Please Don't Pass the Salt - Preventing and Managing Soil Salinity

Transcription

1 10 Please Don't Pass the Salt - Preventing and Managing Soil Salinity Curtis Cavers, Manitoba Agriculture & Food, Soils & Crops Branch, Box 1149 Carman, MB R0G 0J0 ccavers@gov.mb.ca Background: Soil salinity is a condition where plant growth is reduced due to the presence of soluble salts in soil which holds water more tightly than the ability of plants to extract water from the soil. As a result, many plants will exhibit symptoms of droughtiness, but the soil moisture is often relatively moist. Conditions required for soil salinity to occur: - presence of soluble salts in subsoil, groundwater or both; - high water tables (< 6 ft from the surface) that can carry soluble salts into the root zone of the soil; - plants that are susceptible to soil salinity (pulses, vegetables and oilseed crops most sensitive) Figure 1. Origin of soil salinity.

2 11 Salinity can occur in several different forms. The most common type of salinity is that which is described above, having an excess amount of salts of any type in the soil, limiting the availability of water to plants. If sodium salts are the dominant type of salts present, a relatively small amount of sodium salts can negatively affect soil structure and create a sodic soil condition (aka "alkali", "gumbo", etc.) Salt-affected soils can occur locally (only a few square feet in size, scattered over a given landscape) or regionally (large areas several acres in size). Depending on moisture conditions, these areas can increase in size or intensify in salt concentration. Overall outcomes are primarily dependent on the movement, salt content and depth of groundwater. There are no quick or easy solutions to soil salinity. Saline soils can only be reclaimed by leaching the salts down and out of the root zone. This involves adopting water management practices which improve drainage and promote the downward movement of salts through the soil. Measurements to quantify soil salinity: Total Dissolved Solids (TDS) - the total mass of all dissolved ions per unit volume of water (mg/l). A direct measure of soluble salt content in water. Electrical Conductivity (EC) - an index of a soil water extract to infer the presence of soluble salts within the soil. As the concentration of soluble salts increases, the EC of the soil extract also increases (ds/m). TDS = 0.7 X 1000 X EC or EC = 1.4 X X TDS (assumes soil bulk density of 1.1 g/cm 3, saturated moisture content of 40% and soil depth of 0-12") Sodium Adsorption Ratio (SAR) - the ratio of sodium (Na) to calcium (Ca) and magnesium (Mg) in water, calculated as: SAR = [Na + ]/[[Ca ++ + Mg ++ ]/2] 0.5 where cation concentrations units are mmol(+)/l, or meq/l. Soil water extracts with SAR values >13 are indicative of a soil with a sodium problem. High concentrations of Na relative to Ca and Mg results in dispersion of clay particles, soil structural breakdown, soil pore blockage and hard, dense subsoil. These conditions restrict root growth and penetration, reduce infiltration rates and increase erosion potential. Irrigation waters with SAR > 8 pose a potential problem for medium and fine-textured soils receiving the water. SAR values for irrigation water should be < 4 to be considered safe. Consult soils report: - Using detailed soils information (if available), look for indicators of salinity: - Possibility of salinity in soil series description - N and D subclasses in agriculture capability rating - Look for the following symbols on the soil map which indicate saline phases of a given soil series (Table 1).

3 12 Table 1. Soil survey information as indicators of salinity. Soil phase symbol (4 th letter) xxxx xxxs xxxt xxxu Interpretation non-saline (0-4 ds/m) slightly saline (4-8 ds/m) moderately saline (8-15 ds/m) strongly saline (>15 ds/m) - If no detailed soils information available, consult aerial or infrared photos. Saline areas are usually easily detectable and observation of a series of photos over time may indicate changes in salinity patterns. Site visit: - symptoms: poor crop growth; light gray or white colours on soil surface; impassable when wet; growth of salt-tolerant weeds (foxtail barley, kochia, Russian thistle, oak-leaved goose foot). - detection & measurement: a) salinity can be measured indirectly using inductive electromagnetic (EM 38 ) instruments. These instruments measure the apparent conductivity of the ground in ms/m to a depth of about 4 ft in the vertical mode and about 2 ft in the horizontal mode. Because the EC values obtained from the EM 38 are affected by soil texture, soil moisture and soil temperature, calibration of these values with EC values from saturated pastes is required for each salinity investigation site (McKenzie et al, 1989). Table 2. EM 38 data from two field investigations on soybean performance (Manitoba Agriculture & Food, 2002). Soybean Stand Health Soil Type Depth (in) Laboratory EC (ds/m) EM 38 Reading (vertical/horizontal) Good Lakeland Clay loam /80 Marginal Poor (salinity) Poor (wetness) Good Marginal Poor Lakeland Clay loam Lakeland Clay loam Lakeland Clay loam Osborne Heavy clay Osborne Heavy clay Osborne Heavy clay N/A / / / / / /325

