Chapter 37 Plant Nutrition A Nutritional Network energy and materials Every organism is an open system linked to its environment by a continuous exchange of 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 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 However as van Helmont s data suggested mineral nutrients from the soil contribute For example many plants acquire nitrogen in the form of nitrate ions NO3 little to the overall mass of a plant Typically about 80 90 of a plant is water and plants grow mainly by accumulating water in the central vacuoles of their cells The water retained by a plant serves three main functions 1 Acts as a solvent 2 Provides most of the volume for cell elongation 3 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 that is assimilated from the air The dry weight of an organism can be determined by drying it to remove all water About 96 of the dry weight of a plant consists of organic molecules The remaining 4 consists of inorganic molecules Macronutrients and Micronutrients More than 50 chemical elements have been identified among the inorganic substances present in plants However not all of these 50 are essential elements Essential elements are required for the plant to complete its life cycle and reproduce To some extent the chemical elements in a plant reflect the soil composition Some elements are taken up by plant roots even though they do not have any function in the plant To determine which chemical elements are essential elements researchers use hydroponic culture in which plants are grown in mineral to solutions instead of soil 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 list pg 758 Elements required by plants in relatively large quantities are macronutrients There are nine macronutrients in all of 17 including the six major ingredients in organic compounds carbon oxygen hydrogen nitrogen sulfur and phosphorus The other three macronutrients are potassium calcium and magnesium 1 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 A deficiency of a micronutrient can weaken or kill a plant Concept 37 2 Soil quality is a major determinant of plant distribution and growth The texture and chemical composition of soil are major factors determining what kinds of plants can grow well in a particular location along with climate Texture is the general structure of soil referring to the relative amounts of various sizes of soil particles Composition refers to the soil s organic and inorganic chemical 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 terrestrial ecosystems The soil plant interface is a critical component of the chemical cycles that sustain Texture and Composition of Soils 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 The topsoil and other horizons distinct solid layers are often visible in vertical profile organisms and humus remains of partially decayed organic material where there is a road cut or deep hole most important for plant growth coarse sand to microscopic clay particles The texture of topsoil depends on the size of its particles which are classified from The top soil also known as A horizon is the richest in organic material and is therefore 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 containing oxygen that can be used by roots for cellular respiration Inadequate drainage can dramatically impact survival of many plants A teaspoon of soil has about 5 billion bacteria that cohabit with various fungi algae and 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 other protists insects earthworms nematodes and the roots of plants The activities of these organisms affect the physical and chemical properties of soil Bacterial metabolism alters the mineral composition of soil Plant roots can also affect soil composition and texture They can affect soil pH by releasing organic acids and reinforce the soil against erosion Humus important component of top soil consists of decomposing organic material formed by the action of bacteria and fungi on dead organisms feces fallen leaves and other organic refuse 2 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