ABSTRACT
Compaction and fragmentation actions of agricultural machines generally don’t affect the whole volume of Ap horizon. The result is a greater spatial variability of soil structure within the ploughed layer than in untilled horizons, leading to some difficulties to characterize, at field, soil physical conditions.
The method presented here consists of: (i) drawing up a vertical and lateral partition of soil profile, taking into account the depth of action of tillage tools (which definite several anthropic horizons) and the positions where tractor wheels have passed or not; (ii) describing one or several morphological units in each zone then being defined.
Soil tilth in a morphological unit is defined by the combination of 2 criteria: (i) visible porosity of clods: “Δ” is compact, containing only textural pores; “Γ” contains structural pores, aggregates are visible in clods; “Ф” derives from “Δ” but contains cracks; (ii) the way the clods are brought together: 4 types can be defined (“M” when massive, “SD” when clods are very closely packed together, “SF” when they are adherent but can be individualized, “F” when they are not adherent – diameter of clods is noted in F and SF).
A soil tilth map can then been drawn up in each anthropic horizon: it is a usefull guide for physical measurements in such heterogeneous horizons, and for soil-roots interactions studies. This map can be synthetized: (i) evaluation of “Δ” frequency, (ii) 3 main types of horizons are distinguished according to domination of F and SF – with small clods – (“0” type), M and SD (“C” type), M and F – with decimetrical clods separated by large voids – (“B” type).
This approach leads to an understanding of soil structure formation, as a result of climatic agents and agricultural practices interactions; particularly, recent and cumulative compacting effects on soil can be distinguished. “Δ” frequency (which can be obtained also by densimetric screening) is a global index of crop rotation and machines effects on soil structure, in interaction with climate and texture.
