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

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 goals of crop rotation are:


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‐ ond half of the XX century, resulted in a great increase of agriculture means of production, including fertilizers and pesticides. In addition, for the period extending from 1950 to 1970, intensive breeding programs within the "Green Revolution" succeeded first in new wheat and rice varieties, and next other cereals and maize. The main attribute of high-yielding va‐ rieties at that time was the extended capacity to accumulate nitrogen. Soon, the rigid crop sequence rules, based on legumes as a source of nitrogen and assuring its biological protec‐ tion became limiting factors in a sharp yield increase. Consequently, changes in sequences of cultivated crops practiced by farmers were more and more oriented on net income, neglect‐ ing at the same time the efficiency of applied nitrogenous fertilizers, pesticides. This in turn increased the pressure of agriculture on environment [2, 80, 81].

**Crops RLV, cm cm-3 Rd, cm** Been 0.5-2.0 0.50

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cereals

leafy crops

Oil-seed rape - 1.00

Potato 1-2 0.50

Sugar beet 1-2 1.00

Barley 3-4 0.50

Wheat 3-8 0.80

Rye 4-8 1.00

vey light ligth medium heavy soil agronomic cathegory

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

1 in the top-soil layer and mean rooting depth Rd <sup>2</sup>

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

**6. Crop rotation potassium balance – A strategic scale of K management**

The efficient system of potassium management should focus on requirements of the most sensitive crop in a particular crop rotation. The mandatory objective of effective

**Figure 8.** Effect of soil potassium fertility level on yield level of two groups of crop plants; Adapted from [90]

**6.1. System of potassium fertilization – Key assumptions**

very high high medium low very low soil K fertility level

8

Source 1[88], 2[89]

**Table 8.** Root length density (RLV)

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

New paradigms of agriculture development, oriented on sustainable use of resources, can be achieved, provided crop rotation rules are introduced. The modern view on principles of crop rotation arrangement takes into account its flexibility in the selection of crop species, depending on market needs. Nowadays, the objectives of the rational crop sequence should strictly consider i) the farm economic profitability – its adaptability to market oriented changes, ii) the optimization of resource use, both internal (soil) and external (fertilizers, pesticides), iii) the minimization of the impact of agriculture on local and global environ‐ ment (nitrogen, phosphorus) [82, 83]. Therefore, a profitable crop production requires the development of alternative strategies, oriented on a well-thought-out management of water and nutrient resources in a particular crop rotation.

The reported expectations and assumptions regarding crop rotation refer also to potassium management. There are some experimental data supporting the concept of sustainable use of soil potassium, based on crop rotation principles:



**Table 8.** Root length density (RLV) 1 in the top-soil layer and mean rooting depth Rd <sup>2</sup> texture; Adapted from [71]

ond half of the XX century, resulted in a great increase of agriculture means of production, including fertilizers and pesticides. In addition, for the period extending from 1950 to 1970, intensive breeding programs within the "Green Revolution" succeeded first in new wheat and rice varieties, and next other cereals and maize. The main attribute of high-yielding va‐ rieties at that time was the extended capacity to accumulate nitrogen. Soon, the rigid crop sequence rules, based on legumes as a source of nitrogen and assuring its biological protec‐ tion became limiting factors in a sharp yield increase. Consequently, changes in sequences of cultivated crops practiced by farmers were more and more oriented on net income, neglect‐ ing at the same time the efficiency of applied nitrogenous fertilizers, pesticides. This in turn

New paradigms of agriculture development, oriented on sustainable use of resources, can be achieved, provided crop rotation rules are introduced. The modern view on principles of crop rotation arrangement takes into account its flexibility in the selection of crop species, depending on market needs. Nowadays, the objectives of the rational crop sequence should strictly consider i) the farm economic profitability – its adaptability to market oriented changes, ii) the optimization of resource use, both internal (soil) and external (fertilizers, pesticides), iii) the minimization of the impact of agriculture on local and global environ‐ ment (nitrogen, phosphorus) [82, 83]. Therefore, a profitable crop production requires the development of alternative strategies, oriented on a well-thought-out management of water

