STRATEGIES TO MEET POTASSIUM NEEDS OF CROPS FOR OPTIMUM PRODUCTION

2. STRATEGIES TO MEET POTASSIUM NEEDS OF CROPS FOR OPTIMUM PRODUCTION A. Subba Rao Former Director, ICAR-Indian Institute of Soil Science, Nabi Bagh, Berasia Road, Bhopal

3. Introduction • The consumption of potassic fertilizers in India increased from 0.24 million tonnes in 1970-71 to 3.5 million tonnes in 2010-11(Anonymous, 2015) • But it has dropped to level of 1.98 million tonnes in 2013-14 widening the nutrient imbalance (N:P:K ratio widened from 5.0:2.4:1.0 to 8.36:2.76:1.0). • Currently we are importing 100% of the requirement of potassic fertilizers, resulting in a huge burden to the exchequer and a subsidy burden of thousand crores of rupees to the Government of India

4. Role of potassium in plant nutrition • Potassium, a mobile nutrient in plant, has role in electrical balance in ion transport, plant cell-water regulation and enzyme activation, e.g. activates at least 60 enzymes. • Potassium is also needed in transport of sugars produced in photosynthesis and in the synthesis of cellulose and lignin-giving strength to plant and remain erect. • Root and tuber development in crops such as potatoes require adequate amount of K. Crops such as banana, pineapple, papaya, alfalfa and sugarcane have, especially, high K requirement. Potassium plays key role in enhancing the crop produce quality.

5. K status in Indian Soils • In more than 90 percent of districts (combining low and medium categories) in India availability of N and P is critical constraint to maintain soil productivity and health. • The soils are in better position in respect of potassium with 51% samples in different districts in low to medium and 49% samples in high category (Muralidharudu et al.2011). • A comprehensive account of potassium status of Indian soils is available in bulletins brought out by PRII and IPI (Sekhon et al.1992 ;Subba Rao et al. 2011).

6. K reserves in soils • Most of the potassium in soils is held in the primary minerals like micas (biotite and muscovite) and feldspars (orthoclase and microcline) and secondary minerals (illite and vermiculite). • Among the primary minerals, the trioctahedral biotite mica weathers faster than the dioctahedral muscovite mica and feldspars (microcline) and releases K more readily towards crop uptake. • Interlayer K in secondary minerals is a major reservoir of potassium in soils which is rather slowly- available to plants (Sanyal et al. 2009)

7. K forms in soils Potassium in soil occurs in four distinct forms: • Soil solution (0.1-0.2% of the total K)-readily available to plants, • Exchangeable K (1-2% of the total K)-readily available to plant. • Interlayer or fixed or non-exchangeable K (1-10%) which is slowly available to plant, • Structural mineral K (90-98%) which is slowly released in negligible amount over short or medium term period.

8. Fertilizer K Plant uptake Plant residues and manures Soil organic matter (humus) Soil solution K 0.1 – 0.2 % Exchangeable K (Soil colloids) 1-2% Runoff Erosion (Soil) Non-exchangeable K (Secondary minerals) 1-10% Leaching Structural K (Primary minerals K feldspars, micas etc.) 90-98% • Exchangeable K represents the fraction adsorbed on external charge sites and accessible internal surface of clay colloids or on organic matter (humus). • So, the amount of clay, mineralogy and CEC modify the amount of exchangeable K available to plants. • The exchange of K between the soil solution and exchangeable K sites is extremely rapid, with equilibrium between the two pools being restored in minutes or hours after K is removed from the solution phase • Fig1 (modified from Brady and Well 2007)

9. To characterize contribution the K release from non-exchangeable pool and quantify its in short and medium term cropping is a challenge. • As the soil micaceous minerals are exposed to the action of weathering and dissolution, the layers are ‘peeled away’ allowing some substitution of K by other, often larger hydrated cations like Ca, Na etc . • This allows greater access to the K and H3O+ ions that still reside in the peeled or the so called’ wedge zone • The fourth pool of K in the soil is the structural K, which is part of the chemical structure of minerals such as micas and feldspars .The K from this source is very slowly available, with release of K taking months to years.

10. K Fixation • In the process of potassium fixation in soils, the added soluble K is converted to a form that cannot be extracted with a neutral salt solution, commonly employed to extract plant- available form. As a consequence of K fixation, availability of added K to plants decreases to varying extent. • The K fixation is negligible in soils dominant in kaolinite, chlorite and unweathered micas , slight in montmorillonite, substantial in illite and large in vermiculite-dominant soils.

