Notes for Salinity and Irrigation Desert Green 2014 Slide 1. Follow This Presentation with Notes You Can Find Now Slide 2. Title slide. Salinity and irrigation Slide 3. What is salt? We commonly think of sodium chloride or table salt as salt. But in fact we are surrounded by salts. Table salt is a natural product mined from the earth or evaporated from salt water or springs that contain lots of minerals. Table salt is a seasoning as well as a food preservative. The word salad comes from the Latin word for salted which was a Roman practice of salting leafy vegetables. Table salt was so popular that it was used as a form of trade and barter for centuries and was responsible for starting wars. The word salary comes from the Latin word for salt, salarium. We still retain the Greek expression when someone is worthless saying they are not worth their salt. Table salt is used in many manufacturing processes and only about 6% of the salt produced is used for human consumption. Nearly half of the salt produced in the United States is used to manufacture chlorine gas and sodium hydroxide which in turn is used in the manufacturing of paper products, textiles, soaps and detergents and as a drain cleaner. Slide 4. We are surrounded by salts. Most fertilizers are salts. We use salt in our water softeners because sodium chloride, and to a lesser degree potassium chloride, will substitute for the magnesium and calcium salts that contribute to hard water. Because one of salt s properties is to lower the freezing point of water, deicing salts are applied to icy sidewalks and streets in winter. Sodium and potassium-based salts are essential for plant and animal life. Both animals and plants contain many different types of salts. Animals contain much more salts than plants. Therefore composts and soil amendments made from animal waste tend to have much more salt than those made from green waste or are totally plant-based. We now commonly see other types of salts or mixtures of salts in the marketplace. These include sea salt, Himalayan salt, salt scrubs and other value-added products for niche products such as MSG. One of the myths that surround commercial fertilizers is that the salts they contain are harsh on the biology of the soil. The reality is that salt is essential to all of life. Either too much or too little can harm. Slide 5. Where Do Salts Come from? Salts can come from the natural minerals in the soil. These minerals slowly break down over eons of time to give the soil specific plant nutrients. Some of these minerals may be high in magnesium or phosphorus. Others might be high in potassium or calcium. While others may have chemicals that can be toxic to plants such as sodium and boron. Salts are in soil amendments such as compost. Animal byproducts contain more salts than plant byproducts. Therefore compost made from animal byproducts such as manure is higher in salts than those made only from plants or green waste. Other soil amendments such as coconut coir can be high in salts as well.
Most fertilizers are salts. Applying fertilizer to the soil will raise the salt level of that soil. Particular care must be made when applying fertilizers that the soils are moist and not already high in salts. All irrigation water carries various amounts of salts and of all different kinds. Even well water will carry salts of different types depending upon the soil and minerals in the area. Colorado River water has a great deal of salt which increases as the river flows further south. These salts come primarily from the native minerals in the soil but fertilizers and irrigation practices concentrate these salts further. Slide 6. Salt Is Found in Soil. Soil salts are plant nutrients for the most part. Plants take a great deal of these salts out of the soil as they grow. Fertilizers are added to the soil to replenish those soil salts which were used by the plants as they were growing. Slide 7. Salt Is Found in Soil Amendments. Manures, composts and compost tea contain inorganic salts, organic salts, and insoluble organic forms of nutrients. But their salt content may not differ much from fertilizers needed by plants. This is because the volume of organic soil amendments needed compared to fertilizers is much higher even though there nutrient content is much lower. And they contain salts not necessarily needed by the crop. Also, as they decompose organic nutrients convert into salts. Slide 8. Salt Is Found in Fertilizers. Fertilizers are meant to dissolve in water. Some fertilizers dissolve more quickly than others. Since the nutrients in fertilizers are salts, these dissolved nutrients increase the salinity of the soil. Most slow release fertilizers release their fertilizer salts more slowly than rapid or conventional fertilizers. Most slow release fertilizers are safer to use around plants because of their slow release characteristics. Quick release or conventional fertilizers should be applied in smaller amounts and more frequently to reduce potential fertilizer burn to plants. Slow release fertilizers typically can be applied in higher amounts with a much greater degree of safety for plants. Slide 9. Fertilizer Salt Index (SI). The fertilizer salt index (SI) is a guide to how salty fertilizers can be. This guide does not tell you how much fertilizer to apply. Rather it tells you how salty fertilizers are relative to each other; the higher the number, the saltier it is. There are several factors that govern how salty of fertilizer will be once it is applied to the soil. It is hard to judge by the name of the fertilizer what its salt index might be. However, there are a few tips you can follow. The more concentrated the fertilizer, in other words fertilizers with higher numbers in their grade, the less salt damage will occur if applied appropriately. On the other hand, the potential for damaging plants from fertilizer burn (salt) will be greater with these fertilizers if they are not applied correctly.
