**4. Potassium**

Potassium (K) is essential nutrient for plant growth. K concentrations in dry matter of plants vary between 1.0 and 6.0 % and more and are generally higher than those of all other cations. The exact function of K in plant growth has not been clearly defined. By numerous investigations were found that K stimulates early growth, increases protein production, improves the efficiency of water (drought resistance), improves resistance to diseases, insects and stalk lodging (Kovacevic & Vukadinovic, 1992; Rehm & Schmitt, 1997).

Soils mainly containing enormous amounts of K, but depending on soil types, 90-98 percent of total K is unavailable. Slowly unavailable K is thought to be trapped between layers of clay minerals (Johnston, 1987; Rehm & Schmitt, 1997). K deficiency is encountered mostly on light, usually acid soils with a low cation exchange capacity or on soils with a high content of three-layered clay minerals often loess soils with illite clay (Bergmann, 1992).

Soybean requires large amounts of K and K deficiencies are easy to recognize (edge necrosis of leaves – the margins of leaflets turn light green to yellow) and correcting them is inexpensive as K is to lowest-cost major nutrient. K deficiencies as result of strong K fixation and high levels of available magnesium (Mg) were found on heavy hydromorphic soils of Sava valley area in Croatia. By ameliorative KCl fertilization yields of maize and soybean drastically increased due to improved plant nutritional status (Vukadinovic et al., 1988; Kovacevic & Vukadinovic, 1992; Kovacevic 1993; Kovacevic & Grgic, 1995¸ Kovacevic & Basic, 1997).

The K deficiency in soybeans was found on the drained gleyols which had inadequate rates of the exchangeable K and Mg (low K and high Mg status). These soil characteristics affected correspondingly K and Mg status in soybean plants (Tables 3 & 4).


Soybean (the upermost full-developed threfoliate leaf before anthesis) and soil status

Table 3. Plant and soil status (drained gleysol): symptoms of K deficiency in soybean (Kovacevic et al., 1991)


Table 4. Nutritional status of normal and chlorotic (K-deficiency symptoms) soybeans (Kovacevic et al., 1991)

Response of soybeans to ameliorative KCl- fertilization (the upermost full-developed threfoliate leaf before anthesis)


Table 5. Response of soybeans to potassium fertilization (Katusic et al. 1988; cit. Kovacevic & Basic, 1997)

Soybean (the upermost full-developed threfoliate leaf before anthesis) and soil status

Zupanja (1988) 0.35 0.98 1.20 0.73 1930 7.33 6.91 6.6 15.9 Vinkovci (1989) 0.57 1.05 2.22 2.14 780 7.75 6.87 28.0 12.2 Jasinje (1988) 0.38 0.87 1.30 1.11 1410 7.68 7.20 10.6 10.2 N. Gradiska (1990) 0.32 1.16 1.82 0.92 2060 7.76 6.91 7.9 16.6 Table 3. Plant and soil status (drained gleysol): symptoms of K deficiency in soybean

Soybean (the upermost full-developed threfoliate leaf before anthesis) nutritional status The state farm Percent in dry matter mg/kg (ppm) in dry matter and date of sampling P K Ca Mg Zn Mn Fe Al

means of two samples/field Zupanja (June 19, 1987) 0.45 2.02 1.74 2.91 13.0 28.0 609 588

means of two samples/field Zupanja (June 19, 1987) 0.25 2.87 1.69 1.78 12.0 28.0 386 309

Response of soybeans to ameliorative KCl- fertilization (the upermost full-developed

Fertilization The 1986 growing season The 1987 growing season (spring 1986) Yield Leaf (% in dry matter) Yield Leaf (% in dry matter) N P2O5 K2O (t/ha) P K Ca Mg (t/ha) P K Ca Mg 0 0 0 2.43 0.37 0.72 1.84 1.62 1.45 0.35 0.91 1.69 1.35 120 120 180 2.40 0.36 0.87 1.87 1.49 1.48 0.32 0.99 1.37 1.35 120 120 990 2.83 0.35 1.28 1.76 0.74 1.88 0.32 1.29 0.92 0.77 LSD 5% 0.27 ns 0.22 ns 0.25 0.29 ns 0.16 0.30 0.31 Table 5. Response of soybeans to potassium fertilization (Katusic et al. 1988; cit. Kovacevic &

