Management strategies for saline irrigation Considerations for citrus production Peter Rigden, Development Horticulturalist, Department of Agriculture Fisheries and Forestry Queensland. These notes are intended to provide an outline of the strategies that citrus growers in the North Burnett may find useful to understand and manage irrigation water with increased salt levels and to reduce salt accumulation in the soil root zone. This information is NOT intended to provide recommendations because these will depend on a number of site specific factors, including: soil type (texture and drainage characteristics) the salinity of irrigation water (ions and electrical conductivity) how much salt has already accumulated in the soil crop management rootstocks and varieties It is recommended that growers seek assistance from people with specialist knowledge and equipment to help with assessing the situation and to develop an appropriate management strategy. Leaching accumulated salts using saline irrigation water Salt that has accumulated in the soil root zone can be leached out of the root zone by the application of one or more heavy irrigations, which serve to dissolve the salt and move it down the profile to below the root zone. The amount of extra irrigation water required (referred to as the leaching fraction ) depends on the relative concentration of salts both in the irrigation water (EC i ) and the root zone (EC se ). Application of a leaching fraction will assist with movement of accumulated salt down the soil profile even when the irrigation water is more saline than is ideal. Table 1 Terminology Term Use Description EC Water & Soil Electrical Conductivity; indicates level of salt concentration. EC i Water Measure of salt concentration in irrigation water. EC se Soil Measure of salt concentration in soil profile. EC 1:5 Soil Method used to measure EC of soil (1 part soil to 5 parts water). How does leaching with saline irrigation water work? When saline irrigation water is used repeatedly in irrigation cycles that are stopped as soon as the soil in the root zone has reached capacity and before any significant drainage of water out of the root zone occurs, salts accumulate in the root zone and the concentration of salt in the soil (measured as ECse) increases. In a normal irrigation cycle soil moisture monitoring devices such as tensiometers or capacitance probes are used to measure the soil moisture content of the root zone and irrigation is halted when the root zone profile is full. This conserves water, minimises irrigation costs and prevents the leaching of nutrients. 1 7
In contrast when additional water is applied to the saturated root zone, the excess water (i.e. the leaching fraction) dissolves the accumulated salt and flushes it below the root zone. If saline water is used to apply the leaching fraction, the root zone soil will still be saline, however it will be reduced compared to the much higher level of salt that had accumulated prior to application of the leaching fraction. As a result, the health of the trees and yields should improve, provided the leaching fraction irrigation regime is maintained as required. Some points to note: Soil drainage limitations: For a leaching fraction application to work it is essential that drainage below the root zone is not impeded, because the water draining through the root zone must be able to drain into the subsoil below the root zone. If soil drainage is impeded, the extra water applied will just be adding a greater amount of salts to the root zone making the problem worse and will actually drive the ECse reading even higher. In the North Burnett, citrus is grown on a number of different soil types refer to the Soil types of the Burnett River Catchment factsheet. These include an outline of permeability, drainage characteristics and an indication of suitability for management of accumulated salts by application of a leaching fraction. Pre wet the root zone soil profile: To increase the effectiveness of leaching the soil profile it is necessary to pre wet the soil profile. If the profile is not pre wetted the irrigation water will take the path of least resistance, passing only through macro and micro cracks in the soils and as a result not dissolving and flushing salts stored within the soil crumb structure. If irrigation water is in plentiful supply, irrigation cycles to thoroughly wet the root zone profile prior to applying a leaching faction will help to prevent this happening. A leaching fraction irrigation cycle is more likely to be effective after a significant rain event that has wetted the soil profile and closed up any cracks in the soil. It is important to apply the correct leaching fraction to the soil