**2. Potassium impact on crop plant productivity – Physiological backgrounds**

Potassium (K) is one of the 16 elements needed for plant growth. It is essential in nearly all processes required to sustain the adequate plant growth and its reproduction. Potassi‐ um plays a basic role in series of fundamental metabolic and physiological processes in the plant. Plants under potassium deficiency reduce carbon dioxide assimilation and ATP production. Carbon fixation and assimilates transportation to other organs requires potas‐ sium. A sufficient supply of potassium is therefore, a background of efficient solar-energy use [19, 20].

accumulation course was found for winter wheat [23]. In this particular case, the highest ac‐ cumulation rate of K was lower than that observed for leafy crops, achieving 4.4. kg K ha-1 ∙

relative change of K consumption, (x) %


R2

Sustainable Management of Soil Potassium – A Crop Rotation Oriented Concept

Y = 0.01x + 13.76

= 0.39 for n = 24 and P < 0.001

P

http://dx.doi.org/10.5772/53185

141

N K

nutrient


ARRG AKAR


0

2

4

absolute K accumulation rate,

AKAR, kg K ha

MK-

potassium treatments

MK+

HK-

HK+

 -1 d-1

6

8

10

12

Fig. 2. General pattern of K and N accumulation in high-yielding crops, a case of sugar beet;

Fig. 3. Dynamics of K accumulation by sugar beet on the background of root growth; Adapted

28 42 56 70 84 98 126 154 182

days from germination

**Figure 3.** Dynamics of K accumulation by sugar beet on the background of root growth; Adapted from [21, 28]

Fig. 4. Dynamics of dry matter accumulation by maize on the background of potassium

Legend: M, H – levels of soil fertility, medium and high, respectively; K-, K+ - freshly

14 35 55 65 75 83 85 89

growth stages, BBCH scale

29-05 18-06 8-07 28-07 17-08 6-09 26-09 16-10 5-11 calendar dates

**Figure 2.** General pattern of K and N accumulation in high-yielding crops, a case of sugar beet; Source [21]

Fig.1. Effect of relative change of potassium fertilizer consumption on relative wheat yield change (1986-1990 = 100%); source: FAOSTAT, IFADATA, available online 2012-08-07

5

6

d-1 from the beginning of stem elongation up to heading.




0

10

20

30

40

50

60

Legend: HP, I, P – groups of countries.

0

20

40

60

80

realative N and K accumualtion %

100

120

140

Source [21]

from [21, 28]


0

50

100

absolute rot rate growth, ARRG,

 m ha -1 d-1

150

200

250

300

fertilizing; Source [29]



0

20

40

CGR, g m-2 d-1

60

80

100

120

applied potassium; CGR – crop growth rate.

GrzebiszFigures

HP

I

relative change of wheat grain yield, (Y) %

A high-yielding crop takes up large quantities of potassium to cover its requirements during the whole vegetation. The highest accumulation of potassium is generally attributed to root crops such as sugar beet and potato. In fact, the first one yielding at the potential level, i.e., 80 t ha-1, accumulates more than 400 kg K ha-1 [21]. Cereals are considered as low K consum‐ ers. Winter plants yielding at the level of 10 t ha-1, can accumulate at harvest 190 kg K ha-1 [22]. It is necessary to stress that the total amount of K in the crop at harvest is by 1 /10 to 1 /3 lower than its maximum during the growth season. This difference should be taken into ac‐ count when calculating the K recommended rate.

Potassium management by a crop requires an insight into some canopy characteristics: i) quantity of accumulated nutrient, ii) absolute/relative uptake rate, iii) dynamics of nitrogen uptake. The first step in understanding K impact on a crop productivity is to define its sensi‐ tivity to K supply. The general trend of K accumulation during the life cycle of a crop can be described by the sigmoid-like curve (Fig. 2). The same patterns, as shown in Fig. 2, have been found for winter wheat [22, 23] and for oil-seed rape [24]. The well-defined maximum describes the date of the highest amount of K fixed by the canopy followed by a subsequent decrease during maturation. The second important information drawn from Fig. 2 refers to Kmax. As a rule, K accumulation precedes the absolute rate of both dry matter and N accu‐ mulation. Based on the pattern of N and K in-season accumulation, it can be formulated a hypothesis, that K accumulated in *excess* during the vegetative part of the seed crop growth builds-up a nutritional buffer, supporting effective N use during the grain filling period. The best examples are cereals, for which the crucial stage of dry matter production occurs at the end of booting and during heading. This period is decisive for establishing both the number of ears and grains per ear. At these stages, cereals reach the top of K accumulation, which is conclusive for high-yield [22, 23, 25].

