Most terrestrial plants obtain nutrients and water from the soil

A few unusual plants, for example the epiphytes discussed in Chapter 1 (Figure 1.9), obtain necessary water and nutrients without being rooted in the soil. However, the vast majority of plants obtain nutrients and water from the soil through their root systems. As a result, plants have a number of adaptations that help them perform this task.

Soil Nutrients

In addition to the oxygen, carbon, and hydrogen that plants incorporate into carbohydrates to fuel their survival and growth, plants require many other inorganic nutrients including nitrogen, phosphorus, calcium, and potassium to make proteins, nucleic acids, and other essential organic compounds. Whereas oxygen and carbon are available in the air, other nutrients are obtained as ions dissolved in the water held by the soil around plant roots. Nitrogen exists in soil as ammonium (NH₄⁺) and nitrate (NO₃⁻) ions, phosphorus exists as phosphate ions (PO₄³⁻), and calcium and potassium exist as the elemental ions Ca²⁺ and K⁺, respectively. The availability of these and other inorganic nutrients varies with their chemical form in the soil, and with temperature, pH, and the presence of other ions. A scarcity of inorganic nutrients, such as nitrogen, often limits plant production in terrestrial environments. We shall have much more to say about nutrient uptake by plants in later chapters.

Soil Structure and Water-Holding Capacity

To understand how plants obtain water and nutrients, it is first necessary to understand how water behaves in the soil. The movement of water in the soil can be described in terms of its water potential, which is a measure of the water’s potential energy. Water potential affects the movement of water in the soil from one location to another and depends on several factors that include gravity, pressure, osmotic potential (discussed in Chapter 2), and matric potential, so named because the collection of all soil particles is known as the soil matrix. The matric (or matrix) potential is the potential energy generated by the attractive forces between water molecules and soil particles. It exists because water molecules, which have electrical charges, are attracted to the surfaces of soil particles, which also have electrical charges. This attraction explains why soil is able to retain water against the downward pull of gravity.

Because electrical charges are responsible for the attraction between water molecules and soil particles, those water molecules closest to the surfaces of soil particles adhere the most strongly. When water is plentiful, most of the water molecules are not close to the surfaces of soil particles. As a result, these water molecules are not held tightly and plant roots can easily take up the water. As more of the water is used up, however, the water molecules that remain are positioned close to the soil particles and adhere tightly.

As we learned when discussing osmotic pressure in Chapter 2, scientists quantify water potential in units of pressure, called megapascals (MPa). In a soil that is completely saturated with water, as illustrated in Figure 3.1a, the matric potential is 0 MPa. When a saturated soil drains under the force of gravity, the resulting matric potential is about −0.01 MPa. At this point, the force of gravity on the water molecules is equally opposed by the attractive force of soil particles on the water molecules. The maximum amount of water held by soil particles against the force of gravity is called the field capacity of the soil. The field capacity, illustrated in Figure 3.1b, represents the maximum amount of water available to plants.

Figure 3.1 Soil water. (a) Immediately after a rain event, soils can become saturated with water and all spaces between soil particles are filled. (b) The field capacity of soil represents the amount of water remaining after it has been drained by gravity. (c) The wilting point occurs when the opposing attractive forces of the soil particles prevent plants from extracting any more water.

As water becomes less abundant, such as when plants take up some of the water from the soil, the matric potential values become more negative. Water always moves from areas of higher potential (less negative values) to areas of lower potential (more negative values). So, for plants to extract water from the soil, they must produce a water potential that is lower than that of the soil. As soils dry out, they hold the remaining water ever more tightly because a greater proportion of that water lies close to the surfaces of soil particles. Most crop plants can extract water from soils with water potentials down to about −1.5 MPa. At lower soil water potentials, these plants wilt, even though some water still remains in the soil, as shown in Figure 3.1c. Scientists refer to a water potential of −1.5 MPa as the wilting point of soil because this is the lowest water potential at which most plants can obtain water from the soil. The wilting point of −1.5 MPa, however, is only a general rule of thumb. Many species of drought-adapted plants can extract water when the water potential is less than −1.5 MPa.

The amount of water in soil and its availability to plants depends on the physical structure of the soil. It also explains why the amount of water the soil can hold depends on the soil’s surface area; for a given volume of soil, the more surface area that soil has, the more water it can hold. The surface area of soil depends on the sizes of the particles that comprise the soil. Soil particles include sand, silt, and clay, in addition to organic material from decomposing organisms. As shown in Figure 3.2, sand particles are the largest, with diameters exceeding 0.05 mm. Silt particles have diameters of 0.002 to 0.05 mm and clay particles are the smallest, with a diameter of less than 0.002 mm. Rarely is a soil composed of a single particle size. Instead, as illustrated in Figure 3.3, soils are typically composed of mixtures of different ratios of each particle size. For example, a soil composed of 40 percent sand, 40 percent silt, and 20 percent clay is classified as a loam soil. In contrast, a soil containing a higher proportion of silt and a lower proportion of sand is classified as a silt loam soil.

