Flooded soil and Irrigation water quality.pptx

Flooded soil quality

1. Flooded Soils

2. Physical Changes Upon flooding, the pore spaces in soil - saturated with water . T he soil swells & hard clods soften and break into small aggregates. Puddling completely destroys the remaining structural aggregates (clods and clumps) and transforms the soil into a sludge, or soupy mixture . This slows the drying of the soil , since the exchange of air between the atm & soil is impeded, and since the water particles are held by soil particles and prevented from percolating downward and escaping.

3. Biological Changes A bsence of soil air in flooded soils causes a change in the varieties of microbes, or microscopic organisms which live in the soil. Microbes - absence of oxygen -- anaerobic microbes --- slower, less efficient decomposers of organic matter than aerobic cousins. Consequently, the rate of decay of organic matter tends to be slow in flooded soils. E nd products produced by anaerobic decomposition differ, some are toxic to rice , particularly those released during the first two weeks after decomposition begins.

4. Chemical Changes Flooded soils develop two distinct chemical zones . The upper zone, a thin 1-10 mm, absorbs O 2 from the water , turns brown in color, and reacts to nitrogen like an unfl oo ded soil. This zone is called the oxidized zone . The lower zone, which extends down as far as the water, is extremely low in available oxygen , turns dark blue or gray in color, and takes on chemical properties quite different from those of the oxidized layer above . This lower zone is known as the reduced zone .

5. When a soil is flooded, N in the incorporated plant (and animal) residues is changed to NH 4 , which is stable under flooded conditions and will later be used by growing rice plants. If the soil is allowed to dry thoroughly (e.g., when it is drained for plowing), a microbacteriological change takes place during which the NH 4 form of N is changed to NO 3 form . When the soil is later re-flooded , part of the N held in the NO 3 form is changed into N 2 , escapes into the air. Between 20-700 kg/ha of N can be lost through this process, known as denitrification , so it is extremely important to keep the Plot thoroughly flooded at all times after initial irrigation has taken place.

6. Iron Toxicity R esults - chemical imbalances in soil and in p lant severely inhibits the yields . S ymptoms - bronzing of leaves and stems , generally occurs 4-7 weeks after transplanting. R oots are stunted, coarse and reddish brown or dark brown in color (a coating of iron oxides reduces root surface and decreases capacity to absorb soil nutrients ). In extreme cases , black or brown spots appear on leaves and stems, and leaf edges turn dark brown and roll in toward the midrib.

7. Treatment of iron toxicity D eepening peripheral gutters in sandy swamps (to decrease percolation of iron compounds down from the uplands) and improving drainage in peaty swamps (the mineral slicks in peaty swamps are often associated with iron toxicity). Fertilization with large amounts of P & K can be effective , although application of nitrogenous fertilizer generally aggravates the symptoms (since N is relatively available already & the presence of N in the absence of P induces the symptoms). S election of iron-toxicity-resistant varieties .  

8. Irrigation Water Quality

9. It is essential resource for better growth of plants and to get better yields. Poor quality water - water logging, salinity and sodicity problems in soils. Q uantity and quality of water available in rivers, tanks and ground water - influenced by rainfall. Generally rainwater is as pure as distilled water since it is practically devoid of any salts. Only in certain industrial areas the rainwaters during the first showers may contain some pollutants like NO 3, SO 2 etc., depending upon the nature of industries functioning in that area.

10. In agriculture, the quality of irrigation water - decides the plant growth and soil fertility status than the quantity of water available for irrigation. Irrespective of the source, whether it is surface water or ground water, it contains some amount of soluble salts. The continuous use of irrigation water of varying quality in terms of its higher salt content (saline), high electrical conductivity and Sodium Adsorption Ratio (Saline-Sodic) or high Residual Sodium Carbonate (Sodic water) affect the physical and chemical properties of soil over a long period of time. Ultimately these soils become unfit for any agricultural operation . Hence it is essential to assess the quality of irrigation water before using them.

11. Criteria for evaluation of irrigation water Water quality is determined according to the purpose of use . I rrigation water - criteria used include salinity, sodicity and element toxicities . O ther criteria includes taste, colour, odour , turbidity, temperature, hardness, pH, BOD or COD, nutrient elements like N, P etc., and other pathogenic organisms .  

