By Hannah Mathers
Chemical compounds that are taken up by plants and are required for life are termed nutrients. Sixteen elements are essential for plant growth. To be an essential element, the nutrient element must be either directly involved in the metabolism of the plant or it must be necessary for the plant to complete its life cycle. Nine essential elements required in relatively large amounts by the plant are macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), carbon (C), hydrogen (H) and oxygen (O). Seven other essential elements required in small amounts by the plant are micronutrients: iron (Fe), manganese (Mn), zinc (Zn), boron (B), molybdenum (Mo), copper (Cu) and chlorine (Cl).
Nutrient availability to plants is affected more by pH than by any other factor. In high pH soils, ions of aluminum (Al), Fe and Mn precipitate and the availability of these elements decrease. Plants in a high pH soilless medium may express deficiencies of Fe, B, Zn, Mn and Mo. Phosphorus may also become deficient in alkaline conditions as it complexes with Ca to form insoluble calcium phosphates. Deficiencies of most micronutrients can be corrected by adjusting soil pH. However, some deficiencies are not solely nutrient related.
Mineral deficiencies
Mineral deficiencies induced either by the lack or excess of fertilizer, poor site conditions or other physiological problems can disturb biochemical and physiological processes, resulting in reduced growth in ornamentals. Mineral deficiencies, whether in the greenhouse, nursery or landscape, most likely develop early in the growth cycle. Mild deficiencies often go undetected for many years because their effects are usually chronic rather than catastrophic.
Five methods can be used to detect mineral deficiencies. The first method is the use of visual symptoms, such as chlorosis, malformed leaves and dieback of shoots. The second and third methods assess the plant's nutritional status by plant tissue or soil analysis. The fourth method involves conducting biological tests including fertilizer trials to determine the nutritional requirements of specific plants. The fifth method involves conducting an analysis of irrigation water. No one method will be entirely satisfactory and a combination of methods is often necessary.
Visual symptoms
Leaves and stem and root apices are particularly sensitive to mineral deficiencies. Leaves of mineral-deficient plants tend to be small and pale in color (chlorotic) and sometimes have dead areas at the tips and margins or between the veins. One of the most common symptoms is loss of green color caused by breakdown of or interference with synthesis of chlorophyll, which is commonly caused by a deficiency of N, Fe, Mn or Mg. Sometimes sensitive tissues develop in tufts or rosettes, needles of conifers become fused and other abnormalities in shape and color develop that enable experienced observers to diagnose the cause. Other visible symptoms include dieback of stem tips and twigs, lesions in the bark and excessive gum formation. See Table 1 for some deficiency symptoms that can be used to determine if nutrient deficiencies occur in your plants.
Sole reliance on visual symptoms to pinpoint the deficiency has other complications. Some deficiencies can cause reductions in growth before they cause visible symptoms. Sometimes the problem is caused by multiple deficiencies. Detecting a particular deficiency when multiple deficiencies exist is nearly impossible.
Other conditions, not related to nutrients, can complicate the use of visual symptoms for diagnosis. For example, deficient soil aeration, water stress, air pollution and an excess of minerals can cause chlorosis. Genetic factors also produce chlorosis, ranging from mottling to albino seedlings. Bacteria and virus infections may cause symptoms that look like the deficiency symptom of a particular nutrient. Overwatering or high soluble salts may induce similar symptoms. Root damage, caused by poor overwintering or overheating in the summer, may be expressed as deficiency symptoms. By combining the results obtained from Table 1 with the data gathered from the other four methods, you should be able to identify deficiencies in most situations.
Plant tissue analysis
Foliar or leaf analysis is a procedure in which the leaf tissue is analyzed to determine the mineral element content within a plant. Foliar analysis is an important tool for establishing and maintaining a proper nutrition program in woody plants. Foliar analysis should be considered both to diagnose suspected mineral element deficiencies and as a check on the fertilizer program.
For woody plants, it is best to sample the leaves only. All leaves taken must be the same age and have the same position on the plant. This usually means the first fully matured leaves back from the tips or the youngest fully matured leaves. Each sample should contain 30-100 leaves or groups of leaflets should be collected from trees, shrubs and broad-leaved evergreens. The size of the sample depends on size of the tree or shrub being sampled. About 50 terminal cuttings, 2 inches long, should be removed from narrow-leafed evergreens from as many different plants as possible with the same condition.
In general, leaf samples should be taken between mid-June and mid-September or later with evergreens from plants that represent conditions within the planting. When some plants in an area are growing poorly and others are growing well, take two samples. Both samples must be taken of leaves that are at the same stage of growth. The differences between the two samples may point to the cause of the problem. The analytical laboratory you use will give you specific guidelines for preparation of the sample after you collect it. Follow their guidelines closely and always wash your hands before handling the sample.
