Reference Information on the Agronomically Important Properties of Soils

The physical and chemical properties of soils are among the main properties that have a decisive influence on plant growth and development.

According to DSTU 4362:2004, which standardizes the fertility indicators of agricultural land soils, physical and chemical properties include the following indicators: soil solution reaction and the composition of absorbed cations. The composition of absorbed cations determines the soil solution reaction, directly affects the ratio of humic and fulvic acids in humus formation processes, and influences soil microbiota activity, coagulation and peptization of colloids, and water-physical properties. This indicator has an extremely important influence on the mobility, and hence the availability to plant root systems, of mineral nutrients, and on the manifestation of ion antagonism and synergism.

In practice most attention is paid to the soil solution reaction, as one of the important factors influencing plant growth and development, including in connection with the availability of nutrients to plant root systems. The following kinds of soil acidity are distinguished: actual (active) and potential (latent), which appears under certain conditions. In turn potential acidity is divided into exchangeable and hydrolytic by the type of extracting solution used to displace hydrogen ions into solution (fig. 1).

Actual acidity is determined by treating a soil sample with distilled water, and is caused by the presence and dissolving in water of CO₂ with formation of carbonic acid H₂CO₃. However, carbonic acid is an unstable compound and in the soil it is neutralized by absorbed cations. When this acid interacts with calcium or sodium compounds, calcium bicarbonate Ca(HCO₃)₂ or sodium carbonate Na₂CO₃ or sodium bicarbonate NaHCO_3. forms. On their hydrolysis, strong alkalis Ca(OH)₂ or NaOH form, which alkalinize the soil solution. On soils with calcium and magnesium predominating in the absorbing complex, soil pH is in the range 7.0–8.0. On soils with calcium and organic acids predominating in the soil-absorbing complex, 5.0–7.0. Sodium bicarbonate alkalinizes especially strongly. Predominance of sodium in the soil-absorbing complex leads to the formation of a strong base, so such soils have an alkaline reaction.

Thus the actual acidity of the soil is created by carbonic acid H₂CO_3, water-soluble organic acids and hydrolytically acidic salts.

Potential acidity is associated with the presence of hydrogen and aluminum ions in the absorbed state. When a soil sample is treated with a solution of the neutral salt KCl, hydrogen and partly aluminum pass into solution and acidify it. This is the main indicator by which soil acidity is assessed. According to DSTU, the grouping of soil acidity and alkalinity levels by this indicator is given in the table.

Table 1. Grouping of soils by degree of acidity and alkalinity

Hydrolytic acidity

When a soil sample is treated with a solution of the hydrolytically alkaline salt sodium acetate CH₃COONa, hydrogen and aluminum are fully displaced. This indicator thus combines both actual and potential acidity. It is expressed not in pH units but in milligram equivalents per 100 g of soil. The hydrolytic acidity (Hh) indicator is the basis for calculating the need for liming materials in chemical amelioration of acidic soils. The simplest formula is: lime dose = Hh × 1.5.

Content and composition of cations

For soils with increased acidity (pH below 6.0), hydrogen and aluminum predominate in the composition of absorbed cations.

For neutral and slightly alkaline soils (pH 6.0–7.5), calcium and magnesium cations predominate in the soil-absorbing complex.

The cation composition of alkaline soils may be represented by both monovalent and divalent cations; as a rule they are all classed as saline. Unlike acidic soils, which are inherent primarily in certain natural and climatic zones, alkaline (saline) soils belong to the so-called intrazonal soils, because in nature they occur in practically all natural and climatic zones in separate areas. Some of them are linked with human economic activity (secondary salinization).

Saline soils include solonchaks, solonetzes, solonetzic soils and solods. They occur in almost all soil and climatic zones and therefore belong to intrazonal soils. The following varieties of solonetzic soils are distinguished: solonchakous, solonetzic and solodized. Solonchaks are soils in which easily soluble salts accumulate throughout the profile and especially in the upper horizons. Among the salts in solonchaks the most common are CaCO₃, Ca(HCO₃)₂, Na₂SO₄, NaHCO₃, CaSO₄, MgSO₄, NaCl, Na₂CO_3, and in small amounts CaCl₂, MgCl₂, MgCO₃ and others. There are often districts in which, depending on conditions (relief, parent rocks), various salts accumulate, but in each variety of saline soil one or two kinds of salts predominate. If solonchaks contain more sodium carbonates they are called soda solonchaks, and if calcium carbonates are present, meadow-carbonate solonchaks, and so on. The accumulation of salts in the soil also depends largely on their solubility. Chlorides are the most soluble in water, sulfates less so, and calcium and magnesium carbonates the least. The salts that salinize soils can be varied, but most of all they include chlorides (NaCl, MgCl₂), sulfates (Na₂SO₄, CaSO₄), carbonates (Na₂CO₃, MgCO₃, CaCO₃) and bicarbonates (NaHCO₃, MgHCO₃).

On solonchaks plants are strongly suppressed and die as a result of the high osmotic pressure of the soil solution, which exceeds the pressure of the cell sap. Water intake into the plant decreases, transpiration increases, and assimilation and respiration processes are disrupted. In Ukraine the most widespread are weakly solonchakous soils with a salt content up to 0.2%, on which almost all agricultural crops are grown. There are also moderately solonchakous soils with a salt content of 0.2 to 0.4%, on which crops grow much worse than on weakly solonchakous ones.