Chapter 37 Plant Nutrition Nutritional Requirements of Plants

Chapter 37 Plant Nutrition

Nutritional Requirements of Plants • Mineral nutrients- essential chemical elements absorbed from the soil in the form of inorganic ions. (i. e. nitrogen)

Essential Nutrients • Essential nutrient- chemical element that is required for a plant to grow and complete the life cycle. There are 17 elements that are essential to all plants. • Essential nutrients can be dividend into two categories, macronutrients and micronutrients.

Macronutrients • Macronutrients are elements required by plants in relatively large amounts. There are 9 total. They are Carbon, Oxygen, Hydrogen, Nitrogen, Sulfur, Phosphorus, Potassium, Calcium, and Magnesium.

Micronutrients • Micronutrients are elements a plant needs in very small amounts. They are Iron, Chlorine, Copper, Manganese, Zinc, Molybdenum, Boron, and Nickel. • These elements function in plants mainly as cofactors of enzymatic reactions. (i. e. Iron is a metallic component of cytochromes, the proteins that function in the electron transport chains).

Symptoms of Mineral Deficiency • A deficiency of magnesium, which is an ingredient in chlorophyll, causes the yellowing of the leaves, or chlorosis. • If nutrients move freely from one part of the plant to another, symptoms of deficiency will show up in the older parts. • If the nutrients do not move freely, they generally appear in the newer parts of the plant.

The Role of Soil in Plant Nutrition • Factors that will determine what types of plants can grow in the region: 1. Texture 2. Composition 3. Availability of soil water and nutrients 4. Climate

Texture and Composition of Soil • Horizons are layers of soil that are often visible in vertical profiles. • Topsoil is a mixture of particles derived from rock, living organisms, and humus, a residue of partially decayed organic material. • The texture of the topsoil depends on the size of the particles, ranging from coarse sand to microscopic clay particles. • The most fertile soil is loams, made up of roughly equal amounts of sand, silt, and clay.

Availability of Soil Water and Nutrients • Water stays in smaller spaces in the soil because of its attraction for the soil particle which have electrically charged surfaces. • The minerals in the soil-especially the positively charged minerals, such as potassium, calcium, and magnesium-adhere by electrical attraction to the negatively charged surfaces. • Minerals that are negatively charged, such as nitrate, phosphate, and sulfate, are not bound as tight to the soil and are leached away more quickly. • Positively charged minerals are made more available when the hydrogen ions in the soil displace the mineral ions from the clay particles. This process, called cation exchange, is stimulated by the roots, which discrete the hydrogen.

The Availability of Soil Water and Materials

Soil Conservation • Agriculture uses up the nutrients in the soil. To preserve them, farmers must use a variety of techniques to increase the amount on minerals in the soil. • Farmers use fertilizers to enrich the soil. Fertilizers generally contain the three elements most commonly deficient in the soil, nitrogen, phosphorus, and potassium. • The numbers on fertilizer bags represent the percentage of these elements in the fertilizer. (I. e. a bag marked 10 -12 -8 has 10% nitrogen, 12% phosphorous, and 8% potassium) • Farmers must also pay attention to the conditions of the soil to fertilize properly. A plant in a field with a p. H of 8 can absorb calcium, but cannot absorb iron.

Soil Conservation • Irrigation is a problem for the soil. In an arid climate, irrigation can make the soil so salty that it becomes infertile. Eventually, salt left in the lowers the water potential of the soil, which causes the plant to lose water. Farmers now use drip irrigation and are trying to develop varieties of plants that require less water. • Topsoil is lost due to wind and water erosion. To stop this, farmers use trees as wind breakers, and terracing a hill side to prevent soil from washing away during a heavy rain. • The goal is sustainable agriculture, using farming methods that are environmentally safe and profitable.

Soil Bacteria Makes Nitrogen Available to Plants • 80% of the air is nitrogen, yet plants suffer from nitrogen deficiency because they are not able to absorb the N 2. • For plants to absorb the nitrogen, it must be converted to ammonium or nitrate. • Nitrogen-fixing bacteria convert N 2 to NH 3(ammonia) through a metabolic process called nitrogen fixation. N 2 +8 e ¯ +8 H+ +16 ATP -->2 NH 3 +H 2 +16 ADP+ 16 P * Nitrogenase reduces N 2 to NH 3 by adding electrons with hydrogen ions. Nitrifying bacteria oxidize ammonium into NO 3.

Nitrogen-Fixing Bacteria

Symbiotic Relationships • Legumes’ roots have swellings called nodules containing nitrogen-fixing bacteria of the genus Rhizobium. Legumes are peas, beans, soybeans, peanuts, clover, and alfalfa. • The nodules contain bacteroids, which have vessels formed by the root cell. • Farmers use plant rotation to replenish nitrogen. One year they plant a nonlegume plant, such as corn, and the next year, a legume. The legume plant is often mowed down and then allowed to decompose.

Soybean Root Nodule

Mycorrhizae • Mycorrhizae, “fungus roots”, are modified roots consisting of symbiotic associations of fungi and roots. • There are two main types of Mycorrhizae; ectomycorrhizae and endomycorrhizae. • Ectomycorrhizae forms a dense mantle over the surface of a root, which increases the surface area for absorption. • Endomycorrhizae have hyphae, which extend into the root by digesting small patches of the cell wall. They have arbuscles, knotlike structures, where nutrients pass from fungi to plant.

Parasitic and Carnivorous Plants • Some plants get their nutrition by using projections called haustoria to siphon xylem sap from the host tree. Mistletoe, dodder, and Indian pipe are parasitic. * Carnivorous plants make their carbohydrates by photosynthesis, but get their nutrients by killing and digesting insects. This includes the Venus Flytrap and Pitcher plants.