Plant Nutrition of Greenhouse Crops
Essential Plant Nutrients
Except for a few minor crops or specific plant species, there are 16 nutrients that are considered essential for plant growth. Each of these 16 nutrients is equally essential for proper plant growth and development, yet they are required in very different quantities. These differences led to a natural grouping of the elements into three categories: primary plant nutrients (macronutrients), secondary plant nutrients, and micronutrients or trace elements. The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). The secondary nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). Micronutrients or trace elements are those elements essential for plant growth that are needed in only very small (micro) quantities. The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo), and zinc (Zn). Some mineral nutrients have a structural function. For example, nitrogen is an integral part of plant proteins, and nitrogen and magnesium are structural components of chlorophyll molecules needed for photosynthesis. Understanding these basic functions has practical applications because a deficiency of either nutrient causes plants to be chlorotic (that is, yellowish in color). Other mineral nutrients have no structural role, but potassium, for example, is functionally important in causing the stomata in leaves to open and close.
The Concept of the Limiting Factor
The law of the minimum states that plant production can be no greater than that level allowed by the growth factor present in the lowest amount relative to the optimum amount for that factor. This growth factor, be it temperature, nitrogen, sulfur, or water supply, will limit the amount of growth that can occur and is therefore called the limiting factor.
Nutrient Mobility within the Plant
All nutrients move relatively easily from the root to the growing portion of the plant through the xylem. Interestingly, some nutrients can also move from older leaves (the source) into developing flowers or leaves (the sink) if there is a deficiency of that nutrient. Knowing which nutrients are “mobile” (i.e., able to move) is very useful in diagnosing plant nutrient deficiencies because if only the lower leaves are affected, then a mobile nutrient is most likely causing the deficiency.
Antagonistic and Synergistic Effects of Soil Nutrients
Two or more nutrients may interact in plants. Nutrient interactions are generally measured in terms of growth response and change in concentration of nutrients. When an increase in crop yields by the addition of two nutrients is more than adding only one, the interaction is positive (synergistic). If adding the two nutrients together produces fewer yields than the individual ones, the interaction is negative (antagonistic). When there is no change, there is no interaction. A nutrient may interact simultaneously with more than one nutrient. This may induce deficiencies, toxicities, modified growth responses, and/or modified nutrient composition.
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