Plant Nutrition of Greenhouse Crops
Diagnosing Plant Nutrient Deficiencies
The correct diagnosis of nutritional deficiencies is important in maintaining optimum plant growth. The recognition of these symptoms allows growers to “fine tune” nutritional regimes as well as minimize stress conditions. A lot of the uncertainty in identifying nutritional problems can be reduced by keeping accurate, up-to-date records on the day-to-day events in the production process and, specifically, the details about application dates of water, fertilizer, and pesticides; and other factors affecting growth. Despite these efforts, nutritional problems arise even in the most carefully thought-out fertility programs. In most cases, symptoms of nutritional disorders occur in defined patterns and are specific for each nutrient. For instance, elements that are mobile within the plant generally induce deficiencies on the older (lower) leaves first, while immobile elements induce deficiencies on the younger (upper) leaves. In some cases, pesticide toxicity or disease symptoms may resemble nutrient deficiencies or toxicities. In addition, symptoms of nutritional disorders are often species or cultivar dependent. Therefore, growers should become familiar with nutritional deficiencies on a crop-by-crop basis. Several tools are available that allow growers not only to identify and correct nutritional problems when they arise but also to forecast problems. These include visual diagnosis, media testing by commercial/university laboratories, and tissue analysis along with tracking soluble salts and pH and water analysis. Each test provides some information that is not provided by the others. Information from the above listed tests along with cultural records provides the basis for diagnosing nutritional status of a crop. Nutritional disorders of plants rarely occur in well managed greenhouse crops.
Common Deficiency Symptoms
A first step in diagnosing nutrient deficiencies is to describe the symptoms. Each deficiency symptom is related to some function of the nutrient in the plant. Symptoms caused by nutrient deficiencies are generally grouped into five categories: (1) stunted growth, (2) chlorosis, (3) interveinal chlorosis, (4) purplish-red coloring, and (5) necrosis. Stunting is a common symptom for many deficient nutrients due to their varied roles in the plant. For example, when nutrients involved in plant functions such as stem elongation, photosynthesis, and protein production are deficient, plant growth is typically slow, and plants are small in stature.
Monitoring Nutrien Status
There are three basic tools available for monitoring nutrient status: (1) visual symptoms, (2) plant tissue analysis, and (3) media analysis (media testing). It is not possible to rely on visual symptoms of nutrient deficiency as single element since symptoms can often be confused with each other or with disease or other stresses. Also, once the deficiency has manifested itself by way of a symptom, the yield penalty may have already occurred. In many cases plants will not show obvious signs of a deficiency even though crop growth may be restricted.
Visual Diagnosis
Visual diagnosis is probably the least reliable method of fertility monitoring because it requires considerable experience and can vary by crop species, cultivar, and the conditions under which the crop is grown. As a rule, nutrients that are highly mobile through the phloem may be remobilized from older leaves and retranslocated to young growing organs, should supply shortages occur. Hence, the symptoms of deficiency in these nutrients appear first in the older leaves. In contrast, nutrients with poor mobility through the phloem cannot be retranslocated from old to young leaves when their availability decreases. Deficiency symptoms for these nutrients therefore appear first in new vegetation in the upper plant.
Plant Tissue Analysis
Tissue analysis is an analysis of representative plant parts, usually leaves, to determine the concentration of nutrients as well as potentially toxic elements that a plant has taken up. Different plant parts contain different levels of nutrients, but analysis of leaf tissue is most commonly used for diagnosing nutrient availability (uptake) in the media. The mineral content of plants is usually expressed as a percentage on a dry weight basis for the macronutrients (e.g., nitrogen, phosphorus, potassium, etc.). For micronutrients (e.g., iron, manganese, copper, zinc, boron, and molybdenum), mineral content may be expressed as milligram per gram (mg/g) or parts per million (ppm) of plant dry matter.
Media Testing
Growing media analysis at periodic intervals during the production cycle is one of the best ways to track the fertility status of a crop. Samples are taken and sent to a commercial or university lab for analysis of the media nutrient content. Some growers object to media testing because of the cost and time required to collect samples. However, this must be compared with potential crop loss. Usually a compromise can be found between the cost of processing a large number of samples and gaining the information needed to track a crop’s fertility status.
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