The dynamics of potassium uptake by a crop can be described using indices such as the ab‐ solute/relative rate of K accumulation (A/R-RKA). The first one is shown in Fig. 3 for sugar beet. This crop can keep the uptake rate at the level of 10 and 9 kg K ha-1 ∙ d-1 for 7 and 17 consecutive days, respectively. Dynamics of K uptake coincides with the absolute rate of the root system extension, reaching top values at the period of maximum dry matter accumula‐ tion, both in leaves and roots [26]. In oil-seed rape dominates the same pattern of K and N uptake. The uptake rate of K during the period from the rosette stage up to flowering ranges from 3 to 7 kg K ha-1 ∙ d-1, reaching the maximum at booting [24, 27]. The same potassium

Y = 0.01x + 13.76

P

accumulation course was found for winter wheat [23]. In this particular case, the highest ac‐ cumulation rate of K was lower than that observed for leafy crops, achieving 4.4. kg K ha-1 ∙ d-1 from the beginning of stem elongation up to heading. Fig.1. Effect of relative change of potassium fertilizer consumption on relative wheat yield change (1986-1990 = 100%); source: FAOSTAT, IFADATA, available online 2012-08-07 Legend: HP, I, P – groups of countries. relative change of K consumption, (x) %




0

10

20

30

40

50

60

GrzebiszFigures

HP

I

relative change of wheat grain yield, (Y) %

Source [21]

**2. Potassium impact on crop plant productivity – Physiological**

Potassium (K) is one of the 16 elements needed for plant growth. It is essential in nearly all processes required to sustain the adequate plant growth and its reproduction. Potassi‐ um plays a basic role in series of fundamental metabolic and physiological processes in the plant. Plants under potassium deficiency reduce carbon dioxide assimilation and ATP production. Carbon fixation and assimilates transportation to other organs requires potas‐ sium. A sufficient supply of potassium is therefore, a background of efficient solar-energy

A high-yielding crop takes up large quantities of potassium to cover its requirements during the whole vegetation. The highest accumulation of potassium is generally attributed to root crops such as sugar beet and potato. In fact, the first one yielding at the potential level, i.e., 80 t ha-1, accumulates more than 400 kg K ha-1 [21]. Cereals are considered as low K consum‐ ers. Winter plants yielding at the level of 10 t ha-1, can accumulate at harvest 190 kg K ha-1

lower than its maximum during the growth season. This difference should be taken into ac‐

Potassium management by a crop requires an insight into some canopy characteristics: i) quantity of accumulated nutrient, ii) absolute/relative uptake rate, iii) dynamics of nitrogen uptake. The first step in understanding K impact on a crop productivity is to define its sensi‐ tivity to K supply. The general trend of K accumulation during the life cycle of a crop can be described by the sigmoid-like curve (Fig. 2). The same patterns, as shown in Fig. 2, have been found for winter wheat [22, 23] and for oil-seed rape [24]. The well-defined maximum describes the date of the highest amount of K fixed by the canopy followed by a subsequent decrease during maturation. The second important information drawn from Fig. 2 refers to Kmax. As a rule, K accumulation precedes the absolute rate of both dry matter and N accu‐ mulation. Based on the pattern of N and K in-season accumulation, it can be formulated a hypothesis, that K accumulated in *excess* during the vegetative part of the seed crop growth builds-up a nutritional buffer, supporting effective N use during the grain filling period. The best examples are cereals, for which the crucial stage of dry matter production occurs at the end of booting and during heading. This period is decisive for establishing both the number of ears and grains per ear. At these stages, cereals reach the top of K accumulation,

The dynamics of potassium uptake by a crop can be described using indices such as the ab‐ solute/relative rate of K accumulation (A/R-RKA). The first one is shown in Fig. 3 for sugar beet. This crop can keep the uptake rate at the level of 10 and 9 kg K ha-1 ∙ d-1 for 7 and 17 consecutive days, respectively. Dynamics of K uptake coincides with the absolute rate of the root system extension, reaching top values at the period of maximum dry matter accumula‐ tion, both in leaves and roots [26]. In oil-seed rape dominates the same pattern of K and N uptake. The uptake rate of K during the period from the rosette stage up to flowering ranges from 3 to 7 kg K ha-1 ∙ d-1, reaching the maximum at booting [24, 27]. The same potassium

/10 to 1 /3

[22]. It is necessary to stress that the total amount of K in the crop at harvest is by 1

count when calculating the K recommended rate.

which is conclusive for high-yield [22, 23, 25].

