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Fluid Journal : Fluid Journal 1999-2001
2 Fluid Journal Spring 2001 Table 2. Average daily nitrogen, phosphorus, and potassium uptake by field-grown cotton. Days after Nutrient uptake, 10-9 lb inch-1 d-1 Planting N P K 37-49 4.14 0.51 2.47 49-64* 17.48 1.53 9.81 64-87 12.99 1.80 8.96 87-99+ 6.14 2.47 11.26 88-112 7.43 0.43 0.17 * = first square + = peak bloom Schwab et al. 2000 Table 1. Effect of soil bulk density and water content on corn root growth and K uptake.* Bulk Density Water Content Root Growth K Influx g cm-3 %(w/w) inches d-1 10-9 lb inch-1 d-1 1.2 10.7 3.40 3.43 14.2 6.63 5.32 19.0 8.27 6.37 1.4 10.7 4.22 3.71 16.3 6.60 6.90 19.0 7.62 8.54 1.6 10.7 3.81 4.09 14.2 5.54 3.94 19.0 7.89 6.82 * 18 days after planting Seiffert et al. 1995 Figure 1. Corn root proliferation in a fertilizer band. through soil pores and rapidly extend into the profile. The effect on root distribution in a controlled-traffic system can be dramatic. In some cases, soil water content affects root growth and, in turn, nutrient uptake more than soil bulk density (Table 1). Soil water contents above and below optimum both cause problems for roots. In dry soils, mechanical impedance is the dominant stress factor. Problems with loss of soil- root contact, as well as ion imbalance in rhizosphere soil solution, may also occur in dry soil. The effect of water deficit is less stressful for plants with root systems that easily and rapidly penetrate the subsoil where water content is usually greater. Nutrient availability can be a problem, however, given that subsoils are usually less fertile. In wet soils, loss of aeration and accumulation of phytotoxins are dominant stress factors. Oxygen (O2) is necessary for root respiration, as well as the respiration of soil microorganisms. Unfortunately, critical O2 concentrations are difficult to determine in soil, due to various interactive effects. While differences exist among plant species, O2 concentrations of 10 to 15 percent are generally sufficient to provide uninhibited root growth and function. In addition to limited respiration, low molecular-weight solutes that inhibit root growth (e.g., phenolics and short- chain fatty acids released by decomposing organic material) often accumulate in water-logged soils. The effect of soil temperature on root growth varies with species. In general, optimum soil temperatures for root growth are in the range of 68 to 77o F. Minimum temperature for root growth of species native to warm climates is in the range of 46 to 59o F. When changing from cool, suboptimal conditions to conditions near optimum, root/shoot ratios decrease. In cold soils, relatively more roots are needed for the same increase in shoot dry weight. Decreased shoot growth in cold soils is the result of a decrease in water and nutrient uptake. This is the basis of starter use in the spring. Finally, research suggests that the angle of root growth also is affected by temperature, with lower soil temperatures giving more horizontal growth and higher temperatures more vertical growth. When present in sufficiently high concentrations in soil, many elements can adversely affect root growth. Aluminum (Al), manganese (Mn), and hydrogen (H) often have a pronounced effect on root growth of agricultural crops. High H+ concentrations (low pH) per se are usually not the main factor affecting root growth. At low pH, Al, Mn, other toxic metals come into solution and hinder root growth. Aluminum-injured roots are typically stubby because development of lateral roots is poor. Appreciable concentrations of Al usually are present when pH is below 5.2. Soluble Al in
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