4 13 - b) Conduct diagnostic soil testing of affected and unaffected areas. Request analyses for: ph, EC, SAR, soluble cations (Ca ++, Mg ++, Na + ) and soluble anions (Cl -, SO 4 =, HCO 3 - ). - c) Install observation wells and piezometers. Observation wells provide quick inspection of the critical depth of water table, which is less than 6 ft (2 m) from the surface. Piezometers, constructed in close proximity to an observation well, indicate water table movement as recharge (net downward) or discharge (net upward). - d) Confirm whether the type of salinity is primary (natural) or secondary (induced by human activities). Use hydrochloric acid (HCl) to test for the presence of carbonates. The presence of salt crystals can be confirmed by visual inspection, and salt crystals do not react with HCl. Salts are more mobile than carbonates and should be the first component to be leached out of the soil profile under long term recharge conditions. If this is not the case, then refer to the information in Table 3. Table 3. Determination of primary or secondary salinity (Eilers, 1988). Observation Water Movement Comments Carbonates closer to soil surface Recharge site well drained soils than soluble salts net downward movement salts in upper root zone may indicate restricted drainage Carbonates and soluble salts roughly same depth in soil Carbonates and soluble salts roughly same depth in soil BUT concentrated highest near soil surface Soluble salts closer to soil surface than carbonates Laboratory analysis: below Static imperfectly drained soils lack of soil profile development beyond surface carbonates and salts usually near surface little water movement through soil Long term, continuous groundwater discharge (primary salinity) Current groundwater discharge (secondary salinity) obvious accumulations of surface salts soils lack profile development imperfectly to poorly drained net upward movement indicates resalinization strong indication that a significant increase in local water table levels has occurred relatively recent change from net downward to net upward movement of water Many soil testing laboratories use a 1:1 soil:water mixture to extract salts for rapid EC analysis. The standard laboratory practice for EC determination is the saturated paste method, which is more accurate but also more time consuming. The saturated paste method adds enough water to soil to completely saturate the soil sample to more completely extract the salts, but not so much that the water/salt solution becomes diluted. To properly interpret salinity results in Tables 4 and 6, multiply 1:1 EC results by 2 to approximate saturated paste results.

5 14 Interpretations: Table 4. Salinity interpretations. Non-saline Saline Sodic Saline-sodic EC (ds/m) * 0-2 (non-saline) 2-4 (slightly saline for sensitive crops) > 2 (for pulses and vegetable crops) > 4 (for all other crops) < 4 > 4 SAR < 13 < 13 > 13 > 13 Other Normal crop features growth * ds/m = ms/cm=mmho/cm Salt crystals at or near soil surface when dry; little or no plant growth Shiny black when wet; dull grey, hard & cracked when dry; little or no plant growth; ph may be > 8.6 Any combination of these features may be present The above values should be used as a guide for determining the presence and intensity of salinity in soil. These values are not absolute; rather, increases in soil salinity at or near these critical values cause progressive changes in crop performance. The effects of soil salinity are affected somewhat by soil texture, organic matter content, soil moisture, etc. Recommendations: For primary salinity, leave in natural state; re-establish salt-tolerant vegetation if necessary. For secondary salinity: Table 5. Salinity control options according to landscape position (Eilers, 1988). Control Types Recharge Areas Discharge Areas Vegetative high moisture use crops salt tolerant crops continuous cropping Mechanical surface drainage tile drainage salt leaching 1. Reduce summerfallow reduce evaporation (through crop establishment or addition of crop residues) 2. Improve drainage lower the water table; promote leaching of salts ("The only real reclamation procedure for saline soils is to drain the excess water off the bottom and pour fresh water on the top to flush the salts out and away". - Les Henry, Grainews, 1990) 3. Seed crops that have high moisture use requirements (alfalfa, perennial grasses, corn, sunflowers, winter wheat) in groundwater recharge areas this will help prevent water table from rising in groundwater discharge areas. 4. Select salt tolerant crops for groundwater discharge areas in order to reduce evaporation and use soil water. Since crops will not root into the water table (saturated soil), drainage may be required for crops to establish. Refer to crop tolerance to salinity information as cited in Tables 6 and Use manure and crop residues to provide additional organic matter to the soil. Surface residues reduce evaporation and movement of salts into the soil profile. Although increased organic matter does not cure salinity, it improves water infiltration, water holding capacity and tilth of the soil. Apply tons/acre of solid manure once every 3-4 years to saline areas.