**backgrounds**

140 Soil Fertility

use [19, 20].

Fig. 2. General pattern of K and N accumulation in high-yielding crops, a case of sugar beet; **Figure 2.** General pattern of K and N accumulation in high-yielding crops, a case of sugar beet; Source [21]

5

MK-

potassium treatments

MK+

HK-

HK+

6

Fig. 3. Dynamics of K accumulation by sugar beet on the background of root growth; Adapted from [21, 28] **Figure 3.** Dynamics of K accumulation by sugar beet on the background of root growth; Adapted from [21, 28]

Fig. 4. Dynamics of dry matter accumulation by maize on the background of potassium

Legend: M, H – levels of soil fertility, medium and high, respectively; K-, K+ - freshly

14 35 55 65 75 83 85 89

growth stages, BBCH scale

fertilizing; Source [29]



0

20

40

CGR, g m-2 d-1

60

80

100

120

applied potassium; CGR – crop growth rate.

150

 -1 d-1 200

250

300

Based on these sets of data it can be formulated a hypothesis that an efficient supply of po‐ tassium to a crop is a prerequisite of achieving the highest rate of canopy growth. The im‐ portance of potassium management for dry matter accumulation by a maize canopy is presented in Fig 4. The analysis of the course of crop growth rate (CGR) can be used to dis‐ criminate the critical stage of a particular crop response to the supply of potassium. In maize, for example, the elevated rate of dry matter accumulation takes place from tasselling (BBCH 51) and extends up to the blister stage (BBCH 71). This crop shows a very high plasti‐ city to K management. The highest CGR was an attribute of both groups of plants grown on i) a fertile K soil, irrespectively on current K supply, and ii) medium K fertile soil but freshly fertilized with K. Fig. 3. Dynamics of K accumulation by sugar beet on the background of root growth; Adapted from [21, 28] -50 0 50 100 28 42 56 70 84 98 126 154 182 days from germination absolute rot rate growth, ARRG, m ha-4 -2 0 2 absolute K accumulation rate, AKAR, kg K haARRG AKAR

4

6

 -1 d-1

> rate (Deff), which is nutrient specific. Under constant physical conditions, ions with higher Deff are taken faster, resulting in steeper depletion of its concentration in the bulk soil sur‐ rounding the root surface [34]. The occurring processes can be described quantitatively us‐

> > 0.5

r1 = (4/π)0.5 <sup>×</sup> 1/(Lvr

Deff - coefficient of diffusion in soil solution for a particular nutrient, cm2

cm-3

The typical values of coefficients of diffusion for two main nutrients, i.e., nitrate nitrogen

ever, their values may significantly decrease under conditions of low water content down to 10-10 and 10-12 for both nutrients, correspondingly. The depletion zone calculated using typi‐ cal values, and a period of seven days extends from the root surface to 18.5 mm and 4.9 mm, respectively. It is necessary to keep in mind that the competition between two neighboring roots for a given nutrient begins, when their depletion zones overlap. Even though, the question remains, which nutrient is dominant in this process, in turn impacting the whole plant metabolism? Solving this problem requires sets of data concerning root length density, which is variable both between crop species, stage of development and root's distribution in the soil profile. Therefore, K uptake characteristics of winter wheat and sugar beets, were compared at stages with maximum uptake rates, i.e., at heading and in the second half of July, respectively (Table 2). The calculated half distance between neighboring roots, as a re‐

Dw - coefficient of diffusion in water for a particular nutrient, cm2

b - soil buffering capacity for a particular nutrient, unitless

r1 - the mean distance between neighboring roots, cm Dd - degree of nutrient utility in the depletion zone, %

eff d = 2 D )× t( (3)

Sustainable Management of Soil Potassium – A Crop Rotation Oriented Concept

Deff= Dw × θ × f/b (4)

( ) Dd = d/0.5r ×100% <sup>1</sup> (6)

0.5) (5)

http://dx.doi.org/10.5772/53185

143

s-1

s-1

s for nitrates and 2 10-7 s for potassium [35]. How‐

ing the following set of equations:

d - the root depletion zone, cm

t - time since initiation of calculation, days

θ- volumetric soil water content, cm3

Lvr - root length density, cm cm-3

f - tortuosity factor of soil pores, unitless

and potassium, are as follows: 2 10-6 cm2

where:

8

10

12

Legend: M, H – levels of soil fertility, medium and high, respectively; K-, K+ - freshly applied potassium; CGR – crop growth rate.

