FACTORS AFFECTING CROP NEEDS FOR POTASSIUM WESTERN PERSPECTIVE TERRY A. TINDALL AND DALE WESTERMANN MANAGER OF AGRONOMY J.R

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1 FACTORS AFFECTING CROP NEEDS FOR POTASSIUM WESTERN PERSPECTIVE TERRY A. TINDALL AND DALE WESTERMANN MANAGER OF AGRONOMY J.R. SIMPLOT COMPANY USDA-ARS SOIL SCIENTIST SOIL FACTORS--POTATOES Potassium uptake and availability for plant growth and development vary depending on environmental influences associated with a particular set of growing conditions. The soil itself contributes greatly to K availability. In southern Idaho for example, most of the soil parent material in the major growing areas of the state come from loess. Other areas of the west have been affected by alluvial actions that have sorted materials creating extensive coarse textured soil areas. The basis for K responses or subsequent needs for additional K fertilizer applications are actually based upon the minerals contained within the clay fractions of these soil parent materials. The common minerals are smectites, illite, and kaolinite. There are smaller amounts of vermicullite, quartz and feldspars with illite being the dominant clay-sized minerals within most of the silt based soils. Within the sandy coarse textured soils there is an abundance of K-feldspars, micas and even volcanic glass. These minerals are important sources of non-exchangeable K that replenish both exchangeable and solution K-fractions that are removed by plant uptake. The single greatest decline of any nutrient in western agriculture soils and in particular southern Idaho has been K. Many soils that had soil test K concentrations of greater than 400 ppm in the mid-1960 s, are now in the range of ppm today. I have personally seen soil test K levels in the 20 to 30 ppm range as recent as last year. Many of these soils have been historically very production potato or alfalfa producing soils that have had a tremendous requirement for K. Those requirements are not diminishing, but the capability of the soils to provide the necessary levels of K are being affected and cannot be maintained without additional K fertilizer being applied. To make accurate and reliable K recommendations we need to understand the inter-relations of soil- K fractions. It is of interest to determine the magnitude of selected K fractions, and their interrelationships to each other and the standard soil test K concentrations. This is especially true as one compares current academic recommendations for K and those apparent higher K fertilization rates that are being reported by many growers. To help understand the relations between various soil fractions, the interaction between them and soil K recommendations several soil samples, (70) were collected through out the Pacific Northwest by Dale Westermann (Table 1). These samples represented a wide variety of soil textures and types. NaHCO3-extractable K as well as a hot-acid extractable K method to estimate slow-release K analyzed each sample. Potassium isotherms were developed and analyzed for initial soil solution K concentration, equilibrium solution K concentration where the isotherm line crosses Y=0 and the slope of the isotherm line at Y=0.

2 Table 1. Source of soil samples used to determine K isotherms and STKC used in study. Source / Location Number of Samples K Experiments Southern Idaho 20 Columbia Basin 3 Potato K Survey -- Southern Idaho 29 Utah Experimental Station - Logan 1 Aberdeen R & E Center 1 Parma R & E Center 1 ARS & Kimberly R & E center 13 Soil test K Concentration varied from < 80 ppm to over 400 ppm, while slow release K ranged from about 500 ppm to nearly 800 ppm (Fig. 1) There appears to be an upper concentration limit for the slow-release K fraction when the STKC reached 175 to 200 ppm. A linear line can be drawn along the upper points downward to where STKC is zero, 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 re-enter the inter-layer 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. Figure 1. Relationship between STKC and slow-release K from soil extracts.

3 Potassium adsorption isotherms (Fig. 2) were generally linear within the range of initial K solution concentrations used. Slopes were generally steeper for the silt loam soils compared to the sandy coarse textured soils. (E 268 vs. E266 or E270). Steeper slopes indicate greater K sorption and potential buffering of soil solution K concentrations resulting in lower critical levels. There was a general tendency for the isotherm to decrease as STKC increased (slope= *STKC, r ). Figure 2. Potassium sorption isotherms for the five field experiments conducted in southern Idaho between 1992 and There was also a relationship between the K concentration in the initial solution with no K and to the STKC (Fig. 3). In addition the calculated solution K concentration where the isotherm line crosses Y=0 was also related to the STKC. Except for three soils from the Columbia Basin that were able to maintain a much higher soil solution K than the others at a similar STKC. This graph illustrates how K fertilizer additions are distributed between the water-soluble fraction, exchangeable and non-exchangeable forms in proportion to the clay content and dominant soil mineralogy.

