SOIL TILTH - THE DESIRED SOIL STRUCTURAL STATE OBTAINED THROUGH PROPER SOIL FRAGMENTATION AND REORIENTATION PROCESSES

Soil tilth is a term used to describe a given soil structural state an d its direct and indirect effects on the physical, chemical and biolog ical processes occurring in the soil. Although the concept dates back to the initiation of agricultural activity, neither a clear definition nor a useful, quantitative index has been developed for it. Therefore , the term expresses a range of meanings from a simple, descriptive re ference of a given soil structural state, to very complex, qualitative indices, which are sometimes, correlated to crop yields. The treatise presented here attempts to: (a) evaluate the interactions between cha nging soil structural states and the affected soil physical and mechan ical properties; and (b) assess the possibilities of matching crop req uirements and expected yields with the potential for controlling soil environment by tillage-induced soil structure modification. It is show n that an attempt to satisfy seasonal variations of phenological and c limatic affected crop needs by tillage is very difficult. Tillage-indu ced soil structure instability, seasonal variability of weather condit ion and mechanized field activity may cause soil structure deteriorati on and impairment of soil physical conditions. As a result, the abilit y to control soil structure diminishes as the crop grows. Thus, the 'd esired' soil structural state attained is a practical compromise solut ion, which matches expected future crop needs, weather conditions and soil structure changes with soil-tilling capabilities. Several 'desire d' soil structural states may be considered for a given set of future crop needs, expected variations in climate and soil conditions. The ob jective is to evaluate and improve our current understanding of the in teractions mentioned above, and to enable a knowledgeable, quantitativ e selection of a 'desired soil structure' management. Several potentia l research directions are indicated, which may lead to the improved un derstanding of the processes controlling: (a) the soil structure state s and soil fragmentation; (b) the interdependencies between the natura l soil structure state prior to tillage and the stability of the vario us soil structural states produced. and (c) the crop's root system res ponse to the initial state and seasonal changes in soil structural sta tes. It is shown that the current specifications of 'desired soil stru ctural states' and means to attain them are empirically based rather t hen being based on estimated probable future climatic variations. seas onally changing crop needs and deterioration rates of soil structure. (C) 1997 Elsevier Science B.V.