Fig. 4. Dynamics of dry matter accumulation by maize on the background of potassium **Figure 4.** Dynamics of dry matter accumulation by maize on the background of potassium fertilizing; Source [29]

6 fertilizing; Source [29] Legend: M, H – levels of soil fertility, medium and high, respectively; K-, K+ - freshly applied potassium; CGR – crop growth rate. Plants take up potassium as the K+ ions. Its availability and the plant uptake rate is affected by several soil and plant factors: i) K concentration in the soil solution, ii) size of the soil cati‐ on exchange complex, iii) soil properties such as: moisture, soil aeration and oxygen level, temperature, iv) plant crops internal requirements, v) rooting depth [30, 31]. The first two factors are decisive for potassium resources. However, K utility by a particular plant from different soil reservoirs depends on the internal plant requirement, which is defined by the rate of dry matter accumulation, expressed as the biomass ingrowths rate. In fact, it is the basic factor depending on the supply of water and nitrogen. Nevertheless, the evaluation of factor's hierarchy implicitly shows, that the rate of root growth affects the K uptake rate from soil resources the most [32]. The elucidation of the role of root growth requires a deep insight into mechanisms of potassium uptake by a plant root. Its has been well documented that K+ ion's transportation from the soil solution to the root is mainly *via* diffusion. The movement of potassium depends on the water regime and plant root system activity [33]. The rate of any ion transportation to the root surface is governed by its effective diffusion

rate (Deff), which is nutrient specific. Under constant physical conditions, ions with higher Deff are taken faster, resulting in steeper depletion of its concentration in the bulk soil sur‐ rounding the root surface [34]. The occurring processes can be described quantitatively us‐ ing the following set of equations:

$$\mathbf{d}\_{\circ} = \left(\mathbf{\mathcal{D}}\_{\mathrm{off}} \times \mathbf{t}\right)^{0.5} \tag{3}$$

$$\mathbf{D}\_{\rm eff} = \mathbf{D}\_{\rm w} \times \boldsymbol{\Theta} \times \mathbf{f} / \mathbf{b} \tag{4}$$

$$\mathbf{r}\_1 = (4/\pi)^{0.5} \times 1/(\mathbf{L} \,\mathrm{v\_r}^{0.5}) \tag{5}$$

$$\text{Dd} = \left(\text{d}/0.5\text{r}\_1\right) \times 100\% \tag{6}$$

where:

6

Based on these sets of data it can be formulated a hypothesis that an efficient supply of po‐ tassium to a crop is a prerequisite of achieving the highest rate of canopy growth. The im‐ portance of potassium management for dry matter accumulation by a maize canopy is presented in Fig 4. The analysis of the course of crop growth rate (CGR) can be used to dis‐ criminate the critical stage of a particular crop response to the supply of potassium. In maize, for example, the elevated rate of dry matter accumulation takes place from tasselling (BBCH 51) and extends up to the blister stage (BBCH 71). This crop shows a very high plasti‐ city to K management. The highest CGR was an attribute of both groups of plants grown on i) a fertile K soil, irrespectively on current K supply, and ii) medium K fertile soil but freshly

28 42 56 70 84 98 126 154 182

days from germination

Fig. 3. Dynamics of K accumulation by sugar beet on the background of root growth; Adapted


ARRG AKAR


0

2

4

absolute K accumulation rate,

AKAR, kg K ha

MK-

potassium treatments

MK+

HK-

HK+

 -1 d-1

6

8

10

12

Fig. 4. Dynamics of dry matter accumulation by maize on the background of potassium

ions. Its availability and the plant uptake rate is affected

Legend: M, H – levels of soil fertility, medium and high, respectively; K-, K+ - freshly applied potassium; CGR – crop

