Prescription Fertilizers for Trees and Shrubs
Notes to the PowerPoint Presentation
Desert Green 2013
Robert Ll. Morris, Emeritus
University of Nevada
Desert Green is a conference held annually in Las Vegas for landscape professionals. This is a copy of my presentation on PowerPoint and my notes for the presentation.
Slide 1. Title slide with contact information.
Slide 2. Characteristics of desert soils. Desert soils contain very little organic matter and they are chemically and physically undeveloped. Because they contain so little organic matter in them, they have very little structure or are structureless. For this reason they sometimes tend to drain poorly. Their alkalinity or pH is normally high. Frequently they contain high levels of salt of all different types. If these soils have never been developed, they can change rapidly both physically and chemically when water is applied to them.
Slide 3. There are 16 or 17 nutrients that are essential to plants. Several of these are needed in large amounts and we call these major elements or macronutrients. The rest of the nutrients are needed in a much smaller amounts, still just as essential. These are referred to as minor or micronutrients. All of these nutrients are essential to plant life and if any one of them is missing, the plant will die. If any of these are insufficient for the plant, it can display visual symptoms, poor health and subject to increased disease and insect problems. Bags of fertilizer traditionally contain nitrogen, phosphorus and potassium if they are a complete fertilizer. These three nutrients are called NPK and their relative amounts determine the fertilizer ratio. Carbon, hydrogen and oxygen the plant obtains from air and water. Most desert soils contain adequate amounts of calcium, magnesium and sulfur. Of the minor elements, iron, manganese and zinc can be in short supply to plants because of the soil's alkalinity or high pH. In most desert soils were landscape plants are being grown, nitrogen is most commonly found in the greatest need by plants. Second to nitrogen is probably available iron.
Slide 4. The three numbers on the fertilizer bag give an indicator of the amount of nitrogen, phosphorus and potassium contained in the bag, in that order. Sometimes there is a fourth number present. Although not legally required it usually represents the amount of sulfur contained in the fertilizer. The fertilizer ratio tells us the relative amounts of NPK. For instance, if the fertilizer has a ratio of NPK of 3:1:2 then fertilizers such as 12-4-8 and 21-7-14 would represent this ratio.
Slide 5. Although all nutrients are needed for a plant to live, nitrogen represents the one nutrient that is universally in short supply. Nitrogen helps to drive plant growth and causes increases in size. It stimulates increased leaf and stem growth, causes a darkening of leaf color, hastens plant growth after winter dormancy and helps to increase the amount of food manufactured by the plant. If applied too late in the year, it will decrease a plant’s hardiness to freezing temperatures. Most nitrogen fertilizers are pure white in color. Most nitrogen fertilizers dissolve easily in water and moves readily through the soil with irrigation water.
Slide 6. Lack of nitrogen shows up in plants as slow growth, foliage with a light green color and a lack of density in the canopy.
Slide 7. In pine trees a lack of nitrogen shows up in the same way. This translates to a canopy which is not full, small candles and needles occupying the ends of branches while most of the branch is without needles.
Slide 8. If too much nitrogen is applied, plants might grow rapidly with an extremely dense canopy and very dark color. When nitrogen is applied in large amounts it can cause scorching to occur on leaves and tip dieback on needles. In some cases it may cause plant death. Applying too much nitrogen is a waste of money and causes environmental problems as well.
Slide 9. Adequate amounts of phosphorus is most closely related to good root growth and plant establishment, flowering and fruit production, seed and oil production. When soils are cold and wet this can lead to a lack of phosphorus taken up by plant roots. There is a quick recovery by plants as soon as the soils begin to warm. Phosphorus fertilizers are typically dark in color, usually dark gray or brown.
Slide 10. Unlike nitrogen, phosphorus can stay present in the soil for long periods of time, does not dissolve easily in water and does not move through the soil unless the soil is very sandy. Phosphorus levels can build in the soil with repeated applications of high levels. In some cases phosphorus can build to toxic levels with repeated applications. Phosphorus can interfere with other nutrients, iron in particular. Over application is a waste of money and can lead to environmental pollution.
Slide 11. Deficiency of phosphorus can sometimes lead to purple discoloration of the plant which disappears when phosphorus becomes available again. This is very common to many plants in cold, wet soils.
Slide 12. Potassium is sometimes underappreciated in fertilizer applications. Deficiencies are hard to see since a deficient plant gives no outward symptoms. Potassium chloride, a common potassium fertilizer used in mixing fertilizers together, as a reddish-brown color.
Slide 13. Over applying potassium usually will not harm anything and it does not build up in the soil like phosphorus does. Deficiencies of potassium can lead to a plant's decreased tolerance to stresses such as heat, cold, freezing, disease and others.
