5.4 Nutritional Requirements of Plants

Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow. An understanding of plant material can inform more easily how it is broken down in decomposition or digestion.

The Chemical Composition of Plants

Plants are primarily composed of carbohydrates, used as the primary source of energy and supporting structural integrity.

Plant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be composed of either organic or inorganic compounds. An organic compound is a chemical compound that contains carbon, such as carbohydrates, lipids, proteins, and nucleic acids and is made by a living organism. Carbon that was obtained from atmospheric CO₂ is incorporated into organic molecules by plants and as such, composes the majority of the dry mass within most plants. An inorganic compound does not contain carbon (except in the form of CO₂) and is not part of, nor is it produced by, a living organism. Inorganic substances, which form the majority of the soil solution, are commonly called minerals: those required by plants include nitrogen (N) and potassium (K) for structure and regulation.

Essential Nutrients

Plants require only light, water, and about 20 elements to support all their biochemical needs: these 20 elements are called essential nutrients. For an element to be regarded as essential, three criteria are required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the function of the element; and 3) the element is directly involved in plant nutrition.

Macronutrients and Micronutrients

The essential elements can be divided into two groups: macronutrients and micronutrients. Nutrients that plants require in larger amounts are called macronutrients. About half of the essential elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is therefore present in all macromolecules. On average, the dry weight (excluding water) of a cell is 45 percent carbon. Carbon is a key part of plant biomolecules, followed by oxygen (45 percent) and hydrogen (6 percent), which are the next two most abundant elements in plants.

Figure 5.9. Cellulose, the main structural component of the plant cell wall, makes up over thirty percent of plant matter. It is the most abundant organic compound on earth. [Image Description]

Whereas carbon is obtained from the atmosphere through the process of photosynthesis, other macronutrients and micronutrients are obtained from the soil through the roots. Nitrogen in the form of nitrate (NH₃⁺) is critical for the synthesis of amino acids and nucleotides, which make up proteins and nucleic acids found in cells throughout a plant. Phosphorus in the form of inorganic phosphate (PO₄^2-) is important for the synthesis of nucleotides and phospholipids.

Deficiencies in any of these nutrients—particularly the macronutrients—can adversely affect plant growth (Figure 5.10). Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis (yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of cell death.

Figure 5.10. Nutrient deficiency is evident in the symptoms these plants show. This (a) grape tomato suffers from blossom end rot caused by calcium deficiency. The yellowing in this (b) Frangula alnus results from magnesium deficiency. Inadequate magnesium also leads to (c) interveinal chlorosis, seen here in a sweetgum leaf. This (d) palm is affected by potassium deficiency. (credit c: modification of work by Jim Conrad; credit d: modification of work by Malcolm Manners) [Image Description]

Check out this video describing specific nutrient deficiencies in plants:

Video 5.1. Plant Nutrition | Plants | Biology | FuseSchool by FuseSchool – Global Education

The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant’s total weight. Soil is the water source for land plants, and can be an abundant source of water, even if it appears dry. Water flow into root cells occurs through osmosis and is promoted by the active uptake of nutrients from the soil into the plants. Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem. As water vapor is lost from the leaves, the process of transpiration and the polarity of water molecules (which enables them to form hydrogen bonds) draws more water from the roots up through the plant to the leaves (Figure 5.11). Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.

Figure 5.11. Water is absorbed through the root hairs and moves up the xylem to the leaves. [Image Description]

Figure Descriptions

Figure 5.9. The graphic has two levels of detail. Across the top, three pale-green, rope-like cellulose fibers run horizontally, suggesting the bundled strands that reinforce plant cell walls. Below, a close-up cellulose structure depicts six repeating β-glucose rings drawn as hexagons that alternate orientation. Each ring bears –CH₂OH and –OH groups, and the rings are joined by oxygen bridges to form straight β-1,4 glycosidic bonds; square brackets and an “n” on the right indicate the pattern continues for many units. Together the fiber view and the molecular diagram highlight cellulose as long, unbranched chains of glucose that aggregate into strong structural fibers. [Return to Figure 5.9]

Figure 5.10. The composite image arranges four square photographs in a 2 × 2 grid, each labeled with a lowercase letter. (a) shows a grape-tomato plant; one pale green fruit hangs downward with a dark, sunken, leathery spot at its blossom end—classic blossom-end rot caused by calcium deficiency. (b) depicts a cluster of Frangula alnus (alder buckthorn) leaves in partial shade; several leaves display irregular yellow patches while their veins remain darker green, indicating magnesium deficiency. (c) focuses on a single dropped sweetgum leaf lying on soil; the veins form a bold green network while the tissue between them is yellow—a pattern called interveinal chlorosis, also linked to inadequate magnesium. (d) features a fan-shaped palm; many frond tips and outer leaflets are yellowing or browning, symptoms typical of potassium deficiency. Together, the quartet illustrates how different mineral shortages produce distinct visual cues in plant tissues. [Return to Figure 5.10]

Figure 5.11. The illustration depicts a single, beige primary root drawn as a narrow cone tapering to a point. Along its mid-section dozens of fine, hair-like extensions—root hairs—protrude sideways, increasing the surface area for water uptake. At the thick top end, a circular cross-section exposes the vascular core, where a star-shaped bundle labeled xylem occupies the center and a surrounding ring labeled phloem encircles it. Thin, brick-like cortex cells fill the space between the vascular bundle and the outer epidermis. The labels emphasize that water enters through the root hairs and then travels upward through the xylem toward the leaves. [Return to Figure 5.11]

Figure 5.12. The image shows a detailed diagram illustrating the process of nitrogen uptake in plants. On the right is a cross-section of a plant root with root hairs, colored in various earthy tones. To the left, a close-up view depicts the plant cell membrane. Within this section, a blue proton (H+) pump and a red nitrate (NO3-) transporter are shown embedded in the membrane. The proton pump transfers H+ ions from the plant cell into the soil, converting ATP to ADP with the release of inorganic phosphate (Pi). The nitrate transporter takes up nitrate ions from the soil into the plant cell. This entire process is shown as taking place within a simplified plant cell environment, with clear lines indicating each part of the mechanism. [Return to Figure 5.12]