in the fine porosity of soil aggregates (Kirda et al., 1973; van

Genuchten and Wierenga, 1977; Rao et al., 1980a).

 Considerable evidence has shown that gravel contents above 10 to

20% by weight have deleterious effects on root development and soil

penetration (Babalola and Lal, 1977a and 1977b; Vine and Lal, 1981).

Regardless of gravel hydrologic properties, the gravel limits rooting

depth and, therefore, limits the volume of soil from which the plant can

extract immobile nutrients.

 In light of the previously mentioned considerations, any research

effort with respect to the development of management practices may be

most productive if the research is designed to determine the combined

behavior of the processes and their combined effects on soil

productivity, instead of investigating separately the numerous

interdependent processes.