Chapter 15

Plant Nutrition of Greenhouse Crops

Fundamental Aspects of Plant Nutrition

Plants require certain nutrients to grow properly. Sixteen elements are considered to be essential for their growth and development. They are: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, zinc, boron, molybdenum, and chlorine. Plants are nonselective in absorbing nutrient elements from the growing medium. This means that the presence of a particular element in a plant tissue does not indicate that the element is essential for growth. For example silicon, chromium and cobalt have been found in many plant species but it is not known if they are essential for growth. Out of the 16 essential elements, carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are required in relatively large amounts and that is why these elements are referred to as macro or major elements. The remaining elements in the above list are micro-nutrients. They are required in small amounts to carry out different essential functions in the plant.

Absorption of Plant Nutrients

Plants use carbon, hydrogen and oxygen from the air and water in general from the growing medium to make simple foods by the process of photosynthesis. These substances are needed to make amino acids, proteins, and protoplasm. Other elements are taken up by plants through the roots. Moderate amounts are also absorbed through the leaves and stem tissues. Absorption through roots is the major route of nutrient uptake. If the root system is damaged by disease, insects or higher levels of soluble salts in the growing medium, the nutrient uptake is reduced.

Nutritional Interactions

Nutritional problems in greenhouse crops can be common because of the high rate of growth, the different specific requirements of each crop, the limited rooting volume of the container, and the limited nutrient reserve of the medium in which the crop is grown. Both the amount of nutrients being supplied and their balance play important roles in producing high-quality crops. Changing the level of one nutrient in a solution can affect the uptake or transport through the plant of another.

Cation-Exchange Capacity

The actual process of nutrient uptake by plants is controlled by the cation-exchange capacity (CEC) of the growing medium. This action is associated with the clay particles of a mineral soil. Organic materials such as peat moss also have a cation exchange capacity. The clay particle has a negative (-) surface charge and attracts cations (+ charge).

Anion-Exchange Capacity

Plants also need anions for good growth. Nitrates (NO3-), chlorides (Cl-) and sulfates (SO4-) are examples of anions. Negatively charged anions are not attracted by the negative charge of the clay particles. Thus, they are not held like cations.

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