31: Soil and Plant Nutrition

Question 1

What is an inorganic compound?

Answer 1

A chemical compound that does not contain carbon; it is not part of or produced by a living organism.

Question 2

What is a macronutrient?

Answer 2

A nutrient that is required in large amounts for plant growth; carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.

Question 3

What is a micronutrient?

Answer 3

A nutrient required in small amounts; also called a trace element.

Question 4

What is an organic compound?

Answer 4

A chemical compound that contains carbon.

Question 5

What are the major determinants of plant distribution and growth?

Answer 5

Soil quality and climate.

Question 6

What allows plants to grow?

Answer 6

The combination of soil nutrients, water, and carbon dioxide, along with sunlight.

Question 7

How much volume of a plant is water?

Answer 7

The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant’s total weight.

Question 8

How does water move through a plant?

Answer 8

Soil is the water source for land plants, and can be an abundant source of water, even if it appears dry. 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.

Question 9

How do plants use water?

Answer 9

Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.

Question 10

What are nutrients?

Answer 10

Plant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be composed of either organic or inorganic compounds.

Question 11

What constitutes most of the dry mass of plants?

Answer 11

Carbon that was obtained from atmospheric CO₂ composes the majority of the dry mass within most plants.

Question 12

What are minerals?

Answer 12

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.

Question 13

What are the criteria for an element to be regarded as essential?

Answer 13

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.
3. The element is directly involved in plant nutrition.

Question 14

What are the essential micronutrients for plant growth?

Answer 14

• Iron (Fe)
• Manganese (Mn)
• Boron (B)
• Molybdenum (Mo)
• Copper (Cu)
• Zinc (Zn)
• Chlorine (Cl)
• Nickel (Ni)
• Cobalt (Co)
• Selenium (S)
• Silicon (Si)

Question 15

In which types of compounds is carbon found?

Answer 15

Carbon (C) is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is present in all macromolecules.

Question 16

How much of the dry weight of plants is carbon?

Answer 16

On average, the dry weight (excluding water) of a cell is 50 percent carbon.

Question 17

How is nitrogen used in plants?

Answer 17

Behind carbon, nitrogen (N) is the next most abundant element in plant cells. It is part of proteins and nucleic acids. Nitrogen is also used in the synthesis of some vitamins.

Question 18

How is hydrogen and oxygen used in plants?

Answer 18

Hydrogen and oxygen are macronutrients that are part of many organic compounds, and also form water. Oxygen is necessary for cellular respiration; plants use oxygen to store energy in the form of ATP.

Question 19

How is phosphorus used in plants?

Answer 19

Phosphorus (P), another macromolecule, is necessary to synthesize nucleic acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical energy through oxidative phosphorylation. Likewise, light energy is converted into chemical energy to be extracted during respiration.

Question 20

How is sulfur used in plants?

Answer 20

Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also plays a role in photosynthesis as part of the electron transport chain, where hydrogen gradients play a key role in the conversion of light energy into ATP.

Question 21

How is potassium used in plants?

Answer 21

Potassium (K) is important because of its role in regulating stomatal opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a potassium ion pump supports this process.

Question 22

How is magnesium and calcium used in plants?

Answer 22

Magnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate nutrient transport, and to support many enzyme functions. Magnesium is important to the photosynthetic process. These minerals, along with the micronutrients, also contribute to the plant’s ionic balance.

Question 23

What is the result of deficiencies in nutrients?

Answer 23

Deficiencies in nutrients, particularly macronutrients, can adversely affect plant growth. 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.

Question 24

What is hydroponics?

Answer 24

Hydroponics is a method of growing plants in a water-nutrient solution instead of soil. Since its advent, hydroponics has developed into a growing process that researchers often use. Scientists who are interested in studying plant nutrient deficiencies can use hydroponics to study the effects of different nutrient combinations under strictly controlled conditions. Hydroponics has also developed as a way to grow flowers, vegetables, and other crops in greenhouse environments. Hydroponically-grown produce can often be found at grocery stores, where many lettuces and tomatoes are now grown hydroponically.