type to avoid wasting water and also to ensure excess water does not seep lower in the landscape. Premature cessation of leaching fraction application: Incorporation of a leaching fraction into the irrigation cycle should only be stopped once the salt concentration of the irrigation water (ECi) has returned to normal levels. Reverting to a conservative irrigation regime too early will only serve to increase the salt content of the soil root zone, due to discontinued flushing. Before leaching fraction irrigation cycles are stopped do a soil test to confirm that ECse has returned to an acceptable level. How to decide if a leaching faction irrigation cycle is appropriate? Monitoring salt concentrations of water and soil requires specialised equipment and knowledge to interpret the results. It is recommended that growers seek the services of a consultant agronomist to help schedule leaching fraction irrigation cycles. Below is some basic information to assist understanding of the principles involved. To know when application of a leaching faction is required the salt levels in the root zone need to be measured. This can be done using one of the following: sending off samples for laboratory analysis to derive the ECse, measuring the EC 1:5 of the soil and applying a factor based on the soil texture to derive the ECse. 2 7
The salt content of the irrigation water (ECi) also needs to be measured to find out if the irrigation water can be used to leach out the accumulated salts from the root zone. How to measure the EC1:5 and derive the ECse It is possible to determine the salt concentration in the soil from a soil sample using the following method (sourced from the NSW Department Primary Industries Salinity Note 8 How to Texture Soils & Test for Salinity ): To perform the test you will need a soil sample, some distilled water (good rainwater is suitable), a testing container (such as a jar with a lid) and a calibrated salinity meter. 1. Take a soil sample from about 25 30cm deep and leave it to dry as long as possible (leave sample bag or container open for at least a day to let moisture escape). Alternatively, it can be oven dried on a tray in a cool oven. 2. Crush dried sample so there are no large aggregates (clods of soil 2 mm or larger). You may need to crush these aggregates with a mortar and pestle, rolling pin or hammer. Remove any foreign matter, plant material and stones from the sample. 3. The test involves adding one part soil for every five parts water e.g. if you add 50 g of soil (weighed on scales) to the testing container, then you need to add 250 ml of water. 4. Shake the container for three minutes to make sure the salts dissolve. For clay loams and clay soils, more shaking (for one minute every 3 minutes repeated three times) will bring more salts into the solution and increase the accuracy of the test. 5. Allow the solution to settle for a minute before testing. 6. Place the salinity meter in the solution (but not in the soil in the bottom of the jar) and read the display once it has stabilised. This value is the EC 1:5. 7. Wash the meter electrodes and sample jar with distilled or rainwater, and dry. 8. Assess the texture of the soil and determine the appropriate conversion factor from Table 2. 9. Multiply the EC 1:5 by the conversion factor for the soil texture, this is the ECse of the soil. Table 2. Conversion factors for calculation of ECse from EC 1:5 Conversion factor to calculate soil ECse from soil EC 1:5 Soil texture Conversion factor* Sand 17 Sandy loam 13.8 Loam 9.5 Clay loam to light clay 8.6 Medium to heavy clay 7 *Source: NSW Department Primary Industries Salinity Note 8 How to Texture Soils & Test for Salinity The soil sample should be taken from the middle of the main root zone, which for citrus is 25 30cm deep in most soil types in the North Burnett. If possible take another sample from below 60 cm as this will provide an understanding of whether or not salt has accumulated below the main root zone (which will influence decisions made regarding application of a leaching faction). Repeat this test on samples taken at several sites throughout the irrigation block. If the soil type is uniform throughout the block you should find that the results are similar and the leaching fraction can be applied uniformly to the whole block. 3 7
