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

#### **4.1. Soil K mining**

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

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

7

duction mostly relies on soil resources and alternative sources of nutrients, including K mineral fertilizers. The high-input systems, which do not cover, or even replenish plant K needs at critical stages of yield development, result in soil K mining. High year-to-year vari‐ ability and/or yields stagnation is not always recognized as the attribute of the inappropriate K management. Therefore, all potassium mined soils as well as light textured and also or‐ ganic soils should be considered as risky for crop production. For all these groups, recom‐

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

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

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‐

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

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‐

ions concentration in the soil solution results in their movement from the ex‐

ions with different rates, depending on geochemical characteristics

Sustainable Management of Soil Potassium – A Crop Rotation Oriented Concept

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

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ions present in the soil solution, is directly available to the plant. In Pol‐

ions held by negatively charged clay and humus parti‐

mended rates of applied K should be greater than its removal.

**4.2. Soil K pools**

in pools, which release K+

first one, containing K+

changes in K+

ing minerals [Table 5].

ginated from loams and sands, respectively [66].

The second K pool (EXK) contains K+

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‐ duction mostly relies on soil resources and alternative sources of nutrients, including K mineral fertilizers. The high-input systems, which do not cover, or even replenish plant K needs at critical stages of yield development, result in soil K mining. High year-to-year vari‐ ability and/or yields stagnation is not always recognized as the attribute of the inappropriate K management. Therefore, all potassium mined soils as well as light textured and also or‐ ganic soils should be considered as risky for crop production. For all these groups, recom‐ mended rates of applied K should be greater than its removal.