4 Figure 3. Relationship between STKC and solution K initially Y=0 for the isotherm line. Points in parenthesis were not used in the regression analysis. The STKC was also related linearly to the DFFK estimated with the Unocal procedure for the five experimental fields (Fig. 4). 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 a potato plant roots were able to extract 505 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 non-exchangeable K, as there was no apparent relationship to slow release K. In the PNW on native silt loam soils the slow release K concentrations are at least 1200 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 indicate 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 with excess irrigation water being applied. A portion of the applied K will also be fixed in the slow release K fraction, since the equilibria between solution, exchangeable and non-exchangeable K forms is 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.

5 Figure 4. Relationship between STKC and the K diffusion rate (DIFFK) estimated by the Unocal procedure. Potato tuber yield responses to K fertilization were related to STKC during a set of field experiments conducted during 1992 and 1995 by Tindall and Westermann. The original fertilizer K fertilizer guides for Idaho, Oregon and WA had critical STKC of 150 ppm. The Idaho guide had these values established since the 1960 s. Their work indicated that the critical STKC should be raised to 175 ppm K and much higher K fertilization rates were needed for economicallyprofitable potato production (Table 2). Assuming that NaHCO 3 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 K/A 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 STKC and solution K (Fig. 3). The adjusted rates are higher than those derived from the field studies or using the mass balance approach (Table 2, Soil Sorption column). Part of the difference between these two comparisons occur because all 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. Table 2. Fertilizer K recommendations (lbs/ac K) for potatoes by different approaches assuming fertilization of 12 inches of soil.

6 Initial STKC (ppm) 1987 F. G. potato K - Study (95% RY) Soil Sorption (@y=0) Soil Mass The magnitude of the STKC as well as the slow release K fractions in southern Idaho soils and PNW has decreased tremendously with recent cropping history. This necessitates K fertilizer applications for optimum plant growth and to replenish the non-exchangeable K fraction. There was a reasonably close agreement between K fertilization rates from recent field experiments for potatoes and those calculated using a mass balance approach. Potassium fixation could be as high as 27% of the applied K at relatively low STKC. It is our recommendation that potato growers seriously consider a maintenance approach to K fertilization in an attempt to avoid high corrective rates required to over-come K deficiencies that might occur in-season. The new potato K fertilizer guide provides growers with recommended rates of K fertilizer application rates to over-come K deficiency. It is also recommended that higher rates of K should be split applied with at least 50% of the applied K being applied during the fall with the remainder at the time of planting (Table 3). Of those higher concentrations the bulk of the K fertilizer applied for potatoes in the spring should be as K 2 SO 4. Maintenance levels for K application should especially be made for sandy coarse textured soils that might have soil CEC s of less than 10 meq/100 g of soil.

7 Table 3. Final fertilizer K recommendations comparing 1987 rates (K 2 O) developed from the field experiments in southern Idaho. Soil Test K (ppm) 1987 lb K 2 O/A lb K 2 O/A PLANT FACTORS--POTATOES Crops differ in their ability to take up K from a given soil. This is associated with a given plants root system and surface area of the root. Grasses differ from alfalfa and alfalfa differs from potatoes. It all depends on the effective root zone of exploration that a crop has. Potassium in each of these plants is required for many plant enzymatic systems including the transport of starch and sugars. Potassium in potatoes is very specific for osmotic regulation. It is highly mobile in the xylem and phloem tissues of the plant. When potato plants are K deficient, they become stunted, the younger leaf tissue will develop a leathery surface, and the leaf margins will turn downward. Marginal scorch will occur with necrotic spots or necrosis along the leaf margins.

