2. Managing soil structure.

Excessive and improper tillage may seriously impair soil structure. Very often the advantages of the moldboard plow are not so obvious. Disking and some other types of cultivation may be as good or even better. With the inter tilled crops, cultivation should be kept at a minimum not to hasten the decline of tilth.

Practical management of soil structure is generally restricted to the topsoil or promotes more rapid in filtration of water during storms and result in reduced water runoff and soil erosion and greater water storage within the soil. The permanence of peds depends on their ability to retain their shape after being subjected to the disruptive effects of raindrops and tillage. Microbial gum is the most important in promoting ped formation. Soil management practices with frequent additions of organic matter to the soil, as plant residues, or manure will tend to produce more microbial gum and increase pet formation and stability. Calcium added to the soil with chemical amendments (lime or gypsum) is a sound contribution to the formation of soil structure.

3. Particle density and bulk density.

Particle density is the average density of the soil particles (solid phase). Bulk density is the density of the bulk soil in its natural state, including both the particles and pore space.

Particle density is determined by the composition of the solid phase. A soil is composed of mineral and organic particles of varying composition and density. Quarts, a common soil mineral, has a density of 2.65 g/cm³. Feldspars, though not so common, being weathered, have density ranging from 2.56 g/cm³ to 2.76 g/cm³. Soil organic matter's density is 1.60-1.80 g/cm³. The particle density for mineral soils is within 2.40-2.80 g/cm³. Soil cores are collected to determine soil bulk density. They must be representative of field conditions. A sample of soil has the undisturbed density and pore space. Core must be exercised in the collection of soil cores so that the natural structure is preserved. The bulk density is the mass per unit volume of oven dry soil, calculated as follows:

Bulk density = mass oven dry/volume.

The bulk density of a soil is inversely related to porosity. Soils without structure have a bulk density of about 1.6 to 1.7 g/cm³. Development of structure results in the formation of pore spaces between peds. The bulk density of Ukrainian soils heavier then loamy sand are within 1.0 to 1.4 g/cm³.Sandy soils may have their bulk density up to 1.5 g/cm³ and even higher. Optimal bulk density for most crops is within 1.1 to 1.3 g/cm³.

4. Soil porosity and aeration porosity.

Soils have about 50% porosity by volume. Total soil porosity is calculated by the formula:

According to N.A. Kachinsky’s estimation, the values over 65% are excessively high : 65-55% : excellent for the arable layer: 55-50% - satisfactory for the arable layer and <50% - non-satisfactory for the arable layer. For most sandy and loam-sandy soils their total porosity may be too low to be sufficient for most crops.

Porosity is important for soil aeration. Porosity determined for the wet soil is called aeration porosity. It should not be below 15%V at field capacity. Aeration porosity at field capacity when soil capillaries are filled with water can be calculated by the formula:

AP = TSP – FIELD CAPACITY x BULK DENSITY.

Any multiplication of soil wetness (expressed in % by weight of oven dry soil) by the bulk density yields soil wetness in % by volume (%V). This simplified calculation goes well for the soils which do not swell too much on wetting (light textured soils, poor in clay). A well-structured porous soil is always desirable for plant growth as it has a good balance between the retention of water for plant use and an oxygen supply for root respiration. Oxygen deficiencies are created when soils become water saturated. Tomatoes and peas are very susceptible to oxygen deficiency. Some plants are able to make some adjustment. Plants growing on flooded soils or in swamps have special mechanisms for obtaining oxygen.