Effects of Tropical Weeds on Soil Potassium
7.1 The Release of Potassium from Soil Minerals
Potassium in soils is contained in mineral and organic matters, which are not easily available to plant root absorption. Some of K reserves in soil are listed in Table 7.1, which mainly exist as silicate minerals, like feldspars, and organic matters. Potassium is absorbed by plant roots as K+ ions and therefore the structural K in the silicate minerals must be released as K+ to be available to plants.
The mechanism by which K+ ions are released into the soil solution from mineral K is called dissolution or chemical weathering. Similarly, organic K must be released as K+ into the soil solution through a chemical mechanism called decomposition.
Therefore, the soil K in the soil environment is not only supplied by mineral sources but also by organic sources. The root excretion may enhance the soil available K from soil minerals particularly in young soils with enough K-minerals and from soil organic matters containing K. Inputs of organic matters into soils from root
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excretion and plant residues are therefore very important to enhance the soil K availability. Infertile soils may have sufficient K if treated with enough organic matters.
Table 7.1. Plant nutrient reserves in soil and their releasing mechanisms*.
Nutrient
Elements Reserve Forms Releasing
Mechanisms
Dissolved Forms
N Organic Matters
NH4+
in Mica and Vermiculite Interlayers Exchangeable NH4
Decomposition Dissolution Cation Exchange
NH4+
NH4+
NH4+
S Organic Matters
Sulfides (Ex. FeS) Gypsum (CaSO4)
Decomposition Oxidation Dissolution
S2-, SO42-
SO42-
SO42-
P Organic Matters
Al-P, Fe-P, Ca-P
Decomposition Ligand Exchange Dissolution
H2PO4-
, HPO42-
H2PO4-
, HPO42-
H2PO4-
, HPO42-
K, Ca, Mg Silicate Minerals (Ex.
Feldspars)
Carbonates, Sulfates Organic Matters
Weathering, Dissolution Dissolution Decomposition
K+, Ca2+, Mg2+
Ca2+
K+ Fe, Mn, Zn,
Cu
Hydroxide Precipitates Adsorbed by Oxides of Fe, Al, and Mn Chelates Exchangeable
Dissolution Desorption
Dissociation Ion Exchange
Cations and Dissolved Chelates
B, Mo Adsorbed by Oxides of Fe and Al and Clay Minerals
Desorption H3BO3
MoO42-
*Taken from Singer and Munns (1987)
The plant nutrient elements are released as free ions into the soil solution through several mechanisms (Table 7.1) controlled by several factors as listed in
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Table 7.2. Potassium adsorbed on soil solids called exchangeable K is released through desorption process and controlled by equilibrium constant (K), cation exchange capacity (CEC), soil pH, and solid preference towards cations.
Exchangeable K is released when its concentration in soil solution is lowered by some mechanisms like plant root absorption and leaching. The release is more difficult when the soil CEC and pH are high and the soil preference towards K is high. Therefore, the release of the exchangeable K is easily enhanced by lowering the soil pH, increasing the concentration of competing cations, and lowering the soil solution concentration of K+.
Table 7.2. The mechanisms of plant nutrient release and their controlling factors.
No. Chemical
Processes Sources of Free Ions
Dominant Controlling Factors
1 Decomplexation Complex Ions K*
2 Dissolution Precipitates Ksp*
3 Desorption Exchangeable K K*, CEC, pH,
Preference 4 Weathering Mineral Structural K pH, Soil Moisture,
Mineral Structure, Mineral Chemical Composition 5 Decomposition Organic Structural K pH, Soil Moisture,
Enzymatic Activity, Temperature, C/N Ratio
*K is an equilibrium constant related to the related chemical reaction, Ksp is an equilibrium constant related to the dissolution of a precipitate
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The structural K in the soil minerals is more difficult to release. This element is released through mineral weathering which is controlled by several factors among which are soil pH and soil moisture. As those for all primary minerals, the weathering of K containing minerals is enhanced by the presence of water and H+ ion. Therefore, the release of K from soil minerals can be enhanced by lowering soil pH in the presence of water. The elimination of the released K+ from soil solution through some mechanisms like plant root absorption and leaching may also speed up the mineral weathering.
The weathering of orthoclase (KAlSi3O8) may exemplify the above mechanisms.
The weathering of this K-mineral as shown in Fig 1 may progress in the presence of water molecule and H+ ion. Following the mass action law, the reaction may speed up in the presence of high amounts of water molecule (moist soils) and H+ ion (acid soils) and low concentration of K+ (and also those of Al3+ and Si(OH)4). This means that the weathering process can be enhanced by moistening the soil sample and/or lowering the soil pH and/or eliminating K+ (and also Al3+ and Si(OH)4). However, intensive weathering of orthoclase may end up with the existence of more stable secondary minerals not containing K, like montmorillonite, kaolinite, and gibbsite.
Fig. 7.1. The release of K+ in the weathering of K-containing primary minerals.
KAlSi3O8 + 4 H2O + 4 H+ Primary Mineral:
Orthoclase/K-Feldspar
+ Al3+ + 3 Si(OH)4 K+
Low [Si(OH)4] Al3++3 H2O
3H++Al(OH)3
Secondary Mineral: Gibbsite
High [Si(OH)4]:
2 Al3+ + 2 Si(OH)4 + H2O
6 H+ + Al2Si2O5(OH)4
Secondary Mineral: Kaolinite
Gibbsite
Al2Si2O5(OH)4 + 5 H2O
3 Si(OH)4 + 2 Al(OH)3 Secondary Mineral:
Kaolinite Gibbsite
More Intensive Weathering:
Si(OH)4 leached
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There are several forms of K present in the soil environment but not all forms of K in soils are available to plants and are classified into Slowly Available, Slightly Available, and Readily Available. The soil K in the mineral structure is Slowly Available (Table 7.3). Those classified as Slowly Available (primary minerals) is about 100 to 3000 times greater than those on the soil exchange sites (Exchangeable K). Therefore not all K is available and can be absorbed by plant roots. To use this source, this slowly available K must be released from the structure of the minerals through weathering processes.
Table 7.3. The forms and availability of K in the soil environment*.
Sources Classification Concentration
1 Primary Minerals:
- K Feldspars - Muscovit - Glauconit - Biotite
Slowly Available 0.1 – 3%
(20.000 – 60.000 kg ha-1)
2 Fixed-K Slightly Available 0 100 kg ha-1 3 Exchangeable K Readily Available 200 kg ha-1
(Fertile Soils) 4 Dissolved K Readily Available 1–50 ppm
(Soil Solution) (0.4-20 kg ha-1)
Total varies depending on parent materials and weathering stage
Granitic Soils > Calcareous Soils
Old Soils (Latosol) < Young Soils
*Adapted from Corey (1964)
The relationships between all forms of soil K is depicted in Fig. 7.2. As explained previously, the primary minerals and organic matters are the main
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sources of K in the soil environment. Under several controlling factors, these sources encounter weathering and decomposition processes releasing K+ into the soil solution. Potassium ions in the soil solution will then encounter different fates:
absorption by plant roots, leaching, and adsorbed by soil exchange sites. The readily available K+ and the exchangeable K may also shift into the Slightly Available fixed K. The input of K from fertilizers may contribute the dissolved K through reaction similar to those of the primary minerals and organic matters.
Fig. 7.2. The relationship between forms of K in the soil environment (After Salam, 2019).
Dissolved K+
Root Absorption
K Leaching Primary Minerals
Organc Matters Fertilizer s
Exch. K
Fixed K Contact Exchange
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