6 15 6. Avoid deep tillage on saline soils to prevent bringing leached salts back to the soil surface. Zero tillage should be considered for strongly saline soils. For sodic soils, deep tillage may be beneficial to break up the hardpan and improve infiltration, as well as to bring any calcium salts present in the subsoil to the surface. A field investigation should be conducted before attempting deep tillage. Table 6. Effect of Root Zone Salinity on Crop Productivity of Selected Crops (Carter, 1981). Crop Salinity Threshold (saturated paste EC, mmho/cm) % Productivity Decrease per mmho/cm Increase Alfalfa Barley Beans Birdsfoot Trefoil Clover - red Corn - grain Fescue Flax Potatoes Perennial ryegrass Soybeans Strawberry Wheat Wheatgrass - Crested Wheatgrass - Tall Table 7. Relative Salt Tolerance of Manitoba Crops (adapted from McKenzie, 1988). EC Tolerance Field Crops Forages Vegetables Trees/shrubs (ds/m) High (16) Tall wheatgrass Russian wildrye Slender wheatgrass Sea buckthorn Silver buffaloberry Moderate (8) Low (4) 6-row barley 2-row barley Fall rye Winter wheat Spring wheat Oats Flax Canola Sunflowers Soybeans Corn Peas Field beans Birdsfoot trefoil Sweetclover Alfalfa Bromegrass Crested wheatgrass Intermediate wheatgrass Meadow fescue Reed canarygrass Timothy White Dutch clover Alsike clover Red clover Garden beets Asparagus Spinach Tomatoes Broccoli Cabbage Potatoes Carrots Onions Strawberries Raspberries Russian olive Poplar Apple Common lilac Manitoba maple Colorado blue spruce Cottonwood Birch 7. Seed shallow; seed early. Crop tolerance to salinity is influenced by stage of growth (Table 8).

7 16 Table 8. Salt Tolerance at Two Stages of Growth (Soils '84). Crop Growth Stage Germination Established Barley High High Fall rye High Moderate Wheat Moderate Moderate Alfalfa Low Moderate Corn Moderate Low Field beans Very low Very low 8. Use recommended fertilizers, according to soil test information. There are no quick fixes to cure soil salinity by adding chemicals, commercial soil conditioners, etc. Recommended forage mixture for saline soils (see Field Crop Production Guide): - 5 lb/ac tall wheatgrass, 5 lb/ac slender wheatgrass, 3 lb/ac alfalfa, 3 lb/ac sweetclover (hay) - 5 lb/ac tall fescue, 3 lb/ac alfalfa, 3 lb/ac sweetclover (hay) - 4 lb/ac creeping foxtail, 2 lb/ac alfalfa, 2 lb/ac birdsfoot trefoil, 2 lb/ac sweetclover, 2 lb/ac slender wheatgrass (pasture) - 4 lb/ac tall fescue, 2 lb/ac sweetclover, 2 lb/ac slender wheatgrass, 2 lb/ac creeping foxtail, 1 lb/ac alfalfa, 1 lb/ac birdsfoot trefoil (pasture) Forage buffer strips (at least ft wide) immediately adjacent to municipal ditches, field drains and depressional areas are most effective for preventing and reducing the spread/intensification of soil salinity. Follow-up monitoring: Keep annual records of crop yields (yield maps) and growing season precipitation. Using GPS, establish benchmark sites for repeated soil testing to monitor changes in soil salinity. If inspection wells or piezometers were installed, monitor water table levels throughout the growing season. If salinity levels do not improve, then other management strategies may need to be considered through consultation with specialists in the field of soils and hydrogeology. References Agriculture and Agri-Food Canada and Alberta Agriculture Dryland Salinity Investigation Procedures Manual. Assiniboine Community College Prairie Province CCA Study Guide - Soil & Water Management. Bresler, E., B. L. McNeal and D. L. Carter. Saline and Sodic Soils: Principles-Dynamics- Modeling. Springer-Verlag: Berlin, Heidelberg, New York. Eilers, R. G Hydrological interpretations of soil survey data. In Proc of Dryland Salinity Control Workshop, Calgary, AB. pp McKenzie, Tolerance of plants to soil salinity. In Proc of Dryland Salinity Control Workshop, Calgary, AB. pp

8 17 McKenzie, R. C., W. Chomistek and N. F. Clark Conversion of electromagnetic inductancereadings to saturated paste extract values in soils for different temperature, texture and moisture conditions. Can. J. Soil Sci. 69: Saskatchewan Agriculture The Nature and Management of Salt-Affected Land in Saskatchewan. 23 pp. Agdex 518. Manitoba Agriculture & Food Field Crop Production Guide. 98 pp. Manitoba Agriculture Soils '84 - A Manitoba Home Study Course. Lesson 3: Salinity and Drainage.