Plant Nutrition of Greenhouse Crops
Diagnosing Plant Nutrient Deficiencies
Diagnosing nutritional deficiencies in greenhouse crops requires a combination of experience, methodical process, and information on the nutrient status of the crops. However, the symptoms expressed are often dependent on the species of plant grown, stage of growth or other controlling factors. Therefore, growers should become familiar with nutritional deficiencies on a crop-by-crop basis. 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. 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, soil testing, 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. Tracking soluble salts and pH have already been discussed in previous sections in this chapter and water analysis procedures and standards are covered in Chapter 10, Irrigation Water for Greenhouses.
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. While information is available depicting visual symptoms of deficiencies or toxicities of individual nutrients in greenhouse crops, in actual practice, visual diagnosis is often problematic because more than one element can be deficient at the same time.
Growing Media Testing
Growing media testing 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.
Growth media samples should be representative of the bed or bench of a given crop. A sampling strategy should consider crop species, planting time, container size, and environmental parameters such as shading, location in a greenhouse. Ideally, a sample should be taken from plants representing each of the possible variations in these factors. However, circumstances may not allow crop management to differ with each of these variations. Therefore, it is best to select several subsamples from plants that will be managed as a block and submit a composite sample.
All laboratories generate reports for each sample submitted for analysis. All reports will contain the same basic information although individual labs may present this information in their own unique format. Several methods are used in commercial and state agricultural laboratories to test growing media.
Understanding Laboratory Test Results
Media test reports vary from laboratory to laboratory; however, they all report key results of pH, lime test index (LTI) or buffer pH, phosphorous, potassium, and other parameters. These results are used to develop fertilizer recommendations. Other useful measures on the report, such as cation exchange capacity (CEC), organic matter, and base saturation, further define media parameters related to nutrient availability that should be considered as nutrient plans are developed. Table 15.3 lists common tests performed by laboratories and their respective units. The laboratory report may contain terminology which you may not be familiar with. A brief explanation for some of these terms is given below.
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 soil. The mineral content of plants is usually expressed as a percentage on a dry weight basis for the macro-elements (nitrogen, phosphorus, potassium, etc.). For micronutrients (iron, manganese, copper, zinc, boron, molybdenum), mineral content may be expressed as milligram per gram (mg/g) or parts per million (ppm) of plant dry matter.
When sampling leaves for analysis, several considerations must be made in addition to the correct leaf to take. Leaves should be collected at random within the house making sure enough are taken. Usually 15 to 20 leaves will be enough.
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