are present in variable concentrations in all waters, and the salt concentrations influence osmotic pressure of the soil solution: the higher the concentration, the greater the osmotic pressure. • Osmotic pressure in turn affects the ability of plants to absorb water through their roots. Plants can absorb water readily when osmotic pressure is low, but absorption becomes more difficult as the pressure increases.
- 2. • Even if the soil is thoroughly wet, plant roots have difficulty absorbing water when the osmotic pressure is high. When the pressure is unusually high, it may even be impossible for plants to absorb sufficient water for normal plant growth.
- 3. IRRIGATION WATER QUALITY CLASSIFICATIONS A complete water quality analysis will include the determination of: 1 the total concentration of soluble salts, 2 the relative proportion of sodium to the other cat ions, 3 the bicarbonate concentration as related to the concentration of calcium and magnesium, and 4 the concentrations of specific elements and compounds.
- 4. U.S. Salinity Laboratory Classification 1. The total concentration of soluble salts : Low-salinity water (C1) can be used for irrigation on most crops in most soils with little likelihood that soil salinity will develop. • Medium-salinity water (C2) can be used if a moderate amount of leaching occurs. • High-salinity water (C3) cannot be used on soils with restricted drainage. • Very high-salinity water (C4) is not suitable for irrigation under ordinary conditions, but it may be used occasionally under very special circumstances.
- 5. Sodium Hazard: • Low-sodium water (S1) can be used for irrigation on almost all soils with little danger of developing harmful levels of sodium. • Medium-sodium water (S2) may cause an alkalinity problem in fine- textured soils under low-leaching conditions. It can be used on coarse- textured soils with good permeability. • High-sodium water (S3) may produce an alkalinity problem. This water requires special soil management such as good drainage, heavy leaching, and possibly the use of chemical amendments such as gypsum. • Very high sodium water (S4) is usually unsatisfactory for irrigation purposes
- 6. Two types of salt problems Those associated with the total salinity and those associated with sodium. • Soils may be affected only by salinity or by a combination of both salinity and sodium. Salinity Hazard • Water with high salinity is toxic to plants and poses a salinity hazard. • Soils with high levels of total salinity are called saline soils.
- 7. • High concentrations of salt in the soil can result in a “physiologi-cal” drought condition. • That is, even though the field appears to have plenty of moisture, the plants wilt because the roots are unable to absorb the water.
- 8. Sodium Hazard • Irrigation water containing large amounts of sodium is of special concern due to sodium’s effects on the soil and poses a sodium hazard. • Sodium hazard is usually expressed in terms of SAR or the sodium adsorption ratio. • SAR is calculated from the ratio of sodium to calcium and magnesium.
- 9. • The latter two ions are important since they tend to counter the effects of sodium. • For waters containing significant amounts of bicarbonate, the adjusted sodium adsorption ratio (SARadj) is some times used. Continued use of water having high SAR leads to breakdown in the physical structure of the soil. • Sodium is adsorbed and becomes attached to soil particles. The soil then becomes hard and compact when dry.
- 10. WATER INFILTRATION RATE • Relatively high sodium or low calcium content of soil or water reduces the rate at which irrigation water enters soil to such an extent that sufficient water cannot be infiltrated to supply the crop adequately from one irrigation to the next. • An infiltration problem related to water quality occurs when the normal infiltration rate for the applied water or rainfall is appreciably reduced and water remains on the soil surface too long or infiltrates too slowly to supply the crop with sufficient water to maintain acceptable yields.
- 11. Salinity and Growth Stage • Many crops have little tolerance for salinity during seed germination, but significant tolerance during later growth stages. • Some crops such as barley, wheat and corn are known to be more sensitive to salinity during the early growth period than during germination and later growth periods. • Sugar beet and sunflower are relatively more sensitive during germination, while the tolerance of soybeans may increase or decrease during different growth periods depending on the variety
- 12. • Generally forage crops are the most resistant to salinity, followed by field crops, vegetable crops, and fruit crops which are generally the most sensitive. • forage crops field crops vegetable crops fruit crops
- 13. Salinity Management • Leaching : • Soluble salts that accumulate in Soils must be leached below the crop root zone to maintain productivity. • Leaching is the basic management tool for controlling salinity. Water is applied in excess of the total amount used by the crop and lost to evaporation.
- 14. • The strategy is to keep the salts in solution and flush them below the root zone. • The amount of water needed is referred to as the leaching requirement or the leaching fraction. • Excess water may be applied with every irrigation to provide the water needed for leaching.
- 15. How-ever, the time interval between Leaching does not appear to be critical provided that crop tolerances are not exceeded. • Hence, leaching can be accomplished with each irrigation, every few irrigations, once yearly, or even Longer depending on the severity of the salinity problem and salt tolerance of the crop. • An occasional or annual leaching event where water is ponded on the surface is an easy and effective Method for controlling soil salinity. • In some areas, normal rainfall provides adequate leaching
- 16. Subsurface Drainage Shallow water tables complicate salinity managements in see water May actually move Upward in to the root zone, carrying with it dissolved salts. Water is then extracted by crops and evaporation, leaving behind the salts. .
- 17. Shallow water tables also contribute to the salinity problem by restricting the downward leaching o f salts through the soil profile. Installation of a subsurface drainage system is about the only solution available for t his situation
- 18. Seed Placement Obtaining a satisfactory stand is often a problem when furrow irrigating with saline water. Growers sometimes compensate for poor germination by planting two or Three times as much seed as normally would be required.
- 19. 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 maintain same constant moisture content in the soil.
- 20. • Thus, more of the salts are then kept in Solution which aids the leaching process. • Surge flow irrigations Often effective at reducing the minimum depth of irrigation that Can be applied with furrow irrigation systems. • Thus, larger number of irrigations are possible Using the same amount of water.
- 21. Pre-plant 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.
- 22. Changing 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 3-4inches.
- 23. As a result, irrigating more frequently might improve water availability to the crop but might also wastewater. Otherwise a sprinkler or drip irrigations system may be required.
- 24. Chemical Amendment • In sodic soils(or sodium affected soils),sodium ions have become attached to and adsorbed on to the Soil particles. • This causes break down 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.
- 25. • Amendments are composed of sculpture 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.
- 26. • 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.