4.Soil Water Regime.

The soil water regime expresses the gradual changes in the water content of the soil. During the course of the year, the major changes in soil water content include: recharge of dry soil, surplus water and leaching, and depletion of stored water. Water deficits occur when water loss from the soil by evapotranspiration exceeds the precipitation plus stored water for a period of time.

The field-water balance is like a financial statement of income and expenditures. In it’s simplest form, the water balance merely states that, in a given volume of soil the difference between the amount of water added Win and the amount of water with-drawn Wont during a certain period is equal to the change in water content W during the same period:

W=Win – Wout.

Rain or irrigation water applied to the land may. In some cases infiltrate into the soil as fast as it arrives. In other cases, some of the water may pond over the surface. Depending on the slope and micro relief, a portion of this water may exit from the area as surface runoff while the remainder will be stored temporarily as puddles in surface depressions. Some of the latter evaporates and the rest eventually infiltrates into the soil. Of the water infiltrated, some evaporates directly from the soil surface, some is taken up by plants for growth or transpiration, some may drain downward beyond the root zone, where as the remainder accumulates within the root zone and adds to soil moisture storage. From an agricultural or plant ecological point of view, it is generally most appropriate to consider the water balance of the root zone per unit area of field:

(S+V)=(P+I+U)-(R+D+E+T),

wherein S - is change in root zone soil moisture storage, V, increment of water incorporated in the plant, I irrigation, U upward capillary flow into the root zone, R runoff, D downward drainage out of the root zone, E direct evaporation from the soil surface, and Transpiration by plants. The change in root zone soil moisture storage can be obtained by integrating the change in soil wetness over depth and time as follows:

,

where  is the volumetric soil wetness.

The field water balance may seem simple and readily understandable, but it is still rather difficult to measure in practice.

6. Soil Water Management.

In Ukraine it is customary to classify water regimes taking into account the ration between precipitation and evaporation K and water table depth. There are such water regime groups: percolative, periodically percolative, impercolative, evaporative, and stagnant.

Some other groups may be added to this list. But what in really important to any agronomist is a soil water management practice.

There are three basic approaches to water management on agricultural land: (1) conservation of natural precipitation in subhumid and arid regions, (2) removal of water from wetlands, and (3) addition of water to supplement the amount of natural precipitation (irrigation). There are ways to modify the infiltration rate. Surface soils protected by vegetation in a natural forest or grass land tend to have a higher infiltration rate during rains than exposed soils in cultivated fields. Most of the world’s wheat production occurs in region having a subhumid or semi-arid climate. Here, a practice called summer fallowing is used. No crop is grown and all vegetable growth is prevented by shallow tillage or herbicides for a period of time, to store water for use by the next crop. Fertilizer use may increase the efficiency of water use by plants by increasing the rate of plant growth.

Removal of water by drainage provides an aerated root zone for crops. Irrigation is indispensable in many places. But if irrigation water contains soluble salts, some provision (e.g. drainage) must be made to maintain a favorable salt balance to keep irrigated soils permanently productive. The accumulation of exchangeable sodium, greater than about 15% of the adsorbed cations, results in the formation of sodic soils. High sodium saturation results in a deterioration of soil structure and loss of soil water permeability. Sodic soils can be improved by applying gypsum and then leaching the soil can improve sodic soils. Wastewater disposal is also important. Some environmentally acceptable water disposal methods (techniques) are: (1) treatment and discharge into surface water, (2) treatment and reuse, and (3) land disposal via septic tanks and spray irrigation. The personnel must be well trained!

LECTURE EIGHT.

Soil solution and redox processes.

Soil air and aeration regime.

Study outline:

1. Soil solution, a liquid phase of the soil.

1.1. Composition and concentration of soil solution.

1.2. Osmotic pressure of soil solution.

1.3. Redox potential and redox processes in the soils.

2. Soil air, a gaseous phase of the soil.

2.1. Soil air composition and properties.

2.2. Plant requirements to soil aeration.

3. Management of soil redox and aeration regimes.

1. Soil solution, a liquid phase of the soil.

Any amount of water in the soil can not remain pure water, as it interact with both solid and gaseous phases and dissolves their components. Soil solution is a complex system, which being dynamic and active takes part in the processes of soil formation, various biochemical and chemical reactions, and in plant nutrition. The most important among the properties of soil solution are concentration, osmotic

pressure, reaction, buffer capacity, and redox potential.