11. E P I • Potassium ions are adsorbed by clay minerals on the binding sites which differ in their selectivity • In 2:1 clay minerals, such as illites, vermiculites and weathered micas, three different adsorption sites can be distinguished. • These sites are at the planar surfaces (p-position), at the edges of the layers (e-position) and in interlayer space (i-position). The binding selectivity’s for K by organic matter and clays of the kaolinite type are similar to the p-position sites. Here, the K–bond is relatively weak so that K adsorbed may easily be replaced by other cations, and particularly by Ca2+ and Mg2+ ions. The i-position has the maximum specificity for K+. The latter binding sites largely account for K+ fixation in soils.

12. Fixation of K in non-exchangeable form • In smectitite-rich soils, K+ ions are fixed at interlayer sites, which normally take part in exchange. When dehydration occurs, the lattice sheets come closer and the adsorbed cations loose their attendant water molecules. • Then the outer layers of the ‘sheets’ in clay minerals are composed of array of oxygen atoms, which leaves between them hexagonal or ditrigonal holes or cavities of 2.8 Å diameters. • Dehydrated K+ ions have a diameter of 2.66 Å, and therefore can fit snugly into these cavities. Consequently, these K+ ions find themselves imprisoned between two adjacent sheets (Page and Baver 1940).

13. Most commonly used extractant • There have been several extraction methods tested and used for assessing the potassium fertility status of Indian soils (Subba Rao et al.2001). • Water soluble and exchangeable K is 1 M NH4OAc method (Hanway and Heidel 1952. • Dilute salt solutions like CaCl2 and BaCl2at 1:10 soil : solution ratio and 30 minute equilibration can be used to estimate predominantly water- soluble K (Woodruff and McIntosh, 1960). • Water soluble K can be estimated in saturation (paste) extract or 1:5 and 1: 10 soil: water extraction after 5 or 30 minutes equilibration time (Jackson1973; MacLean 1961). • The amounts of K estimated by the multi-nutrient extractants like Mehlich 3 (Mehlich 1984) are almost equal to ammonium acetate method. • Liu and Bates (1990) found AB-DTPA to be slightly better than ammonium acetate and Mehlich 3 to evaluate available K for Lucerne.

14. Limitations of ammonium acetate and nitric acid extractants • Studies conducted in India and elsewhere clearly showed that NH4OAc method was inadequate to estimate plant- available K ! M 1 ! 1M NH4OAc method was found inadequate to estimate K especially in illitic soils where non-exchangeable K contributes to K uptake substantially. • The 1M boiling HNO3method (Wood and De Turk 1941) is the most widely used procedure for studying soil K supplying capacity from non-exchangeable pool to meet the plants need. • 1 M HNO3 action on soil is harsh as it destroys soil minerals (Al-Kanani et al 1984) and extracts more K than the actual plant uptake even under exhaustive cropping (Moody and Bell 2006) .

15. Development of sodium tetra phenyl boron (NaTPB) • The sodium tetra phenyl boron (NaTPB) procedure was developed by Scott et al (1960) and Smith and Scott (1966) to account for release of interlayer K in micaceous minerals and soils. • The BPh4 anion combines with K in solution and precipitates as potassium tetra phenylboron (KTPB). Therefore, NaTPB can mimic the action of plant roots by depletion of soil solution K as KTPB and causes further release of exchangeable and non-exchangeable K (Cox et al.1999). • However, the NaTPB is less able to remove K from the three-dimensional structure of K-feldspars (Song and Huang 1988). Among the methods to assess the non-exchangeable K reserves viz., strong acids, NaTPB and H-resins, the NaTPB method appears to be better a choice. • But the limitations of the method are long equilibrium time ,tedious analytical procedure and the cost of the NaTPB chemical itself. Jackson’s(1985) test took 16hours with 0.03MNaTPB to measure reserve K in NEW Zealand soils and was superior to IM NH4OAc method for estimating K availability to ryegrass (Lolium perenne L.).

16. Cox et al (1996,1999) modified the original procedure of Smith and Scott (1966) to make it suitable for routine analysis. Cox et al (1996) and Cox and Joern (1997) outlined a new variation of the NaTPB method that used neither acetone (volatile and flammable) nor mercury (volatile and hazardous), but employed copper to destroy the TPB precipitate. • 1-h extraction time was suitable and effective (Moody and Bell 2006). The NaTPB method may serve as good indicator of K supply from inter layers in case of illite or vermiculite dominant soils (Cox etal.1999) but may not reflect better K status than NH4OAc method in kaolinite mineral dominant highly weathered and highly developed soils (Darunsontaya et al. 2010).

17. Biphasic release of K Recent studies employing NaTPB for soil test extraction procedures or K release kinetics, recognized two categories of non-exchangeable K-one the fast release or intermediate K from the so- called wedge zone and the slow release K from interlayer zone. Carey and Metherell (2003) reported a biphasic pattern of K release comprising more rapidly available (1-16 h incubation) and longer-term (48 to 168 h) slowly available K. Beckett (1971) introduced the concept of “intermediate K as a function of the non-exchangeable K that is held around the edges and wedge zones of micaceous minerals .

