Potassium chloride in fertigation

Potassium chloride in fertigation H. Magen (1) (1) ICL Fertilizers Paper presented at the 7th International Conference on Water and Irrigation, 13-16 May, Tel Aviv, Israel Abstract The rapid proliferation of fertigation in modern agriculture has brought about a growing demand for factory mixed fertilizer solutions, as well as wide-scale dissolution of solid fertilizers by farmers in the field. Rules of chemistry and physics control the rate of dissolution and, hence, the nutrient content in the fertilizer solution. Potassium chloride (KCl) is the cheapest potassium-containing fertilizer. Data concerning the dissolution of potassium chloride and other potassium sources are presented below. Introduction Fertigation is the application of solid or liquid mineral fertilizers via pressurized irrigation systems, creating nutrient-containing irrigation water. Although the practice of commercial fertigation started only in the mid - 2th century, there is evidence that the concept of irrigation with dissolved nutrients in water was well known in the past. The first reported example dates back to ancient Athens (4 BC) where city sewage was used to irrigate tree groves (Young and Hargett, 1984). One of the major factors in promoting modern fertigation was the development of micro-irrigation systems (MIS) such as drip irrigation, jets and micro-sprinklers. Field experiments in Israel in the early 196s showed that when localized sections of a field are irrigated, as in MIS, standard broadcasting of fertilizers is ineffective. The limited root zone and the reduced level of mineralization in the restricted wetted zone are the main reasons for the reduced nutrient availability to the plant. When this was recognized, fertigation was integrated in almost all MIS. Israel is an unmatched example of the use of fertilizers by fertigation. The Israeli farmer uses an average of 1, 55 and 75 kg/ha/y of N, P 2 O 5 and K 2 O, respectively. Over 5% of the N and P 2 O 5 is applied by fertigation. Of the 33, tons used annually, approximately 1, tons are applied as clear liquid N-P-K, N-K or P-K solutions or soluble complex fertilizers, and another 5,-1, tons are applied as solid KCl dissolved in the field. Fertigation is by far the most common, and in some cases the only, method of fertilizing in greenhouses, orchards, vegetables and drip irrigated field crops such as cotton, maize, jujube, etc. Various sources have shown the advantage of applying K through irrigation water. Potassium ions are adsorbed at the cation exchange sites of soil colloids, but it was shown that lateral and downward mobility of potassium occur 1

when applied via drip irrigation (Goode et al. 1978; Kafkafi and Bar-Yosef 198). Haynes (1985) showed that the distribution of potassium was more uniform than that of either nitrate or phosphate. Preplanting fertilizer application usually include 15-25% of the recommended N, 2-3% of the recommended K and 1% of the recommended P, Ca, Mg and micro-elements (Sanders 1991). Further nitrogen and potassium is given by fertigation during the growing period to complete the plant requirements. Potassium is applied through fertigation by using various sources of K salts such as potassium chloride (KCl), potassium sulfate (K 2 SO 4 ), potassium nitrate (KNO 3 ) and mono-potassium phosphate (KH 2 PO 4 ). Among the less common K fertilizers are potassium carbonate (K 2 CO 3 ) and potassium silicate (K-Si). The K fertilizer is chosen by price, solubility, anion type and ease of use. This paper discusses various features of KCl fertigation. Compatibility of Fertilizers with Fertigation There are no standards regarding fertigation fertilizers currently in force, therefore, the following are suggested: complete solubility (<.2% insolubles in water), high nutrient content in the saturated solution, fast dissolution in irrigation water, insolubles of non-clogging mineral and bacterial type only, no chemical interactions between the fertilizer and irrigation water, and absence of undesired anions Solubility of K fertilizers. Solubility is defined as the amount of salt (grams) per volume (liter). Potassium chloride (KCl) is the most soluble potassium fertilizer up to a temperature of about 2 C (fig. 1); at higher temperatures potassium nitrate (KNO 3 ) is more soluble. Both salts have an endothermic reaction when dissolved (the solution cools as the fertilizer dissolves). This phenomenon limits the solubility of KNO 3 more than that of KCl. The solubility of fertilizers is reduced when two or three fertilizers are mixed together. The maximum concentration can be determined by using triangular diagrams, from which any ratio can be calculated for a given temperature (Wolf et al. 1985). Solubility (gr. / liter) 5 4 3 2 1 KNO 3 KCl KH 2 PO 4 K 2 SO 4 5 1 15 2 25 3 35 Temp ( C) Fig. 1: Solubility of various K fertilizers at different temperatures. 2