Often, I encounter growers who discount the value of a growing medium analysis and believe most nutritional problems can be solved and corrected based on a plant tissue analysis only. This approach ignores the influence elemental excesses in the growing media can have on the uptake by the plant of one or more totally different, yet essential, elements. Table 2 contains a list of common antagonists and indicates why tissue and soil analyses should always be conducted together. Neither detection method should stand-alone.
Biological tests
Biological tests include fertilizer trials in the field and in pots in the greenhouse. These trials are sometimes necessary to learn which elements produce the most improvement in growth. The diversity of crops within ornamental operations is tremendous and sometimes the kind of exact information growers are looking for can only be found by conducting your own trial. Correctly designing an experiment that can answer your questions is vital.
There are three categories of experiments or trials: preliminary, demonstrational and critical.
Preliminary experiments are carried out with many treatments. They are usually conducted to focus the researcher. The most promising treatments from the preliminary trials are investigated in future critical experiments.
Demonstrational trials are probably the most familiar to growers. In nutrition demonstrational trials a new fertilizer may be compared to the standard fertilizer treatment at the nursery.
Demonstration trials are not replicated or randomized and do not offer precise information. Many growers make changes in their production systems on the basis of non-replicated, non-randomized, demonstrational trials. Without replication and randomization, however, the grower does not know if the observed effect is due to chance or real. A replicated and randomized comparison of a new product with a standard would yield precise information.
Soil analysis
Analyses of soils are used widely to determine the kinds and amounts of fertilizer to be added. They also indicate the existence of mineral deficiencies before planting. In field culture, a soil test should be done every year to ensure that the crop will have sufficient nutrients and the correct pH for good growth. Soil tests can be done at any time when the soil is not frozen. The best time is early spring. Early-spring testing will allow you time to correct any nutrient deficiencies before plant growth advances. Use a local private laboratory for the soil analysis.
For field culture, it is important to know some soil fertility standards for your nursery crops. Very little information is available on soil fertility standards for specific ornamental plants. See Table 3 for some general standards.
As in field culture, to maximize growth and quality of container stock produced, fertilization is extremely important. Essentially all plant health depends on proper nutrition. In container culture, you need to know the relative nutrient levels needed for optimum growth (see Table 4). These are essential if problems in container culture are to be avoided.
In container culture it is also important to realize the importance and consequence of various chemical supplements in your media. Most manufactured media for container-grown plants will require some form of chemical supplement to adjust the pH and to augment the available nutrients. A chemical analysis of the medium and its components should be obtained, unless this information is known. Depending upon the mix, analysis results and the length of time that plants are to be grown in the containers, the additional nutrients needed may best be added using slow- or controlled-release fertilizers at the time of planting.
A soil analysis has its greatest value when used in conjunction with a foliar analysis. With woody plants, because the root system can be quite large and deep, it is difficult to sample the soil to represent the area where the root system absorbs its nutrients. A poor correlation may exist, therefore, between a soil test and a leaf analysis for a given nutrient. Foliar analyses also do not indicate soil pH, which is key to nutrient availability. For these reasons soil tests should always accompany foliar analysis of plants and be used together to diagnose suspected mineral deficiencies. Media tests should always also accompany foliar analyses of plants grown in containers, because container pH can change rapidly. Diagnosis of almost all nutritional problems experienced in container production is by a growing media analysis at some point.
Water analysis
The application of water to nursery and greenhouse plants is the most universal treatment, the most important treatment for crop success and the most discounted and neglected. Water quality has a major influence on nursery, greenhouse and landscape plant nutrition, growth and quality. Because the impact of water quality is so significant, it should be the first step taken when designing your fertilizer program and perhaps the first detection method used when nutrient deficiencies are observed.
You should have a water test done at least once a year. It may be good to have two water tests done the first time you are investigating your irrigation source. The two tests would be conducted as follows: 1) in the spring, after all the rains; and, 2) in late summer, before the rains begin. The spring test will give you the best synopsis of your irrigation water and the test in late summer your worst scenario. Water-quality tests should also be conducted more than once a season if any of the following conditions apply:
1. It has been an exceptionally dry or wet growing season.
2. It has been a period of abnormally high or low water usage.
3. The irrigation water comes from various sources, including a city or municipal source.
When reviewing the results of a water analysis, note excess or minimal levels first, before studying the balance of parameters measured. See Table 5 for some ornamental irrigation water-quality guidelines. The characteristics in set one should definitely be monitored. They are the minimum set of analyses that should be done on a regular basis. Set two characteristics are desirable but not as essential.
The thing that I find most helpful when I'm trying to diagnosis a plant problem is keeping an open mind. Don't be swayed by other people's opinions or previous consultants' recommendations. Try to get as complete a picture as you can before making a decision, which may include conducting all five of the detection methods mentioned above and a plant submission to the plant clinic for biotic diagnosis.
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