Soil porosity and water retention
Posted: Fri Jul 17, 2009 3:36 pm
Today we have the advanced lesson, but I'll try to include some pictures
Porosity is the easy part. Total soil porosity is simply the percentage of the soil volume which is air. Here are some typical values for various soil components:
Peat: 80%-95%
Clay: 50%-70%
Loam: 40%-45%
Sand/grit: 25%-40%
Perlite: 80%-95%
Pumice: 70%-85%
Calcined clay: 60%-70%
Calcined diatomaceous earth: 70%-80%
One easy thing to see is that the maximum possible water content in a saturated soil is equal to the total porosity. A simplistic view is that a certain fixed amount of this "drains" and the soil is left holding a fixed amount of water for plants. The amount that drains is then replaced by air. This air percentage should typically be 10% or more for good plant health, preferably higher for succulents.
In fact the amount of water that is retained in soil depends on the pressure or suction being applied to the water, with more and more water draining away until a fixed amount remains even at extremely high pressures. This can be shown in a water retention curve. From left to right is shown the fraction of water in the soil by volume, and the vertical axis shows the pressure or suction on the water, so at the bottom is saturated soil and at the top is soil which is fully drained (but not dry).
Water which remains in soil at very high pressures cannot be extracted by plant roots. This pressure is called the wilting point because when soils reach this level of dryness then leafy plants will wilt. This is typically at about the 1,000 mark on the water retention graph. Note that although clay soils can hold a lot of water, a relatively large amount is unavailable to plants. In fact, certain loam soil mixtures typically hold the highest level of water available for plants. Coarse fluffy peats as shown on this graph also hold very large amounts of available water, although finer and more decomposed peats behave more like a clay soil.
In real soil, the pressure is applied by gravity and so water will drain to a certain point depending on the height of the water column. The water retention curve is often drawn with the pressure axis labelled as a height or depth and each unit on this axis corresponds to 10cm or 4". In open ground, typical average water retention corresponds to the 10kPa mark on the graph, but in a pot they correspond to somewhat below the 1kPa mark. You can see immediately why potting soils can get so soggy, because there simply isn't enough pressure to drain off a meaningful amount of water. Even pure sand doesn't drain off more than a few percent of the water, although coarse grits are better. Halfway through this document there is a comparison of different peat materials and you can see that fresh sphagnum moss peat would drain quite well in a pot, but older more decomposed peats are very poor:
http://nrs.fs.fed.us/pubs/jrnl/1968/nc_ ... er_001.pdf
You can see that some quite extraordinary substances are needed to allow more than 10% of the water to drain away from a pot. These substances are things like perlite and the various calcined mineral granules. Here is a graph comparing calcined diatomaceous earth granules (Axis and Isolite) with sand. The vertical portion of the sand curve to 10cm means there will be little drainage in a pot, although in open ground most of the water will drain away from sand. The sloped curves of the other diatomaceous earth products means there will be significant drainage even in a pot. Note also the high level of water which remains unavailable to plant roots and is only released very slowly by evaporation. Perlite has similar drainage behaviour, but doesn't strongly retain as much water and will become completely dry more quickly. Coarse grit also drains quickly in a pot but has a low total porosity and so is less able to retain both water and air.
In case it looks like hardly anything will provide drainage and so some air retention in a pot, there is a hysteresis effect in most soils. That is, they do not become completely saturated until a particular water pressure is applied for some time or until excess water pressure is applied to force the soil to become wet. You can read about this in detail, but the main thing to know is that the water retention curve is steeper as the soil is being watered and so contains more air than you might expect. Lengthy soaking from below, standing in water, or blocked drainage holes will overcome this effect.
Porosity is the easy part. Total soil porosity is simply the percentage of the soil volume which is air. Here are some typical values for various soil components:
Peat: 80%-95%
Clay: 50%-70%
Loam: 40%-45%
Sand/grit: 25%-40%
Perlite: 80%-95%
Pumice: 70%-85%
Calcined clay: 60%-70%
Calcined diatomaceous earth: 70%-80%
One easy thing to see is that the maximum possible water content in a saturated soil is equal to the total porosity. A simplistic view is that a certain fixed amount of this "drains" and the soil is left holding a fixed amount of water for plants. The amount that drains is then replaced by air. This air percentage should typically be 10% or more for good plant health, preferably higher for succulents.
In fact the amount of water that is retained in soil depends on the pressure or suction being applied to the water, with more and more water draining away until a fixed amount remains even at extremely high pressures. This can be shown in a water retention curve. From left to right is shown the fraction of water in the soil by volume, and the vertical axis shows the pressure or suction on the water, so at the bottom is saturated soil and at the top is soil which is fully drained (but not dry).
Water which remains in soil at very high pressures cannot be extracted by plant roots. This pressure is called the wilting point because when soils reach this level of dryness then leafy plants will wilt. This is typically at about the 1,000 mark on the water retention graph. Note that although clay soils can hold a lot of water, a relatively large amount is unavailable to plants. In fact, certain loam soil mixtures typically hold the highest level of water available for plants. Coarse fluffy peats as shown on this graph also hold very large amounts of available water, although finer and more decomposed peats behave more like a clay soil.
In real soil, the pressure is applied by gravity and so water will drain to a certain point depending on the height of the water column. The water retention curve is often drawn with the pressure axis labelled as a height or depth and each unit on this axis corresponds to 10cm or 4". In open ground, typical average water retention corresponds to the 10kPa mark on the graph, but in a pot they correspond to somewhat below the 1kPa mark. You can see immediately why potting soils can get so soggy, because there simply isn't enough pressure to drain off a meaningful amount of water. Even pure sand doesn't drain off more than a few percent of the water, although coarse grits are better. Halfway through this document there is a comparison of different peat materials and you can see that fresh sphagnum moss peat would drain quite well in a pot, but older more decomposed peats are very poor:
http://nrs.fs.fed.us/pubs/jrnl/1968/nc_ ... er_001.pdf
You can see that some quite extraordinary substances are needed to allow more than 10% of the water to drain away from a pot. These substances are things like perlite and the various calcined mineral granules. Here is a graph comparing calcined diatomaceous earth granules (Axis and Isolite) with sand. The vertical portion of the sand curve to 10cm means there will be little drainage in a pot, although in open ground most of the water will drain away from sand. The sloped curves of the other diatomaceous earth products means there will be significant drainage even in a pot. Note also the high level of water which remains unavailable to plant roots and is only released very slowly by evaporation. Perlite has similar drainage behaviour, but doesn't strongly retain as much water and will become completely dry more quickly. Coarse grit also drains quickly in a pot but has a low total porosity and so is less able to retain both water and air.
In case it looks like hardly anything will provide drainage and so some air retention in a pot, there is a hysteresis effect in most soils. That is, they do not become completely saturated until a particular water pressure is applied for some time or until excess water pressure is applied to force the soil to become wet. You can read about this in detail, but the main thing to know is that the water retention curve is steeper as the soil is being watered and so contains more air than you might expect. Lengthy soaking from below, standing in water, or blocked drainage holes will overcome this effect.