The NSW Department Primary Industries Salinity Note 8 How to Texture Soils & Test for Salinity has more details at: http://www.dpi.nsw.gov.au/ data/assets/pdf_file/0008/168866/texturesalinity.pdf Some capacitance probes, used to monitor soil moisture for irrigation scheduling purposes, also measure soil salinity. How to measure ECi Use a pocket size salinity meter; this is accurate enough to determine the salinity of water. This is not as accurate as sending samples to a laboratory but should be accurate to within 10% of a laboratory test. Test water samples using the following method (sourced from the NSW Department Primary Industries Salinity Note 4 How do I test water Salinity? ): 1. Thoroughly mix the water to be tested before taking a sample. 2. Dip a water sampling container into the water being tested and rinse thoroughly. 3. Allow the jar to half fill with water. 4. Remove the protective cap from the salinity meter and turn the unit on. 5. Immerse the salinity meter into the sample up to the raised mark (about 25 mm), and slowly swirl the meter. The two electrodes of the meter must be covered. (If testing free water in the paddock, do not rest the end of the meter in sediment on the bottom). 6. Allow the displayed value to stabilise (it takes several seconds to compensate for the temperature of the sample). 7. Read the number on the meter, then convert it to ds/m EC units (1dS/m is equal to 1,000µS/cm) and record the reading. 8. Wash off the lower part of the meter with fresh water (especially the electrodes). The NSW Department Primary Industries Salinity Note 4 How do I test water Salinity? has more details: http://www.dpi.nsw.gov.au/ data/assets/pdf_file/0006/168882/water salinity.pdf Salinity thresholds for citrus The following table shows the root zone salinity thresholds at which a 0%, 10% and a 25% yield reduction could be expected for citrus crops. Table 3. Soil root zone salinity levels corresponding to citrus yield reduction Crop Root zone salinity ECse (ds/m) for yield reduction of: 0% 10% 25% Orange 1.7 2.3 3.3 Lemon 1.0 not available not available Grapefruit 1.8 2.4 3.4 Source: Salinity management handbook. 2 nd Edition (2011). Department of Environment and Resource Management, Qld. 4 7
In practice these thresholds are variable because the effect of irrigation water salt content on the soil and plant is modified by several factors, including: Climate: the regularity of leaching rainfall events that remove salt from the root zone. Soil type: different soils leach salts more easily than others e.g. sandy soils leach much more readily than clayey soils. Rootstocks: some rootstocks are more tolerant than others to salinity. Irrigation method: drip, under tree sprinkler and overhead sprinkler application. Plant maturity: young plants are more susceptible to salinity problems and damage. Irrigation management: if irrigation control is tight and scheduled to just maintain soil moisture in the root zone then no leaching of salts out of the root zone occurs and there is a steady build up of salt in the root zone. It is not possible to be specific about the effect on yield of the irrigation of saline water unless the factors above are considered at each site. How much water to apply The reduction in the root zone salt concentration ECse will depend on the salt content of the irrigation water (ECi) and the amount of water applied in the leaching fraction. An indication of how much the ECse can be reduced, based on varying ECi levels, for three leaching fractions is provided in the Table 4. This information, developed by the South Australian Research and Development Institute (SARDI), is based on a scenario involving average inherent root zone ECse levels and leaching volumes required for a 1 metre root zone, under sprinkler irrigation. Important: This information is provided as a general illustration, multiple factors (climate, soil type etc.) influence concentration of salts in the root zone and conditions in the North Burnett are different to those in which SARDI developed the information given in Table 4. Therefore it strongly recommended that ECse is measured prior to deciding on the application of a leaching fraction and then monitored to determine the need for continued application. Table 4. Root zone salt concentrations (ECse) for irrigation water of varying salt concentrations (ECi) under three different leaching fraction regimes Irrigation water salinity (ds/m) Average root zone salinity with 10% leaching (ds/m) Average root zone salinity with 15% leaching (ds/m) Average root zone salinity with 20% leaching (ds/m) 0.2 1.1 0.8 0.7 0.4 2.1 1.7 1.4 0.6 3.2 2.5 2.1 0.8 4.3 3.3 2.8 1.0 5.3 4.1 3.5 1.5 8.0 6.2 5.3 2.0 10.6 8.2 6.9 2.5 13.3 10.3 8.6 Source: Salinity Management Practice Guidelines (National Program for Sustainable Irrigation) at: lwa.gov.au/files/products/national program sustainable irrigation/pn22225/pn22225.pdf 5 7
