3. Morphological features of the soil profile.

A soil profile is diagnosed by the morphological features which are the evidence of the processes of soil formation. In this country the morphological features of a soil horizon include (1) the depth of a horizon in cm, (2) soil color, (3) soil texture, (4) soil structure, (5) soil consistence, (6) new formation, (7) inclusions,& (8) line and character of transition to the next horizon in a profile.

Soil texture is the relative proportions of sand, silt, and clay in a soil. It is determined by one of the field methods. The soil must be moistened to the consistence of putty. Loamy soils form a good cast when moist. Clay loam forms a ribbon that breaks somewhat easily, and clay forms a long flexible ribbon. Sand is easily discernible. In Ukraine the procedure is different. Sandy soils do not even form a ball. Loamy sands form some easily breakable balls but do not mold in to wires. Sandy loams’ wires are thick & break to pieces during the process of molding. Clay loams may be shaped into wires which break when turned round the finger in an attempt to make a "ring". Loamy clays show cracks but do not break to pieces. And clays make "rings" without cracks.

The soil may be structureless (with no observable aggregation). The structure may be weak (with poorly formed indistinctive peds, barely observable in place. Moderate – well formed distinctive peds. Strong – durable peds that are quite evident in undisturbed soil. By the shape of aggregates the structure may be single grained, massive, granular, platy, blocky, prismatic, columnar, etc.

Consistence is the resistance of the soil to deformation or rupture. In this country there is a rule to determine or just describe qualitatively the extent of soil compactness & wetness: wet, moist, dry, or slightly moist just to cool the hand.

New formations are the material evidence of a given pedogenic process. Amorphous SiO₂ "destiny" may be an evidence of podzolic or sodicic (solonetzic by Canadian terminology) processes. The accumulations of lime, gypsum and soluble salt crystals testily to the development of soddy, chestnut or saline processes. Mostly spots may signify the development of gleization & so on.

Inclusions of biological or other character are not directly connected with the processes of soil formation.

The transition from one horizon to the other, underlying it, may be gradual, visible, clear (discernible) and sharp (or very distinct). The border line between the horizons may be of different shape (strait, wavy, pocket-like, etc.).

4. Soil ecology.

Soil is a complex ecologic system & a vast number of organisms live in it. The greater proportions of these belong to plant life. Plants are the producter of organic matter, while the animals and microorganisms are its consumers & decomposers. Plants give two-thirds of the material needed for the formation of soil humus, and microorganisms give roughly one-third of it, and soil animals – just some decimal part of a metric ton. The larger animals of the soil are chiefly (1) rodents & insectivore, (2) insects, (3) millipedes, (4) sow bugs (wood lice), (5) mites, (6) slugs & snails, (7) centipedes, (8) spiders, & (9) earthworms. The activity of rodents results in the pulverization, granulation, & transfer of considerable quantities of soil. Insect – eating animals, especially moles are equally impotent in soil formation, though the activities of these animals are usually unfavorable to agricultural operations. Their burrows serve to aerate & drain the land. In some cases they so thoroughly disturb the various soil horizons as to make the original nature of soil profile difficult to ascertain.

Ants, beetles & springtails apparently affect the humus constituents either by translocation or by digestion. Millipedes, sowbugs, mites, slugs, & snails use more or less undecomposed plant tissue as food. They thus serve to initiate the decomposition processes that are continued by bacteria & fungi. Ch. Darwin wrote that "earth worms have played a much more important role in the history of the earth than the average person can think of". He compared their activity to that of a plow.

The “Lumbricus terrestris”, a reddish organism, & “Allolobephora caliginosa”, pale pinkish in color, are very common in Europe & in the U.S. The amount of soil that these creatures pass through their bodies annually "may amount to as much as 15 tons of dry earth per acre" - Ch. Darwin . Digestive enzymes enrich the earthworm casts with OH^- & Ca²⁺, make them less acid. The casts are definitely higher in OM, total & nitrate N^-, exchangeable Ca²⁺ & Mg²⁺, available P & the percentage base saturation. The holes left in the soil serve to increase aeration & drainage. They also mix & granulate the soil. They prefer a moist habitat, are found in medium and heavy soils rather than in those of a sandy & droughty nature, though they have certain lime – secreting glands. The earth worms feel better under mulches & residues.

Nematodes, threadworms or eelworms are found in almost all soils, often in surprisingly large numbers.

Protozoa probably are the simplest form of animal life. They are known to "make younger" microbial populations by eating up certain bacteria.

Most soil algae are chlorophyll – bearing organisms & must live at or very near the surface of the soil. There is some evidence that algae favor the activity of Azotobacter. The roots of higher plants are not only the source of OM. They influence the equilibrium of the soil solution & have something to do (in a direct way) with nutrient availability. Organic acids are formed at the root surfaces & hence become effective solvents.

The excretion of readily decomposable compounds such as amino acid as well as the sloughing of root tissue stimulate the microflora to intensive action.

Soil fungi are divided into three groups: (1) yeasts, (2) molds, & (3) mushrooms. Molds & mushrooms are considered important in soil. Molds play a role infinitely more important than the mushroom fungi. They are mostly favored by a lowered pH, and are noticeably abundant in acid soils. They are especially important in decomposing the organic residues of acid forest soils.

The mycelia of the fungi infest the plant roots giving an association called “mycorhyzae”, a term meaning «fungus root». The fungi in this respect can be ectotrophic & endotrophic. The first do not enter the calls. Endotrphic fungi actually penetrate the epidermal & cortex cells of the roots. The fungi secrete appropriate enzymes which permit the penetration of the plant to which the fungus is adapted. This association is of mutual benefit.

Soil bacteria are single-cell organisms, one of the smallest forms of life. They multiply rapidly by elongating & dividing into parts. The numbers of bacteria present in soil are variable. They fluctuate markedly with season. Many of them are able to produce spores. Most soil bacteria are heterotrophic. Their energy & carbon both come directly from the SOM. The general-purpose decay & ammonifying bacteria are all heterotrophic in character.

Nitrogen fixation. Two groups of bacteria participate in it: the nodule organisms, especially those of legumes & the so called free-fixing bacteria of several kinds.

The legume bacteria use the carbohydrates of their hosts as an energy source, fix the N & pass part of it on to the infected host. Some is left in the root tissue. This is likely to appear later in the soil.

The free-fixing soil bacteria acquire their energy from the SOM, fix the free N & make it a part of their own tissue. When they die and decay, ammonification & nitrification render at least a part of this N available to higher plants.

LECTURE TWO.

Soil as a medium for plant growth.