Water moves through soil due to the force of gravity, osmosis and capillarity. At zero to one-third bar suction, water moves through soil due to gravity; this is called saturated flow. At higher suction, water movement is called unsaturated flow. Water infiltration into soil is controlled by six factors: Soil texture Soil structure. Fine-textured soils with granular structure are most favourable to infiltration of water. The amount of organic matter. Coarse matter is best and if on the surface helps prevent the destruction of soil structure and the creation of crusts. Depth of soil to impervious layers such as hardpans or bedrock The amount of water already in the soil Soil temperature. Warm soils take in water faster while frozen soils may not be able to absorb depending on the type of freezing. Water infiltration rates range from 0.25 cm (0.098 in) per hour for high clay soils to 2.5 cm (0.98 in) per hour for sand and well stabilised and aggregated soil structures. Water flows through the ground unevenly, called “gravity fingers”, because of the surface tension between water particles.  Tree roots create paths for rainwater flow through soil by breaking though soil including clay layers: one study showed roots increasing infiltration of water by 153% and another study showed an increase by 27 times.  Flooding temporarily increases soil permeability in river beds, helping recharge aquifers.

The amount of water remaining in a soil drained to field capacity and the amount that is available are functions of the soil type. Sandy soil will retain very little water, while clay will hold the maximum amount. The time required to drain a field from flooded condition for a clay loam that begins at 43% water by weight to a field capacity of 21.5% is six days, whereas a sand loam that is flooded to its maximum of 22% water will take two days to reach field capacity of 11.3% water. The available water for the clay loam might be 11.3% whereas for the sand loam it might be only 7.9% by weight. Wilting point, field capacity, and available water capacity of various soil textures Soil Texture Wilting Point Field Capacity Available water capacity Water per foot of soil depth Water per foot of soil depth Water per foot of soil depth  % in.  % in.  % in. Medium sand 1.7 0.3 6.8 1.2 5.1 0.9 Fine sand 2.3 0.4 8.5 1.5 6.2 1.1 Sandy loam 3.4 0.6 11.3 2.0 7.9 1.4 Fine sandy loam 4.5 0.8 14.7 2.6 10.2 1.8 Loam 6.8 1.2 18.1 3.2 11.3 2.0 Silt loam 7.9 1.4 19.8 3.5 11.9 2.1 Clay loam 10.2 1.8 21.5 3.8 11.3 2.0 Clay 14.7 2.6 22.6 4.0 7.9 1.4 The above are average values for the soil textures as the percentage of sand, silt and clay vary within the listed soil textures.

The forces with which water is held in soils determine its availability to plants. Forces of adhesion hold water strongly to mineral and humus surfaces and less strongly to itself by cohesive forces. A plant’s root may penetrate a very small volume of water that is adhering to soil and be initially able to draw in water that is only lightly held by the cohesive forces. But as the droplet is drawn down, the forces of adhesion of the water for the soil particles make reducing the volume of water increasingly difficult until the plant cannot produce sufficient suction to use the remaining water. The remaining water is considered unavailable. The amount of available water depends upon the soil texture and humus amounts and the type of plant attempting to use the water. Cacti, for example, can produce greater suction than can agricultural crop plants. The following description applies to a loam soil and agricultural crops. When a field is flooded, it is said to be saturated and all available air space is occupied by water. The suction required to draw water into a plant root is zero. As the field drains under the influence of gravity (drained water is called gravitational water or drain-able water), the suction a plant must produce to use such water increases to 1/3 bar. At that point, the soil is said to have reached field capacity, and plants that use the water must produce increasingly higher suction, finally up to 15 bar. At 15 bar suction, the soil water amount is called wilting percent. At that suction the plant cannot sustain its water needs as water is still being lost from the plant by transpiration; the plant’s turgidity is lost, and it wilts. The next level, called air-dry, occurs at 1000 bar suction. Finally the oven dry condition is reached at 10,000 bar suction. All water below wilting percentage is calledunavailable water.

Water is retained in a soil when the adhesive force of attraction of water for soil particles and the cohesive forces water feels for itself are capable of resisting the force of gravity which tends to drain water from the soil. When a field is flooded, the air space is displaced by water. The field will drain under the force of gravity until it reaches what is called field capacity, at which point the smallest pores are filled with water and the largest with water and air. The total amount of water held when field capacity is reached is a function of the specific surface area of the soil particles. As a result, high clay and high organic soils have higher field capacities. The total force required to pull or push water out of soil is termed suction and usually expressed in units of bars (105pascal) which is just a little less than one-atmosphere pressure. Alternatively, the terms “tension” or “moisture potential” may be used.

Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth. Water is essential to plants for four reasons: It constitutes 85%-95% of the plant’s protoplasm. It is essential for photosynthesis. It is the solvent in which nutrients are carried to, into and throughout the plant. It provides the turgidity by which the plant keeps itself in proper position. In addition, water alters the soil profile by dissolving and redepositing minerals, often at lower levels, and possibly leaving the soil sterile in the case of extreme rainfall and drainage. In a loam soil, solids constitute half the volume, air one-quarter of the volume, and water one-quarter of the volume, of which only half will be available to most plants.