The reported expectations and assumptions regarding crop rotation refer also to potassium management. There are some experimental data supporting the concept of sustainable use

**a.** soil, taking into account the whole profile, must be sufficiently reach in K to supply suf‐ ficient amount of potassium to a high-yielding crop within an extremely short period of growth – the critical period of yield components formation, to assure maximization of

**b.** leafy crops, for example sugar beet, oilseed rape, to cover K requirements during crucial

**c.** root system of leafy crops is much weaker in comparison to cereals (Table 8), being a

**d.** demand of cereals for potassium is much lower than leafy crops; root density is at the same time much higher, hence a higher efficiency in K uptake (Fig. 8; Table 8),

**e.** both leafy crops and cereals respond more to soil fertility K level than to freshly applied

**f.** all crops respond to current potassium fertilization in years with stress, mostly related to water shortage and site specific diversification of K management [47, 55, 86, 87].

stages of growth, need to explore a thick layer of the soil profile [28, 84],

increased the pressure of agriculture on environment [2, 80, 81].

158 Soil Fertility

and nutrient resources in a particular crop rotation.

of soil potassium, based on crop rotation principles:

its yielding potential exploration [21, 24],

fertilizer K [27, 66, 85, 86],

prerequisite of higher level of available potassium,

Fig. 8. Effect of soil potassium fertility level on yield level of two groups of crop plants; **Figure 8.** Effect of soil potassium fertility level on yield level of two groups of crop plants; Adapted from [90]

#### **6. Crop rotation potassium balance – A strategic scale of K management**

#### **6.1. System of potassium fertilization – Key assumptions**

Adapted from [90]

6000

7000

8000

The efficient system of potassium management should focus on requirements of the most sensitive crop in a particular crop rotation. The mandatory objective of effective

8

strategy of any crops fertilization with potassium is to cover K demands of the current‐ ly growing crop during the period of their highest growth rate. However, crop plants grown in a fixed crop rotation present different sensitivity to current level of attainable soil K. Thereby, the primary objective of rational K management should focus on fulfill‐ ing the requirements of the most sensitive crop within the given crop sequence. It de‐ mands for K determines the top level of the critical range of soil available K for the whole sequence of growing crops. The basic target is the non-limited supply of soil po‐ tassium to plants during stages of the highest biomass increase, which coincides with the end of the linear period of dry matter accumulation. The degree of the K require‐ ment covering by the most sensitive crop is decisive for both: i) water-use efficiency, ii) nitrogen utilization use efficiency. Any increase of these two indices results in the de‐ gree of yield component's development, considered as a crucial for a yield increase. The secondary objective is to select production measures essential for reaching the required level of available K [66].

**b.** determination of the current level of available K,

In practice, the balance sheet operates on the equation, which in a simple way quantifies K

Rearrangement of the equation No. 14 allows to calculate the potassium application rate:

f up plr nl ni fym <sup>v</sup> <sup>2</sup> K = P ±P + K – K + K ±dK kg K O ha [ ] é ù <sup>ù</sup>

a

In agronomic practice, some components of the K balance sheet such as natural input or losses may be omitted due to their minor importance as a source of K. The minimal set of data required

**a.** unit K uptake (specific K uptake) by each crop cultivated in a given rotation, i.e., K ac‐ cumulation in the main crop product unit and its respective amounts in the by-prod‐

The first parameter shows a certain level of variability, according to soil, crop and produc‐ tion technology. The critical issue of the proposed concept relates to the management of the Kplr component of the Eq. No 15. All vegetative plant organs, such as straw or sugar beet tops, are very important sources of potassium. The environmentally and economically

when constructing the balance sheet for a particular crop sequence is as follows (Table 9):

ucts, for example, in straw (expressed in kg K2O ∙ t-1 of the main product,


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<sup>û</sup> <sup>é</sup> · <sup>ë</sup> ë û (15)

**c.** correction of the current K level.

processes occurring at the field level:

Kup - K uptake by the main yield, kg ∙ ha-1

Kplr - K accumulation in plant residues, kg ∙ ha-1

Knl - natural K losses (erosion, leaching), kg ∙ ha-1

Kfym - K supply in organic manure, kg ∙ ha-1;

**b.** crop sequence in the fixed crop rotation,


Kni - natural K input (dry and wet deposition), kg ∙ ha-1

δKav - intended/required change of soil available K, mg kg ∙soil-1

**c.** methods of a specific management of by-products at the farm,

sound solution is to incorporate all harvested by-products into soil.