18. Table 1. Threshold levels of potassium Critical value of K • Non-Exch.K is released when K concentration in soil solution approaches a certain critical value, known as the ‘threshold concentration’ (Datta and Sastry,1989). Threshold values for potassium release and fixation in different soils are furnished in Table 1. • The threshold levels were higher for K fixation and much lower for K release

19. In a similar study employing NaTPB with different times of extraction(15 minutes, 1, 4 and 16 hours), the K extracted at 15 minutes showed better relationship with cumulative dry matter yield and K uptake of crops compared to other extraction periods (Sharma and Swami 2000). • A segmented regression model gave the best characterization of soil K release to the Ca-resin and of non-exchangeable K release to plants in K exhaustion study (Nilawonk et al 2008) from Thailand. • The green house study yielded fast and slow release rates from the non-exchangeable K pools of 0.45 to 0.85 and 0 mg kg-1d-1, respectively in kaolinitic soils (no slow release K) and 1.60 to 1.98 and 0.27 to 0.52 mg kg-1d-1, respectively in smectitic soils. • Based on this study, in kaolinitiic soils the NH4OAc method may be suitable to determine plant-available K. Based on extraction of potassium with NaTPB at two periods, 15 minutes and 60 minutes extraction, fast release (NaTPB15-Exch.K) and slow release (NaTPB60-exch.K) non-exchangeable were categorized in the soils of Queensland, Australia(Moody and Bell,2006).

20. These studies indicate a trend and possibility that only a part of interlayer K which can be termed ‘fast release K’ is plant- available in short to medium term. • Hence there is need to evaluate the NaTPB with short equilibration time say one hour or 15 minutes, HNO3, HCl and H2SO4 acids at lower strength say at 0.5M , 2M and 3 N strength or lower , respectively for their suitability to predict crop response to applied K fertilizer. Long term K supplying power of soils can be assessed through the estimation of step-K and constant rate-K using 1 N boiling nitric acid (Haylock,1956 ) or other repeated acid/resin extraction methods.

21. Some Extractants for K determination • Some recent studies showed that in place of IM HNO3 extraction procedure , some methods based on lower strength of acids such as 3 N H2SO4 (Hunter and Pratt,1957), cold 2N HCl (Saarela et al. 2003) and O.5N HNO3 (Oommen, 1962) methods gave better relationship with crop response compared to higher concentrations of those acids . • In a study involving 14 agricultural soils from Czech Republic, several soil tests and two plant tests were evaluated for their suitability to assess plant-available K, the methods based on non-exchangeable K i.e. Step K and NaTPB gave the best relations with K uptake in exhaustion study. • The change in 3M H2SO4 extractable K showed higher correlation coefficients with K release rate constant (b) and K release constant (a) compared to 1M NH4OAc (original and minimum) in swell-shrink soils of Central India(Srinivasa Rao et al. 1998).

22. Some other Extractants for K determination • In a three- year study the K extracted by strong acids of different strengths with K uptake in different years showed interesting differences (Ogaard and Krogstad,2005). • Potassium extracted with boiling 1M or 2M HNO3 was significantly related to the K yield (uptake) in the 2nd and 3rd years. • Potassium extracted with cold 2M HCl, 0.1M HNO3 and 0.5M HNO3 was significantly related to the K yield in all 3 years. • Among these extractants, 0.1M and 0.5M HNO3-extractable K were better predictors of K uptake than 2M HCl.

23. ApproachesforMeetingtheCrop K Needs General Fertilizer Recommendation General recommendations to crops are based on response to fertilizer application of a particular crop in a given agro-climatic region, irrespective of nature of soil. Later on they are adjusted for soil fertility status by advocating general fertilizer dose to a medium fertility category soil and then applying 25-30 % more dose of nutrient to a soil of low fertility and decreasing by the same quantity in case of a soil of high fertility. These are, then, called soil test based recommendations.

24. Critical soil test value approach • Cate and Nelson (1965) proposed a graphical method for dividing the Bray’s percent yield versus soil test values from a good number of pot or field trails covering a range of soil test values into two classes • those which had a high probability of getting a large response to fertilizer use and • those which had a low probability of response. • The soil test value separating the two groups was termed as critical level.

25. SubbaRao and Sammi Reddy (2005) compiled critical limits for K based on NH4OAc method from several sources for many crops on diverse soils. The critical limits are especially high for Vertisols and associated soils and deltaic alluvial soils. SubbaRao et al(.2001) compiled critical limits for some important crops determined using boiling nitric acid method (Table2) .In red soils or Alfisols the values are much low but in alluvial soils the values much exceeded 1000 mg kg-1 soil.

