**4.2. Soil K pools**

7

YG/L WLY

Fig. 5. Response of maize grown on soils differing in texture to increased fertilization level in

NP. NPK NPKMg NPKMgNa NP. NPK NPKMg NPKMgNa loamy sand, L sandy loam, M soil textural class, fertilizing treatments

a

<sup>b</sup> <sup>b</sup>

c

a

Fig. 6. Effect of soil K fertility level on maize yield in two weather contrastive years; Adapted

100 140 180 100 140 180 100 140 180 100 140 180 K- K+ K- K+ 2001 2003 years and fertilizing treatments

Legend: WLY – water limited yield; YG/YL – yield loss/gain, K-, K+ - K fertilizing

As described in the first part, the consumption of K fertilizers in many parts of the world has significantly decreased. The annual shortage of K in the global scale is calculated at the level of 20 kg ha-1 [63]. Decades of cropping without sufficient replacement of K removed by harvested plant portions depleted soil K resources to the yield-limiting level. The long-last‐ ing negative K balance is nowadays considered as the second factor of agricultural soil pro‐ ductivity degradation, following soil acidity. On the average, 18.6% world soils is extremely poor in potassium. The worst situation occurs in South-East Asia (43.5%), followed by Latin America (39.3), Sub-Saharan Africa (29.7%), East Asia (19.8%) [64]. Central Europe and countries originated from Former Soviet Union are also threatened by soil mining, because

A minimum of 300 kg ha-1 of available potassium is required for a good growth of highyielding crops, assuming 33% of its utilization by crop [66]. In low-input systems, crop pro‐

Legend: WLY – water limited yield; YG/YL – yield loss/gain, K-, K+ - K fertilizing treatments; 100, 140, 180 kg N ha-1.

**Figure 6.** Effect of soil K fertility level on maize yield in two weather contrastive years; Adapted from [62].

**4. Soil potassium resources – K availability to crop plants**

25% of arable soils present low content of potassium [65].

means with the same letter are not significantly different at α=0.05 (Tukey test).

2006 (dry year); Source [55]

0,000

2,000

a

a a

4,000

6,000

yield of grain, t ha-1

8,000

10,000

12,000

a

152 Soil Fertility

from [62]

**4.1. Soil K mining**

yield of grain, t ha-1

treatments; 100, 140, 180 kg N ha-1.

The total content of soil potassium in the top-soil (layer 0-0.2 m) ranges, depending on soil tex‐ ture from *ca* 1 000 to 50 000 kg K ha-1 [67]. Therefore, it can be concluded, that whole reserves of K in the rooted soil profile (down to 1.0 m) are several times larger. However, most of the soil potassium is not directly attainable for currently growing crop. Soil K resources are distributed in pools, which release K+ ions with different rates, depending on geochemical characteristics of a particular pool. Based on chemical extraction procedures and probability of K uptake by a meanwhile grown crop, four operational K pools/forms have been defined: i) water-soluble (WSK) ii) exchangeable (EXK), iii) non-exchangeable (NEXK, iv) structural/mineral (MIK). The first one, containing K+ ions present in the soil solution, is directly available to the plant. In Pol‐ ish soils, it content ranges from about 60 to 90 kg K ha-1 for the light and heavy soil, respectively (Fig. 7]. This form of potassium is at its highest level in spring and decreases throughout the growth season as plant takes it up. It covers plant needs at early stages of growth, but not in the high-season. This K pool is also sensitive to leaching, which in temperate regions of the world takes place in autumn and winter, provided water saturation of the whole soil profile. The amount of leached K is inversely related to soil texture, ranging from 1 to 8 kg K ha-1 for soil ori‐ ginated from loams and sands, respectively [66].