12. EC at 25 o C ( dS m -1 ) Salinity Class Remarks < 0.25  Low ( C1) Low salinity water - can be used for irrigation with most crops on most soils. 0.25 – 0.75   Medium ( C2)   Medium salinity water - can be used if a moderate amount of leaching occurs. Moderate salt tolerant crops can be grown. 0.75 – 2.25   High ( C3) High salinity water - cannot be used on soils with restricted drainage. Special management for salinity control may be required. Good salt tolerant crops can be grown. >2.25 Very High ( C4) Very high salinity water - unsuitable for irrigation under ordinary conditions. Soil must be permeable with good drainage. Salt tolerant crops can be grown. Irrigation water to be applied in excess. Salinity hazard or total soluble salt concentration It can be classified in terms of EC and expressed as dS m -1 at 25 o C. Based on EC, the following classification of waters are made as given by USDA Classes C 1 and C 2 - suitable (no problem) for irrigat ion . Classes C 3 and C 4 - not suitable for irrigation purpose (severe problem)

13. The effect of salt on crop growth is largely osmotic in nature . Higher proportion of soluble salts in soil solution increases its osmotic potential and thereby decreases water absorption, leading to physiological drought. If excessive quantities of soluble salts accumulate in root zone , the crop has difficulty in extracting water from the salty solution, thereby affecting the yields adversely.

14. Total dissolved solids (TDS) – ppm Salinity hazard and effects on management < 160 Very low hazard. No detrimental effects on plants and no soil build up expected 160 -480 Low hazard. Sensitive plants may show stress; moderate leaching prevents salt accumulation in soil 480 – 1280 Medium hazard. Salinity may adversely affect plants. Requires selection of salt tolerant plants, careful irrigation, good drainage and leaching 1280 – 1920 Medium-high hazard. Requires careful management to raise crops >1920 High hazard. Generally unacceptable for irrigation, except for very salt tolerant plants when there is excellent drainage, frequent leaching and intensive management. Total dissolved solids (TDS) EC in dS m -1 X 640 = Total Soluble Salt Content (ppm)

15. Sodicity hazard High conc. of Na is undesirable in water because Na adsorbs onto the soil cation exchange sites, causing soil aggregates to break down (deflocculation) sealing the pores of the soil and making it impermeable to water flow. The tendency of Na to increase in proportion on the cation exchange sites at the expense of other types of cations is estimated by the ratio called Sodium Adsorption Ratio (SAR).

16. Sodium hazard Sodicity class SAR value Remarks Low S 1 0 -10 Little or no hazard Medium S 2 10-18 Appreciable hazard, but can be used with proper management High S 3 18-26 Unsatisfactory for most of the crops Very high S 4 > 26

17. Na hazard expressed through SAR does not take into account of the anionic composition. Adjusted SAR – account the removal of Ca and Mg in their precipitation with HCO 3 and CO 3 ions in the irrigation water added. U sed to predict sodicity hazard more correctly for those water which contained appreciable amounts of HCO 3 Adjusted SAR should be evaluated for water which has EC > 1.5 and < 3.0 dSm -1 . Adjusted SAR

18. pHc = (pk 2 ’ – pkc ’) + p[HCO 3 - ] + p[Ca 2+ + Mg 2+ ] (pk 2 ’ – pkc ’) = Concentration of Ca 2+ , Mg 2+ and Na + ions in mel -1 p(HCO 3 - ) = Concentration of carbonate and bicarbonate in mel -1 p(Ca 2+ +Mg 2+ ) = Concentration of Ca 2+ and Mg 2+ ions in mel -1

19. Puri’s salt Index (PSI) This explains the relationship between Na and Ca conc . in waters. It is used for predicting sodium hazard. PSI = (Total Na + - 24.5 ) - (Total Ca 2+ - Ca as CaCO 3 ) x 4.85 Where all the quantities are being expressed in ppm If negative : Good quality water Positive : Poor quality water and harmful for irrigation.

20. RSC value (meL -1 ) Water quality < 1.25 Water can be used safely 1.25-2.5 Water can be used with certain management >2.5 Unsuitable for irrigation purposes Alkalinity hazard - Residual Sodium Carbonate (RSC) HCO 3 occur in low salinity water and its conc. usually decreases with an increase in EC. P roportion of HCO 3 ions higher than Ca ions is considered to be undesirable , because after evaporation of irrigation water HCO 3 ions tend to precipitate Ca ions. Hence the effect of HCO 3 together with CO 3 ions is to be evaluated through RSC. RSC is used to evaluate the quality of irrigation water and is expressed in meL -1 . RSC (meL -1 ) = (CO 3 2- + HCO 3­ - ) – (Ca 2+ + Mg 2+ )

21. RSBC value (meL -1 ) Water quality < 3 Water can be used safely 3-9 Water can be used with certain management >9 Unsuitable for irrigation purposes Residual Sodium Bicarbonate (RSBC) HCO 3 - anion is important in irrigation water. This brings about a change in the SSP in the irrigation water and therefore an increase of the Na hazard. Since CO 3 ions do not occur very frequently in appreciable conc. and because HCO 3 ions do not precipitate Mg ions, Gupta suggested that alkalinity hazard should be determined through the index called RSBC. RSBC = HCO 3 - - Ca 2+ , all ions expressed as mel -1

22. Soluble sodium percentage This explains proportion of sodium in relation to total cations in water Soluble sodium percentage (SSP) = Na + x 100 Ca 2+ + Mg 2+ + Na + where all the soluble cations are in meqL -1 . Irrigation waters having SSP of 60 and above are harmful .

