2. Soil Properties as Effected by Exchangeable Cations.

• Exchangeable Ca²⁺ & Mg²⁺.

• Exchangeable Na⁺& their alkaline cations.

• Exchangeable H⁺& Al³⁺.

Two groups of adsorbed cations (S and Hh) tend to have opposing effects on soil acidity & alkalinity.

Ca²⁺ & Mg²⁺ tend to dominate in neutral soils or slightly alkaline ones like calcareous and slightly sodicity – affected soils. Ca²⁺ is especially important in forming soil structure and an accumulative type of soil profile rich in organic matter in the upper horizons. Such soils being well granulated possess favorable physical properties, optimum water-holding capacity and infiltrability. Mg²⁺, if there is not much of it (Ca²⁺/Mg²⁺>4), is calcium’s ally in this respect. Sufficient soil porosity and water-proof aggregates of soil structure prevent the antagonism between soil air & water typical for the soils with poor granulation.

Exchangeable Na⁺ and other alkaline cations are responsible for soil alkalinity. Sodium-affected soils with 15 percent or more of the CEC saturated with Na⁺ are called sodic and are highly alkaline. When a soil contains Na₂CO₃ the pH may be as high as 10 or more.

Adsorbed H⁺ contributes directly to the H⁺ ions concentration in soil solution. Al³⁺ ions do so indirectly - through hydrolysis. This may be illustrated as follows:

Al³⁺+ H₂O→Al(OH)²⁺+H⁺

Al(OH)²⁺+H₂O→Al(OH)⁺₂+H⁺

Adsorption of H⁺ at the edge of silicate clay minerals makes aluminum unstable & it exits from the edge of the clay lattice. Consequently, acidity in soil stimulates the development of additional acidity through aluminum hydrolysis and aluminum hydrolysis becomes a very important source of H⁺ when soils become acid.

Soils rich in exchangeable H⁺ & Al³⁺ are podzolic or podzolized, with profiles divided into elluvial & illuvial parts. Their structure & physical properties are poor. They may have aluminum, manganese and other toxicities when they become waterlogged or otherwise wet.

3. Soil Acidity & Acid Soil Amendment.

Soil acidity may be defined as the ability of the soil to yield hydrogen cations to water or salt solutions. Active soil acidity is determined by titration or pH-measurement of the water extract with distilled water to soil ratio 5:1 (our country’s standard). This form of acidity is close to the pH-requirements of crops and other plants. Adsorbed (exchangeable) H⁺& Al³⁺ (Al(OH)²⁺,Al(OH)₂⁺) cations do not come to soil-water extract. The so-called exchangeable soil acidity is that determined by pH measurement or titration of 1M KCl soil extract with the same (1:5) soil to solution ratio. Exchangeable soil acidity includes active soil acidity as its component. pH_KCl in this country still remains the most widespread criterion to determine whether a given soil requires liming. Another of these criteria is - base saturation percent (BSP).

By pH KCl the soils of Ukraine are divided into 5 groups:

pH KCl	Lime requirement
<4.0	Needs urgent liming in all types of crop rotation
4.0-4.5	The same
4.5-5.0	Needs liming in vegetable and fodder crop rotations on sandy loams and loams; a moderate need in liming may appear in field crop rotations on sandy soils
5.0-5.5	A serious need in liming of loamy-sand and loam soils especially in crop rotations with grasses, forage & vegetable crops. Last to be limed are sandy & loamy-sandy soils
5.5-6.0	Some sandy-loam and loamy sand soils may need a selective liming in crop rotations with demanding crops
>6.5	The soils do not need liming

By PBS (V,%) the division is as follows:

V,%	Lime requirement
<50	An urgent need in liming
50-70	Strong to moderate need in liming depending on the crops & soils
70-90	There is some selective need in liming for some crops & levels of fertilizing
>90	No need in liming

Common end points of soil acidity titration are pH 7 or pH 8.2, although soils in the field are rarely limed above pH 6 or 6.8.The value 8.2 was chosen historically by Kappen because it approximates the pH of soil containing free CaCO₃ in equilibrium with the normal CO₂ content (0.003 mole fraction) of the atmosphere. This pH also corresponds closely with the pH of complete neutralization of soil hydroxy aluminum compounds. This last point is in this country realized by the determination of hydrolytic soil acidity employing by the Kappen procedure (titration of 1N CH₃COONa extract obtained with the ratio soil solution=1:2.5). Hydrolytic acidity (Hh, m-eq/100g soil) is used in liming rate computations.

A major problem of managing acid soils is to estimate the quantity of lime required to raise the soil pH to a certain level. Plant species vary considerably in their response to soil pH.

The most theoretically satisfying way to estimate the lime requirement of acid soil is to titrate a sample of soil with a standard base, determining the quantity of base required to raise soil pH to a specified level. But each titration step must allow sufficient time for the added base to react thoroughly with the soil. Both exchangeable and titratable acidity will be neutralized during the titration process.

In Ukraine each equivalent of acidity is neutralized by an equivalent of lime (CaCO₃).So the so-called full rate of liming by hydrolytic acidity may be computed by the equation:

D_CaCO3, mt/ha=0.05 H_hhdv,

where H_h=hydrolytic acidity, m-eq/100g soil; h= depth, cm, of a plow layer or any other depth if a lime is not plowed down; and d_v= the bulk density of the soil, g/cm³.

American authors of the Textbook of Soil Chemistry (H.L.Bohn, et al) believe the primary effect of lime to be the provision of hydroxyl ions and adequate soil calcium:

CaCO₃ +H₂ O=Ca²⁺+ HCO₃^- +OH^-

The hydroxyl ions produced by the lime neutralize soil acidity, raise soil pH, and thus provide the most important effects of the liming.

According to this notion, American rates of liming should be twice those of Ukrainian. According to our notion, the lime reacts with the acid soil by the reactions:

CaCO₃ +H₂ O+CO₂→Ca(HCO₃)₂

Ca(HCO₃)₂+2H⁺(soil solution)→Ca²⁺+2H₂O+2CO₂↑

[SAC^2-]2H⁺+ Ca(HCO₃)₂↔[SAC^2-]Ca²⁺+2H₂O+2CO₂↑

Field liming reactions are generally incomplete, because of incomplete mixing & require considerable time.