Legend: M, H – levels of soil fertility, medium and high, respectively; K-, K+ - freshly

by several soil and plant factors: i) K concentration in the soil solution, ii) size of the soil cati‐ on exchange complex, iii) soil properties such as: moisture, soil aeration and oxygen level, temperature, iv) plant crops internal requirements, v) rooting depth [30, 31]. The first two factors are decisive for potassium resources. However, K utility by a particular plant from different soil reservoirs depends on the internal plant requirement, which is defined by the rate of dry matter accumulation, expressed as the biomass ingrowths rate. In fact, it is the basic factor depending on the supply of water and nitrogen. Nevertheless, the evaluation of factor's hierarchy implicitly shows, that the rate of root growth affects the K uptake rate from soil resources the most [32]. The elucidation of the role of root growth requires a deep insight into mechanisms of potassium uptake by a plant root. Its has been well documented

 ion's transportation from the soil solution to the root is mainly *via* diffusion. The movement of potassium depends on the water regime and plant root system activity [33]. The rate of any ion transportation to the root surface is governed by its effective diffusion

**Figure 4.** Dynamics of dry matter accumulation by maize on the background of potassium fertilizing; Source [29]

14 35 55 65 75 83 85 89

growth stages, BBCH scale

fertilized with K.

from [21, 28]

CGR, g m-2 d-1

growth rate.

that K+


0

50

100

absolute rot rate growth, ARRG,

142 Soil Fertility

 m ha -1 d-1

150

200

250

300

fertilizing; Source [29]

Plants take up potassium as the K+

applied potassium; CGR – crop growth rate.


Deff - coefficient of diffusion in soil solution for a particular nutrient, cm2 s-1


Dd - degree of nutrient utility in the depletion zone, %

The typical values of coefficients of diffusion for two main nutrients, i.e., nitrate nitrogen and potassium, are as follows: 2 10-6 cm2 s for nitrates and 2 10-7 s for potassium [35]. How‐ ever, their values may significantly decrease under conditions of low water content down to 10-10 and 10-12 for both nutrients, correspondingly. The depletion zone calculated using typi‐ cal values, and a period of seven days extends from the root surface to 18.5 mm and 4.9 mm, respectively. It is necessary to keep in mind that the competition between two neighboring roots for a given nutrient begins, when their depletion zones overlap. Even though, the question remains, which nutrient is dominant in this process, in turn impacting the whole plant metabolism? Solving this problem requires sets of data concerning root length density, which is variable both between crop species, stage of development and root's distribution in the soil profile. Therefore, K uptake characteristics of winter wheat and sugar beets, were compared at stages with maximum uptake rates, i.e., at heading and in the second half of July, respectively (Table 2). The calculated half distance between neighboring roots, as a re‐ sult of root length density variability, increased with the soil profile depth. The degree of nitrate's utilization by wheat was the highest in the top-soil, several times exceeding its po‐ tential to deliver the required amount of nitrogen. The most spectacular is the fact that in the 7th-day period, the depletion of the nitrate-nitrogen zone extended down to 150 cm. At the same time, the K depletion zone occurred at the depth of 60 cm. In the case of sugar beet, the depletion zone for nitrates reached down to 90 cm, whereas for potassium only to 30 cm.

gans. Therefore, the rate of plant growth, taking into account its aerial part, is determined by nitrogen availability, especially nitrate ions. It is recognized, that higher soil moisture content usually means greater availability of nutrients to plants. Nitrogen supply to roots is *via* the transpiration stream of water (mass flow). Processes leading to the decrease of soil water con‐ tent are the main reasons increasing the importance of diffusion as the core mechanism of nu‐ trient transportation towards roots [30, 33, 38]. Nitrogen fertilizer use by a crop is related to the soil K fertility level. It has been documented, that insufficient supply of K results in lower, than expected, uptake rate of nitrate-nitrogen, which in turn decreases the rate of aerial biomass growth. This specific phenomenon is explained by the fact, that potassium accumulated at the root surface controls nitrogen inflow into the root. The rate of nitrate's transport through roots into the shoot depends on K concentration in the soil solution, governed also by K soil fertility level. At the same time, malate is produced in the shoot and part of the K-malate undergoes re‐ cycling through the root system [39]. Therefore, external and internal K sources are responsible for effective uptake of nitrogen from its soil pool. Insufficient supply of potassium from the soil solution significantly restricts the uptake of nitrates, reducing in turn their concentration in the root and consequently their transportation into leaves, where they undergo reduction. This al‐ so means, that the plant is not able to take up adequate amounts of N, when K is in limited sup‐ ply. It can be concluded, that high-yielding crops require excessive supply of K in order to

Sustainable Management of Soil Potassium – A Crop Rotation Oriented Concept

http://dx.doi.org/10.5772/53185

145

match their demand for N in critical stages of yield component's development.