Generally speaking, phosphate fertilizers have a lower salt index than nitrogen fertilizers. Also, some slow release fertilizers will cause less plant damage than quick release or conventional fertilizers. Urea is a soluble fertilizer that isn t a salt. Nevertheless, its high solubility means it will act like a salt just like any other fertilizer. It also quickly decomposes to form ammonium, a salt. Elemental sulfur is neither salt nor soluble but it turns into sulfates in the soil, a salt. Slide 10. Most salts dissolve in water. Some salts like table salt dissolve quickly and completely in warm water. Once dissolved in water, the crystals disappear after a short time. When table salt is dissolved in water the sodium and chloride atoms separate. Some fertilizer salts like ammonium sulfate also dissolve quickly and completely in warm water. In fact, ammonium sulfate dissolves so easily in water it is frequently used as a fast-acting, liquid nitrogen fertilizer that can be applied to the leaves of plants, rapidly absorbed through the leaf surface, and causing a very rapid color response in plant leaves. Slide 11. Salt is in irrigation water. Salts are in our irrigation water coming from the Colorado River and even well water. The Colorado River water carries a large amount of different types of salts, much more than most water from wells that contain drinking water. In fact, Colorado River water carries about 1 ton of salt for every 300,000 gallons of water. Irrigating with straight Colorado River water is equivalent to applying about 600 or 700 pounds of salt to a 1000 ft.² tall fescue lawn every year. The good thing about Colorado River salts is they are primarily not toxic in themselves. In other words, the levels of sodium, chlorides, boron, fluorides and carbonates and bicarbonates are fairly low. Problems occur if salts from these waters cannot move through the soil. In cases like these salts begin to build in the soil. These salts begin to compete with plants for water. If water can drain through the soil, this type of water will not become a problem. Reuse water from municipalities along the Colorado River have salt levels 125% higher than Colorado River water. Because these salt levels are so much higher, even more care must be used when using these kinds of waters. Managing the soils for good drainage, applying water through drip systems and watering more frequently are management practices used when irrigating with reuse water. Slide 12. Salt competes with plants for water. Fertilizers are salts. As these fertilizer salts dissolve in water they form soluble ions. As these fertilizer ions increase they become more concentrated in the soil water or soil solution. This increase in salinity pulls on the same soil water the plant needs, essentially competing for the same water. Even though the soil is moist, the plant reacts to this competition as if the soil is dry. This is why plants affected by fertilizer burn look about the same as if they had been stricken by drought. They can t get the water, because there s too much salt in it.
Slide 13. How are soil salts measured? Salts can be measured in the soil fairly inexpensively with salinity meters that cost between $100 -$200. Soil samples can be sent to soil testing laboratories but the turnaround time can be rather long. Samples can be taken from the field back to a field laboratory where the soil water containing the salts can be extracted. All salinity meters need to be calibrated against salinity standards before measurements are taken. In some cases the soil water must be diluted to bring the concentration of salts into a range that the meter can detect. Commonly these dilutions are 1:1, 1:2 and 1:5 with distilled water as noted by some University soil testing laboratories. Once the measurement is made it must be re-adjusted to whatever dilution was made to the water extracted from the soil. Slide 14. Plant Damage Results When Total Salts Are Too High. Salts in the soil compete for water that plants need. If these salts are generally not toxic themselves, then as the salt levels increase, the amount of competition for that water increases for the plant as well. Higher salt levels act exactly like the soil is getting drier and dryer. Plants in good health are better able to handle higher salt levels than plants that are in poor health. Salt damage to plants first occurs as a yellowing on the leaves furthest from the veins. This is usually on the tips or margins of the leaves. Slide 15. Salinity units of measurement. Typically on soil test reports salinity units may be as total dissolved solids (TDS) in parts per million (ppm) or in electrical conductivity (EC) and expressed as millimhos per centimeter (mmhos/cm) or decisiemens per meter (ds/m). Frequently electrical conductivity measurements are expressed in a form that must be converted using the metric system to a known form. Slide 16. Some salts are bad guys. Salts containing sodium, chlorides, boron or sulfates may cause extra problems for plants at fairly low salt concentrations. Boron is an essential nutrient for plants. Plants will die if they don t receive boron. The amount of boron that plants need is very tiny, one or 2 ppm is enough to keep plants healthy. But if boron salts are too high, over 5 to 10 ppm, we will see damage to many plants. The damage will look very similar to drought or damage when general salt levels are too high in the soil. Plant damage from boron can only be discovered through soil or tissue testing. In irrigation water, carbonates and bicarbonates can pose a big problem for the soil chemistry. Slide 17. Some Salts Are Bad Guys. Some salts which can be particularly troublesome for plants at fairly low concentrations include boron, sodium, chlorides and fluorides. Fluorides can be particularly troublesome for many interior plants. Because most interior plants are from tropical rain forests, they are not very tolerant of salts or specific kinds of salts. In cases like these, distilled water should be used that has been fortified with a small amount of fertilizer.