The second K pool (EXK) contains K+ ions held by negatively charged clay and humus parti‐ cles. In Polish soils, the amount of the EXK ranges from about 200 to 650 kg K ha-1, for very light and heavy soils, respectively. For this K form a threshold content is fixed at the level of 100 mg kg-1 [67], i.e., 360 kg K2O ha-1. The first two K pools are in a dynamic equilibrium, enforced by the presence of the plant root. According to the Le Chatelier*-*Braun principle of *contrariness,* any changes in K+ ions concentration in the soil solution results in their movement from the ex‐ changeable to the soil solution pool. The reverse process occurs in response to K fertilizer's ap‐ plication. Both pools, when not replenished with K in fertilizers or manures, undergo depletion, decreasing the capacity to match plant demand in time and space [68, 69]. Under lack and/or insufficient K delivery from external sources to currently grown crop, which even in the high cropping systems is not exception, but a rule, its growth and productivity depends on the non-exchangeable soil resources (NEXK). This pool is several times larger than the EXK one, as shown in Fig. 7. For this K form, the threshold level Is fixed at 400 mg kg-1 [70], i.e., 1440 kg K2O ha-1. The fourth pool (MIK) represents K in soil rocks and minerals. This pool is consid‐ ered as long-term K reservoir, highly dependent on the type and the weathering rate of K bear‐ ing minerals [Table 5].


Source 1[72], 2[73], 3[74]

**Table 5.** Potassium content in K-bearing minerals1, rocks2 and soil3

Fig. 7. Distribution of potassium among pools in Polish soils at the background of soil texture; Adapted from [71] **Figure 7.** Distribution of potassium among pools in Polish soils at the background of soil texture; Adapted from [71]

#### 110 **4.3. Factors affecting potential availability of the NEX-K to crops**

Adapted from [90]

40

50

60

70

80 90 100 relative yield level, % cereals In the majority of cropping systems, harvested yields depend on the non-exchangeable K pool [68, 69]. The yielding impact of this K form increases in most systems, where K removal by crop is not fully replenished. In order to elucidate the importance of this K pool for plant production, an example of four different fertilizing systems on spring barley yields is short‐ ly described. The status of K forms in black earth after twelve cycles of three-course rotation,

Fig. 8. Effect of soil potassium fertility level on yield level of two groups of crop plants;

very high high medium low very low soil K fertility level

8

leafy crops as presented in Table 6, showed a significant decrease in the content of both available and slow-released K forms. The EXK pool was much below the standards (100 mg K kg soils-1), irrespective of the fertilizing system. The NEXK was several times larger, exceeding the threshold level in three of four treatments, i.e., in K fertilized ones. The described study im‐

In Poland, the official recommendation for K is based on the Egner-Riehm extraction proce‐ dure (Doppel-Lactat, pH 3.55). The harvested yield of barley grain also showed a significant

This type of relationship between yield and available K means, that potassium supply limit‐ ed the yield of grain to a certain value, which in this particular case was fixed at 111.5 g K kg-1 soil. This value implicitly indicates the FYM treatment as optimal for the maximum yield of barley. In the third step of evaluating the yield forming effect of K, the ERK was regressed against K content in other K pools. The applied stepwise regression implicitly re‐ vealed its significant dependence on the NEXK, The reliability of the ERK pool prediction

Control 7 17 378 1045 50

NPK 16 33 648 949 84

FYM 19 28 695 1140 114

1/2NPK + ½FYM 10 33 565 855 61

**Table 6.** Effect of 36 years of continuous fertilizing systems on the distribution of potassium forms

<sup>2</sup> Y = -0.456 + 0.008NEXK; R = 0.69; n = 16. (10)

Sustainable Management of Soil Potassium – A Crop Rotation Oriented Concept

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

155

2 2 Y = -0.84 + 0.107ERK – 0.00048 ERK ; R = 0.55; n = 16 (11)

<sup>2</sup> ERK = -5.7 + 0.157NEXK; R = 0.51 and P 0.001 £ (12)

**WSK EXK NEXK MIK ERK mg kg-1**

**K pools Egner-Riehm K**

<sup>2</sup> ERK = -17.9 + 0.12EXK + 0.12NEXK; R = 0.71 and P 0.001 £ (13)

plicitly revealed the significant effect of the NEXK pool on yield of spring barley (Y):

dependence on the content of available K (ERK), following the quadratic model:

was improved by implementing the EXK into the model:

**Potassium treatments**

Source [68]

as presented in Table 6, showed a significant decrease in the content of both available and slow-released K forms. The EXK pool was much below the standards (100 mg K kg soils-1), irrespective of the fertilizing system. The NEXK was several times larger, exceeding the threshold level in three of four treatments, i.e., in K fertilized ones. The described study im‐ plicitly revealed the significant effect of the NEXK pool on yield of spring barley (Y):

**Minerals 1 Formula g ∙ kg-1 Rocks <sup>2</sup> g ∙ kg-1** K feldspar KAlSi3O8 140.3 Sanstone 12.3 Leucite KAlSi2O6 178.9 Clays 23.3 Nepheline (Na,K)AlSiO4 130.0 Shales 20.4 Kalsilite KAlSiO4 246.8 Limestons 2.6 Muscovite KAl3Si3O10(OH)2 90.3 Chernozem3 8.4-22.0 Biotite K2Fe6Si6Al2O20(OH)4 76.2 Cambisols3 11.4-20.9 Phlogopite K2Mg6Si6Al2O20(OH)4 93.8 Vertisols3 16-28.5

Fig. 7. Distribution of potassium among pools in Polish soils at the background of soil

**Figure 7.** Distribution of potassium among pools in Polish soils at the background of soil texture; Adapted from [71]

In the majority of cropping systems, harvested yields depend on the non-exchangeable K pool [68, 69]. The yielding impact of this K form increases in most systems, where K removal by crop is not fully replenished. In order to elucidate the importance of this K pool for plant production, an example of four different fertilizing systems on spring barley yields is short‐ ly described. The status of K forms in black earth after twelve cycles of three-course rotation,

**4.3. Factors affecting potential availability of the NEX-K to crops**

vey light ligth medium heavy soil agronomic cathegory

Fig. 8. Effect of soil potassium fertility level on yield level of two groups of crop plants;

very high high medium low very low soil K fertility level

Source 1[72], 2[73], 3[74]

154 Soil Fertility

**Table 5.** Potassium content in K-bearing minerals1, rocks2 and soil3

texture; Adapted from [71]

0

1000

2000

3000

4000

K content, kg ha-1

5000

6000

7000

8000

Adapted from [90]

40

50

60

70

80

relative yield level, %

90

100

110

$$\text{Y} = -0.456 + 0.008 \text{NEXK}; \text{R}^2 = 0.69; \text{n} = 16. \tag{10}$$

In Poland, the official recommendation for K is based on the Egner-Riehm extraction proce‐ dure (Doppel-Lactat, pH 3.55). The harvested yield of barley grain also showed a significant dependence on the content of available K (ERK), following the quadratic model:

$$\text{Y = -0.84 + 0.107ERK - 0.00048 \,ERK^2; \, R^2 = 0.55; n = 16\tag{11}$$

This type of relationship between yield and available K means, that potassium supply limit‐ ed the yield of grain to a certain value, which in this particular case was fixed at 111.5 g K kg-1 soil. This value implicitly indicates the FYM treatment as optimal for the maximum yield of barley. In the third step of evaluating the yield forming effect of K, the ERK was regressed against K content in other K pools. The applied stepwise regression implicitly re‐ vealed its significant dependence on the NEXK, The reliability of the ERK pool prediction was improved by implementing the EXK into the model:

$$\text{ERK} = \text{-5.7} + 0.157 \text{NEXK}; \text{R}^2 = 0.51 \text{ and } \text{P} \le 0.001 \tag{12}$$

$$\text{ERK} = -17.9 + 0.12 \text{EXK} + 0.12 \text{NEXK}; \ R^2 = 0.71 \text{ and } P \le 0.001 \tag{13}$$


