36 Plant Nutrition

36 Plant Nutrition 36.1 How Do Plants Acquire Nutrients? 36.2 What Mineral Nutrients Do Plants Require? 36.3 What Are the Roles of Soil? 36.4 How Does Nitrogen Get From Air to Plant Cells? 36.5 Do Soil, Air, and Sunlight Meet the Needs of All Plants?

Bladderwort, Utricularia intermedia, traps mosquito larvae

Long-leaf sundew, Drosera anglica, traps horsefly

Venus Flytrap, Dionaea

36.1 How Do Plants Acquire Nutrients? Nutrients: the major ingredients for macromolecules: carbon, hydrogen, oxygen, nitrogen. Carbon enters the living world from CO 2 in the atmosphere through fixation by photosynthesis. Hydrogen and oxygen enter as water. These three elements are in plentiful supply. Nitrogen is in relatively short supply.

36.1 How Do Plants Acquire Nutrients? Mineral nutrients required include sulfur for proteins and phosphorus for nucleic acids. Chlorophyll has magnesium, many compounds require iron. In the soil, these and other minerals are dissolved in the soil solution. Plants take up most minerals from the soil solution in ionic form.

36.1 How Do Plants Acquire Nutrients? Autotrophs: make their own organic molecules from simple inorganic nutrients. Plants, some protists, and some bacteria, are autotrophs. Heterotrophs required pre-formed organic molecules and depend directly or indirectly on autotrophs for food.

36.1 How Do Plants Acquire Nutrients? Most autotrophs are photosynthesizers; light is the source of energy to synthesize organic compounds. Some are chemolithotrophs, using energy from reduced inorganic compounds such as H 2 S. All are bacteria.

36.1 How Do Plants Acquire Nutrients? Plants cannot move around to obtain nutrients, but they can extend themselves by growth. As roots grow through the soil, they mine it for new sources of minerals and water. Growth of leaves helps plant obtain more sunlight and CO 2.

36.1 How Do Plants Acquire Nutrients? As they grow, plants must deal with variable microenvironments. Microenvironmental conditions can encourage or discourage growth of roots. For example, animal droppings create local high concentrations of nitrogen.

36.2 What Mineral Nutrients Do Plants Require? Except for nitrogen, all the minerals required by plants are derived from rock. They are all usually taken up from the soil solution. An essential element is one required for the plant to complete its life cycle, and cannot be replaced by another element.

36.2 What Mineral Nutrients Do Plants Require? Two categories of essential elements: Macronutrients: need at least 1 g per kg of dry plant matter. Micronutrients: need less than 100 mg per kg.

Table 36.1 Mineral Elements Required by Plants

36.2 What Mineral Nutrients Do Plants Require? If a plant is deficient in an essential element, it displays characteristic deficiency symptoms. The symptoms help in diagnosis of problems, and appropriate treatment can be applied in the form of fertilizer.

Table 36.2 Some Mineral Deficiencies in Plants

36.2 What Mineral Nutrients Do Plants Require? Nitrogen deficiency is the most common. In natural environments, plant growth slows to match the availability of nitrogen. Nitrogen deficiency symptoms include uniform yellowing of older leaves; nitrogen is needed for chlorophyll.

36.2 What Mineral Nutrients Do Plants Require? Iron deficiency also causes yellowing, but of the youngest leaves. Nitrogen can be transported from older leaves into younger, actively growing leaves. Iron cannot be readily distributed, so younger leaves show symptoms first.

Figure 36.1 Mineral Deficiency Symptoms

36.2 What Mineral Nutrients Do Plants Require? Some elements have several roles. Magnesium is essential for chlorophyll, and as a cofactor for many enzymes. Phosphorus is essential for nucleic acids, ATP, and phosphorylation of enzymes. Calcium affects membranes and cytoskeletal activity, spindle formation, and has many other functions.

36.2 What Mineral Nutrients Do Plants Require? Essential elements were identified by growing plants hydroponically—without soil. Macronutrients were easily identified, but micronutrients were more difficult. In the ninteenth century, chemicals were so impure, they often supplied the micronutrients.

Figure 36.2 Identifying Essential Elements for Plants

36.2 What Mineral Nutrients Do Plants Require? For some micronutrients, there is enough stored in the seed for the plant’s lifetime. Iron was the first micronutrient to be identified (1840s), nickel was the last (1983).

Figure 36.3 Is Nickel an Essential Element for Plant Growth? (Part 1)

Figure 36.3 Is Nickel an Essential Element for Plant Growth? (Part 2)

36.3 What Are the Roles of Soil? Soils are complex systems with living and nonliving components. Living components include plant roots, bacteria, fungi, protists, and animals such as insects and earthworms. Nonliving components include rock fragments from large rocks and pebbles through to sand, silt, and clay.