Slide 18. Structural Damage from Salts. Some salts in soils can be problematic for steel and concrete. These minerals or salts can be destructive and include sulfates, chlorides and bicarbonates. Slide 19. Plant Reaction to Salts. We can categorize the damage to plants as either direct damage that we can see or indirect damage that is more subtle in its effects. Slide 20. Salt s Direct Damage. Direct damage to plants include wilting when temperatures are high or sunlight is intense. In the early stages we typically might see leaf drop or needle drop. As these conditions do not improve the overall quality of the plant starts to decrease. The plant fails to put on a lot of new growth, the leaves may become damaged from yellowing and scorching and leaf drop causes the plants to have an open canopy. Slide 21. Salt s Direct Damage. The early stages of salt damage looks exactly like drought. In flowers and vegetables we may see the leaves beginning to wilt during the day. With lawn grasses we start to see gray green color appear in spots usually between sprinklers. This gray green color is due to the folding or rolling of the leaf blades from drought caused by salinity. Slide 22. Salt s Direct Damage. Intermediate stages of salt damage include leaves beginning to yellow usually around the margins are edges first or on the leaf tips. Next the leaves begin to burn back or scorch on the tips and margins which are furthest from the leaf veins that carry water. Sometimes it is confused with general plant yellowing such as iron chlorosis or lack of iron. On pine trees the needles begin to scorch on the tips with needle die back. This is a very common occurrence in Las Vegas on p you ines. Slide 23. Salt s Direct Damage. These are more pictures of plants suffering from intermediate stages in salt damage. Slide 24. Salt s Direct Damage. Long-term damage to plants from salinity include a thinning of the canopy because of poor growth and leaf or needle drop. Looking at the branches themselves you will see a slowing of the growth each year. In pines this shows up as a thinning of the crown or canopy of the tree because there is much less new growth to fill in the voids between branches. Slide 25. Salt s Indirect Damage. Indirect damage to plants can t be seen very well but contributes to the direct damage of the plant. Salt damage causes the plant to produce less carbohydrates through photosynthesis and has less energy for growth. Plants affected by salts are less healthy and their tolerance to high and low temperatures decreases. Their ability to recover from insect and disease damage decreases.
Slide 26. Confusion with Other Problems. Salt damage is confused with other types of damage that decrease the amount of water reaching the leaves. The first is drought. Before assuming the plant has salt damage check the soil moisture using a soil probe or shovel. Check for plugged emitters or bubblers. Make sure the irrigation clock is working properly and the irrigation schedule is appropriate for the weather conditions. Salinity damage can be confused with a micronutrient deficiency such as iron chlorosis or deficiencies of manganese and zinc. Apply minor elements to the plants along with irrigation in anticipation of these types of problems. Salinity damage can be confused with insect and disease problems. Damage to the trunk by mowers or line trimmers, damage by boring insects such as flat headed borers, collar rot and Verticillium wilt can give the plant symptoms similar to salt damage. Root damage from borers and nematodes can also be confused with salt damage. Slide 27. What Can You Do? Don t assume it is salt damage until you have eliminated other possibilities and have confirmed it with a soil test or water test for salinity. If you have narrowed it down to a salt problem, determine where the salts might be coming from. Slide 28. What Can You Do? If you have determined that the salts have come from the soil, the only thing you can do is improve the drainage of that soil and increase the irrigation around the roots. This means increasing the volume of water that s applied at each irrigation and increasing the uniformity or the number of places where water is delivered to the plant. Trees and shrubs this might mean vertical mulching, creating drainage holes in the soil. In lawns this means aerating the lawn to improve drainage and increasing the amount of applied water. Using soil wetting agents helps to move water through the soil profile and remove salts. Check which fertilizers you are using and their salt index. Pick fertilizers that have a lower salt index. Slide 29. What Can You Do? If the salt is in the irrigation water see if you can switch to a different source of water, blend higher-quality water with the lower quality water or follow an irrigation of poor quality water with an application of good quality water. If you are using overhead irrigation, consider switching to drip irrigation. Water applied through drip irrigation can be higher in salts than the same water applied through overhead sprinklers but cause less plant damage. Improve the uniformity of the irrigation system so that water is applied more evenly over the lawn or the soil surface. Increase the amount of water applied so that soil salts are leached below the root zone of the plants. Water more often. As soils become dryer, the salts in the soils become more concentrated and cause more damage. By watering more often the soils will not dry out as much and cause less damage to the plant. Make sure the soil is moist before the heat of the day. Plants can handle higher salt levels with moist soils than dry soils. If you have to, switch to more salt tolerant plants in the landscape. Slide 30. End