**Table 6.** Effect of 36 years of continuous fertilizing systems on the distribution of potassium forms

8

KMI NEXK EXK WSK

cereals

leafy crops The efficient use, i.e., exploitation of the non-exchangeable K pool in crop production re‐ quires to use specific agronomic methods. The most farming efforts are focused on in‐ creasing both amounts of plant available potassium and crop accessibility to this particular soil pool. There are numerous processes involved in the equilibrium between exchangeable potassium (EXK) and non-exchangeable potassium (NEXK) pools. The basic way in reaching both goals simultaneously is to fix the soil pH at a level adequate for the most sensitive crop in the crop rotation system. The application of lime induces a series of interrelated processes, resulting in the improvement of fundamental growth conditions for crop plants. Therefore, their demand for nutrients, including potassium, increases propor‐ tionately. Aluminum (Al3+) neutralization is the primary effect of lime application, which in turn creates a chemical and physical milieu for better growth of roots. This action is the key agronomic practice responsible for increasing the accessibility of a given crop to po‐ tassium resources in the soil profile. Other processes induced by lime results in increasing amounts of available potassium in the soil solution. The key one is directly related to the disturbance of the K+ /Ca2+ equilibrium at the interface soil solution/EXK pool. The sudden increase of Ca2+ ions concentration in the ambient soil solution is attributed to the acceler‐ ated rate of K+ displacement from the cation exchange capacity (CEC). The another conse‐ quence of liming is the proliferation of soil fauna, which increases the rate of organic matter decomposition. The induction of microorganisms activity results in series of secon‐ dary processes affecting:

The importance of externally incorporated microorganisms to arable soil in raising up soil fertility as described above was mostly limited to laboratory experiments. The study conducted with the application of bio-fertilizers in Poland showed, in general, a signifi‐ cant increase of mineral nitrogen content, indirectly stressing on the accelerated rate of organic matter decomposition. Much larger amounts of released nitrates in response to increasing fertilizers application and bio-fertilizer indicate an efficient rate of ammonia

acidification, which results in a significant increase of cations and phosphorus contents. The highest increase of the latter ones suggests a multifunctional action of soil applied

N 24.3 1 55.1 1 36.2 91.6 76.6

N + biofertilizer 26.6 72.1 59.2 111.7 97.0

NPK + biofertilizer 28.4 107.4 91.5 140.4 88.9

**Table 7.** Effect of a bio-fertilizer on the post-harvest content of available nutrients in soil cropped with potato1

Crop rotation describes a sequence of crop plant species cultivated on the same field within a fixed time. Three classical principles of crop rotation include: i) an appropriate choice of cultivated species, ii) crop frequency, taking into account some biological limitation, iii) fixed crop sequence. Crop rotation, in fact, under a particular climate and soil agronomic properties of a field, defines the structure and management of applied inputs. The main

**b.** amelioration of the resistance of growing plants to stress, mostly of biological origin,

All these goals were rigorously guarded by farmers up to the end of the first half of the XX century. The technical progress, which started at the beginning, but accelerated in the sec‐

**c.** optimization of the use of soil resources, with respect to water and nutrients.

**5. Crop rotation – The background of soil fertility management**

**a.** yield stability, as a basis of a long-term stabilization of farm economy,

**N-NH4 <sup>+</sup> N-NO3 - P2O5 K2O Mg kg ha-1 mg kg soil-1**

. The formulated hypothesis assumes a local soil

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

157

Sustainable Management of Soil Potassium – A Crop Rotation Oriented Concept

nitrification, which in turn generates H+

Source [78], 1extracted in 0.01 M CaCl2,

microorganisms (Table 7).

**Treatments**

goals of crop rotation are:


The processes reported in positions *a* and *d* are of a great importance for the current and long-term soil K economy, respectively. However, both are efficient the most under condi‐ tions of a slight acid pH. At neutral pH, the elevated concentration of Ca2+ slows down the effect of H+ on K+ displacement. With respect to the third process (position *c*), the build-up of soil organic matter content, oriented on increasing soil CEC is the long-term strategy of K management. The increased size of CEC should be considered as the extended reservoir for cations, both potentially threatened by leaching from the soil solution and/or dissolved from the non-exchangeable K pool.

