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Fluid Journal : Fluid Journal 2002-2004
2 Fluid Journal Summer 2002 the interaction between them and soil K recommendations, samples of several soils were collected throughout the Pacific Northwest (PNW). Soil Test K concentration varied from less than 80 ppm to over 400 ppm, while slow release K ranged from about 500 ppm to nearly 800 ppm (Figure 1). There appears to be an upper concentration limit for the slow- release K fraction when the soil test K concentration (STKC) reached 175 to 200 ppm. A linear line can be drawn along the upper points downward to where STKC is zero, and crosses the y- axis at about zero for slow-release K. Those samples that have a STKC of greater than 250 ppm may have recently had soluble K applied to them or were recently manured. It is not known how easily fertilizer K that has been applied to a field can reenter the interlayer mineral lattice and become part of the slow-release K fraction of the soil. It would be important to note that when K is applied to a soil it can displace Ca, Mg, or Na on the exchange complex. This relationship may impact the availability of these ions to growing plants. This would normally be a transient relationship, but it could exist around a dissolving K fertilizer granule in the soil-solution system. The STKC was also related linearly to the K diffusion rate (DFFK) estimated with the Unocal® procedure for the five experimental fields (Figure 2). For a STKC concentration of 150 ppm, the calculated diffusion rate would be 1.77 ppm K/day or about 6.4 lbs K/day for an acre-foot of soil. An available classification would rank this rate as low and predict a response to K fertilization. Even if the plant's roots were able to extract 50% of the K supplied to the soil solution by this mechanism, it would barely be able to keep up with K tuber demands since tubers growing at 700 cwt/acre/day require about 3 lbs K/A-day. Sodium bicarbonate should extract all the soluble and most of the exchangeable K fractions within the soil. It did not appear to extract a significant portion of the non- exchangeable K, as there was no apparent relationship to slow release K. Slow release K concentrations on Table 1. Final fertilizer K recommendations comparing 1987 rates (K2O) developed from 1992-95 field experiments in southern Idaho. Soil test K (ppm) lbs K2O/A (1987) lbs K2O/A (1992-95) 25 250 600 50 200 500 75 150 400 100 100 300 125 50 200 150 0 100 175 0 0 native, uncropped silt loam soils in the PNW were at least 1,200 ppm. Whenever crop history entered the equation the slow release K fraction was much smaller. This was particularly true of the coarse- textured soils. Isotherm slopes were also smaller for these sandy soils, which indicates that K fertilization response will occur on sandy coarse-textured soils at a higher STKC than for silt loam soils. There is also an indication that leaching of available K will take place on sandy soils where excess irrigation water is applied. A portion of the applied K will also be fixed in the slow release K fraction, since the equilibria among solution, exchangeable, and non- exchangeable K forms are reversible. This means that plants may actually recover only small amounts of fertilizer K that is applied during a given season during that season of application. This may necessitate even larger amounts of K fertilizer applications for adequate plant growth on these soils. Potato tuber yield responses to K fertilization were related to STKC during a set of field experiments conducted during 1992 and 1995. The original fertilizer K guides for Idaho, Oregon, and Washington had a critical STKC of 150 ppm. The Idaho guide had these values established since the 1960s. Their work indicated that the critical STKC should be raised to 175 ppm K and that much higher K fertilization rates were needed for economically profitable potato production to be achieved. Assuming that NaHCO3 will extract all soluble and most of the exchangeable K fractions, and that added K fertilizer contributes to only those two fractions, about 380 lbs/A of K would be required to change the STKC 100 ppm in an acre of soil. However, if conversion of any of the added K fertilizer to a non- exchangeable form might occur, the increase of K fertilization would be even higher. An adjustment of the fertilization rate of K was made by using the relationship between the isotherm slope and the STKC, and solution K concentrations. The adjusted rates are higher than those derived from the field studies or using the mass balance approach. Part of the difference between these two comparisons occurs because all of the soil particles and surfaces are exposed during isotherm equilibrium, while in the field studies only the soil around the dissolving fertilizer particle would be exposed to higher K concentrations. Plant Plants differ in the ability to take up K from a given soil. This is associated with a given plant's root system and surface area of the root. When the potato plant is K deficient it becomes stunted, the younger leaf tissue develops a leathery surface, and leaf margins turn downward. Marginal scorch will occur with necrotic spots or necrosis along the leaf margins.
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