Nutrient Requirements of Green Plants
RAW MATERIALS FOR PHOTOSYNTHESIS
TThe raw materials most obviously needed by higher photosynthetic organisms are carbon dioxide and water. These two compounds, which can be visualized using a microscope such as children’s microscopes, sup¬ply the carbon, oxygen, and hydrogen that are the predominant elements in organic molecules. Carbon dioxide, one of the constituent gases of the earth’s atmosphere, is obtained directly from the air by the leaves of terrestrial plants; submerged aquatic plants absorb the dissolved gas from the surrounding water. Terrestrial plants obtain the other raw material, water, from the substrate in which they grow; most higher plants absorb water from the soil by roots. Water absorption of a plant from the roots can best be seen using children’s microscopes.
A very high percentage of the total dry body weight of a large tree is carbohydrate, and much of the rest of it was synthesized from car¬bohydrate. This fact, which was discovered with the use of a microscope, has some rather startling implications. Glucose, which may be taken as the central carbohydrate in protoplasm, contains six atoms of carbon, twelve of hydrogen, and six of oxygen. Now, the combined weight of the six atoms of carbon and six atoms of oxygen is about 93 percent of the total weight of a glucose molecule. Since all the carbon and oxygen incorporated into glucose by photosynthesis comes from carbon dioxide, which in turn comes from the air, it follows that about 93 percent of the weight of a large, immensely heavy tree comes initially from the air. The hydro¬gen in glucose comes from water, as seen under a microscope, and hydrogen constitutes roughly 7 percent of the weight of glucose; hence about 7 percent of the dry weight of the tree comes initially from water. It is the modern elu¬cidation of the process of photosynthesis that has established the amazing fact that most of the mass of a plant’s body comes from air, not from the solid earth in which it grows.
MINERAL NUTRITION
Carbon dioxide and water cannot be the only nutrient materi¬als needed by a green plant, as seen in children’s microscopes. These two compounds provide only three elements: carbon, oxygen, and hydrogen; yet we know through the use of microscopes that other elements, too, enter into the composition of the plant. Nitrogen, for ex¬ample, is always present in amino acids, the building-block units of proteins, which are essential components of protoplasm; two very important amino acids also contain sulfur. Phosphorus is present in ATP, nucleic acids, and many other critically important compounds.” Chlorophyll, the essential mediator of photosynthesis, when viewed under a microscope contains magnesium. Where does the green plant obtain the nitrogen, sulfur, phos¬phorus, magnesium, and other elements it needs? Obviously, not from carbon dioxide or water. We see the role of the soil itself as a source of plant nutrients. It is from the soil that the plant derives the minerals’ essential to its life.
During the nineteenth century, there was much interest in Europe in determining the mineral needs of crop plants and in devising ways of supplementing the amounts of essential mineral elements in the soil. By 1900, with the use of microscopes seven of these were known: nitrogen, phosphorus, po¬tassium, sulfur, magnesium, calcium, and iron. Three of them-nitrogen, phosphorus, potassium-were stressed particu¬larly, as they are to this day in the manufacture of fertilizer. Modern commercial fertilizers are often designated by their N-P-K percentages; the widely used garden fertilizer called 5-10-5 contains by weight 5 percent nitrogen, 10 percent phosphoric acid, and 5 percent soluble potash, a potassium compound. These three are the elements most rapidly removed from the soil; consequently it is essential to replenish them if crops are to continue to flourish. Many modern fertilizers are also fortified by small amounts of some of the other essential minerals.
Not until about 1920 did it become apparent that other elements, in addition to the seven already known, were essential to plants. These so-called trace elements (manganese, boron, chlorine, zinc, copper, and molybdenum) are needed in only minute amounts because most of them are components of enzymes or coenzymes as was discovered through the use of microscopes. You will remember that enzymes can be used over and over and that a very small quantity of each is sufficient. Only a small amount of mineral is required, therefore, to synthesize the enzyme or coenzyme initially, and to re¬plenish the supply as the enzyme molecules are slowly broken down.