Question 25

What is the A horizon?

Answer 25

It consists of a mixture of organic material with inorganic products of weathering.

Question 26

What is the B horizon?

Answer 26

A soil layer that is an accumulation of mostly fine material that has moved downward.

Question 27

What is bedrock?

Answer 27

Solid rock that lies beneath the soil.

Question 28

What is the C horizon?

Answer 28

A layer of soil that contains the parent material, and the organic and inorganic material that is broken down to form soil; also known as the soil base.

Question 29

What is clay?

Answer 29

Soil particles that are less than 0.002 mm in diameter.

Question 30

What is a horizon?

Answer 30

A soil layer with distinct physical and chemical properties, which differs from other layers depending on how and when it was formed.

Question 31

What is humus?

Answer 31

Organic material of soil; made up of microorganisms, dead animals, and plants in varying stages of decay.

Question 32

What is loam?

Answer 32

Soil that has no dominant particle size.

Question 33

What is mineral soil?

Answer 33

A type of soil that is formed from the weathering of rocks and inorganic material; composed primarily of sand, silt, and clay.

Question 34

What is the O horizon?

Answer 34

A layer of soil with humus at the surface and decomposed vegetation at the base.

Question 35

What is organic soil?

Answer 35

A type of soil that is formed from sedimentation; composed primarily of organic material.

Question 36

What is parent material?

Answer 36

Organic and inorganic material in which soils form.

Question 37

What is the rhizosphere?

Answer 37

The area of soil affected by root secretions and microorganisms.

Question 38

What is sand?

Answer 38

Soil particles between 0.1–2 mm in diameter.

Question 39

What is silt?

Answer 39

Soil particles between 0.002 and 0.1 mm in diameter.

Question 40

What is a soil profile?

Answer 40

A vertical section of a soil.

Question 41

What is soil?

Answer 41

The outer loose layer that covers the surface of Earth.

Question 42

What does soil quality depend upon?

Answer 42

Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.

Question 43

How does soil develop?

Answer 43

Soil develops very slowly over long periods of time, and its formation results from natural and environmental forces acting on mineral, rock, and organic compounds.

Question 44

How can soil be categorized?

Answer 44

Soils can be divided into two groups: organic soils are those that are formed from sedimentation and primarily composed of organic matter, while those that are formed from the weathering of rocks and are primarily composed of inorganic material are called mineral soils. Mineral soils are predominant in terrestrial ecosystems, where soils may be covered by water for part of the year or exposed to the atmosphere.

Question 45

What are the major components of soil?

Answer 45

Soil consists of these major components:

• Inorganic mineral matter, about 40 to 45 percent of the soil volume
• Organic matter, about 5 percent of the soil volume
• Water and air, about 50 percent of the soil volume

The amount of each of the four major components of soil depends on the amount of vegetation, soil compaction, and water present in the soil. A good healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life.

Question 46

How does humus improve soil quality?

Answer 46

Humus improves soil structure and provides plants with water and minerals.

Question 47

What is the source of the inorganic material of soil?

Answer 47

The inorganic material of soil consists of rock, slowly broken down into smaller particles that vary in size.

Question 48

What is the ideal physical composition of soil?

Answer 48

Soil should ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should contain water, and the other half should contain air.

Question 49

What is the role of organic material in soil?

Answer 49

The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms–bacteria, algae, or fungi–are dormant in dry soil, but become active once moisture is available.

Question 50

Which factors account for soil formation?

Answer 50

Parent material, climate, topography, biological factors, and time.

Question 51

How does parent material account for soil formation?

Answer 51

The organic and inorganic material in which soils form is the parent material. Mineral soils form directly from the weathering of bedrock, the solid rock that lies beneath the soil, and therefore, they have a similar composition to the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured material, such as clay.