Slide 14. Nutrients needed in smaller amounts. Of the seven nutrients needed in much smaller amounts, iron, manganese and zinc are the ones most likely to be in short supply for plants growing in alkaline soils. Of the three, iron is by far the one found most efficient in plants. This is odd because iron is one of the most abundant minerals on earth. The key to its limited availability to plants is the pH or alkalinity of our soils.
Slide 15. Desert soils and available nutrients. This is a chart showing the availability of plant nutrients as the pH of the soil changes from acid to alkaline. The pH ranges at the bottom of the chart from 4.0 on the left to 10 on the far right. The red rectangle on the chart represents the range of soil pH of our desert soils. The bars running vertically across the graph represent each of the nutrients. As this bar becomes thinner, that particular nutrient is less available. Nutrients which decrease as the soil pH increases (becomes thinner from left to right inside the red rectangle) include phosphorus, iron, manganese, boron, copper and zinc. Of these nutrients, the ones more commonly deficient to plants in our soils include iron, manganese and zinc. Iron is found deficient in about 90% of the times a minor element is deficient in our soils.
Slide 16. Another problem with our desert soils is the lack of decaying organic material. Most productive soils contain anywhere from 3 to 5% organic matter. Our soils contain far less than 1/10 of 1% organic matter. Decomposing organic matter helps to lower soil pH and adds nutrients to the soil. As it is decomposing it improves the chemistry of our soils and its physical structure. Organic matter helps to preserve soil moisture and increases the amount of earthworms, soil fungi and bacteria and other decomposing organisms. During the decomposition process, natural chelates are released which help to improve plant health.
Slide 17. Micronutrients. The three micronutrients we are most concerned with when applying fertilizers to our soils are iron, manganese and zinc.
Slide 18. When iron is not available to plants they display very obvious outward symptoms. These symptoms include the yellowing of new foliage, called chlorosis, while the veins of the leaves remain green if the chlorosis is mild. In advanced stages of iron chlorosis or yellowing due to a lack of iron, the leaves may turn entirely bright yellow without green veins.
Slide 19. The use of rock mulches decreases soil organic matter and the release of natural chelates in the soil. Plants intolerant of these types of soils developed iron chlorosis in 4 to 5 years after they have been planted.
Slide 20. Plants with purple leaves such as purple leaf plum developed leaves which are pink in color since their leaves are red to begin with. If you look closely at the leaves, the veins are darker color but they are dark red instead of dark green. Plants that have severe iron chlorosis are intolerant of excessive heat and cold and the leaf margins typically scorch when temperatures are high. Normally people relate the scorching of the edges of the leaves as a lack of water and so they water more. Increased watering frequency can lead to root death and increased leaf scorch and yellowing.
Slide 21. Causes of iron chlorosis include high soil pH, continuously wet soils, the use of rock mulches around plants which require soils with higher organic matter, continuous applications of high phosphorus fertilizers, mechanical damage to the roots or trunk from lying trimmers, mowers or borers.
Slide 22. Iron fertilizers and strategies. The typical strategies used to correct iron deficiency include adding additional iron, lower the soil pH usually with the addition of sulfur, applying soil chelates that help to keep iron available and the addition of organic soil amendments which breakdown and improve the soil. Iron fertilizers can be applied to the soil, sprayed on the foliage or injected into the trunk or large limbs.
Slide 23. What is a chelate? A chelate is a chemical which surrounds the nutrient and keeps it available for plants to use. Without the use of chelates these nutrients are changed into forms that the plant cannot use.
Slide 24. Desert soils and the best chelates. There are many different kinds of chelates used in agriculture. These include EDTA, DTPA, EDDHA and citrates. Some chelates are better at keeping nutrients like iron available to plants than others. The diagram on the right shows how different chelates begin to drop or lose the iron as the pH of the soil increases with one exception. At the top of the diagram you can note that the chelate known as EDDHA does not drop the iron through the entire range of pH from 4 to 10. When the pH of the soil is not known and you are applying iron chelate to the soil, it is best to select a fertilizer with the iron chelated with EDDHA. Unfortunately, this is also the most expensive of the iron chelates. If the soil pH is 7.5, the iron chelate DTPA will be adequate and it is less expensive. Frequently iron chelates used in fertilizers are iron EDTA which becomes ineffective when the pH exceeds 6.0. When using iron chelates as a foliar spray it is very important to make sure that the pH of the spray mix is appropriate for the chelate being used or buffer the water to the appropriate pH or the iron will drop out of the solution and not be effective.