Nutrient content of K fertilizers. Nutrient content is defined as the value received by multiplying solubility times the percentage of the nutrient in the fertilizer. KCl yields the highest nutrient content at 1 C (fig. 2), achieving a concentration of 15% K 2 O, compared to only 8% with KNO 3, and even less with K 2 SO 4 and KH 2 PO 4. Fast dissolution. This parameter is important when considering industrial dissolution processes dissolution at field level for the calculation of irrigation timing and intervals. Elam et al. (1995) showed the difference between KCl, KNO 3 and K 2 SO 4 dissolution rates and the change in temperature after dissolving the fertilizers (fig. 3). The graph shows that the dissolution time (t 9, the time needed to dissolve 9% of the salt added, in minutes) of KCl is much shorter and the K 2 O content is much higher, about 13% for KCl in 8 minutes, as compared to 4% for K 2 SO 4 in 25.2 min. and 9% for KNO 3 in 15.6 min. Solubility (gr./l) 6 45 3 15 MOP KNO3 MKP SOP 16 14 12 1 8 6 4 2 % of plant food (K2O; anion) Solubility K2O (%) Anion (%) Fig. 2: Solubility and nutrient concentration at saturation of K fertilizers at 1 C (5 F); MOP = muriate of potash (KCl), MKP = mono-potassium phosphate (KH 2 PO 4 ), SOP = sulfate of potash (K 2 SO 4 ). Insolubles of non-clogging type KCl is marketed either as white or red MOP (muriate of potash). The source of the red color is the presence of insoluble (in water) Fe-Oxide compounds in the material. When dissolving red potash, It is clearly seen that the saturated solution contains a red colored fraction of insoluble material, thus raising the question of its ability to be applied through fertigation Chemical interactions between the fertilizer and irrigation water The formation of precipitates in irrigation water due to the addition of fertilizer, is one of the most common problems farmers encounter at field level. The most common precipitates are Ca-P compounds at ph>7., when P fertilizers are added. Fertilizing with K and N fertilizers are limited when irrigation water contain high concentration of Ca and or SO 4. At such conditions, salting out of K 2 SO 4, CaSO 4 and NH 4 SO 4 occur. Since chloride salts are highly soluble, precipitation of its salts practically does not exist in such systems. A more accurate method of predicting precipitation under various conditions of ph, and concentrations of Ca, Mg, Fe and PO 4 is by the use of the computer program GEOCHEM-PC (Parker et al., 1995). The program can predict the precipitation of any salts in irrigation water, and thus plays an important role in fertigation management. 3

Laboratory experiments with nutrient solutions showed a good correlation between the program s predictions and the actual results (Magen, 1995). Fraction Dissolved 1.8.6.4.2 12 1 8 6 4 Temperature (C) KCl K2SO4 KNO3 2 1 2 3 4 5 Time (min) at equilibrium KCl K 2 SO 4 KNO 3 % K 2 O 12.9 4.3 9. t 9 8. 25.2 15.6 % Salt 2.4 8. 19.2 Fig. 3: Dissolution kinetics (full points) and change in temperature (hollow points) when dissolving K fertilizers (8% saturation, 2 C, 1 rpm). Presence of undesired anions Table 1 shows types of anions and their relative consumption by plants. Table 1: Cations and anions from different K fertilizers and their uptake by plants Fertilizer Cation Anion Anion absorbed by plants as... nutrient KCl K+ Cl - secondary K 2 SO 4 K+ -2 SO 4 secondary KNO 3 K+ - NO 3 macro KH 2 PO 4 K+ - H 2 PO 4 macro Chloride is consumed by plants at low quantities, therefore, under heavy fertilization with Cl containing fertilizers and sub-optimal leaching conditions, it will accumulate and create salinity problems. Specific guidelines and tolerance levels were assessed (Maas, 1986), enabling research and farmers to monitor and adopt proper management to minimize this problem. 4