26. Critical levels of available (NH4OAc-extractable) K in different soils for different crops

27. Critical levels for non-exchangeable (1M HNO3) K in soils and crops

28. Estimation of K Recommendation for a Yield Target In targeted yield approach for formulating fertilizer recommendations, the basic data required are (i) nutrient requirement in kg/q of produce, grain or other economic produce, (ii) the percent contribution from the soil available nutrients (% CS) and (iii) the percent contribution from the applied fertilizer nutrients (% CF) (Ramamoorthy et al. 1967). These basic data are transformed into workable adjustment equation as follows: • Fertilizer dose = (Nutrient requirement in kg q-1 of grain/ % CF) x 100 xT- (%CS/%CF) x STV • = a constant x yield target (q/ha) – b constant x soil test value (kg/ha) Where T is target yield (q/ha)

29. The basic data have been derived for various crops and soil types from the soil test crop response field experiments conducted under the ICAR coordinated Soil Test Crop Response correlation (STCR) project at 15 State Agriculture Universities and 2 ICAR institutes. Fertilizer adjustment equations and prescriptions or ready-reckoners have been developed from the basic data. These adjustment equations help the farmers to choose a particular yield target depending upon their resources and apply fertilizers on the basis of soil test values to obtain economically profitable yields(SubbaRao and Srivastava 2001;Muralidharudu et al.2011).

30. Soil test based fertilizer potassium (K2O) requirement for wheat on different soils of India

31. Buildup and maintenance of K • The build up and maintenance approach is based on applying nutrients in excess of crop removal initially for a few years as a means of increasing the soil test values to a nonresponsive level. • Once the soil test reaches the adequate level, nutrients are applied based on estimated crop removal so that the soil nutrient level is maintained at an adequate level. • This method of nutrient application is suitable to less mobile nutrients like P and K. But in India we have hardly generated any information in this line especially for K .

32. …. … Buildup and maintenance of K • In case of K the luxury consumption of K in high fertility soils may create problem with implementation of this approach. • Most of the time the recommended rates of fertilizer K application to crops is 1/3 to ¼ of the actual K uptake by crops, giving little scope for buildup of K in soil. Also how to complementarily utilize the potentially available non-exchangeable K in meeting the K requirement of crops partly or fully is crucial.

33. Basic cation saturation • Basic cation saturation ratio (BCSR) method of fertilizer recommendation, originally conceived by F.E Bear in 1945, promotes the concept that maximum yield is only achieved by creating an ideal ratio of Ca ,Mg, and K . • It is proposed that the ideal percent saturation of cations on exchange complex was 65% Ca,10% Mg and 5% K and 20% H. • Graham (1959) suggested the range could be from 65-85% for Ca,6-12% for Mg and from 2.5 to 5% for K. • This concept to some extent can work well in highly weathered soils with a low to moderate CEC, low base status and low pH and needed adjustment in cations for providing proper basic cation nutrition. In other neutral to alkaline soils adequate amounts of K, Ca and Mg individually may be sufficient rather than their ratios.

34. Research Needs • It is of utmost importance to develop and/or standardize the soil test methods like NaTPB based on extraction of the interlayer K especially from the wedge zone and its availability to crop plants both in short term and medium term field experiments . • There is need for evaluating promising strong acid based extractants at lower or reduced concentrations for their suitability to assess the fast release non- exchangeable K using green house exhaustion cropping of ryegrass etc. • Some more promising extractants also need to be evaluated for their suitability to assess in medium term the utilization of the fast non-exchangeable K under field conditions. Under field conditions, in addition to normal chemical analysis, the percentage of soil volume exploited by plants need to be assessed

35. Conclusions • The key strategies for making fertilizer recommendations for optimum crop nutrition are: maintain soil test K at an adequate level, • Estimate crop K requirement for a desired targeted yield, • Apply the amount of K that is the difference between the crop requirement and the amount supplied by soil based on soil test. • Also see whether the ratio among Ca, Mg and K is more or less maintained or individually whether all the three cationic nutrients are adequate amounts to meet the crop need.

36. Conclusions • Organic recycling through return of crop residues or manures and composts application may substantially reduce the fertilizer K requirement. • In order to fully understand the contribution of non-exchangeable in short to medium term, new soil test crop response calibration need to be developed for the fast non-exchangeable K on different soils and under important production systems not only to provide optimum K nutrition to crops but to raise yield level and sustain productivity. Also we have to fertilizer the crops for quality produce of cereals, pulses, fruits and vegetables.

37. Thanks