Like a skilled chef, a professional landscape manager takes pride in providing a well-balanced diet for those under his care. Furthermore, as a chef can be the backbone of a restaurant's reputation, skilled landscape technicians can build the reputation of their companies, too.
The key to proper plant nutrition is knowledge of plant fertility requirements and environmental influences. Even plants of the same species have different fertility needs based on sunlight, temperature, and soil. A single landscape has many different types of plants in a wide assortment of environmental conditions. Details make the difference between a grouping of plants and a colorful, effective landscape.
The critical chemical elements required by plants to thrive can be boiled down to less than a dozen. These elements must be present and available for biochemical reactions to take place properly...reactions that allow the plant to use sunlight to create energy to grow.
These elements must also be in balance. Too much or too little of one element can reduce the usefulness of other elements. Ultimately, the trick is getting the right amounts of the right elements in the right place at the right time.
First you have to appreciate that nutrients delivered to a plant through the soil can be rendered useless by the soil. The first soil consideration is moisture. Plant roots accept chemicals and gases in certain forms. Moisture is necessary to convert fertilizers applied by the contractor into the forms that can pass through the root membrane into the plant. Water also carries these nutrients from their point of application to the root zone.
The second soil consideration is acidity and alkalinity, measured as pH, the balance of hydrogen and hydroxyl ions determined by a soil test. A pH of 7 is considered neutral. The soil is categorized as alkaline above 7 and acid below 7. High or low pH can stop the chemical reactions necessary to make nutrients in the soil available to the plant.
Plants should be grouped according to their optimum soil pH range, just as they should be for irrigation and light requirements. This makes fertilization more effective and subsequently provides better plant performance.
Correction of acid soil is usually accomplished over time with limestone (crushed or dolomite). Alkaline soils are corrected with applications of granulated sulfur. Fertilizers and irrigation water can have an impact on soil pH. A variety of fertilizers contain sulfur and can be used as part of a program to modify alkaline soils. Irrigation water should be tested for pH if fertility problems are diagnosed.
Soil tests also provide another gauge of the chemical reactivity of the soil, cation exchange capacity (CEC). A cation is a positively charged ion. Examples of fertilizer nutrients that are cations are calcium, magnesium, and potassium. Soils that contain higher levels of organic matter and/or clay, have higher CECs. This means they provide a better environment for chemical reactions. Low CECs mean that chemical reactions are less likely to take place and fertilizer elements might not be available to the plants, even though they are present in the soil.
Another common cause of poor fertilizer utilization is salinity, the concentration of salts in the soil solution. High salinity, brought about by use of saline irrigation water or overuse of fertilizer salts, prevents the plant roots from absorbing the water they require to carry on physiological processes. The common way to modify saline soils is to over irrigate the area to leach the salts out of the root zone. This requires good drainage. Salinity tends to be a regional problem and requires the attention of an expert using materials such as gypsum and sulfur.
The Key Elements
Most of the research and published work on fertilization has focused on turfgrasses and nursery production. Recent advice that supplemental potassium, iron, and sulfur have a place in annual fertilization programs has some relevance to ornamental shrubs, flowers and trees in the landscape. Some have suggested that nitrogen rates are too high in many cases and can be reduced by relying more on slow-release synthetic and organic fertilizers and micronutrient supplements. To make this adjustment without harm to the health of landscape plants, you need more than a basic understanding of fertilizers and how they interact with one another. You need to confirm your changes with soil testing and document them with site reports.
In many cases, soils are fairly well balanced and require only supplemental nitrogen, the element consumed in greatest quantities by plants. Slow-release synthetic and organic forms of nitrogen prevent leaching and help keep the element in the root zone. Yellowing of foliage is the primary sign of nitrogen deficiency.
Phosphorus, important primarily for establishment, is not mobile in the soil and does not leach out to any great degree. Many landscapes have sufficient or excess phosphorus. Excess phosphorus can cause deficiency of some micronutrients, such as manganese.