In the last decade, a lot of scientific projects dealt with microorganisms, considered as a tool for increasing the availability of K from non-exchangeable potassium (NEXK) and that oc‐ cluded in rocks and minerals (MIK) pools. The study carried out with plant growth-promot‐ ing Rhizobacteriaceae (PGPR) showed, that some bacterial strains such as *Bacillus edaphicus*, *Bacillus mucilaginosus* are capable to release potassium from silicate minerals. Their action in K-bearing minerals is *via* H+ ions, and/or by organic acids (citric, tartaric, oxalic), active in divalent cations complexion [72, 75, 76, 77]. A similar effect is expected when plants such as cotton, grasses, legumes, crucifers were used.

The importance of externally incorporated microorganisms to arable soil in raising up soil fertility as described above was mostly limited to laboratory experiments. The study conducted with the application of bio-fertilizers in Poland showed, in general, a signifi‐ cant increase of mineral nitrogen content, indirectly stressing on the accelerated rate of organic matter decomposition. Much larger amounts of released nitrates in response to increasing fertilizers application and bio-fertilizer indicate an efficient rate of ammonia nitrification, which in turn generates H+ . The formulated hypothesis assumes a local soil acidification, which results in a significant increase of cations and phosphorus contents. The highest increase of the latter ones suggests a multifunctional action of soil applied microorganisms (Table 7).

The efficient use, i.e., exploitation of the non-exchangeable K pool in crop production re‐ quires to use specific agronomic methods. The most farming efforts are focused on in‐ creasing both amounts of plant available potassium and crop accessibility to this particular soil pool. There are numerous processes involved in the equilibrium between exchangeable potassium (EXK) and non-exchangeable potassium (NEXK) pools. The basic way in reaching both goals simultaneously is to fix the soil pH at a level adequate for the most sensitive crop in the crop rotation system. The application of lime induces a series of interrelated processes, resulting in the improvement of fundamental growth conditions for crop plants. Therefore, their demand for nutrients, including potassium, increases propor‐ tionately. Aluminum (Al3+) neutralization is the primary effect of lime application, which in turn creates a chemical and physical milieu for better growth of roots. This action is the key agronomic practice responsible for increasing the accessibility of a given crop to po‐ tassium resources in the soil profile. Other processes induced by lime results in increasing amounts of available potassium in the soil solution. The key one is directly related to the

increase of Ca2+ ions concentration in the ambient soil solution is attributed to the acceler‐ ated rate of K+ displacement from the cation exchange capacity (CEC). The another conse‐ quence of liming is the proliferation of soil fauna, which increases the rate of organic matter decomposition. The induction of microorganisms activity results in series of secon‐

ions from both the EXK and NEXK pools,

The processes reported in positions *a* and *d* are of a great importance for the current and long-term soil K economy, respectively. However, both are efficient the most under condi‐ tions of a slight acid pH. At neutral pH, the elevated concentration of Ca2+ slows down the

soil organic matter content, oriented on increasing soil CEC is the long-term strategy of K management. The increased size of CEC should be considered as the extended reservoir for cations, both potentially threatened by leaching from the soil solution and/or dissolved from

In the last decade, a lot of scientific projects dealt with microorganisms, considered as a tool for increasing the availability of K from non-exchangeable potassium (NEXK) and that oc‐ cluded in rocks and minerals (MIK) pools. The study carried out with plant growth-promot‐ ing Rhizobacteriaceae (PGPR) showed, that some bacterial strains such as *Bacillus edaphicus*, *Bacillus mucilaginosus* are capable to release potassium from silicate minerals. Their action in

divalent cations complexion [72, 75, 76, 77]. A similar effect is expected when plants such as

displacement. With respect to the third process (position *c*), the build-up of

ions, and/or by organic acids (citric, tartaric, oxalic), active in

/Ca2+ equilibrium at the interface soil solution/EXK pool. The sudden

disturbance of the K+

156 Soil Fertility

dary processes affecting:

**b.** the displacement of K+

effect of H+

**c.** the build-up of soil CEC,

on K+

the non-exchangeable K pool.

K-bearing minerals is *via* H+

cotton, grasses, legumes, crucifers were used.

**a.** the release of K from organic matter,

**d.** the dissolution of non-exchangeable K from clay particles.


**Table 7.** Effect of a bio-fertilizer on the post-harvest content of available nutrients in soil cropped with potato1