Slide 25. Manganese. The visual symptoms of manganese deficiency can be confused with iron deficiency. When either are deficient they both produce chlorotic or yellowing leaves with green veins on new growth. About 90% of the time or more the chlorosis will be due to iron, not manganese. A spray bottle containing an iron or manganese chelate and buffered to the appropriate pH can be sprayed on the foliage as a test. Response to a foliar applied chelate is rapid and you should see results in less than 24 hours. And this will confirm which nutrient is in short supply. Manganese chlorosis is treated the same way it is iron but using a source of manganese instead. Manganese chelates are available as well as manganese sulfate. Applications include soil applied, foliar sprays and injection.
Slide 26. Zinc. Zinc deficiency is relatively rare but when it does occur it results in what we call “little leaf” and a rosette or cluster of leaves around the end of the branch. Correction of zinc deficiency can be with zinc chelates or zinc sulfate applied to the soil or to the foliage.
Slide 27. What is the perfect fertilizer? The perfect fertilizer bills a reservoir of nutrients in the soil or in plant tissue. It also replaces nutrients taken from the soil and used by the tree. The perfect fertilizer maintains the plant in good health so it is resistant to harsh environments and disease. Once you become familiar with the soils of a geographic region and to begin having success using certain fertilizers, soil tests become less important. But when coming into a new area it is best to either perform some soil or foliar tests or talk with your local farm or extension agent or NRCS representative.
Slide 28. Liebigs law of the minimum. The growth of plants from the application of fertilizer is limited by the nutrient in the least supply to the plant. If nitrogen is limiting the growth of trees, adding phosphorus fertilizers will not stimulate growth. If phosphorus is limiting growth, adding more nitrogen will not increase plant growth. One fertilizer or nutrient cannot be substituted for a deficiency of another.
Slide 29. What is the perfect ratio? The amount of nitrogen, phosphorus or potassium to apply is dictated by the type of plant, previous fertilizers that were applied and any soil tests that were done. Plants that are grown for their beauty such as flowering trees may have a higher requirement for phosphorus than non-flowering trees. In previous fertilizer applications, if fertilizers containing high levels of phosphorus were used, phosphorus applications might be eliminated entirely for a couple of seasons. Soil tests or foliar tests are the only certain way of knowing what is lacking in the soil or in the plant. The problem is that these can be expensive and adequate interpretation for landscape trees and shrubs is lacking. Generally speaking fertilizers fruit trees and shrubs should be high in nitrogen, moderately high to high and potassium but phosphorus levels should be low. The same fertilizer ratios recommended for turfgrass is is also adequate for nonflowering trees and shrubs. Flowering trees and shrubs require higher amounts of phosphorus fertilizer.
Slide 30. What is the perfect timing? General fertilizer applications to trees and shrubs is normally done in late winter or early spring, preferably just before new growth begins. Trees and shrubs that have a higher aesthetic value could be fertilized with a split fertilizer application; half of the annual amount of fertilizer applied in late winter and the other half just after flowering or fruiting. All plants that are winter tender, subject to potential freeze damage, should not receive any nitrogen fertilizer applications after August 1.
Slide 31. What is the perfect amount? Adjust the rates of nitrogen application to the growth observed in previous years. If good or excessive growth was observed, then reduce your nitrogen application by half. If the plant has been in poor health in previous seasons, increase the amount of fertilizer you might normally apply. Generally, rates of application are equivalent to about 1 to 2 pounds of nitrogen applied for every 1000 ft.² under the canopy of the tree. By using ratios mentioned earlier, the proper amounts of phosphorus and potassium will be applied. Foliar applications of fertilizer are applied after new growth has begun but before hot weather. About 1 - 11/2 pounds of nitrogen is mixed in 100 gallons of spray solution and sprayed to run off. One foliar application is typically not enough but three applications about two to three weeks apart would be adequate. Iron chelates can be mixed in the spray solution and should be applied multiple times if applied to the foliage. Correcting chlorotic foliage with iron will require multiple applications about one week apart. Foliar applications of fertilizers require wetting agents mixed in the spray solution.
Slide 32. How to apply fertilizers. It is best to apply fertilizers in the same locations where water is applied to the tree. If the tree is receiving water through drip emitters, apply the fertilizer close to the drip emitters. Irrigations will move the fertilizer into the rootzone. If the tree is growing in the lawn, the fertilizer can be applied in concentric circles around the tree about 2 feet apart. The fertilizer is placed 6 to 8 inches below the lawn to minimize uptake of the fertilizer by the grass yet shallow enough so that the fertilizer is not placed beneath the root system of the tree. Foliar applications of a fertilizer require a wetting agent in the spray solution and multiple applications if this is the only source of fertilizer. Foliar applied fertilizers are short-lived and follow-up applications should be made 3 to 4 weeks apart.
Slide 33. Late fall fertilization. There is some evidence that late fall fertilization of trees and shrubs can substitute for a late winter or early spring application. The timing of this application is late enough in the season so that new growth is not stimulated but well before leaf drop in the fall. Approximate dates might be around mid-October.
Slide 34. Resources.