Vinkovci (June 13, 1986) 0.59 1.13 1.48 1.25 17.0 26.0 260 Jasinje (June 13, 1986) 0.59 1.06 1.25 1.11 18.0 248.0 180 Mean 0.48 1.69 1.47 1.38 15.7 100.7 275 Table 4. Nutritional status of normal and chlorotic (K-deficiency symptoms) soybeans

Vinkovci (June 13, 1986) 0.73 0.66 1.16 1.65 19.0 25.0 600 Jasinje (June 13, 1986) 0.52 0.70 1.11 1.29 27.0 200.0 220 Mean 0.57 1.13 1.34 1.95 19.7 84.3 476

Chlorotic soybeans (K-deficiency symptoms ):

Normal soybeans (oasis in the chlorotic soybeans):

Grain yield

**P K Ca Mg** (kg/ha) **H2O KCl P2O5 K2O** 

Soil status

(0-30 cm of depth)

pH mg/100 g

(AL-method)

(means of four fields)

(Kovacevic et al., 1991)

(Kovacevic et al., 1991)

Basic, 1997)

threfoliate leaf before anthesis)

farm (year) Leaf status

Soybean

(K-deficieny symptoms)

(precent in dry matter)

The state


Table 6. Response of soybean plants to potassium fertilization (Kovacevic & Vukadinovic 1992)

Katusic et al., (1988; cit. Kovacevic & Basic, 1997) applied increasing rates of KCl on Cerna drained gleysol. Soybean responded by yield increases for 16 % and 30 %, for the first and the second year testing, respectively. Soybean under unfertilized and usual fertilization was contained in mean 0.82 % K (acute K-deficiency with correspondingly symptoms) and 1.49 % Mg. Soybean nutritional status was considerable improved by K fertilization (mean 1.29 % K and 0.76 % Mg) – Table 5.

Kovacevic & Vukadinovic (1992) tested response of soybean and maize to increasing rates of potassium application in KCl form on silty clay gleysol developed on calcareous loess. Low levels of exchangeable K, high levels of exchangeable Ca and Mg and strong K fixation were found by the soil test (Vukadinovic et al., 1988; Kovacevic & Vukadinovic, 1992). Also, clay fraction (35.2 % of soil) composition was as follows: vermiculite/chlorite 30 %, smectite 30 %, mixed layer minerals 20 %, illite 15 % and kaolinite 5 % (Richter et al., 1990). By ameliorative K fertilization soybean yields were increased drastically (3-y means: 1286 and 2607 kg/ha, for the control and the highest rate of K) and they were in close connection with improvement of leaf K and Mg status (Table 6 and Fig. 1).

Fig. 1. Soybean status (middle of July 1989) on the control (left) and the highest rate of K (2670 kg K2O/ha in spring 1987) application (right) – the data in Table 6 (photo V. Kovacevic)

In Ontario, Canada, studies looked at the response of soybeans to potassium fertilizer as related to K leaf tissue levels (Reid and Bohner, 2007). The data collected during that study formed the basis for updated critical and normal values for potassium in soybeans. Below a leaf K concentration of 2.0% (on dry matter basis), most of the plots showed a response to added K fertilizer. Above this level, most of the plots were unresponsive. Based on the results of these experiments and other similar studies, the critical concentration for K in soybean tissue was established at 2.0% and the maximum normal concentration from 2.5 to 3.0%. According this criterion, in our investigations under strong K-fixing conditions (Table 5) only by application of enormous K rates leaf-K concentrations were increased to normal level. However, in spite of considerable improvement of soil and plant K status, yields of high-yielding soybean cultivar were less than 3.0 t/ha (Table 5).