Question 52

How does climate affect soil formation?

Answer 52

Temperature, moisture, and wind cause different patterns of weathering and therefore affect soil characteristics. The presence of moisture and nutrients from weathering will also promote biological activity: a key component of a quality soil.

Question 53

How does topography affect soil formation?

Answer 53

Regional surface features (familiarly called “the lay of the land”) can have a major influence on the characteristics and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steep soils are more prone to erosion and may be thinner than soils that are relatively flat or level.

Question 54

How do biological factors affect soil formation?

Answer 54

The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can produce pores and crevices, and plant roots can penetrate into crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root, in an area known as the rhizosphere. Additionally, leaves and other material that fall from plants decompose and contribute to soil composition.

Question 55

How does time affect soil formation?

Answer 55

Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil.

Question 56

What are the layers of soil profile?

Answer 56

The soil profile has four distinct layers:

• O horizon
• A horizon
• B horizon, or subsoil
• C horizon, or soil base

Question 57

What are some characteristics of the O horizon?

Answer 57

The O horizon has freshly decomposing organic matter–humus–at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients and enhances soil moisture retention. Topsoil–the top layer of soil–is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Topsoil is rich in organic material; microbial processes occur there, and it is the “workhorse” of plant production.

Question 58

What are some characteristics of the A horizon?

Answer 58

The A horizon consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. This horizon is typically darkly colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries materials from the surface.

Question 59

What are some characteristics of the B horizon?

Answer 59

The B horizon is an accumulation of mostly fine material that has moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate.

Question 60

What are some characteristics of the C horizon?

Answer 60

The C horizon, or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place, or transported from elsewhere to its present location. Beneath the C horizon lies bedrock.

Question 61

How do soil profiles differ between locations?

Answer 61

Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable, and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers.

Question 62

What do soil scientists study?

Answer 62

A soil scientist studies the biological components, physical, and chemical properties, distribution, formation, and morphology of soils.

Question 63

What education do soil scientists need?

Answer 63

Soil scientists need to have a strong background in physical and life sciences, plus a foundation in mathematics.

Question 64

What does the work of soil scientists involve?

Answer 64

Soil scientists may work for federal or state agencies, academia, or the private sector. Their work may involve collecting data, carrying out research, interpreting results, inspecting soils, conducting soil surveys, and recommending soil management programs. Many soil scientists work both in an office and in the field.

Question 65

What skills do soil scientists need?

Answer 65

According to the United States Department of Agriculture (USDA), “a soil scientist needs good observation skills to analyze and determine the characteristics of different types of soils. Soil types are complex and the geographical areas a soil scientist may survey are varied. Aerial photos or various satellite images are often used to research the areas. Computer skills and geographic information systems (GIS) help the scientist to analyze the multiple facets of geomorphology, topography, vegetation, and climate to discover the patterns left on the landscape.”

Question 66

What impact do soil scientists have?

Answer 66

Soil scientists play a key role in understanding the soil’s past, analyzing present conditions, and making recommendations for future soil-related practices.

Question 67

What is an epiphyte?

Answer 67

A plant that grows on other plants but is not dependent upon other plants for nutrition.

Question 68

What is an insectivorous plant?

Answer 68

A plant that has specialized leaves to attract and digest insects.

Question 69

What is nitrogenase?

Answer 69

An enzyme that is responsible for the reduction of atmospheric nitrogen to ammonia.

Question 70

What are nodules?

Answer 70

Specialized structures that contain Rhizobia bacteria where nitrogen fixation takes place.

Question 71

What is a parasitic plant?

Answer 71

A plant that is dependent on its host for survival.

Question 72

What are rhizobia?

Answer 72

Soil bacteria that symbiotically interact with legume roots to form nodules and fix nitrogen.

Question 73

What is a saprophyte?

Answer 73

A plant that does not have chlorophyll and gets its food from dead matter.

Question 74

What is a symbiont?

Answer 74

A plant in a symbiotic relationship with bacteria or fungi.

Question 75

Are all plants autotrophic?

Answer 75

No. Some plants are heterotrophic: they are totally parasitic and lacking in chlorophyll. These plants, referred to as holo-parasitic plants, are unable to synthesize organic carbon and draw all their nutrients from the host plant.

Question 76

What are some examples of symbiotic relationships between plants and other organisms?

Answer 76

Particular species of bacteria and fungi have evolved along with certain plants to create a mutualistic symbiotic relationship with roots. This improves the nutrition of both the plant and the microbe. The formation of nodules in legume plants and mycorrhization can be considered among the nutritional adaptations of plants.

Question 77

What is the largest source of nitrogen?

Answer 77

Atmospheric nitrogen, which is the diatomic molecule N₂, or dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems. However, plants cannot take advantage of this nitrogen because they do not have the necessary enzymes to convert it into biologically useful forms.

Question 78

What is nitrogen fixation?

Answer 78

Nitrogen can be “fixed”, which means that it can be converted to ammonia (NH₃) through biological, physical, or chemical processes. Biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃), exclusively carried out by prokaryotes such as soil bacteria or cyanobacteria. Biological processes contribute 65 percent of the nitrogen used in agriculture.

Question 79

What is the equation that represents BNF?

Answer 79

N₂ + 16ATP + 8e⁻ + 8H⁺ ⇒ 2NH₃ + 16ADP + 16P_i + H₂

Question 80

What is the most important source of BNF?

Answer 80

The most important source of BNF is the symbiotic interaction between soil bacteria and legume plants.

Question 81

How is the ammonia that is produced by BNF converted by plants?

Answer 81

The NH₃ resulting from fixation can be transported into plant tissue and incorporated into amino acids, which are then made into plant proteins.

Question 82

What are some examples of the products of BNF?

Answer 82

Some legume seeds, such as soybeans and peanuts, contain high levels of protein, and serve among the most important agricultural sources of protein in the world.

Question 83

What are the benefits of using rhizobia to fertilize plants?

Answer 83

Using rhizobia is a natural and environmentally friendly way to fertilize plants, as opposed to chemical fertilization that uses a nonrenewable resource, such as natural gas. Through symbiotic nitrogen fixation, the plant benefits from using an endless source of nitrogen from the atmosphere. The process simultaneously contributes to soil fertility because the plant root system leaves behind some of the biologically available nitrogen. As in any symbiosis, both organisms benefit from the interaction: the plant obtains ammonia, and bacteria obtain carbon compounds generated through photosynthesis, as well as a protected niche in which to grow.

Question 84

What are nutrient depletion zones?

Answer 84

A nutrient depletion zone can develop when there is rapid soil solution uptake, low nutrient concentration, low diffusion rate, or low soil moisture.

Question 85

Why do mycorrhizae form?

Answer 85

Nutrient depletion zones are very common; therefore, most plants rely on fungi to facilitate the uptake of minerals form the soil.

Question 86

How do mycorrhizae form?

Answer 86

Fungi form symbiotic associations called mycorrhizae with plant roots, in which the fungi actually are integrated into the physical structure of the root. The fungi colonize the living root tissue during the active plant growth.

Question 87

How do plants and fungi benefit from mycorrhization?

Answer 87

Through mycorrhization, the plant obtains mainly phosphate and other minerals, such as zinc and copper, from the soil. The fungus obtains nutrients, such as sugars, from the plant root.

Question 88

How do mycorrhizae increase nutrient uptake by plants?

Answer 88

Mycorrhizae help increase the surface area of the plant root system because hyphae, which are narrow, can spread beyond the nutrient depletion zone. Hyphae can grow into small soil pores that allow access to phosphorus that would otherwise be unavailable to the plant. The beneficial effect on the plant is best observed in poor soils.

Question 89

How much carbon can fungi obtain in mycorrhizae?