Figure 3.2 Soil particle size. (a) Soil particles are separated into three sizes: clay, silt, and sand. (b) Each soil particle attracts a surface film of water around it. The greater surface area of small clay particles holds a greater total amount of water than the much larger sand particles, which have a much smaller surface area relative to their volume.

Figure 3.3 Combinations of soil particle sizes used to categorize soils. Most soils are composed of different percentages of sand, silt, and clay. Each name represents a category with a specific composition of the three particle sizes.

Smaller particles have a larger surface area relative to their volume compared to larger particles. As a result, the total surface area of particles in a given volume of soil increases as particle size decreases. Therefore, soils with a high proportion of clay particles hold more water than soils with a high proportion of silt particles, which hold more water than soils with a high proportion of sand particles. Soils with a high proportion of sand particles tend to dry out because water quickly drains away, leaving many tiny pockets of air between the large sand particles. Clay soils represent the opposite extreme; each tiny particle of clay can attract a thin film of water on its surface, leaving little space for air pockets. Although clay soils retain a lot of water, clay particles can hold water molecules so tightly that it can be difficult for plants to extract the water from the soil.

In Figure 3.4, we can see how soil particle size affects the amount of water in the soil, measured in terms of the percent of the soil volume occupied by water. As we move from sand to silt to clay, there is an increase in field capacity. However, there is also an increase in the wilting point. The difference between the field capacity and the wilting point is the amount of water available to plants. Thus, even when precipitation is frequent, sandy soils cannot retain much of the water that enters the soil. At the other extreme, clay soils can retain a lot of water, but if precipitation is not frequent enough for the soil to reach its field capacity, most of the water in the soil will be unavailable. This means that soils high in sand or high in clay are both poor soils for growing many plants including crops that humans rely on for food. Instead, soils that contain a mixture of clay, silt, and sand particles—such as loam—are some of the best soils for growing plants.

Figure 3.4 Water-holding capacity of different soils. Soils composed of different combinations of sand, silt, and clay differ in their water-holding capacity. Soils containing large amounts of sand have a low field capacity and a low wilting point. In contrast, soils containing large amounts of clay have a high field capacity and a high wilting point.

Osmotic Pressure and Water Uptake

In Chapter 2 we noted that osmotic forces cause water molecules to move from areas of low solute concentration to areas of high solute concentration. At the same time, ions and other solutes diffuse through water from regions of high solute concentration to regions of low solute concentration. In the case of a plant, if a root cell has a higher solute concentration than the soil water, osmotic forces can draw the water into the root. It is this osmotic potential in plant roots that causes water to enter the roots from the soil against the attractive forces of soil particles and the downward pull of gravity.

Without any other adaptations, we would expect the solute concentrations within the root cell and in the soil water to eventually come into equilibrium. At this point, the osmotic potentials of the root cell and its surroundings would be equal, and there would be no net movement of water into the plant. However, root cells possess two adaptations that prevent this equalization. First, semipermeable cell membranes prevent larger solute molecules from leaving the plant’s root. Second, cell membranes can actively transport ions and small molecules against a concentration gradient into the root cells. These two adaptations maintain high solute concentrations inside the roots and allow strong osmotic forces to continue.

As noted earlier, plants growing in places with very negative water potentials typically have adaptations to help them extract water beyond −1.5 MPa. Plants living in deserts, for example, can lower the water potential of their roots to as much as −6 MPa, thereby overcoming soil water potentials down to −6 MPa. Plants living in very salty environments can also meet the challenge of extracting water from an environment that contains unusually high concentrations of solutes in the form of salt ions. In both situations, plants have evolved adaptations that allow them to increase the concentrations of amino acids, carbohydrates, or organic acids in their root cells. Maintaining these high concentrations of dissolved substances, however, comes at a high metabolic price for the plants because they must divert some of the energy that would normally be used for growth and use it instead to manufacture these additional organic compounds.