Long-term studies conducted on integrated nutrient management in soybean-wheat system (Singh & Swarup, 2000) revealed that continuous use of FYM along with recommended NPK for 27 crop cycle not only restricted K mining by reducing non-exchangeable K contribution to grain formation but also enchanced K uptake to the system (Table 7).


Table 7. Removal and addition of K during 27 crops cycle of soybean-wheat-maize (fodder) cropping system (Singh and Swarup, 2000)

Morshed et al. (2009) applied six treatment of potassium (unfertilized, 50%, 70%, 100 % , 125% and 150% of recommend rate based on soil test) on equal N, P and S fertilization in Dhaka (Bangladesh) during Rabi season 2004-2005. By application of the highest K rate grain yield of soybean was increased for 83%. Slaton et al., (2009) found close connection of soybean response to K fertilization (five rates from 0 to 148 kg K/ha) and Mehlich-3 extractable soil K in eastern Arkansas. Experiments were established on silt loams at 34 site-years planted with a Maturity Group IV or V cultivar. Mehlich-3-extractable soil K ranged from 46 to 167 mg K/kg and produced relative soybean yields of 59 to 100% when no K was applied. Eleven sites had Mehlich-3-extractable K < 91 mg K/kg and all responded positively to K fertilization. Soybean grown in soil having 91 to 130 mg K/ g responded positively at nine of 15 sites. Mehlich-3 soil K explained 76 to 79% of the variability in relative yields and had critical concentrations of 108 to 114 mg K/kg, depending on the model. Based on these investigations, Mehlich-3-extractable K is an excellent predictor of soil K availability for soybean grown on silt loams in eastern Arkansas.

In Ontario, Canada, studies looked at the response of soybeans to potassium fertilizer as related to K leaf tissue levels (Reid and Bohner, 2007). The data collected during that study formed the basis for updated critical and normal values for potassium in soybeans. Below a leaf K concentration of 2.0% (on dry matter basis), most of the plots showed a response to added K fertilizer. Above this level, most of the plots were unresponsive. Based on the results of these experiments and other similar studies, the critical concentration for K in soybean tissue was established at 2.0% and the maximum normal concentration from 2.5 to 3.0%. According this criterion, in our investigations under strong K-fixing conditions (Table 5) only by application of enormous K rates leaf-K concentrations were increased to normal level. However, in spite of considerable improvement of soil and plant K status, yields of high-yielding soybean cultivar were less

Long-term studies conducted on integrated nutrient management in soybean-wheat system (Singh & Swarup, 2000) revealed that continuous use of FYM along with recommended NPK for 27 crop cycle not only restricted K mining by reducing non-exchangeable K

Fertilizer K added in 27 crop rotation, total K uptake, available and non-exchangeable K

Potassium ( kg K /ha) Contribution of

After

status Total K non-echangeable K

<sup>1999</sup>uptake kg K/ha %

contribution to grain formation but also enchanced K uptake to the system (Table 7).

Before 1971

a) Control 0 370 252 3247 3129 96.4 b) 100 % N 0 370 263 4418 4311 97.6 c) 100 % NP 0 370 235 10067 9932 98.7 d) 100 % NPK 2117 370 308 11826 9647 81.6 e) d + 5 t FYM/ha 4142 370 324 14094 9906 70.3 Table 7. Removal and addition of K during 27 crops cycle of soybean-wheat-maize (fodder)

Morshed et al. (2009) applied six treatment of potassium (unfertilized, 50%, 70%, 100 % , 125% and 150% of recommend rate based on soil test) on equal N, P and S fertilization in Dhaka (Bangladesh) during Rabi season 2004-2005. By application of the highest K rate grain yield of soybean was increased for 83%. Slaton et al., (2009) found close connection of soybean response to K fertilization (five rates from 0 to 148 kg K/ha) and Mehlich-3 extractable soil K in eastern Arkansas. Experiments were established on silt loams at 34 site-years planted with a Maturity Group IV or V cultivar. Mehlich-3-extractable soil K ranged from 46 to 167 mg K/kg and produced relative soybean yields of 59 to 100% when no K was applied. Eleven sites had Mehlich-3-extractable K < 91 mg K/kg and all responded positively to K fertilization. Soybean grown in soil having 91 to 130 mg K/ g responded positively at nine of 15 sites. Mehlich-3 soil K explained 76 to 79% of the variability in relative yields and had critical concentrations of 108 to 114 mg K/kg, depending on the model. Based on these investigations, Mehlich-3-extractable K is an excellent predictor of soil K availability for soybean grown on silt loams in eastern

status in soil (maize crop was discontinued in the system since 1995)