Potassium, the third and final component of complete fertilizers, is not as stable as phosphorus and is now being recognized for its role in helping plants withstand environmental stresses, such as traffic, heat, and drought. Many complete fertilizers today contain potassium at one-half or more of the amount of nitrogen for this reason. Supplemental potassium and iron can be applied during the summer when high temperatures could cause nitrogen to burn turf and other plants.
Nitrogen is most likely to be deficient in well-drained, sandy soils that are irrigated regularly. Sandy soils can also result in shortages of potassium and micronutrients, especially sulfur and iron. Where sandy soils are needed to solve drainage and compaction problems, fertilizing is a much greater challenge requiring frequent application of low rates of nutrients.
Before applying nitrogen in the summer or during periods of drought, check the salt index of the material. Select nitrogen sources with lower salt indexes, such as organic materials, calcium nitrate or ammonium sulfate. Ammonium nitrate and sodium nitrate have high salt indexes. Foliar applications of ferrous sulfate can solve summer nitrogen deficiencies while also providing a boost of color from iron.
Phosphorus deficiency is evidenced by a wilted, purple appearance in the foliage. Superphosphate, diammonium phosphate or monoammonium phosphate can correct the problem. Monoammonium phosphate is recommended for the cure on alkaline soils.
Since potassium can be leached through the soil, it needs to be applied regularly to most irrigated soils. Poor drought tolerance, increased incidence of disease, and slow grow-in are signs of potassium deficiency. Applications of potassium sulfate offer a low salt index and an acidifying boost from iron during the summer. Potassium chloride is not recommended for summer application due to its higher salt index. Potassium nitrate ranks in the middle for burn potential.
Iron is perhaps the element next likely to be deficient. Chlorosis is the term for the yellow foliage when iron is not available. Iron is easily oxidized and tied up in the soil, rendering it unavailable. Supplements need to be reapplied every few weeks for maximum effect. Applications of granular chelated iron or foliar soluble ferrous ammonium sulfate or ferrous sulfate are the most common corrective solutions. Iron is preferred over nitrogen during the summer to create green foliage for a special event.
Sulfur deficiency is more common than phosphorous deficiency. Not only is sulfur required by plants in significant amounts, it also can improve the availability of other nutrients in alkaline soils. It improves the utilization of nitrogen in the plant and therefore has a greening effect. Fortunately, some sources of nitrogen, potassium and iron contain sulfur as well. They include ammonium sulfate, potassium sulfate and ferrous sulfate. Granular sulfur can be applied following soil tests to correct deficiencies. Sulfur can be used to help leach out sodium in saline soils.
Calcium is another element that is part of fertilization programs. Both lime and gypsum provide calcium for the soil. Like sulfur, calcium can be used to leach out sodium from saline soils. Rarely is it deficient. Dolomitic limestone provides magnesium in addition to calcium.
Micronutrient deficiencies are caused primarily by soil alkalinity or acidity. Alkaline soils reduce the availability of nitrogen, phosphorus and micronutrients, such as iron, manganese, zinc, copper and boron. It is nearly impossible to gets seeds germinated and seedlings established in alkaline soils. Acid soils reduce the availability of nitrogen, phosphorus, potassium and the micronutrients sulfur, calcium, magnesium and molybdenum.
Correcting soil pH is much more difficult than supplementing nutrients. Combinations of micronutrients are available to improve plant health while soil problems are being addressed. Foliar applications enable the landscape manager to avoid soil problems, however they need to be repeated every few weeks to maintain adequate levels in the plant tissue.
Maintenance contractors often do not have the control to properly correct soil problems. They are forced to use nutritional tricks to deliver a healthy, thriving landscape instead. In the long run, the goal should be to build landscapes with full consideration of soil conditions and to retrofit those that were built incorrectly. But first, you must earn the trust of the client by demonstrating your expertise with plant nutrition.