ABSTRACT
Minerals that are required for plant growth and development are termed “essential�” These include carbon (C), hydrogen (H), and oxygen (O) as we described earlier are part of the photosynthesis process� These come from the air and water� The remaining essential elements come from the soil or nutrient solution in the case of hydroponic culture� These include nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg), which are required in relatively large amounts and therefore termed macro-or major elements� The others needed in very small amounts are termed micro-, minor, or trace elements� These include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl)� Nickel (Ni) is now believed to also be an essential element� Other elements accumulate in some plants and may be used in their growth� These are silicone (Si), aluminum (Al), cobalt (Co), vanadium (V), selenium (Se), and platinum (Pt)� However, when we speak of the elements that we must provide for our plants, whether in soil or hydroponics, they are the six macro-and seven microelements (eight if we include nickel) listed above�
Each of these essential elements has specific functions within the plant� It is helpful if we understand what these functions are to assist us in recognizing nutritional disorders that may occur in the plants�
Nitrogen-Part of organic compounds, including proteins, nucleic acids, and chlorophyll�
Phosphorus-Plays a role in respiration and cellular division and is used in the synthesis of energy compounds-adenosine triphosphate and adenosine diphosphate�
Potassium-Usually found in the meristems (tips of plants) where it activates many enzymes�
Calcium-Vital part of cell walls holding them together, maintains membrane integrity and acts in the movement of substances through cell membranes�
Magnesium-Essential component of the chlorophyll molecule and activates many enzymes�
Sulfur-Part of many amino acids� Iron-Essential for chlorophyll synthesis, enzyme activator, and acts as an
electron carrier in photosynthesis and respiration� Manganese-Enzyme activator� Boron-Involved in calcium ion use�
Zinc-Enzyme activator� Copper-Acts as an electron carrier and is part of certain enzymes� Molybdenum-An electron carrier in conversion of nitrate to ammonium� Chlorine-Acts as an enzyme activator in photosynthesis� Nickel-Essential for urease enzyme activity�
If any of these elements is in deficiency or excess, disorders will occur in the plants� These disorders will be expressed as symptoms� Symptoms (specific colors or deformities) will give you a clue that your plants are under stress and must be corrected to avoid loss in production�
The ability of the soil or hydroponics to provide adequate nutrition through the availability of the essential elements to plant roots depends on the amounts of the various elements present, their solubility (presence in the soil water or nutrient solution in a solution and not just a suspension), and the pH of the soil or nutrient solution� Soil nutrients exist in complex, insoluble compounds, and soluble forms readily available to plants� In hydroponics, highly soluble compounds are dissolved in water to obtain the nutrient solution that has the elements readily available to the plants� The reaction of the soil or hydroponic solution (pH) determines the availability of the various elements to the plant (Figure 7�1)� The pH is a measure of the acidity
or alkalinity� If the solution pH is less than seven it is acidic, seven is neutral, and greater than seven is alkaline� Most plants prefer a pH between 6�0 and 7�0 for optimum nutrient uptake regardless of whether it is the soil solution or a nutrient solution� Specific crops require different optimum pH ranges� For example, lettuce likes a pH between 5�5 and 5�8, whereas tomatoes, peppers, and cucumbers prefer a pH from 6�0 to 6�4�
If you encounter plant symptoms pointing to a lack of a specific element, be sure to check the pH of the solution in case the element may be present in ample amount, but, unavailable to the plant due to the incorrect pH� The pH can be tested with indicator papers, dye solutions, and pH meters� In hydroponic systems, the pH must be tested at least once a day� If it is too low, raise it with a base such as potassium hydroxide, or on a small-scale use bicarbonate of soda� With high pH values lower the pH with an acid such as sulfuric (battery) acid or phosphoric acid� You could also use vinegar (acetic acid) or citric acid for home use� The pH of cider vinegar at normal strength is 4�25-5�00� White vinegar is stronger with approximately 5%–8% acetic acid in water� It has a pH of approximately 2�4� An even easier method is to purchase a “pH Up” or “pH Down” solution from a hydroponic retail outlet or from an Internet website of hydroponic suppliers� Always remember to add acid to water to avoid splashing or fumes� It is best to use eye protection and wear gloves when using strong acids or bases to avoid burns�
