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Chapter 37

BIOL 1030 Chapter 37: Chapter 37 Plant Nutrition

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Biological Sciences
BIOL 1030
Scott Kevin

Chapter 37 Plant Nutrition Lecture Outline Outline: A Nutritional Network • Every organism is an open system linked to its environment by a continuous exchange of energy and materials. • In ecosystems, plants and other photosynthetic autotrophs perform the crucial step of transforming inorganic compounds into organic ones. • Plants need sunlight as the energy source for photosynthesis. • They also need inorganic raw materials such as water, CO2, and inorganic ions to synthesize organic molecules. • Plants obtain CO2 from the air. Most vascular plants obtain water and minerals from the soil through their roots. • The branching root and shoot systems of vascular plants allow them to draw from soil and air reservoirs of inorganic nutrients. • Roots, through fungal mycorrhizae and root hairs, absorb water and minerals from the soil. • CO2 diffuses into leaves from the surrounding air through stomata. Concept 37.1 Plants require certain chemical elements to complete their life cycle • Early ideas about plant nutrition were not entirely correct and included: • Aristotle’s hypothesis that soil provided the substance for plant growth. • van Helmont’s conclusion from his experiments that plants grow mainly from water. • Hale’s postulate that plants are nourished mostly by air. • In fact, soil, water, and air all contribute to plant growth. • Plants extract mineral nutrients from the soil. Mineral nutrients are essential chemical elements absorbed from soil in the form of inorganic ions. • For example, many plants acquire nitrogen in the form of nitrate ions (NO3?). • However, as van Helmont’s data suggested, mineral nutrients from the soil contribute little to the overall mass of a plant. • About 80–90% of a plant is water. Because water contributes most of the hydrogen ions and some of the oxygen atoms that are incorporated into organic atoms, one can consider water a nutrient. • However, only a small fraction of the water entering a plant contributes to organic molecules. • More than 90% of the water absorbed by a field of corn is lost by transpiration. • Most of the water retained by a plant functions as a solvent, provides most of the mass for cell elongation, and helps maintain the form of soft tissues by keeping cells turgid. • By weight, the bulk of the organic material of a plant is derived not from water or soil minerals, but from the CO2 assimilated from the atmosphere. • The dry weight of an organism can be determined by drying it to remove all water. About 95% of the dry weight of a plant consists of organic molecules. The remaining 5% consists of inorganic molecules. • Most of the organic material is carbohydrate, including cellulose in cell walls. • Carbon, hydrogen, and oxygen are the most abundant elements in the dry weight of a plant. • Because some organic molecules contain nitrogen, sulfur, and phosphorus, these elements are also relatively abundant in plants. • More than 50 chemical elements have been identified among the inorganic substances present in plants. • However, not all of these 50 are essential elements, required for the plant to complete its life cycle and reproduce. • Roots are able to absorb minerals somewhat selectively, enabling the plant to accumulate essential elements that may be present in low concentrations in the soil. • However, the minerals in a plant also reflect the composition of the soil in which the plant is growing. • Some elements are taken up by plant roots even though they do not have any function in the plant. Plants require nine macronutrients and at least eight micronutrients. • Plants can be grown in hydroponic culture to determine which mineral elements are actually essential nutrients. • Plants are grown in solutions of various minerals in known concentrations. • If the absence of a particular mineral, such as potassium, causes a plant to become abnormal in appearance when compared to controls grown in a complete mineral medium, then that element is essential. • Such studies have identified 17 elements that are essential nutrients in all plants and a few other elements that are essential to certain groups of plants. • Elements required by plants in relatively large quantities are macronutrients. • There are nine macronutrients in all, including the six major ingredients in organic compounds: carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus. • The other three macronutrients are potassium, calcium, and magnesium. • Elements that plants need in very small amounts are micronutrients. • The eight micronutrients are iron, chlorine, copper, zinc, manganese, molybdenum, boron, and nickel. • Most of these function as cofactors, nonprotein helpers in enzymatic reactions. • For example, iron is a metallic component in cytochromes, proteins that function in the electron transfer chains of chloroplasts and mitochondria. • While the requirement for these micronutrients is modest (e.g., only one atom of molybdenum for every 60 million hydrogen atoms in dry plant material), a deficiency of a micronutrient can weaken or kill a plant. The symptoms of a mineral deficiency depend on the function and mobility of the element. • The symptoms of a mineral deficiency depend in part on the function of that nutrient in the plant. • For example, a deficiency in magnesium, an ingredient of chlorophyll, causes yellowing of the leaves, or chlorosis. • The relationship between a mineral deficiency and its symptoms can be less direct. • For example, chlorosis can also be caused by iron deficiency because iron is a required cofactor in chlorophyll synthesis. • Mineral deficiency symptoms also depend on the mobility of the nutrient within the plant. • If a nutrient can move freely from one part of a plant to another, then symptoms of the deficiency will appear first in older organs. • Young, growing tissues have more “drawing power” than old tissues for nutrients in short supply. • For example, a shortage of magnesium will initially lead to chlorosis in older leaves. • If a nutrient is relatively immobile, then a deficiency will affect young parts of the plant first. • Older tissue may have adequate supplies, which they can retain during periods of shortage. • For example, iron does not move freely within a plant. Chlorosis due to iron deficiency appears first in young leaves. • The symptoms of a mineral deficiency are often distinctive enough for a plant physiologist or farmer to make a preliminary diagnosis of the problem. • This can be confirmed by analyzing the mineral content of the plant and the soil. • Deficiencies of nitrogen, potassium, and phosphorus are the most common problems. • Shortages of micronutrients are less common and tend to be geographically localized due to differences in soil composition. • The amount of micronutrient needed to correct a deficiency is usually quite small. Care must be taken, because a nutrient overdose can be toxic to plants. • One way to ensure optimal mineral nutrition is to grow plants hydroponically on nutrient solutions that can be precisely regulated. • This technique is practiced commercially, but the requirements for labor and equipment make it relatively expensive compared with growing crops in soil. • Mineral deficiencies are not limited to terrestrial ecosystems or to plants. • Photosynthetic protists and bacteria can also suffer from mineral deficiencies. • For example, populations of planktonic algae in the southern oceans are limited by iron deficiency. • In a trial in relatively unproductive seas between Tasmania and Antarctica, researchers demonstrated that dispersing small amounts of iron produced large algal blooms that pulled carbon dioxide out of the air. • Seeding the oceans with iron may help slow the increase in carbon dioxide levels in the atmosphere, but it may cause unanticipated environmental effects. Concept 37.2 Soil quality is a major determinant of plant distribution and growth Soil texture and composition are key environmental factors in terrestrial ecosystems. • The texture and chemical composition of soil are major factors determining what kinds of plants can grow well in a particular location. • Texture is the general structure of soil, including the relative amounts of various sizes of soil particles. • Composition is the soil’s organic and inorganic components. • Plants that grow naturally in a certain type of soil are adapted to its texture and composition and are able to absorb water and extract essential nutrients from that soil. • Plants, in turn, affect the soil. • The soil-plant interface is a critical component of the chemical cycles that sustain terrestrial ecosystems. • Soil has its origin in the weathering of solid rock. • Water that seeps into crevices and freezes in winter fractures rock. Acids dissolved in soil water also help break down rock chemically. • Organisms, including lichens, fungi, bacteria, mosses, and the roots of vascular plants, accelerate the breakdown by the secretion of acids and the expansion of roots in fissures. • This activity eventually results in topsoil, a mixture of particles from rock; living organisms; and humus, a residue of partially decayed organic material. • Topsoil and other distinct soil layers, called horizons, are often visible in a vertical profile through soil. • Topsoil, or the A horizon, is richest in organic material and is thus the most important horizon for plant growth. • The texture of topsoil depends on the size of its particles, which are classified from coarse sand to microscopic clay particles. • The most fertile soils are loams, made up of roughly equal amounts of sand, silt (particles of intermediate size), and clay. • Loamy soils have enough fine particles to provide a large surface area for retaining minerals and water, which adhere to the particles. • Loams also have enough course particles to provide air spaces that supply oxygen to the root for cellular respiration. • Inadequate drainage can dramatically impact survival of many plants. • Plants can suffocate if air spaces are replaced by water. • Roots can also be attacked by molds that flourish in soaked soil. • Topsoil is home to an astonishing number and variety of organisms. • A teaspoon of soil has about 5 billion bacteria that cohabit with various fungi, algae and other protists, insects, earthworms, nematodes, and the roots of plants. • The activities of these organisms affect the physical and chemical properties of soil. • For example, earthworms aerate soil by burrowing and add mucus that holds fine particles together. • Bacterial metabolism alters the mineral composition of soil. • Plant roots extract water and minerals. They also affect soil pH by releasing organic acids and reinforce the soil against erosion. • Humus is the decomposing organic material formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, and other organic refuse. • Humus prevents clay from packing together and builds a crumbly soil that retains water but is still porous enough for the adequate aeration of roots. • Humus is also a reservoir of mineral nutrients that are returned to the soil by decomposition. • After a heavy rainfall, water drains away from the larger spaces of the soil, but smaller spaces retain water because of water’s attraction for the electrically charged surfaces of soil particles. • Some water adheres so tightly to hydrophilic particles that plants cannot extract it, while water that is bound less tightly to the particles can be taken up by roots. • Many minerals, especially those with a positive charge, such as potassium (K+), calcium (Ca2+), and magnesium (Mg2+), adhere by electrical attraction to the negatively charged surfaces of clay particles. • Clay in soil prevents the leaching of mineral nutrients during heavy rain or irrigation because of its large surface area for binding minerals. • Minerals that are negatively charged, such as nitrate (NO3?), phosphate (H2PO4?), and sulfate (SO42?), are less tightly bound to soil particles and tend to leach away more quickly. • Positively charged mineral ions are made available to the plant when hydrogen ions in the soil displace the mineral ions from the clay particles. • This process, called cation exchange, is stimulated by the roots, which secrete H+ and compounds that form acids in the soil solution. Soil conservation is one step toward sustainable agriculture. • It can take centuries for soil to become fertile through the breakdown of soil and the accumulation of organic material. • However, human mismanagement can destroy soil fertility within just a few years. • Soil mismanagement has been a recurring problem in human history. • For example, the Dust Bowl was an ecological and human disaster that occurred in the southwestern Great Plains of the United States in the 1930s. • Before the arrival of farmers, the region was covered with hardy grasses that held the soil in place in spite of long recurrent droughts and torrential rains. • In the 30 years before World War I, homesteaders planted wheat and raised cattle, which left the soil exposed to wind erosion. • Several years of drought resulted in the loss of centimeters of topsoil that were blown away by the winds. • Millions of hectares of farmland became useless, and hundreds of thousands of people were forced to abandon their homes and land. • To understand soil conservation, we must begin with the premise that agriculture is not natural and can only be sustained by human intervention. • In natural ecosystems, mineral nutrients are recycled by the decomposition of dead organic material. • In contrast, when we harvest a crop, we remove essential elements. • In general, agriculture depletes minerals in the soil. • To grow 1,000 kg of wheat, the soil gives up 20 kg of nitrogen, 4 kg of phosphorus, and 4.5 kg of potassium. • The fertility of the soil diminishes unless minerals are replaced by fertilizers. • Most crops require far more water than the natural vegetation for that area, making irrigation necessary. • The goals of soil conservation include prudent fertilization, thoughtful irrigation, and prevention of erosion. • Complementing soil conservation is soil reclamation, the return of agricultural productivity to damaged soil. • A third of the world’s farmland suffers from low productivity due to poor soil conditions. • Farmers have been using fertilizers to improve crop yields since prehistory. • Historically, these have included animal manure and fish carcasses. • In developed nations today, most farmers use commercial fertilizers containing minerals that are either mined or prepared by industrial processes. • These are usually enriched in nitrogen, phosphorus, and potassium, the macronutrients most often deficient in farm and garden soils. • Fertilizers are labeled with their N-P-K ratio. A fertilizer marked “10-12-8” is 10% nitrogen (as ammonium or nitrate), 12% phosphorus (as phosphoric acid), and 8% potassium (as the mineral potash). • Manure, fishmeal, and compost are “organic” fertilizers because they are of biological origin and contain material in the process of decomposing. • The organic material must be decomposed to inorganic nutrients before it can be absorbed by roots. • However, the minerals that a plant extracts from the soil are in the same form whether they came from organic fertilizer or from a chemical factory. • Compost releases nutrients gradually, while minerals in commercial fertilizers are available immediately. • Excess minerals are often leached from fertilized soil by rainwater or irrigation and may pollute groundwater, streams, and lakes. • Genetically engineered “smart plants” have been produced. These plants produce a blue pigment in their leaves to warn the farmer of impending nutrient deficiency. • To fertilize judiciously, a farmer must maintain an appropriate soil pH. pH affects cation exchange and influences the chemical form of all minerals. • Even if an essential element is abundant in the soil, plants may starve for that element if it is bound too tightly to clay or is in a chemical form that the plant cannot absorb. • Adjustments to soil pH of soil may make one mineral more available but another mineral less available. • The pH of the soil must be matched to the specific mineral needs of the crop. • Sulfate lowers pH, while liming (addition of calcium carbonate or calcium hydroxide) increases pH. • A major problem with acidic soils, particularly in tropical areas, is that aluminum dissolves in the soil at low pH and becomes toxic to roots. •
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