Food is an essential commodity that separates prosperous nations from struggling ones. For instance, North Korean agriculture met that entire country's food needs until about a decade ago. The country's farmers were highly efficient and productive. Its food crisis began with the collapse of the Soviet Union, which had provided North Korea with chemicals and petroleum. This loss of support was followed by three years of drought, hailstorms, and floods. Today, North Korea is a starving country with a failed farming system.
Why should a desperate shortage of chemicals and petroleum affect a nation's agriculture? Crop production depends on several factors, but the one that is most commonly limiting is a supply of nitrogen in a form usable by plants. All plants require the element nitrogen, which is an abundant component of proteins and nucleic acids as well as chlorophyll and many other important biochemical compounds. If a plant cannot get enough nitrogen, it cannot synthesize these compounds at a rate adequate to keep itself healthy. To meet their crops' need for nitrogen and other minerals, farmers in all parts of the world apply fertilizers of one kind or another. The industrial production of fertilizers is an energy-intensive process, and the energy needed is most commonly obtained from petroleum. Without petroleum, North Korea cannot begin to provide the fertilizer needed to restore its crop production.
In addition to nitrogen, plants need other materials from their environment. In this chapter, we will explore the differences between the basic strategies of plants and of animals for obtaining nutrition. Then we will look at what nutrients plants require and how they acquire them. Because most nutrients come from the soil, we will discuss the formation of soils and the effects of plants on soils. As any farmer can tell you, nitrogen is the nutrient that most often limits plant growth, so we will devote a section specifically to nitrogen metabolism in plants. The chapter concludes with a look at carnivorous and parasitic plants, which supplement their nutrition in special ways.
Nitrogen is Essential for Plant Growth
In this experimental wheat field in Bangladesh, nitrogen was withheld from the plot on the left.The resulting plants were stunted and unhealthy.
Every living thing must obtain raw materials from its environment. These nutrients include the major ingredients of macromolecules: carbon, hydrogen, oxygen, and nitrogen. Carbon and oxygen enter the living world in the form of atmospheric carbon dioxide through the carbon-fixing reactions of photosynthesis. Hydrogen enters living systems through the light reactions of photosynthesis, which split water. For carbon, oxygen, and hydrogen, photosynthesis is the gateway to the living world, and these elements are in plentiful supply.
In the remainder of this chapter, we shall focus our attention on nitrogen, which is in relatively short supply for plants. The movement of nitrogen into organisms begins with processing by some highly specialized bacteria living in the soil. Some of these bacteria act on nitrogen gas, converting it into a form usable by plants. The plants, in turn, provide organic nitrogen (and carbon) to animals, fungi, and many microorganisms.
In addition to nitrogen, other mineral nutrients are essential to living organisms. The proteins of organisms contain sulfur (S), and their nucleic acids contain phosphorus (P). There is magnesium (Mg) in chlorophyll, and iron (Fe) in many important compounds, such as the cytochromes. Within the soil, these and other minerals dissolve in water, forming a solution—called the soil solution—that contacts the roots of plants. Plants take up most of these mineral nutrients from the soil solution in ionic form.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.