Bulk Density, Porosity, Percolation and Salinity Control in Shallow, Freely Draining, Potting Soils1

Abstract

Ten mixtures of soil, sphagnum peat moss, pinebark, almond hulls, and sheep manure, and 8 combinations of clay loam and sand, 20cm deep, were examined for relevant physical properties and possible use as greenhouse potting composts under Cypriot greenhouse conditions. There were significant variations of bulk density, porosity, moisture content, air space and percolation rates. Comparison of these data with other workers’ information showed a linear relationship between bulk density and total pore space regardless of specific gravity. The rate of salt removal, as measured by effluent conductivity, was predictable from a knowledge of the starting salinity concentration, regardless of the potting mixture. The higher the percolation rate, the less efficient was salt removal, with the removal rate decreasing as the initial salt concentration decreased. Percolation rate showed an exponential relationship with the ratio of porosity to maximum moisture content, with percolation increasing markedly above a ratio of 1.8. Mixtures in excess of 0.70cm3 cm−3 total porosity showed markedly high percolation rates, requiring large water volumes to reduce salinity. It was the opinion of these authors that very porous potting mixtures would unnecessarily increase water utilization in a semi-arid climate, with problems in controlling salinity with trickle irrigation systems likely to be exacerbated.

Leaching of Container Media

“…Except for percolation rate, the properties of the standard mixes were similar to those reported elsewhere (4,5,7,8,15,17). Percolation rates measured in this study ranged from 41 to 5436 cm hr"1, as compared to a previous high in the literature of 987 cm h r'1 for a mix of soil : 1 almond hulls : 1 bark (by volume) (8).…”

“…Except for percolation rate, the properties of the standard mixes were similar to those reported elsewhere (4,5,7,8,15,17). Percolation rates measured in this study ranged from 41 to 5436 cm hr"1, as compared to a previous high in the literature of 987 cm h r'1 for a mix of soil : 1 almond hulls : 1 bark (by volume) (8). A mix with a percolation rate of 41 cm hr"1 required 36 min for 500 ml of effluent to pass through a column, whereas a m ix w ith a percolation rate o f 5 4 3 6 cm hr'1 required only 0.3 min.…”

“…Electrical conductivity (EC) of the effluent was continuously recorded. (8). xAir porosity = total porosity -water porosity.…”

“…(total porosity) = 0.98 -0.362 (bulk density) [2] adapted from (8). Container capacity (17) was the maximum volume of water held by the medium in the column after drainage.…”

“…The miscible displacement theory recognizes the changes in the leaching process brought about by variation in the physical properties of the substrate (Item 1 above). To our knowledge, one study (8) has related leaching in greenhouse mixes to physical properties. Recommendations (3,10,11,16) for the leaching requirement of potting mixes appear inadequate.…”