Plants, like animals and other forms of life, require a great diversity of compounds. Molecules of these compounds are used to build and maintain cells and to perform other necessary life processes, such as growth and reproduction, as well as respiration and photosynthesis.
Plants manufacture, by the process of photosynthesis, simple sugars. From these are formed polysaccharides (starches, cellulose) and, eventually, a variety of lipids (fats, oils, and waxes), hundreds of different protein molecules, and many other organic compounds.
Normal functioning of plants requires that they absorb, usually by roots from soil, a variety of chemical elements. From these elements, which are combined with water, carbon dioxide, and other compounds, the more complex compounds are formed.
Plant nutrition involves the uptake from the environment of the raw materials needed for performing a variety of biochemical processes, the conduction of these substances to all parts of the plant, and their use in growth and metabolic processes.
Most of these materials are in the form of ions dissolved in soil water. The water occupies the spaces between solid soil particles and is absorbed through the roots and then conducted upward by means of xylem tissue located in roots, stems, and leaves.
Present in soil water, and therefore in a plant growing in that soil, are more than sixty chemical elements. Some of these, however, are present only incidentally and perform no known function. Others, known as essential elements, are used in some particular way by the plant. These elements have been the object of study by many plant physiologists.
By the mid-1800’s, chemistry had become sufficiently advanced that botanists could analyze the chemical content of plants. They discovered that in the ash remaining after a plant had been burned were a variety of minerals.
In order to be considered an essential element, that element must be necessary for normal metabolic processes, growth, and reproduction; another element cannot replace it. Essential elements are classified as either macronutrients or micronutrients. Although both types of nutrients are required, macronutrients are needed in much larger amounts.
To recognize the difference between the two categories of minerals, one may consider the following. When freshly harvested plants are heated in an oven to remove the water, the dry matter remaining can be analyzed.
Macronutrients are those required in concentrations of at least 1,000 milligrams per kilogram of dry plant matter. In contrast, micronutrients are required in much smaller or even trace amounts, generally less than 100 milligrams per kilogram of dry matter.
Among the macronutrients, nitrogen is a key element needed in large amounts to form proteins, nucleotides, chlorophyll, and coenzymes. Phosphorus is also required to build nucleic acids, but it is also necessary to form adenosine triphosphate (ATP) and adenosine diphosphate (ADP), energy-carrying compounds vital to all cells.
Among the several functions of calcium are its roles as a component of cell walls and in changing the permeability of cell membranes. Magnesium is necessary to activate certain enzymes; also a single atom of magnesium occupies a central position in each molecule of chlorophyll. Sulfur is essential to synthesizing proteins, coenzyme A, thiamine, and biotin.
Manganese is involved in activating many enzymes. Boron is believed to be involved in carbohydrate synthesis; also it is required for nucleic acid synthesis. Zinc is required for the synthesis of the plant hormone auxin; it also is involved in enzyme activation.
Copper is present in the active site of redox enzymes and electron carriers. Nickel plays a role in enzyme functioning in the metabolism of nitrogen. Molybdenum is required for nitrogen fixation, the process by which free nitrogen (N2) in the air is converted into nitrates.
Identifying Essential Elements
To identify a particular mineral as essential, a classic protocol (series of steps) is followed. The method used today was developed by Julius von Sachs, an early German plant physiologist, in 1860: Two seedlings of the same kind of plant are grown in separate containers.
One container contains what is considered to be a complete growth medium. The other container contains all but a single mineral. If the growth of the seedling in the latter container is abnormal, or if its normal flowering and seed production are not successful, the missing element is determined to be essential.
Just as animals suffer symptoms when certain vitamins or minerals are absent in their diets, so plants are affected when they suffer a deficit in a particular essential element.
Such symptoms are used by farmers and gardeners to determine which types of fertilizer need to be added to the soil in order to correct the problem. Some of the more common mineral deficiencies include nitrogen, iron, magnesium, calcium, and phosphorus deficiencies.
For example, because plants require large amounts of nitrogen for growth, this mineral is often not present in sufficient amounts. A typical symptom is a uniform yellowing of the older leaves, a condition called chlorosis. An iron deficiency is diagnosed when the youngest leaves turn yellow.
In the case of a magnesium deficiency, the older leaves are yellow between the veins. A calcium deficiency causes the growing points to die back; young leaves are yellow and crinkly. Plants with a phosphorus deficiency turn dark green and have leaves with purple veins.
Alternative Nutrient Sources
Whereas most plants, both wild or cultivated, absorb required nutrients from soil water, some plants obtain nutrients by other means. Plants such as Venus’s fly trap, the pitcher plant, and various other insectivorous plants supplement soil nutrients by trapping insects or other small animals.
Once an animal is caught, its protein is digested, yielding nitrogen that is made available to the plant. Intricate traps and other means of attracting and capturing insects have evolved over long periods of time.
The traps are commonly modified leaves. Carnivorous plants generally inhabit sunny habitats with soils that have low levels of nitrogen. Thus nitrogen from animal protein is necessary to supplement that obtained in the more usual way from the soil.
Because of the special role of the proteins in plant (and animal) cells, nitrogen, a key element required to form proteins, is of special concern. Many plant species, especially those of the pea or legume family (Fabaceae) have root nodules, or mycorrhizae, filled with bacteria that are able to convert free nitrogen into compounds of nitrogen called nitrates.
This process, called nitrogen fixation, makes nitrates available to the plant (as well as to the bacteria) as a usable source of nitrogen. Also, the surrounding soil is enriched, making nitrogen available to other plants growing in the soil. Thus, nitrogen taken from the soil in large quantities, especially by cultivated crops, is replaced.
The common agricultural practice of rotating crops, in which a leguminous crop such as alfalfa or clover is alternated with corn or wheat, which removes large amounts of nitrogen from the soil, takes advantage of the nitrogen that has been fixed by legumes.
Hydroponic cultures allow for the growing of plants without soil. The plants survive on solutions that contain water and essential minerals. Plants are typically grown in greenhouses with their roots bathed in circulating water containing the minerals.
Provision must be made to keep the water aerated. Hydroponic plants can be grown year-round, but the added expense of hydroponic culture limits its use to fruits and vegetables made available to specialty markets.