Treatment K added Available K

in 27 cycles

cropping system (Singh and Swarup, 2000)

Arkansas.

than 3.0 t/ha (Table 5).

Gill et al. (2008) reported that imbalance and inadequate nutrient supply particularly devoid of K is main reason for low productivity and quality of soybean in India.

Yin and Vyn (2004) conducted field experiments at three locations in Ontario, Canada from 1998 through 2000 to estimate the critical leaf K concentrations for conservation-till soybean on K-stratified soils with low to very high soil-test K levels and a 5- to 7-yr history of no-till management. For maximum seed yield, the critical leaf K concentration at the initial flowering stage (R1) of development was 2.43 %. This concentration is greater than the traditional critical leaf K values for soybean that are being used in Ontario and in many U.S. Corn Belt states.

Nelson et al. (2005) compared response of soybean to foliar-applied K fertilizer and preplant application. Potassium fertilizer (K2SO4) was either broadcast-applied at 140, 280, and 560 kg K/ha as a preplant application or foliar-applied at 9, 18, and 36 kg K/ha at the V4, R1-R2, and R3-R4 stages of soybean development. Soybean grain yield increased 727 to 834 kg/ha when K was foliar-applied at 36 kg/ha at the V4 and R1-R2 stage of development in 2001 and 2002. Foliar-applied K at the R3-R4 stage of development increased grain yield but not as much as V4 or R1-R2 application timings. Foliar K did not substitute for preplant K in this research. However, foliar K may be a supplemental option when climatic and soil conditions reduce nutrient uptake from the soil.

Numerous studies investigated fertilization effects on soybean grain yield, but few focused on oil and protein concentrations. Haq & Mallarino (2005) determined fertilization effects on soybean grain oil and protein concentrations in 112 field trials conducted in Iowa from 1994 to 2001. Forty-two trials evaluated foliar fertilization (N-P-K mixtures with or without S, B, Fe, and Zn) at V5-V8 growth stages. Seventy trials evaluated preplant broadcast and banded P or K fertilization (35 P trials and 35 K trials). Replicated, complete block designs were used. Foliar and soil P or K fertilization increased (P < 0.05) yield in 20 trials. Foliar fertilization increased oil concentration in one trial and protein in one trial but decreased protein in two trials. Phosphorus fertilization increased oil concentration in two trials and protein in five trials but decreased oil in five trials and protein in two trials. Potassium fertilization increased oil in four trials and protein in two trials but decreased oil in two trials and protein in two trials. Total oil and protein production responses to fertilization tended to follow yield responses. Fertilization increased oil production in 20 trials and protein production in 13 trials. Fertilization that increases soybean yield has infrequent, inconsistent, and small effects on oil and protein concentrations but often increases total oil and protein production.

Potassium is known to play an important role in protecting the plants against drought stress. Quantity and distribution of rainfall in the major soybean regions in India is responsible for yield fluctuations about plus/minus 20% among years in comparison with national average yield of 1 t/ha. For example, K fertilization in level of 112 kg K2O/ha resulted by soybean yield increases for 0. 2 t/ha in normal year (1980) and for 1.2 t/ha under drought stress conditions (1981). Profit from K fertilization was 44 and 259 USD/ha, for 1980 and 1981, respectively (Johnson, 1984). For this reason, K fertilization can help in curtailing the yield loss on account of drought.

There are several materials available to supply K to the soil and potassium chloride is the most economical form. However, certified organic soybean production is limited to the use of potassium sulfate or manures to supply K.
