Compaction of the urban soil decreases total pore space and reduces
the proportion of large pores. It is within these pores that saturated water flow
occurs, as does most of the gaseous diffusion of oxygen and carbon dioxide
after the soil has been drained by gravity. Since water flow and gaseous
57diffusion are controlled by the least permeable (most compacted) horizon, even
a compacted subsoil horizon may affect water movement and aeration of the
entire profile. The nearer the least permeable horizon to the surface, the
greater the negative influence on plant growth and vigor. A compacted surface
horizon is extremely detrimental.
Covering the soil surface with impervious material cuts off water
infiltration and gaseous diffusion. These processes are confined to the
uncovered surface. Lateral movement of water, and especially gaseous
diffusion, is limited. Therefore, a tree placed in a pit surrounded by concrete or
asphalt underlain by compacted soil is supplied with very little natural
precipitation in summer, followed by too much water and too little oxygen in the
dormant season.
Confinement of water movement and gaseous diffusion by curbing, pipe
traces, foundation walls, subway and parking garage ceilings must also be
considered as being restrictive. Here again, planning, design, and
maintenance operations can overcome many of the difficulties.
Compaction of the urban soil decreases total pore space and reducesthe proportion of large pores. It is within these pores that saturated water flowoccurs, as does most of the gaseous diffusion of oxygen and carbon dioxideafter the soil has been drained by gravity. Since water flow and gaseous57diffusion are controlled by the least permeable (most compacted) horizon, evena compacted subsoil horizon may affect water movement and aeration of theentire profile. The nearer the least permeable horizon to the surface, thegreater the negative influence on plant growth and vigor. A compacted surfacehorizon is extremely detrimental.Covering the soil surface with impervious material cuts off waterinfiltration and gaseous diffusion. These processes are confined to theuncovered surface. Lateral movement of water, and especially gaseousdiffusion, is limited. Therefore, a tree placed in a pit surrounded by concrete orasphalt underlain by compacted soil is supplied with very little naturalprecipitation in summer, followed by too much water and too little oxygen in thedormant season.Confinement of water movement and gaseous diffusion by curbing, pipetraces, foundation walls, subway and parking garage ceilings must also beconsidered as being restrictive. Here again, planning, design, andmaintenance operations can overcome many of the difficulties.
การแปล กรุณารอสักครู่..