**d.** type of farm (crop, dairy, mixed) as related to manure production.

where:

Kf

In general, the crop-oriented K fertilizing strategy relies on the view that crops such as sugar beet, potato, oil-seed rape, (grain) legumes are significantly more sensitive to K supply than cereals, when grown subsequently in a fixed crop rotation. Therefore, the economically suc‐ cessful and environmentally sound K fertilization system should based on five pillars, as‐ suming that leafy crops:


The crop rotation-oriented strategy of K management also assumes a maximized recycling of internal, i.e., soil available K sources and field resources (plant residues). Therefore, the amounts of fertilizer K needed to cover its losses due to exports from the field in harvested products or/and leaching processes, depend also on the management of crop by-product (residues). The mentioned concept is in accordance with the Ideal Soil fertility (ISF) ap‐ proach [91]. Potassium timing seems to be of secondary importance taking into account the strategic goals of K management. Natural growth conditions, mostly related to stressing fac‐ tors, can only modify potassium fertilizer timing.

#### **6.2. Potassium balance sheet in crop rotation – An operational procedure**

Following the theoretical assumptions it appears that the main problem of adequate fertilization of the leafy crop with potassium is to develop an appropriate system of managing the potassium rotation-oriented system. The principal farmer's question is, how to achieve the target K availa‐ bility range? The efficient K system development may consist of three basic steps:

**a.** preparation of the K balance sheet for all crops in the fixed cropping sequence,


In practice, the balance sheet operates on the equation, which in a simple way quantifies K processes occurring at the field level:

$$\mathbf{K}\_{\rm up} + \mathbf{K}\_{\rm plz} + \mathbf{K}\_{\rm ul} = \mathbf{K}\_{\rm ul} + \mathbf{K}\_{\rm lym} + \mathbf{K}\_{\rm l} \qquad \left[ \text{kg } \mathbb{K}\_2 \text{O} \bullet \text{ha}^{-1} \right] \tag{14}$$

Rearrangement of the equation No. 14 allows to calculate the potassium application rate:

$$\mathbf{K}\_t = \left\{ \mathbf{P}\_{\rm up} \pm \mathbf{P}\_{\rm plt} + \mathbf{K}\_{\rm ul} \right\} - \left\{ \mathbf{K}\_{\rm nl} + \mathbf{K}\_{\rm bym} \right\} \pm \mathbf{d} \mathbf{K}\_{\rm av} \qquad \left[ \text{kg } \mathbf{K}\_2 \text{O} \cdot \text{o } \text{ha}^{-1} \right] \tag{15}$$

where:

strategy of any crops fertilization with potassium is to cover K demands of the current‐ ly growing crop during the period of their highest growth rate. However, crop plants grown in a fixed crop rotation present different sensitivity to current level of attainable soil K. Thereby, the primary objective of rational K management should focus on fulfill‐ ing the requirements of the most sensitive crop within the given crop sequence. It de‐ mands for K determines the top level of the critical range of soil available K for the whole sequence of growing crops. The basic target is the non-limited supply of soil po‐ tassium to plants during stages of the highest biomass increase, which coincides with the end of the linear period of dry matter accumulation. The degree of the K require‐ ment covering by the most sensitive crop is decisive for both: i) water-use efficiency, ii) nitrogen utilization use efficiency. Any increase of these two indices results in the de‐ gree of yield component's development, considered as a crucial for a yield increase. The secondary objective is to select production measures essential for reaching the required

In general, the crop-oriented K fertilizing strategy relies on the view that crops such as sugar beet, potato, oil-seed rape, (grain) legumes are significantly more sensitive to K supply than cereals, when grown subsequently in a fixed crop rotation. Therefore, the economically suc‐ cessful and environmentally sound K fertilization system should based on five pillars, as‐

**a.** are grown in rotation with other crops, mostly with cereals,

**d.** can explore a thick volume of the soil profile,

tors, can only modify potassium fertilizer timing.