Answer 89

The benefit to fungi is that they can obtain up to 20 percent of the total carbon accessed by plants.

Question 90

Besides nutrient uptake, what other benefits are there for plants in mycorrhization?

Answer 90

Mycorrhizae functions as a physical barrier to pathogens. It also provides an induction of generalized host defense mechanisms, and sometimes involves production of antibiotic compounds by the fungi.

Question 91

What are the different types of mycorrhizae?

Answer 91

There are two types of mycorrhizae: ectomycorrhizae and endomycorrhizae.

Question 92

What are ectomycorrhizae?

Answer 92

Ectomycorrhizae form an extensive dense sheath around the roots, called a mantle. Hyphae from the fungi extend from the mantle into the soil, which increases the surface area for water and mineral absorption. This type of mycorrhizae is found in forest trees, especially conifers, birches, and oaks.

Question 93

What are endomycorrhizae?

Answer 93

Endomycorrhizae, also called arbuscular mycorrhizae, do not form a dense sheath over the root. Instead, the fungal mycelium is embedded within the root tissue. Endomycorrhizae are found in the roots of more than 80 percent of terrestrial plants.

Question 94

How are some plants heterotrophic?

Answer 94

Some plants cannot produce their own food and must obtain their nutrition from outside sources. This may occur with plants that are parasitic or saprophytic. Some plants are mutualistic symbionts, epiphytes, or insectivorous.

Question 95

What are the types of parasitic plants?

Answer 95

Holoparasites completely depend on their host, while hemiparasites are fully photosynthetic and only use the host for water and minerals.

Question 96

What is an example of a parasitic plant?

Answer 96

The dodder, which has no leaves, has a weak, cylindrical stem that coils around the host and forms suckers. From these suckers, cells invade the host stem and grow to connect with the vascular bundles of the host. The parasitic plant obtains water and nutrients through these connections. It is a holoparasite.

Question 97

How many species of parasitic plants are there?

Answer 97

There are about 4100 species of parasitic plants.

Question 98

What are some characteristics of saprophytes?

Answer 98

A saprophyte is a plant that does not have chlorophyll and gets its food from dead matter, similar to bacteria and fungi (note that fungi are often called saprophytes, which is incorrect, because fungi are not plants). Plants like these use enzymes to convert organic food materials into simpler forms from which they can absorb nutrients. Most saprophytes do not directly digest dead matter: instead, they parasitize fungi that digest dead matter, or are mycorrhizal, ultimately obtaining photosynthate from a fungus that derived photosynthate from its host. Saprophytic plants are uncommon; only a few species are described.

Question 99

What are lichens?

Answer 99

Fungi form symbiotic associations with cyanobacteria and green algae (called lichens). Lichens can sometimes be seen as colorful growths on the surface of rocks and trees.

Question 100

How do lichens benefit each partner in the association?

Answer 100

The algal partner (phycobiont) makes food autotrophically, some of which it shares with the fungus; the fungal partner (mycobiont) absorbs water and minerals from the environment, which are made available to the green alga. If one partner was separated from the other, they would both die.

Question 101

What are the types of epiphytic roots?

Answer 101

Epiphytes have two types of roots: clinging aerial roots, which absorb nutrients from humus that accumulates in the crevices of trees; and aerial roots, which absorb moisture from the atmosphere.

Question 102

What is an example of an insectivorous plant?

Answer 102

The Venus flytrap is popularly known for its insectivorous mode of nutrition, and has leaves that work as traps. The minerals it obtains from prey compensate for those lacking in the boggy (low pH) soil of its native North Carolina coastal plains. There are three sensitive hairs in the center of each half of each leaf. The edges of each leaf are covered with long spines. Nectar secreted by the plant attracts flies to the leaf. When a fly touches the sensory hairs, the leaf immediately closes. Next, fluids and enzymes break down the prey and minerals are absorbed by the leaf. Since this plant is popular in the horticultural trade, it is threatened in its original habitat.