Base saturation (V) is the sum of the absorbed bases expressed as a percentage of the cation exchange capacity (T).
The cation exchange capacity is equal to the sum of absorbed cations (S), such as Ca2+, Mg2+, K+, NH4+ and others, and cations H+, Al3+, Fe3+, Mn2+, causing hydrolytic acidity (H), is (in mg-eq/100 g of soil):
T = S + H.
The degree of saturation of bases is determined by the formula:
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Base saturation of the soil is an indicator of the need to lime the soil. The lower it is, the higher the need for lime application. Thus, at identical hydrolytic acidity (H) of the two soils, for example, 5 mg-eq/100 g soil, but different values of the capacity of cation exchange (T), for example, the first – 10 mg-eq/100 g, the second 20 mg-eq/100 g, the degree of saturation with bases (V) in the first case is 50%, the second – 75%. Thus, with an equal value of hydrolytic acidity, the first soil is more acidic, as 50% of the capacity of cation exchange accounts for acidifying cations and it is more in need of replacing them with bases. If the values of cation exchange capacity are equal, the soil with a higher value of hydrolytic acidity will first need liming.
Soil buffering
Soil buffering is the ability to withstand a change in environmental reaction. Buffering capacity is characterized by the value of cationic absorption capacity (T), the composition of absorbed cations and cationic-anionic composition of the soil solution. The indicator is used to calculate the optimal doses, forms, timing and methods of applying fertilizers and ameliorants to crops. The higher is the value of cation exchange capacity, the higher is the buffering capacity of the soil.
Buffering properties against acidity increases with increasing saturation of soils with bases and with the transition from neutral to alkaline reaction of the environment. When hydrogen ions appear in the soil, for example, as a result of nitrification or physiological acidity fertilizer NH4NO3, they are exchanged with the cations of the soil absorbing complex (SAC), resulting in the formation of neutral salt and the reaction of the solution does not change:
[SAC](Ca, Mg) + 2 HNO3 → [SAC](H2, Mg) + Ca(NO3)2.
Buffering properties against alkalization increase in neutral soils with increasing hydrolytic acidity, with decreasing degree of base saturation and with transition from neutral soils to acidic soils. When hydroxide ions such as Ca(OH)2 appear in such soils as a result of introducing physiologically alkaline Ca(NO3)2, calcium cation is displaced from SAC an equivalent amount of hydrogen ions, resulting in the formation of water and the reaction of the solution does not change:
[SAC](H2, Ca) + Ca(OH)2 → [SAC]Ca2 + 2H2O.
Under the influence of acidifying and alkalizing factors the reaction of soil solution can change, but the rate of change in soils with low cation exchange capacity, such as sandy, sandy loam, podzolic soils, is much higher than in high capacity ones, such as loamy black earths.
In the soil solution, buffering is created by the presence of weak organic and mineral acids and their salts:
(CH3COO)2 + 2HNO3 = 2CH3COOH + Ca(NO3)2;
Ca(HCO3)2 + 2HNO3 = 2H2O + 2CO2 + Ca(NO3)2;
2CH3COOH + Ca(OH)2 = (CH3COO)2Ca + 2H2O;
Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 + 2H2O.
Soil buffering also manifests itself in resistance to temporary changes in soil solution concentration caused by lack of moisture, irregular or periodic application of fertilizers and ameliorants. Soils with high buffer capacity, cation exchange capacity and diverse composition of absorbed ions easily retain maximum permissible single doses of ameliorants and fertilizers in the absorbed state without significant increase in concentration of soil solution.
Low buffered, low capacity soils cannot retain large single doses of ameliorants and fertilizers without increasing soil solution concentration and losses of elements from leaching, so on such soils fertilizers are applied fractionally.
Application of organic and mineral fertilizers in combination with periodic application of ameliorants allows to increase cation exchange capacity, regulate the composition of absorbed cations, increase soil buffering.
Sources
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.