**b.** have a substantially weaker root system as compared to cereals,

**c.** express considerably higher quantitative requirements for K at critical stages,

**6.2. Potassium balance sheet in crop rotation – An operational procedure**

bility range? The efficient K system development may consist of three basic steps:

**a.** preparation of the K balance sheet for all crops in the fixed cropping sequence,

**e.** are, in consequence, much more than cereals sensitive to the level of available soil K.

The crop rotation-oriented strategy of K management also assumes a maximized recycling of internal, i.e., soil available K sources and field resources (plant residues). Therefore, the amounts of fertilizer K needed to cover its losses due to exports from the field in harvested products or/and leaching processes, depend also on the management of crop by-product (residues). The mentioned concept is in accordance with the Ideal Soil fertility (ISF) ap‐ proach [91]. Potassium timing seems to be of secondary importance taking into account the strategic goals of K management. Natural growth conditions, mostly related to stressing fac‐

Following the theoretical assumptions it appears that the main problem of adequate fertilization of the leafy crop with potassium is to develop an appropriate system of managing the potassium rotation-oriented system. The principal farmer's question is, how to achieve the target K availa‐

level of available K [66].

160 Soil Fertility

suming that leafy crops:

Kup - K uptake by the main yield, kg ∙ ha-1

Kplr - K accumulation in plant residues, kg ∙ ha-1


Kfym - K supply in organic manure, kg ∙ ha-1;

Kf - fertilizer K, kg ∙ ha-1

δKav - intended/required change of soil available K, mg kg ∙soil-1

In agronomic practice, some components of the K balance sheet such as natural input or losses may be omitted due to their minor importance as a source of K. The minimal set of data required when constructing the balance sheet for a particular crop sequence is as follows (Table 9):


The first parameter shows a certain level of variability, according to soil, crop and produc‐ tion technology. The critical issue of the proposed concept relates to the management of the Kplr component of the Eq. No 15. All vegetative plant organs, such as straw or sugar beet tops, are very important sources of potassium. The environmentally and economically sound solution is to incorporate all harvested by-products into soil.


rate of K supply to a crop during important stages of yield formation. For cereals, the re‐ quired K level is much lower. In general, 100 mg KEX kg soil-1 can be considered as the up‐

Fig. 9. A graphical method of assessing the critical available soil K for reaching maximum

**Figure 9.** A graphical method of assessing the critical available soil K for reaching maximum yield of storage roots by

The annual loss of potassium from the cropped soil due to intended export or leaching ranges from about 20 to 33%, depending on the crop [66]. Only in the case of leafy vegetable and fodder crops, its loss is substantially greater. Therefore, the required amount of potassium to be applied in rotation with leafy crops may be calculated using the equation No. 16. However, taking into account plant residues and their contribution to the required K amount, the needed quantity of purchased K fertilizer can be substantially, even by 3-times lower [Eq. No 17, Table 9]. Data con‐ cerning fertilizers value of crop residues can be obtained directly or calculated using constant re‐

lationships between K content in the main product and its concomitant by-product [66].

– ( ) – / .[ ] [ - == ´ **<sup>1</sup>**

Legend: L-CP, Q-CP – critical point (quantity of available K) as determined by the linear (L) and quadrate (Q) regression

Legend: L-CP, Q-CP – critical point (quantity of available K) as determined by the linear (L) and quadrate (Q) regression models, respectively; OP- optimum content of available K.

0 50 100 150 200 250 300 350 soil available K, mg K2O kg-1

L-CP Q-CP

OP

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– / . [ ] ( ) - = ´ **<sup>1</sup> K K K 3 0 9 K O ha f ar ca <sup>2</sup>** (16)

**g f ar ca rec <sup>2</sup> dK K K K 3 K 0 9 K O ha** (17)

yield of storage roots by sugar beet; Adapted from [92]

models, respectively; OP- optimum content of available K.

