Micronutrients (Trace Elements) Micronutrients (trace elements or microelements) are essential elements utilized by plants in very small amounts. Low-analysis fertilizers such as 5-10-5 have trace elements as impurities. High-analysis fertilizers may be fortified with micronutrients. BORON (B) Boron is absorbed by plants as borate (BO 2 4 ). Mobile in the plant system, it affects flowering, fruiting, cell division, water relations (translocation of sugars), and other processes in the plant. When deficient, symptoms appear at the top of the plant. Terminal buds die, producing growth described as witches’-broom. Lateral branches grow and form rosettes; young leaves thicken and become leathery and chlorotic. Stems become hollow and may crack. IRON (FE) Though more abundant in most soils than other trace elements, iron deficiency occurs in alkaline or acidic soil. It can be absorbed through leaves or roots as Fe 2+ ions (and also as Fe 3+ ions to a much smaller extent since availability is reduced by being bound in plant tissue). Iron chelates can also be absorbed. Iron is a component of many enzymes and a catalyst in the synthesis of chlorophyll. Iron deficiency shows up as interveinal chlorosis of young leaves. In severe cases, leaves may become whitish, since iron plays a role in photosynthesis, as indicated previously. Iron is immobile and thus deficiency appears first in younger leaves. MOLYBDENUM (MO) Vegetables, cereals, and forage grasses are among a number of species that are known to show very visible symptoms when molybdenum is deficient in the soil. This element is unavailable to plants grown under very low pH (highly acidic) conditions. In such cases, liming is employed as a corrective measure. Molybdenum is involved in protein synthesis and is required by some enzymes that reduce nitrogen. The leaves of cauliflower and other cruciferous plants become narrow (whiptail) when the element is lacking in the soil. Plant leaves may also become pale green and roll up. MANGANESE (MN) Manganese, absorbed as Mn 2+ ions, is crucial to photosynthesis because of its role in chlorophyll synthesis. It is also important in phosphorylation, activation of enzymes, and carbohydrate metabolism. It is not mobile in plants. When deficient, interveinal chlorosis is observed in younger leaves, just as in iron deficiency. ZINC (ZN) Zinc is an enzyme activator. It is absorbed as Zn 2+ ions by plant roots and tends to be deficient in calcareous soils that are high in phosphorus. When deficient, plant leaves are drastically reduced in size and internodes shortened, giving a rosette appearance. Interveinal chlorotic mottling may occur in young leaves. Kalanchoe is particularly susceptible to zinc deficiency as a greenhouse plant. In species such as peach and citrus, deficiency of zinc produces a type of chlorosis called mottled leaf. COPPER (CU) Soils that are high in organic matter are prone to copper deficiency. Copper is important in chlorophyll synthesis and acts as a catalyst for respiration and carbohydrate and protein metabolism. Younger leaves may show interveinal chlorosis while the leaf tip remains green; with time, the leaf blade becomes necrotic. Terminal leaves and buds die, and the plant as a whole becomes stunted. Copper sulfate or copper ammonium sulfate may be administered to leaves or soil to correct deficiency problems. CHLORINE (CL) Chlorine is absorbed by plants as chloride ions (Cl ). Deficiency in the field is rare. An excessive level of chlorine is more often a problem than its absence. When deficiency occurs in the soil, plants may be stunted and appear chlorotic, with some necrosis. Micronutrient A chemical element that is required in small amounts (usually less than 1 ppm) for the growth and development of plants. 4.3.2 SOIL ORGANIC MATTER Organic matter in the soil may result from plant or animal materials. Plant residue or green manure crop incorporated into the soil by tilling the decaying plant roots is a good source of organic matter. Plant matter such as dried leaves on the surface of the soil is not 4.3 Belowground (Soil) Environment 113

Soil Erosion The wearing away of the land surface by geological agents such as water, wind, and ice. considered organic matter until it is incorporated into the soil. Soil erosion depletes soil organic matter. Organic matter is important to soil productivity since it is a source of nutrients when it decomposes. It improves soil structure by binding together mineral particles into aggregates for better aeration and drainage. It helps to buffer soils against rapid changes in pH. Organic matter increases the water-holding capacity of soils and gives them their characteristic dark brown or black color. Microorganisms (e.g., bacteria, fungi, and actinomycetes) are responsible for decomposing plant parts for easier incorporation into the soil. Sugars, starches, proteins, cellulose, and hemicellulose decompose rapidly, whereas lignin, fats, and waxes are slow to decompose. Organic matter acts as a slowrelease fertilizer, since its nutrients are released gradually over a particular period. Humus is a very stable part of the soil organic matter. Much of humus is formed from two general biochemical processes. The chemicals in the plant residue undergo decomposition by microbial action to produce simpler products. These breakdown products undergo synthesis, by which the simpler products are enzymatically joined to make more complex products such as polyphenols and polyquinones. These synthetic products interact with nitrogen-containing amino compounds to produce a great portion of resistant humus. Further, the synthetic process is aided by the presence of colloidal clays. Humic particles (or humic micelles) carry a large amount of adsorbed cations (e.g., Ca 2+ , Mg 2+ ,H + , and Na + ) as clay micelles. 114 Chapter 4 Plant Growth Environment 4.3.3 SOIL REACTION AND NUTRIENT AVAILABILITY A soil test showing that adequate amounts of a nutrient are present does not indicate its availability to the plant. In addition to adequate amounts, the presence of moisture is critical, because water is the medium in which solutes are transported through the plant. Other factors that interfere with nutrient availability are soil temperature and soil reaction, or pH. Plant processes are generally slowed down by low temperatures. Soil reaction, or pH, is a measure of the hydrogen ion concentration as an indication of the soil’s degree of acidity or alkalinity. A pH of 7 is neutral. Values above 7 are considered alkaline, and values below 7 are acidic. The pH scale is logarithmic (Figure 4–11), meaning that a soil pH of 5 is 10 times more acidic than a soil pH of 6 and a pH of 4 is 100 times more acidic than a pH of 6. Most horticultural crops tolerate a soil pH within the range of 4 to 8. Soil pH regulates nutrient availability. Figure 4–12 shows the relationship between pH and nutrient availability to plants. A pH of7±1appears to be a safe range for most nutrient elements in the soil. Only iron is available at a strongly acidic pH. Conversely, iron is deficient in the soil under alkaline conditions. Sensitive plants (such as bluegrass [Poa pratensis]) develop iron-deficiency symptoms called iron chlorosis, a condition in which young leaves lose their green color and become yellowish. The difference between this kind of chlorosis and that associated with nitrogen deficiency is that iron chlorosis occurs between the veins of the leaves (interveinal chlorosis) and nitrogen causes a more uniform yellowing of leaves. Soil pH affects the biotic population of soil. Fungi tend to prefer highly acidic conditions (pH of 4 to 5), and nitrogen-fixating bacteria (Rhizobia) prefer a pH range of between 6 and 8. Table 4–8 shows the pH requirements of various horticultural plants. Factors That Affect pH Soil pH may rise in a soil that experiences low rainfall or is poorly drained. Salts tend to accumulate under these conditions. Soils formed on calcareous parent material have high alkalinity. Acidic soils (low pH) occur when soils are exposed to heavy rainfall and good drainage such that the bases are leached into lower depths or washed away in the runoff. Correcting pH Low soil pH may be corrected in practice by adding limestone (CaCO 3 ) or gypsum (CaSO 4 ) to the soil to raise the pH. The choice depends on the soil pH and other characteristics. To lower soil pH, sulfur compounds are added to the soil. Nitrogen fertilizers also tend to