Soil Analysis

determination of the composition and properties of soil. Mechanical, chemical, mineralogical, and microbiological analyses are performed. The results are used to compile soil maps, including agrochemical cartograms.

A mechanical, or granulometric, analysis is performed to determine the quantity of particles of different diameters. It is done with a sieve or with a pipette, making use of the relationship between particle diameter and the rate at which the particles settle in still water. Soil is classified by its granulometric composition as medium loam, sandy loam, and so on, depending on the amount of physical clay (particles < 0.001 mm in diameter) and physical sand (> 0.01 mm in diameter).

Chemical analysis establishes the chemical composition and properties of soil. It consists of four main parts. The bulk, or element, analysis reveals the total content of C, N, Si, Al, Fe, Ca, Mg, P, S, K, Na, Mn, Ti, and other elements. Analysis of a water extract, the basis for studying saline soils, gives some idea of the content of water-soluble substances, such as calcium, magnesium, and sodium sulfates, chlorides, and carbonates. The adsorbing capacity of soil is determined by chemical analysis. Finally, the supply of nutrients—the quantity of readily soluble nitrogen, phosphorus, potassium, and other compounds available to plants—is established, and the fertilizer requirements of the fields are determined from the results of the analysis. Considerable attention is also given to the fractional composition of organic matter and to the types of compounds of the main soil components, including trace elements. There are field, expeditionary, and laboratory chemical analyses. Whereas field analyses are performed by simplified methods, laboratory analyses are generally done by instrumental methods, including spectroscopy, flame photometry, and atomic adsorption.

Mineralogical analysis reveals the content of primary and secondary minerals in order to study the genesis and physicochemical properties of soil. The distribution of minerals in the soil profile is investigated by the section method, and their quantitative correlation and change during soil formation are studied by the immersion method. The clay and colloidal fractions are investigated by thermal, roentgenographic, electronographic, and other methods. Chemical methods are used to determine the composition of clay minerals: a bulk analysis is made, and the adsorbing capacity of the fractions under study is determined.

Microbiological analysis reveals the composition of the soil microflora, which determines the soil’s biochemical properties and biological activity. Counts are made of the main groups of soil microorganisms (in thousands per 1 g of dry soil): bacteria (separate counts of azotobacters, nitrifying and denitrifying bacteria, and ammonifiers), actinomycetes, fungi, soil algae, and the main types of protozoans (amoebas and infusorians).

To obtain reliable results, samples must be taken in the most typical place in the field and properly stored in an air-dried condition. To study the genesis of a soil, samples may be taken from each horizon and subhorizon of the soil profile or from several points in the field. After mixing the soil from the various parts of the field an average sample is used to study the soil’s agrochemical properties.