**Kf** – potassium fertilizer rate, kg K2O ha-1

9

per range of this plant response to soil K.

1.0 = 72.5 t ha-1

0,5

sugar beet; Adapted from [92]

where:

0,6

0,7

0,8

0,9

relative yield of storage roots, (Y) %

1

1,1

1,2

1main product, fresh weight;

<sup>2</sup>root residues + stubble - sugar beets ≈ 5%, cereals ≈ 15%, oil-seed rape ≈ 20% DW of aboveground biomass;

3content of K in plant residues;

<sup>4</sup>K recovery from plant residues/manure in the four-course rotation ≈ 90%; 5fresh weight, K2O content = 7 kg t-1.

**Table 9.** An example of potassium balance, the 4-course rotation, kg K2O ∙ ha-1

#### **6.3. Determination of optimum soil K level**

The efficient management of potassium in a given field depends on cultivated crop species and their cropping sequence. The main operational objective is to assess the degree of each crop sensitivity in the fixed rotation to the amount of soil available + fertilizer K. The graphi‐ cal procedure of the optimum K range determination assumes, that the target crop shows the response to the applied potassium fertilizer, when soil K supply is too low to harvest 95% of the maximum yield. Based on data obtained from on-farm experiments and farmers experiences, it is possible to determine the perfect range of available K. As presented in Fig. 9, the applied statistical procedure, specifically the linear-plateau and quadratic regression models, allowed to fix the critical K point (limit), amounting to 170 mg K2O kg-1. However, as resulted from the analysis of the quadratic model, yield of the tested crop increased fur‐ ther up to 250 mg K2O kg-1 (Fig. 9). In the case of sugar beets, the ideal level of soil K has been fixed at high level (clearly defined range), irrespectively of the site (soil) and year. All other leafy crops also require a fixed, in general, high level of soil available K during critical stages of yield formation. This level of soil attainable K content is the basis of the needed rate of K supply to a crop during important stages of yield formation. For cereals, the re‐ quired K level is much lower. In general, 100 mg KEX kg soil-1 can be considered as the up‐ per range of this plant response to soil K.

Legend: L-CP, Q-CP – critical point (quantity of available K) as determined by the linear (L) and quadrate (Q) regression models, respectively; OP- optimum content of available K.

Fig. 9. A graphical method of assessing the critical available soil K for reaching maximum yield of storage roots by sugar beet; Adapted from [92] **Figure 9.** A graphical method of assessing the critical available soil K for reaching maximum yield of storage roots by sugar beet; Adapted from [92]

Legend: L-CP, Q-CP – critical point (quantity of available K) as determined by the linear (L)

and quadrate (Q) regression models, respectively; OP- optimum content of available K. The annual loss of potassium from the cropped soil due to intended export or leaching ranges from about 20 to 33%, depending on the crop [66]. Only in the case of leafy vegetable and fodder crops, its loss is substantially greater. Therefore, the required amount of potassium to be applied in rotation with leafy crops may be calculated using the equation No. 16. However, taking into account plant residues and their contribution to the required K amount, the needed quantity of purchased K fertilizer can be substantially, even by 3-times lower [Eq. No 17, Table 9]. Data con‐ cerning fertilizers value of crop residues can be obtained directly or calculated using constant re‐ lationships between K content in the main product and its concomitant by-product [66].

$$\mathbf{K}\_t - \left(\mathbf{K}\_{\mu} - \mathbf{K}\_{\alpha}\right) \times \mathbf{3} / 0.9 \text{ (K}\_2\text{O ha}^{-1}\text{)}\tag{16}$$

9

$$\text{pH}\_{\text{g}} - \text{K}\_{\text{f}} = \left| \left( \text{K}\_{\text{m}} - \text{K}\_{\text{m}} \right) \times \text{3} \right.\\ \left. - \text{K}\_{\text{m}e} / 0.9 \left( \text{K}\_{2} \text{O} \text{ ha}^{-1} \right) \text{} \tag{17}$$

where:

**Components of balance sheet Yield**

tops + root residues2

straw + root residues

straw+ root residues

straw + root residues

**Crop products** Sugar beet storage roots1

grain

seeds

grain

1main product, fresh weight;

3content of K in plant residues;

Spring barley

162 Soil Fertility

Oilseed rape

Winter wheat

**t ∙ ha-1**

6.0 6.0 + 1.8

4.0 10 + 2.8

8 9 + 2.55

**Table 9.** An example of potassium balance, the 4-course rotation, kg K2O ∙ ha-1

**6.3. Determination of optimum soil K level**

K net balance I - -836.6 -253 Manure 34.05 +214.4 0.0 K net balance II - -622.4 -253 K fertilizer needs, kg K2O ∙ ha-1 622.2 253 K fertilizer needs, kg K2O ∙ ha-1 ∙ year-1 155.6 63.25

60 250 + 153

36 72 + 27

40 200 + 42

40 126 + 38.3

Total - 946.3 109.7 946.3 693.3

<sup>2</sup>root residues + stubble - sugar beets ≈ 5%, cereals ≈ 15%, oil-seed rape ≈ 20% DW of aboveground biomass;

<sup>4</sup>K recovery from plant residues/manure in the four-course rotation ≈ 90%; 5fresh weight, K2O content = 7 kg t-1.

The efficient management of potassium in a given field depends on cultivated crop species and their cropping sequence. The main operational objective is to assess the degree of each crop sensitivity in the fixed rotation to the amount of soil available + fertilizer K. The graphi‐ cal procedure of the optimum K range determination assumes, that the target crop shows the response to the applied potassium fertilizer, when soil K supply is too low to harvest 95% of the maximum yield. Based on data obtained from on-farm experiments and farmers experiences, it is possible to determine the perfect range of available K. As presented in Fig. 9, the applied statistical procedure, specifically the linear-plateau and quadratic regression models, allowed to fix the critical K point (limit), amounting to 170 mg K2O kg-1. However, as resulted from the analysis of the quadratic model, yield of the tested crop increased fur‐ ther up to 250 mg K2O kg-1 (Fig. 9). In the case of sugar beets, the ideal level of soil K has been fixed at high level (clearly defined range), irrespectively of the site (soil) and year. All other leafy crops also require a fixed, in general, high level of soil available K during critical stages of yield formation. This level of soil attainable K content is the basis of the needed

**Management of plant residues**





**exported from the field left at the field**

**losses input4 losses input**

60 250 + 15

36 72 + 27

40 200 + 42

40 126 + 38.3 - 238.5




**Kf** – potassium fertilizer rate, kg K2O ha-1

**Kar** - soil available K content required by the most sensitive crops in a fixed crop rotation, mg K2O kg-1 soil, the critical range

consecutive exploitation and use of: i) natural soil K reserves, ii) recycled organic K - plant

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residues, and in the last step, iii) K mineral fertilizers.

**Photo 1.** Potassium deficiency symptoms on maize; Author: Witold Grzebisz

Witold Grzebisz, Witold Szczepaniak, Jarosław Potarzycki and Remigiusz Łukowiak

Department of Agricultural Chemistry and Environmental Biogeochemistry, Poznan

[1] FAOSTAT. Available online at: http://faostat.fao.org/default.aspx. (accessed

**Author details**

**References**

2012-08-07).

University of Life Sciences, Poland

**Kca** - current level of soil available K, mg K2O kg-1 soil

**Krec** - K recycled in plant residues and/manure, kg K2O ha-1

3 - coefficient for converting soil K into K rates.

It is possible, based on specific K requirements to assign all cultivated crops into a particular soil available K classes. This has been reported in Table 10 for key crops cultivated in Po‐ land. By using this operational scheme, the farmer can define a right place for crops grown in a particular crop sequence with respect to the required level of soil available K. This table can be considered as the first step in the development of the K fertilizing plan, oriented on K requirements of the most sensitive crop in the given crop sequence.
