Water and soil - how it works

Soil Water Storage
The soil water storage or soil water content can be quantified on the basis of its volumetric or gravimetric water content. The volumetric water content is the volume of water per unit volume of soil, expressed as a percentage of the volume. The gravimetric water content is the mass of water per unit mass of dry (or wet) soil. The volumetric water content is equal to the gravimetric water content times the soil's bulk density (on a dry soil basis).
Factors that affect the soil water storage are:

• Total Porosity or Void Space
• Pore-size and Distribution and Connectivity
• Soil Water Pressure Potential or Energy Status of the Soil Water

The total porosity or void space ultimately establishes the upper limit of how much water can be stored in a given volume of soil. When all the pores are filled with water the soil is saturated, and cannot store any more water. The total porosity is a function of the soil's particle size, particle uniformity and packing or structure because the void space that remains between the solid particles determines the extent and distribution of pore sizes and their connectivity.
If one fills the same volume with sand and clay sized particles, the total porosity of the clay is somewhat higher, about 50-55% of the volume compared to about 35-40% for sand. The spaces between the sand particles will have larger voids, but there will be fewer of them. The total porosity of medium textured loamy soils is generally around 50% because the smaller silt and clay particles fill some of the voids between the larger sand particles. Soils with good structure will have somewhat higher total porosity than soil that has been compacted (i.e., where the soil particles are forced closer together).
The important influence of pore-size and distribution on soil water storage is in regards to how different pore sizes respond to energy forces or the soil water pressure potential. Under saturated conditions, large pores drain more easily in response to gravity potential. Also, when the soil is unsaturated, large pores are less subject to capillary (or matric potential) forces. In unsaturated soil conditions, the soil water pressure potential becomes negative (suction), and the degree to which this occurs greatly influences the soil water storage (retention) or water content in different sized pores.
The soil water characteristic (retention) curve defines the relationship between the soil water pressure potential or energy status (matric or suction potential) and the soil water content. It's important to note that soil water moves in direct response to the energy or pressure potential forces acting upon it (i.e., moving from a higher to lower energy status), and not necessarily in response to different soil moisture contents (i.e., from higher to lower soil moisture content).
The soil water characteristic curve(s) and definitions are used to establish and further refine and quantify the general availability of soil water which is often referred to as (1) gravitational water(water subject to drainage), (2) capillary water (water available to plants), and (3) hygroscopic water (water that is not available to plants). The following figure shows general soil water characteristic curves for various soil types.
Differences in soil water pressure potentials from one point to another in the soil and throughout the larger landscape determine how water will move. For water movement in soil, the water table is used as a convenient reference because below the water table the total porosity of the soil is saturated, and above the water table, the soil porosity is unsaturated (the soil water content is less than the total porosity).
The water table is defined as the upper surface of groundwater (saturated zone) or that level in the ground below the soil surface where the water is at (and in equilibrium with) atmospheric pressure. At the water table reference, the pressure potential is set equal to zero. Thus, below the water table, the pressure potential becomes positive, and above the water table the pressure potential becomes negative. This negative pressure in unsaturated soil is termed matric, tension or suction pressure potential so as not to confuse it with positive pressures.

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image source: NRCCA Soil and Water Management Study Guide​

 
Field Capacity
The field capacity is the amount of water remaining in the soil a few days after having been wetted and after free drainage has ceased. The matric potential at this soil moisture condition is around - 1/10 to – 1/3 bar. In equilibrium, this potential would be exerted on the soil capillaries at the soil surface when the water table is between 3 to about 10 feet below the soil surface, respectively. The larger pores drain first so gravity drainage, if not restricted, may only take hours, whereas in clay soils (without macropores); gravity drainage may take two to three days. The volumetric soil moisture content remaining at field capacity is about 15 to 25% for sandy soils, 35 to 45% for loam soils, and 45 to 55% for clay soils.
Permanent Wilting Point
The permanent wilting point is the water content of a soil when most plants (corn, wheat, sunflowers) growing in that soil wilt and fail to recover their turgor upon rewetting. The matric potential at this soil moisture condition is commonly estimated at -15 bar. Most agricultural plants will generally show signs of wilting long before this moisture potential or water content is reached (more typically at around -2 to -5 bars) because the rate of water movement to the roots decreases and the stomata tend to lose their turgor pressure and begin to restrict transpiration. This water is strongly retained and trapped in the smaller pores and does not readily flow. The volumetric soil moisture content at the wilting point will have dropped to around 5 to 10% for sandy soils, 10 to 15% in loam soils, and 15 to 20% in clay soils.
Available Water Capacity
The total available water (holding) capacity is the portion of water that can be absorbed by plant roots. By definition it is the amount of water available, stored, or released between field capacity and the permanent wilting point water contents. The average amount of total available water in the root zone for a loam soil is indicated by the area between the arrows in the table on page 13.
The soil types with higher total available water content are generally more conducive to high biomass productivity because they can supply adequate moisture to plants during times when rainfall does not occur. Sandy soils are more prone to drought and will quickly (within a few days) be depleted of their available water when evapotranspiration rates are high. For example, for a plant growing on fine sand with most of its roots in the top foot of soil, there is less than one inch of readily available water.
A plant transpiring at the rate of 0.25 inches per day will thus start showing stress symptoms within four days if no rainfall occurs. Shallow rooted crops have limited access to the available soil water, and so shallow rooted crops on sandy soils are particularly vulnerable to drought periods. