Re: What type of soil is the most porous?

You seem to be asking two different questions. When dealing with 
soils, "porous" usually refers to how fast water enters a soil. Water enters 
sands faster than it does other soil types because the pores in sand are 
large. However, sands have less total pore space by volume, or porosity, than 
clays, loams or silts. 

Here are some soil porosities, by volume, (Hausenbuiller 1972):

Sand: 	     42%
Loam: 	     55%
Silt loam:   56%
Clay: 	     60%

Of clay, silt and sand, clay soils hold the most water. Loam soils, a mix of 
sand, silt and clay also hold a lot of water and may actually have more 
available water for a plant. Soils vary so even two loams may hold different 
amounts of water. Here are some data from a soil science textbook 
(Hausenbuiller 1972).

Clay 			40% water by volume, 15% available water
Sandy loam 		 7% water by volume,  4% available water
Silt loam 		32% water by volume, 22% available water

A good loam soil contains by volume roughly 50% solids, 25% water and 25% air 
after it has been irrigated and drainage has stopped. This is termed field 
capacity.

In a pot, the same soil may have 50% solids and almost 50% water by volume 
after irrigation and drainage. This is termed container capacity. Therefore, 
the water content varies widely depending on whether the soil is in the field 
or in a pot. 

Potting soils or potting media in a shallow container may have 70 to 80% water 
by volume at container capacity. 

This 
soil textural triangle calculates the percentage water at field capacity, 
saturation (total porosity) and available water when you fill in the percent 
by weight of sand and clay in the soil. 


References

Hausenbuiller, R.L. 1972. Soil Science: Principles and Practices. Dubuque, IA: 
Wm. C. Brown.

Hershey, D.R. 1990. Container-soil physics and plant growth. BioScience 40:685-
686.


FUNDAMENTALS OF CONTAINER MEDIA MANAGEMENT: PHYSICAL PROPERTIES