Chapter 37: Plant NutritionA 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- 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.- 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 elongation3) 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. About96% of the dry weight of a plant consists of organic molecules. The remaining 4% consists ofinorganic 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 ofplants 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.- The soil-plant interface is a critical component of the chemical cycles that sustain terrestrial ecosystems.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 soilwater 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 (remains of partially decayed organic material).- The topsoil and other horizons (distinct solid layers) are often visible in vertical profile where there is a road cut or deep hole.- The top soil, also known as A horizon, is the richest in organic material and is therefore most important 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 (particlesof 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 canbe used by roots 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. 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