8 The crop may also have an increased susceptibility to various plant diseases with low to moderate K tissue concentrations. High yields of potatoes that are being achieved in western agriculture can remove over 240 lbs/ac K 2 0 in the tuber. This high demand of K acts as a nutrient sink for carbohydrates and mobile nutrients during tuber bulking. If the nutrients are not readily available from the above ground bio-mass or roots yield and quality are affected. For highly mobile nutrients like K the harvested tubers may contain more than 90 % of the total nutrient uptake while only 10-20% of the immobile nutrients are contained in the tubers. Most growers make split application of nutrients for potato production. Therefore in-season applications of K must take place based on crop nutrient demands. The in-season application of K allows a grower to manage more intensely the crop nutrient needs directly related to a specific field or even an area of the field. This requires the grower to be aware of critical tissue concentrations of nutrients including K. The crop advisor also needs to be aware of the fundamental nutrient status of the potato plant. Potassium this status can be defined as the ratio between the total plant K uptake (stems, leaves, and roots) divided by the K tuber K uptake rate. When this ratio or balance is greater than 1.0, there is more nutrient uptake than required for tuber growth. Therefore, nutrients tend to accumulate in other vegetative portions of the plant or are available for additional growth. However, when the ratio is less than 1.0, uptake is less than that required for tuber growth and mobile nutrients will be translocated out of the vegetative portions of the plant to the developing tubers. This approach can be used for all other mobile nutrients and can be especially applied as a K fertility factor for Russet Burbank potatoes or other varieties commonly produced in western agriculture. The problem with excessive K in the plant is that this excessive K can be translocated to the tubers causing a decrease in dry matter because of increasing water content. While low K concentrations decrease tuber dry matter via metabolic reduction in starch formation as well as reducing photosynthate needed for growth. The optimum tuber K concentration for highest dry matter is 1.8 % on a dry matter basis of 22%. At this concentration, 0.48 lb K 2 0/ac is required to grow 100 lb of fresh weight tubers. This relates to at least 3.4 lbs K 2 O/day for tuber growth during bulking. The rest of the plant would require additional K. An additional concern would be the efficiency of the K fertilizer being applied. Our data would indicate that K efficiency is between 30 and 40 %. This could result in a crop availability of between 6 to 8 lbs of K 2 O/ac/day to meet the demands of the tuber and the plant. Therefore, the goal of a K fertilization program for potatoes is to provide sufficient available K to achieve this concentration during the entire tuber bulking stage. Field experiments were conducted to determine a K balance relationship between Russet Burbank potatoes from several

9 field trials in southern Idaho and northern Utah. There was significant tuber yield and quality response to K fertilization. Planted samples consisted of petioles pulled from the fourth leaf as well as tops, roots and tubers. Petiole concentrations began at levels of about % during the first portion of the season. Potassium concentrations decreased with time and after tuber initiation (Fig. 5). The rate of decrease depends on plant/tuber growth rate and the amount of available K. Petiole concentrations can be very high initially when soil K availabilities are high or where large amounts of preplant K were applied. Fertilizer materials of greater solubility give higher petiole concentrations. Petiole K concentrations also respond nicely to fertigation materials containing K. Figure 5 Typical changes in petiole K concentrations with time during Russet Burbank tuber growth We found that the petiole K concentration was linearly related to the K concentration in the active leaves. Potassium was also linearly related to the K concentrations in the above ground biomass and tubers. Therefore, the K concentration associated with the fourth petiole is a good indicator of plant K status. The relationship between the petiole K concentration and K balance showed the average K concentration was about 6.5% when the K balance was 1 (Fig. 6). The K balance is much better determined on an individual field basis than looking at an average across all experiments. This is because K balance is dependent on tuber growth rate. The K balance approach would indicate that a Russet Burbank potato has a petiole concentration requirement of at least 7.0%.

10 Therefore, crop advisors monitoring the K scheduling of additional in-season applications of K, should never allow petiole K concentrations to drop below 7.0 %. There is a lag time between K applications and tissue response of about 15 to 20 days and crop advisors and their growers should keep this in mind in scheduling K applications. Figure 6. Relationship between petiole K concentrations and K balance during Russet Burbank tuber growth. The lines on both sides of 6.4 indicate the range of K balances in seven individual experiments. Future potassium predictions could be made by plotting downward trends of petiole K for each field. When it appears that the K concentration will fall below a sufficiency level additional K through fertigation should be made. For maximum quality tubers and highest yields crop advisors should manage their fields so that K concentrations never fall below 7.0 % (above K balance concentration) until at least 20 days prior to harvest.