Soil resistivity is a measure of a soil’s ability to retard the conduction of an electric current. The electrical resistivity of soil can affect the rate of galvanic corrosion of metallic structures in contact with the soil. Higher moisture content or increased electrolyte concentration can lower resistivity and increase conductivity, thereby increasing the rate of corrosion. Soil resistivity values typically range from about 2 to 1000 Ω·m, but more extreme values are not unusual.

Soil colour is often the first impression one has when viewing soil. Striking colours and contrasting patterns are especially noticeable. The Red River (Mississippi watershed) carries sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma. The Yellow River in China carries yellow sediment from eroding loess soils. Mollisols in the Great Plains of North America are darkened and enriched by organic matter. Podsols in boreal forests have highly contrasting layers due to acidity and leaching. In general, colour is determined by organic matter content, drainage conditions, and the degree of oxidation. Soil colour, while easily discerned, has little use in predicting soil characteristics. It is of use in distinguishing boundaries within a soil profile, determining the origin of a soil’s parent material, as an indication of wetness and waterlogged conditions, and as a qualitative means of measuring organic, salt and carbonate contents of soils. Colour is recorded in the Munsell color system as for instance 10YR3/4. Soil colour is primarily influenced by soil mineralogy. Many soil colours are due to various iron minerals. The development and distribution of colour in a soil profile result from chemical and biological weathering, especially redox reactions. As the primary minerals in soil parent material weather, the elements combine into new and colourful compounds. Iron forms secondary minerals of a yellow or red colour, organic matter decomposes into black and brown compounds, and manganese, sulfur and nitrogen can form black mineral deposits. These pigments can produce various colour patterns within a soil. Aerobic conditions produce uniform or gradual colour changes, while reducing environments (anaerobic) result in rapid colour flow with complex, mottled patterns and points of colour concentration.

Soil temperature regulates seed germination, root growth and the availability of nutrients. Soil temperatures range from permafrost at a few inches below the surface to 38°C (100°F) in Hawaii on a warm day. The colour of the ground cover and its insulating ability have a strong influence on soil temperature. Snow cover will reflect light and heavy mulching will slow the warming of the soil, but at the same time they will reduce the fluctuations in the surface temperature. Below 50 cm (20 in), soil temperature seldom changes and can be approximated by adding 1.8°C (2°F) to the mean annual air temperature. Most often, soil temperatures must be accepted and agricultural activities adapted to them to: maximize germination and growth by timing of planting optimise use of anhydrous ammonia by applying to soil below 10°C (50°F) prevent heaving and thawing due to frosts from damaging shallow-rooted crops prevent damage to desirable soil structure by freezing of saturated soils improve uptake of phosphorus by plants Otherwise soil temperatures can be raised by drying soils or the use of clear plastic mulches. Organic mulches slow the warming of the soil.

Consistency is the ability of soil to stick together and resist fragmentation. It is of rough use in predicting cultivation problems and the engineering of foundations. Consistency is measured at three moisture conditions: air-dry, moist and wet. The measures of consistency border on subjective as they employ the “feel” of the soil in those states. A soil’s resistance to fragmentation and crumbling is assessed in the dry state by rubbing the sample. Its resistance to shearing forces is assessed in the moist state by thumb and finger pressure. Finally, a soil’s plasticity is measured in the wet state by moulding with the hand. The terms used to describe a soil in those three moisture states and a last state of no agricultural value are as follows: Consistency of Dry Soil: loose, soft, hard, extremely hard Consistency of Moist Soil: loose, friable, firm, extremely firm Consistency of Wet Soil: non-sticky, sticky or non-plastic, plastic Consistency of Cemented Soil: weakly cemented, indurated (cemented) Soil consistency is useful in estimating the ability of soil to support buildings and roads. More precise measures of soil strength are often made prior to construction.

Pore space is that part of the bulk volume that is not occupied by either mineral or organic matter but is open space occupied by either air or water. Ideally, the total pore space should be 50% of the soil volume. The air space is needed to supply oxygen to organisms decomposing organic matter, humus, and plant roots. Pore space also allows the movement and storage of water and dissolved nutrients. There are four categories of pores: Very fine pores: < 2 microns Fine pores: 2-20 microns Medium pores: 20-200 microns Coarse pores: 200 microns-0.2 mm In comparison, root hairs are 8 to 12 microns in diameter. When pore space is less than 30 microns, the forces of attraction that hold water in place are greater than those acting to drain the water. At that point, soil becomes water-logged and it cannot breathe. For a growing plant, pore size is of greater importance than total pore space. A medium-textured loam provides the ideal balance of pore sizes. Having large pore spaces that allow rapid air and water movement is superior to smaller pore space but has a greater percentage pore space. Tillage has the short-term benefit of temporarily increasing the number of pores of largest size, but in the end those will be degraded by the destruction of soil aggregation.