Note: 10% leaching means that 10% of the total water applied drains below the root zone and removes accumulated salt, similarly for the 15% and 20% leaching. Using Table 4, if the irrigation water has an ECi of 0.8 ds/m, then a programmed application of a 20% leaching fraction can reduce the root zone salt concentration to 2.8 ds/m. This means that if the soil has an ECse of 6.0 ds/m, then by using a series of 20% leaching fraction applications it should be possible to reduce the ECse to 2.8 ds/m using irrigation water with an ECi of 0.8 ds/m. Leaching soils with impeded drainage Leaching will be severely reduced or fail if the soil has poor drainage characteristics present in or just below the root zone. In this situation install sub surface drainage below the water table, prior to use of a leaching fraction irrigation cycle, to remove the saline water from the root zone. Gypsum can also assist where soil have permeability issues. Rainfall events A high rainfall event can provide a good leaching fraction and can flush out accumulated salts from the root zone into the subsoil. Refer to the Soil types of the Burnett River Catchment factsheet for information on the leaching characteristics of the soil types found in the north Burnett. Such suitable high rainfall events are infrequent and most rainfall events may not actually be sufficient to leach accumulated salt out of the root zone. However, a leaching fraction of irrigation water applied after (or during) a limited rainfall event that wets the soil profile can take advantage of and complete the leaching process that has been started by the rain. In addition, the rainwater in the soil profile will dilute the irrigation water salt concentration, making the leaching fraction more effective. It is important to monitor salt levels in the root zone (ECse) to determine the need for continued application of leaching fractions. Mixing irrigation water Where more than one source of irrigation water, such as a river, bore or dam are available with different salt concentrations, it may be possible to mix together highly saline water with less saline water to reduce the overall salt concentration of the irrigation water that is applied to the crop. Use of the reduced saline water may slow the accumulation of salts in the root zone or can be used to maximise the effectiveness of a leaching fraction irrigation cycle. Irrigation systems (pros and cons) The three types of irrigation used in citrus orchards are: under tree micro sprinklers, drip sprinklers, over head sprinklers. All have some advantages and disadvantages in respect of salt management. Drip irrigation systems minimise the evaporation rate of irrigation water, thus minimising the amount of water used and reducing the build up of salt in the root zone. Because there is no contact of the saline irrigation water with crop foliage, there is less leaf damage caused by high levels of dissolved chloride in the water. However it is more difficult to apply a leaching fraction to reduce the 6 7
root zone salt concentration using drip irrigation systems, because of the low application rate and limited wetting front. Under tree micro sprinklers are more suitable than drip systems for applying a leaching fraction, but have the disadvantage of wetting low hanging foliage and causing leaf damage. Skirting trees, removing low hanging branches and foliage, can reduce the amount of damage caused by saline irrigation water contacting leaf foliage. Micro sprinkler nozzles can be restricted or changed to reduce water throw. Overhead sprinklers can be useful for applying leaching fractions, but where saline water is used, leaf damage can be extensive. Managing increasing chloride Levels of chloride (Cl) in irrigation water can cause poor growth and damage leaves, especially if the irrigation water falls onto the leaves. Water with chloride levels of less than 140 mg/l (ppm) is regarded as being acceptable for use with drip irrigation systems, and under tree micro sprinklers that do not wet the foliage. For overhead sprinkler irrigation systems or where micro sprinklers wet the foliage, chloride levels of less than 70 mg/l are required to avoid damaging foliage. Night irrigation Applying irrigation water at night will help to minimise the damage caused through use of water with high chloride levels. Humidity generally increases at night and winds decrease, reducing the rate of evaporation. This in turn reduces the concentration of salts in the soil surface and on leaves that are contacted by irrigation water from sprinklers. Night time irrigation can therefore reduce or eliminate chloride (and sodium) toxicity due to less foliar absorption and foliar deposits. If irrigation has to be done during the day, avoid irrigating on hot or windy days when evaporation rates are particularly high. Acknowledgements: Department of Natural Resources & Mines, Burnett Mary Regional Group & Chris Searle (consultant agronomist). 7 7