23. Potential salinity This refers to judging quality of irrigation waters considering Cl and SO 4 ion concentration in waters. Potential salinity (PS) = ½ SO 4 2‑ + Cl ‑ If PS between 5 and 20 m.e L -1 – can be used only in sandy soils If PS between 3 and 5 m.e L -1 – can be used in medium textured soils. If PS between 1-3 m.e L -1 - can be used in fine textured soils .

24. Permeability Index (PI) = Na +  HCO 3 X 100 Ca + + Mg + Na Permeability Index This refers to proportion of sodium and bicarbonate in relation to cations in water. Magnesium adsorption ratio This refers to magnesium content in relation to total divalent cations , since Mg adsorption by soils affect their physical properties. A harmful effect on soils appears when Ca: Mg ratio declines below 50. Mg 2+ Mg adsorption ratio = Ca 2+ + Mg 2+ Magnesium hazard on irrigation water - Mg:Ca ratio more than 1 . If Permeability Index is 60, it is unsuitable for irrigation

25. Sodium ratio Na + Sodium ratio = Ca 2+ + Mg 2+ (Concentration in meqL -1 ) The application of SAR to the group of water which have EC >5 dSm -1 and Mg / Ca ratio > 1 is obviously questionable. For the ground water having EC greater than 5 dSm -1 and dominance of magnesium over calcium, SAR is calculated as Na + / Ca 2+ The classification of SAR / SCAR was given by Gupta (1986) and he proposed following six classes of sodicity . Sodium to calcium activity ratio (SCAR) For good quality water, the ratio should not exceed one.

26. Class Particulars Ratio 1 Non sodic water <5 2 Normal water 5-10 3 Low sodicity water 10-20 4 Medium sodicity water 20-30 5 High sodicity water 30-40 6 Very high sodicity water >40

27. Specific ion toxicities Besides the above parameters, concentration of specific ion if exceeds certain limits in water then the water may be unsuitable for irrigation. Excess of any ion (CO 3 , HCO 3 Cl, SO 4 , NO 3 , B, Na, F) including heavy metals like nickel, lead, cadmium etc is injurious to plants. Boron is excessively available is some locations which makes the water quality poor.

28. Chloride concentration ( m.e /L) Water quality <4 Excellent 4-7 Moderately Good 7-12 Slightly suitable 12-20 Not suitable for irrigation purposes >20 Not suitable for irrigation purposes Cl - + NO 3 - Chloride concentration ( m.e /L) = CO 3 2-- + HCO 3 - + SO 4 2-- + Cl - + NO 3 - Chloride

29. Magnesium hazard Magnesium hazard will be noticed if Mg: Ca ratio is more than 1.0 Sulphate Eaton proposed three classes of sulphate < 4 meq L -1 : Excellent water 4-12 meq L -1 : Good to injurious > 12 meq L -1 : Injurious to unsatisfactory Sulphate salts are less harmful compared to chlorides . This is because, when both the ions occur in this conc. only half of the sulphates get precipitated as CaSO 4 while the other half remains in soluble forms as Na-MgSO 4 in the soil.

30. Boron class Boron concentration (ppm) Remarks Sensitive crops Semi Tolerant crops Tolerant crops Very low <0.33 <0.67 <1.00 Can be used safely Low 0.33-0.67 0.67-1.33 1.00-2.00 Can be used with management Medium 0.67-1.00 1.33-2.00 2.00-3.00 Unsuitable for irrigation purpose High 1.00-1.25 2.00-2.50 3.00-3.75 Very high >1.25 >2.50 >3.75 Boron Boron is essential for the normal growth of the plant, but the amount required is very small.

31. Fluorine Unlike Cl , fluorides are only sparingly soluble & are present in most natural water in only small amounts. The concentration of fluoride ranges from traces to more than 10.0 ppm in ground water, although surface water do not exceed 0.3 ppm, unless they are polluted from other sources. <1 ppm = Good >1 ppm = Problematic

32. . Nitrate concentration (meqL -1 ) Very frequently groundwater contains high amount of nitrate . When such water is used continuously for irrigation, physical properties of soil gets affected. < 5 : No problem 5 – 30 : Intensity of problem is moderate >30 : Intensity of problem is severe.  Lithium Lithium is an important trace metal which may found in saline ground waters and irrigated soils. It has been found that small concentration of 0.5 to 1 ppm produce toxic effects on the growth of citrus crops. Saline soils of varying degrees found in India contain upto 2.5 ppm Li .