37 Plant Nutrition

37 The Acquisition of Nutrients All living things need raw materials from the environment. These nutrients include carbon, hydrogen, oxygen, and nitrogen. Carbon comes from photosynthetic organisms or from CO 2 in the air. Hydrogen comes from water. Carbon, oxygen, and hydrogen are plentiful, and enter the living world through photosynthesis.

37 The Acquisition of Nutrients Nitrogen is in relatively short supply for plants. Nitrogen enters living forms first in bacteria, which can convert N 2 in air to forms that are useful to plants. Other mineral nutrients essential for life include sulfur, phosphorus, potassium, magnesium, and iron. Plants take up most nutrients as dissolved solutes in the water of the soil. Nitrogen- fixing bacteria form a nodule in clover root.

37 The Acquisition of Nutrients Plants are autotrophs. They make their own organic molecules from CO 2, H 2 O, and minerals. Heterotrophs require organic compounds as food and depend ultimately on autotrophs. Most autotrophs photosynthesize. Plants obtain energy from sunlight, carbon dioxide from the atmosphere, and nitrogen- containing ions and minerals from nutrients from the soil.

37 Mineral Nutrients Essential to Plants Plants that are deficient in a particular essential element show characteristic deficiency symptoms. These symptoms can be used to determine which elements are lacking. Appropriate fertilizers can be applied after diagnosing the specific deficiencies. A fertilizer is an added source of mineral nutrients. Effect of iron on bean folige.

37 Soils and Plants Soil provides the minerals plants need. Soil provides water, mechanical support, microorganisms, and oxygen for roots. Soils also contain organisms that are harmful to plants.

37 Soils and Plants Soils are complex mixtures of living and nonliving components, including bacteria, fungi, earthworms and other animals, particles of rock, clay, water, dissolved minerals, air spaces, and dead organic matter. The air spaces are a crucial source of oxygen to plant roots.

37 Soils and Plants The structure of many soils changes with depth, revealing a soil profile. Most soils have two or more horizontal layers. Minerals tend to leach, or be carried away by water from the upper horizons, and sink into deeper layers. Soil scientists recognize three major horizons:  A, the topsoil  B, the subsoil  C, the parent rock

Figure 37.3 A Soil Profile Most organic matter, roots, earthworms, insects, and microorganisms. Zone of accumulation of leached materials from above. Parent rock from which soil is derived.

37 Soils and Plants Agricultural soils often require fertilizer because irrigation and rainwater leach minerals from the soil, and the crop harvest removes nutrients. Three elements commonly used in fertilizers are nitrogen, phosphorus, and potassium. N-P-K percentages are often labeled on fertilizer bags. A fertilizer has 5% nitrogen, 10% phosphate (P 2 O 5 ), and 10% potash (K 2 O) by weight.

37 Soils and Plants Organic fertilizers, such as manure, release nutrients slowly, which results in less leaching than occurs with inorganic fertilizers. Organic fertilizers also contain residues of plant or animal materials that help improve the structure of the soil. Inorganic fertilizers provide an immediate supply of plant nutrients, and can be formulated to the requirements of a particular crop or soil type.

37 Soils and Plants The availability of nutrient ions is influenced by soil pH. pH 6.5 is optimal for most crops. In the process of liming, compounds such as calcium carbonate, calcium hydroxide, or magnesium carbonate are added to acidic soil to raise the pH. The pH of soil can be lowered by adding sulfur, which soil bacteria convert to sulfuric acid.

37 Nitrogen Fixation Earth’s atmosphere is about 78% nitrogen in the form of N 2 gas. N 2 is very stable. A great deal of energy is required to break the triple bond. Some bacteria have an enzyme that allows them to break the bond and convert N 2 into a more usable form, NH 3. The process is called nitrogen fixation. There are only a few species of these essential nitrogen fixers - Most nitrogen fixation is done by bacteria. Some nitrogen-fixing bacteria live free in the soil.

37 Nitrogen Fixation Some nitrogen fixers live in close association with plant roots in a mutualistic relationship. In symbiosis, some bacteria fix nitrogen in association with fungi as lichens. Rice farmers increase fixed nitrogen by growing the water fern Azolla in their rice paddies. Some species of bacteria fixes nitrogen in close association with plant roots, forming root nodules.

37 Nitrogen Fixation Nitrogen fixation is the reduction of nitrogen gas by the stepwise addition of three pairs of hydrogen atoms to the N 2.

37 Nitrogen Fixation Most plants take up both ammonium ions and nitrate ions. Nitrifying bacteria oxidize ammonia to nitrate. Plants take up nitrate and reduce it back to ammonia. Denitrifying bacteria return N2 to the atmosphere, completing the biological nitrogen cycle.

37 Carnivorous and Heterotrophic Plants Some plants that grow in acidic, nitrogen-poor environments trap and digest insects to help augment nitrogen and phosphorus supplies. These carnivorous plants include sundews, Venus flytraps, and pitcher plants. Carnivorous plants can survive without feeding on insects, but they grow much faster in their natural habitats when they succeed in capturing insects.

37 Carnivorous and Heterotrophic Plants Some plants are heterotrophic parasites that get nutrients directly from other plants. Parasitic plants can be serious problems for commercial crops and timber. Tree heavily infected with mistletoe, a parasitic plant. Dodder is a parasitic plant which obtains all its food via absorptive organs which invade its host’s vascular tissue.