Prepared liquid fertilizers Clear liquid fertilizers used for fertigation include urea, ammonium nitrate, ammonium sulfate, either individually or in combination as the N source, orthophosphate as the P 2 O 5 source and KCl, K 2 SO 4 and KH 2 PO 4 as the K 2 O sources. By mixing two or three different nutrients, the solubility of each nutrient declines (table 2). Table 2 presents some of the characteristics of selected liquid fertilizers of industrial preparation. Under field conditions, mixing and heating are limited, resulting in a much lower nutrient content. An experiment describing the maximal nutrient content in field prepared solution, was conducted in our laboratory, in order to develop a simple mixing tables for the end user. At the present work, we have examined the preparation of KCl solutions with minimal stirring, at a controlled temperature of 1 C. Different amounts of fertilizer grade potassium chloride (61% K 2 O) (4%, 6%, 7%, and 1% K 2 O concentration w/w) were added to 1 ml of tap water at 1 C in a vessel jacketed with cooling liquifd at 1 C. After addition of the fertilizer the solution was stirred for one minute and the stirring then stopped. The temperature change with time was recorded as well as the time to reach complete dissolution. The temperature changes with time for four different amounts of KCl is shown in table 3. For the first three additions clear solutions were obtained after 5-8 minutes. The fourth addition, 1% K 2 O, did not dissolve completly even after one hour, although the temperature had returned to 1 C after 17 minutes. Table 2: Selection of various formulas of liquid fertilizers (source: Sne, Ministry of Agriculture. Israel, 1989). Fertilizers Formula Salting out Temperature (C ) ph (1:1) NH 4 NO 3 +H 3 PO 4 +KCl 19-5- 6. 7-7-7 15 3.1 4--12 5 4.5 Urea+NH 4 NO 3 + H 3 PO 4 +KCl -1-1 5.3 8-8-8 13.6 8--12 12 7.6 NH 4 NO 3 +H 3 PO 4 +KNO 3 7--7 14 3.5 6-6-6 3 3.5 3--9 12 3.5 5

Table 3: Temperature change with time to reach clear K solution Time 4% K 2 O 6% K 2 O 7% K 2 O 1% K 2 O (minutes) Temp (C) 1 1 1 1.5 7 6 5 2 1 8 6 5 3 2 8 7 6 5 3 8 8 7 6 4 8 8 8 6 5 9# 8 8 6 6 9 8# 8 7 7 9 8 9 8 8 9 9 9# 8 9 9 9 9 8 1 1 9 9 8 15 1 1 1 9 17 1## # - clear solution ## - undissolved material present after one hour On the basis of these results, the most concentrated solution of KCl that can be prepared in simiulated field conditions, at 1 C with minimal stirring, is --7, and this will take approximately 8 minutes to reach full dissolution. Conclusion KCl was compared with other K fertilizers regarding their suitability to fertigation. The chemical characteristics of KCl were discussed and assessed. KCl s high nutrient percentage in saturated solution, as well as its rapid dissolution and compatibility with other fertilizers applied in stock solutions, makes it ideal for fertigation use. In some cases, monitoring of Cl - content in soil and plant is recomended to eliminate its accumulation. Acknowledgments Thanks are due to Dr. M. Lupin, IMI (TAMI) Institute for research & Development Ltd., who conducted and summarized the KCl dissolution at field conditions experiment. 6

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