Yield can be defined as the end-product of yield component's expression of a particular crop during growth and development. According to the concept *multiple limitation hypothesis* [MLH, 17], water and nitrogen supply plays a decisive role in assimilates partitioning among main crop plant organs [40, 41, 42]. These two nutrients affect both the rate of dry matter accumulation and yield component's development. In order to understand their in‐ fluence on the rate of dry matter accumulation of the yield, the whole life cycle of a crop can be divided into three main periods: i) yield foundation (YFP), ii) yield construction (YCP), iii) yield realization (YRP) [43]. The shortage of potassium can affect plant growth in each of the above indicated periods. The key problem remains, about how potassium improves

Potassium concentration in plant biomass varies from 1 to 5% of dry matter weight. When soil potassium is deficient, plant growth is reduced, resulting in smaller, dull bluish-green and wavy leaves and thinner stems. Plants often tend to wilt. Visual symptoms of potassium deficiency are easily recognized, but only those under severe potassium shortage, (Photo 1). In early stages of growth and also under hidden K deficiency, its visible symptoms are even‐ tually confused with nitrogen deficiency. The main reason is the slow rate of the aerial bio‐ mass growth. The shortage of potassium is the reason of basic physiological processes disturbance, which in turn negatively affect the development of yield components. Potassi‐

**3. Potassium as a water-stress ameliorative agent**

yield forming effect of water and nitrogen?

**3.1. Plant growth stages – Yield forming function of potassium**


**Table 2.** Effect of root length density distribution in the soil profile on the degree of potassium and nitrate-nitrogen depletion at the critical stage of potassium accumulation by two crops

These two sets of data outline some important information concerning the management of both nutrients. Firstly, at the critical period of each crop development, nitrogen should be considered as a nutrient significantly limiting plant growth. In the case of wheat as a crop accumulating a significant amount of nitrogen in grain, an external supply of this nutrient is required at heading to fulfill this goal. A quite different strategy should be recommended for sugar beet, since it reaches at the critical stage of K accumulation maximum rates of both dry matter and nitrogen accumulation [26, 37]. This crop in subsequent stages of growth re‐ lies on soil N resources, which uptake is governed by K supply. The second information re‐ fers to un-depleted resources of potassium, present in deeper soil layers. These reserves can be considered as the basic source of K supply during critical stages of beet growth and/or during any kind of growth disturbance due to stress. It is worth mentioning, that water shortages first limit nutrient flow in the topsoil, and then extend down the soil profile. Therefore, soil K reserves present in deeper soil layers are important for the exploitation of the plant yielding potential or to protect its growth under stress.

The effective transformation of solar energy into plant biomass depends on the supply of nitro‐ gen, which is crucial for both carbon fixation and its subsequent partitioning among plant or‐ gans. Therefore, the rate of plant growth, taking into account its aerial part, is determined by nitrogen availability, especially nitrate ions. It is recognized, that higher soil moisture content usually means greater availability of nutrients to plants. Nitrogen supply to roots is *via* the transpiration stream of water (mass flow). Processes leading to the decrease of soil water con‐ tent are the main reasons increasing the importance of diffusion as the core mechanism of nu‐ trient transportation towards roots [30, 33, 38]. Nitrogen fertilizer use by a crop is related to the soil K fertility level. It has been documented, that insufficient supply of K results in lower, than expected, uptake rate of nitrate-nitrogen, which in turn decreases the rate of aerial biomass growth. This specific phenomenon is explained by the fact, that potassium accumulated at the root surface controls nitrogen inflow into the root. The rate of nitrate's transport through roots into the shoot depends on K concentration in the soil solution, governed also by K soil fertility level. At the same time, malate is produced in the shoot and part of the K-malate undergoes re‐ cycling through the root system [39]. Therefore, external and internal K sources are responsible for effective uptake of nitrogen from its soil pool. Insufficient supply of potassium from the soil solution significantly restricts the uptake of nitrates, reducing in turn their concentration in the root and consequently their transportation into leaves, where they undergo reduction. This al‐ so means, that the plant is not able to take up adequate amounts of N, when K is in limited sup‐ ply. It can be concluded, that high-yielding crops require excessive supply of K in order to match their demand for N in critical stages of yield component's development.
