Green plants use the sun's energy for only certain stages in the build - up of carbohydrates, but from these all the other compounds necessary for protoplasm and cell walls can be made. The sugar molecules are linked up into longer molecules to make cellulose. Additional elements such as nitrogen and sulphur are combined with carbohydrates to make proteins.
The necessary energy for these processes comes from respiration, and they go on continuously in the plant, day and night.
Essential elements. Besides carbon, hydrogen and oxygen assimilated during photosynthesis, the plant takes up other elements from the soil. The chlorophyll molecule needs magnesium; many enzyme systems need phosphorus, calcium is used in the material between cell walls and potassium plays some part in controlling the rates of photosynthesis and respiration. These Clements and the nitrogen and sulphur used in making proteins are obtained in the form of soluble salts from the soil. They are absorbed in very dilute solution by the root system. The plant must be able to collect these salts from the soil and concentrate them. The following are examples of soluble salts containing the essential elements mentioned above, but these are not necessarily the ones that occur in the soil:
Potassium nitrate contains potassium and nitrogen.
Magnesium sulphate contains magnesium and sulphur.
Potassium phosphate contains potassium and phosphorus.
Calcium nitrate contains calcium and nitrogen.
Many other elements, e.g. copper, manganese, boron, in very small quantities are also needed for healthy growth. These are often referred to as trace elements.
Water cultures. Since a plant can make all its Vital substances from carbon dioxide, water and Salts, it is possible to grow plants in water containing the necessary salts. These solutions are called water cultures, and by carefully controlling the salts present it is possible to and out the relative importance of particular elements in the growth of the plant.