Plants lacking the appropriate adaptations grow poorly when exposed to salty conditions. For example, in the deserts of the American southwest, a large amount of land is irrigated to grow crops, including cotton and orchard trees. Most well water, however, contains small amounts of salt. As this well water is sprayed over the fields, it moves down into the soil and dissolves any salts. If the irrigation system uses large amounts of water, the water can move deeply into the soil and the salts can be flushed away. If irrigation uses smaller amounts of water—just enough to feed the plant roots—the water stays near the soil’s surface. As much of this water is then taken up by plants or evaporates, the salt is left behind on the soil’s surface. With repeated irrigation events, the salt concentration of the soil continually increases. After many years, the soil can have such a high solute concentration that many crop plants cannot create a lower water potential than the soil, and therefore cannot obtain sufficient water. The process of repeated irrigation that causes increased soil salinity is known as salinization. High-salt soils occur across 831 million hectares (ha) of land and in 100 countries of the world. It is a particular problem throughout arid areas of the world where irrigation typically is limited to small amounts of water, which concentrates the salts at the soil surface.

Transpiration and the Cohesion-Tension Theory

We have seen how osmotic potential draws water from the soil into the cells of plant roots. How does that water get from the roots to the leaves? As you might recall from a previous biology course, plants conduct water to their leaves through tubular xylem elements, which are the empty remains of xylem cells in the cores of roots and stems, connected to form the equivalent of water pipes. The movement of water through these xylem cells depends on the cohesion of water molecules and differences in water potential between the leaves and roots.

The cohesion of water is the result of mutual attraction among water molecules. The attraction of hydrogen bonds causes a water molecule moving up the xylem of a plant to pull other water molecules with it. Cohesion of water helps the entire column of water move through the long vessels of a tall tree. The process, shown in Figure 3.5, begins when osmotic potential in the roots draws water from the soil into the plant and creates a root pressure that forces water into the xylem elements. However, this pressure is counteracted by gravity and the osmotic potential inside living root cells. Because of these two important counteracting forces, root pressure under the best circumstances can raise water to a height of no more than about 20 m, even though the tallest trees can achieve heights of more than 100 m.

Figure 3.5 Water movement in plants by cohesion and tension. Differences in water potential, also known as tension, cause water to move from the soil into the roots, from the roots into the stem, and from the stem into the leaves. The cohesion of water causes the water molecules to adhere to each other and to move as a single column of water up the xylem cells.

Fortunately for plants, leaves can also generate water potential as water evaporates from the surfaces of leaf cells into the tiny air spaces inside the leaf, ultimately moving out of the leaf and into the air. This process is known as transpiration. The column of water in a xylem element is continuous from the roots to the leaves, since it is held together by hydrogen bonds between the water molecules. Thus, low water potentials in leaves can literally draw water upward through the xylem elements against the osmotic potential of both the living root cells and the pull of gravity. The water potential is low enough under most conditions to pull water up through the roots, xylem, and leaves. The water potential from transpiration creates a continuous gradient of water potential from leaf surfaces in contact with the atmosphere down to the surfaces of root hairs in contact with soil water. The movement of water is due to both water cohesion and water tension (which is another name for differences in water potential). This mechanism of water movement from roots to leaves due to water cohesion and water tension is known as the cohesion-tension theory.

Based on the cohesion–tension theory of water transport in plants, very tall plants should have a more difficult time moving water up their stems because the movement of a tall column of water in the plant is being opposed by the force of gravity. Recent research estimates that this system limits plants to a maximum height of 130 m. In support of this prediction, the tallest tree that has been reliably measured was a 126-m Douglas fir tree.

Although transpiration generates a powerful force that moves water through a plant, when the soil reaches the wilting point, water lost from the leaves of a plant can no longer be replaced by new water moving into the roots. To prevent further water loss from the leaves, plants have various adaptations for controlling transpiration. Most of the cells on the exterior of a leaf are coated with a waxy cuticle that retards water loss. As a result, gas exchange between the atmosphere and the interior of the leaf primarily occurs through small openings on the surface of the leaves, called stomata (Figure 3.6). The stomata (stoma, singular) are the points of entry for CO₂ and the points of exit for water vapor escaping to the atmosphere by transpiration. When plants experience a scarcity of water, they can reduce water loss to the atmosphere by closing their stomata. As leaf water potential weakens, the guard cells that border a stoma collapse slightly, causing the guard cells to press together and close the stoma. Although closing the stomata provides the important benefit of reducing water loss, it comes at the cost of preventing CO₂ needed for photosynthesis from entering the leaf. As we will soon see, plants living in hot, dry environments have evolved additional adaptations for dealing with this undesirable side effect.

Figure 3.6 Stomata. Stomata are pores in the surfaces of leaves, each bordered by two guard cells. Under conditions of low water availability, the guard cells close the opening and prevent the loss of water from the leaves.

Photo by Callista Images Cultura/Newscom.