36.3 What Are the Roles of Soil? Soil also contains water and dissolved minerals, air spaces, and dead organic matter. The air spaces are important sources of oxygen for plant roots.

Figure 36.4 The Complexity of Soil

36.3 What Are the Roles of Soil? Soils are not static but change through the effects of weather, activities of plants and animals, and human practices such as agriculture.

36.3 What Are the Roles of Soil? Structure of a soil with depth is the soil profile. Most soils have recognizable layers or horizons. Mineral nutrients tend to be leached (dissolved and carried downwards) from the upper horizons.

36.3 What Are the Roles of Soil? A horizon: topsoil. Minerals may be leached from this layer. Has most dead and decaying organic matter, and most living organisms. Successful agriculture depends on topsoil. This layer is what blew away during the great Dust Bowl of the 1930s.

36.3 What Are the Roles of Soil? Topsoils have different proportions of sand, silt, and clay. Sand has abundant air spaces, but does not hold water well. Clay binds water tightly, but the tiny particles pack close together, leaving no air spaces. Clay particles have charged surfaces that bind minerals and make them available to plants.

36.3 What Are the Roles of Soil? Loam: a type of topsoil with equal proportions of sand, silt, and clay, plus organic matter. Loams are the best soils for agriculture.

36.3 What Are the Roles of Soil? B horizon: subsoil. Materials may accumulate here from the layer above. C horizon: parent material that is breaking down to form soil. Roots rarely enter the C horizon.

Figure 36.5 A Soil Profile

36.3 What Are the Roles of Soil? Soil type depends on type of parent rock, climate, landscape, organisms present, and length of time the soil-forming processes have been at work. Amount and type of clay particles determine many soil properties. Clay particles bind minerals and aggregate into larger particles.

36.3 What Are the Roles of Soil? Rocks are broken down by: Mechanical weathering: physical breakdown by wetting, drying, freezing. Chemical weathering:  Oxidation by atmospheric oxygen  Hydrolysis (reaction with water)  Acids (carbonic acid)

36.3 What Are the Roles of Soil? Availability of minerals depends on clay particles. Negatively charged surfaces of clay particles bind cations: K +, Mg 2+, Ca 2+, NH 4+.

36.3 What Are the Roles of Soil? Roots release CO 2 through cellular respiration—reacts with soil water to form carbonic acid. The protons bind to clay particles, trading places with mineral cations: ion exchange.

Figure 36.6 Ion Exchange

36.3 What Are the Roles of Soil? Soil fertility—capacity to support plant growth—is in part dependent on the ion exchange capacity. Cations are retained in the A horizon, but anions such as SO 4 2– and NO 3 – can be leached rapidly. Most nitrogen in the A horizon is in organic matter.

36.3 What Are the Roles of Soil? Irrigation and rain water leach mineral nutrients from the soil, and harvesting of crops removes nutrients. Mineral nutrients can be replaced by organic and inorganic fertilizers.

36.3 What Are the Roles of Soil? The three elements most commonly added are N, P, and K. Commercial fertilizers are characterized by their N-P-K content, (e.g., is 5 percent N, 10 percent P 2 O 5, and 10 percent K 2 O by weight). They are formulated for specific crops. Nutrients in inorganic fertilizers are immediately available to plants.

36.3 What Are the Roles of Soil? Organic fertilizers such as manure or crop residues release nutrients slowly, so less leaching occurs than with inorganic fertilizers. Organic fertilizers also improve soil structure, providing space for air movement, root growth, and drainage.

36.3 What Are the Roles of Soil? Soil pH affects nutrient availability. Different crops have different pH optima. Rainfall and decomposition of organic matter can lower pH. Acidification can be reversed by liming— addition of lime (calcium carbonate, calcium hydroxide, magnesium carbonate).

36.3 What Are the Roles of Soil? If soil pH is too high for acid-loving plants, sulfur can be added—soil bacteria convert elemental sulfur to sulfuric acid. Iron and some other elements are more available to plants at a slightly acidic pH.

36.3 What Are the Roles of Soil? Nutrient solutions can be sprayed on the leaves of plants. Plants take up more copper, iron, and manganese from foliar sprays. But most fertilizers are applied to the soil.

36.3 What Are the Roles of Soil? Plants also have effects on soils. Plant litter—dead roots, fallen leaves and twigs—is the source of material that breaks down to form humus—dark- colored organic matter. Soil bacteria and fungi produce humus by breaking down plant litter, animal feces, dead organisms, etc.

36.3 What Are the Roles of Soil? Humus is rich in mineral nutrients, especially nitrogen. Humus improves soil structure and traps water and oxygen for roots. Plants also affect soil pH by excreting H + ions, or OH –, or HCO 3 – ions to maintain electric charge balance. Roots also excrete organic acids, such as citric acid and malic acid.