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Fluid Journal : Fluid Journal 1999-2001
3 Fluid Journal Spring 2001 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Fertilized Soil Fraction Corn Soybean Wheat Roots in P-treated/Total Root Length Figure 2. Stimulation of root growth by localized P placement (after Yao and Barber, 1986). Dashed line is in 1:1 relation, indicating no placement affect. 0 10 20 30 40 51015202530 SOLUTION P, mol / L NET P INFLUX, nmol / m2 Cmin E Km l MAX U s Figure 3. Relation of phosphorus in soil solution to root phosphorus absorption rate. subsoils can be a challenge, since lime cannot be used to correct the problem. The toxicity of Al varies with plant species. For example, cranberry and blueberry tolerate high levels of exchangeable Al, whereas cotton and wheat do not. Manganese toxicity is not prevalent and tends to affect shoot growth more than root growth. It is most common when a combination of low pH and reducing conditions exist. When supply of nutrients in the rooting zone is variable, plant roots often proliferate in areas where optimum concentrations exist, such as near fertilizer bands (Figure 1). Research has shown that phosphate, ammonium, and nitrate stimulate development of first- and second-order lateral roots. Therefore, when nitrogen (N) or phosphorus (P) is placed in some fraction of the root zone, root distribution between the fertilized fraction and unfertilized fraction is not proportional. This effect has been characterized for several economic species (Figure 2). The growth stimulation may be due to a competitive advantage that the roots with a more adequate supply of nutrients have over other roots on the same plant. Root growth vs nutrient uptake Nutrients move through the soil to root surfaces by mass flow and diffusion. Mass flow is associated with the convective flow of water to the root. Diffusion is driven by a concentration gradient that develops near the root surface when nutrients are removed from soil solution by roots. After nutrients arrive at the root surface, absorption can occur. For many essential nutrients, the uptake rate relative to nutrient concentration in soil solution can be described by a Michaelis-Menten relation (Figure 3). As solution concentration increases, influx rate increases until some root and/or nutrient-specific maximum (Imax) is reached. The affinity of root cells for a nutrient ion is reflected by the Km value, the concentration at which influx is half that at Imax. When a root is absorbing at the maximum rate, further increases in the concentration of the nutrient in solution have little effect on the absorption rate. However, recent research with several perennial range species has shown that when the roots of these plants grow into nutrient-rich microsites, nutrient uptake rates per unit length of root significantly increase. This characteristic allows plants to take advantage of localized areas of fertile soil. Although data are limited, root systems of crop species may react the same way. For many crop species, it is not clear which fraction of the plant's entire root system is active in nutrient absorption. In general, absorption and translocation of N, P, and potassium (K) occur in all parts of the root system, whereas calcium (Ca) tends to be absorbed in areas near root tips. Studies with winter wheat suggest that N and K uptake rates vary considerably among the seminal and nodal roots on the same plant. There also is evidence that some species have five or more types of roots, each with distinct developmental and physiological characteristics. Differences in root growth among cultivars or hybrids within a species also lead to differences in nutrient uptake. Genetic variation in both root length, and length and number of root hairs is common. To further complicate the issue, some species rapidly accumulate nutrients early in the growing season, while others change the demand for nutrients throughout the season. For example, N, P, and K absorption rates peak early in the growing season for corn, but for cotton maximum P and K influx per unit root length occurs near peak bloom (Table 2). Maximum nutrient influx occurs later in the growing season for soybeans as compared to corn. Influx rates of both crops are significantly greater than those of cotton, suggesting that cotton roots are less efficient at extracting soil nutrients. These differences must be considered in developing an effective nutrient management program. Exploiting root zone An adequate supply of nutrients in the surface soil promotes greater top growth and encourages a more vigorous and extensive root system. Since plants absorb nutrients only from soil in which roots are active, fertilizer should be placed in the volume of soil where the roots will grow and where further stimulation of root growth is desirable. If the plant produces a vigorous taproot early in the season, the presence of nutrient-rich soil directly under the plant is desirable. Hence, fertilizer should be placed under or near the seed for optimum use. For plants with a fibrous root system, fertilizer placement to the side of the seed would allow developing lateral roots to take advantage of the added nutrients. In soils that frequently experience periods of drought, deep placement of fertilizer may be most effective. Within any specific crop production system, optimum fertilizer placement will necessarily depend on both the ability of the soil to supply nutrients and the ability of the root system to absorb the nutrients present.
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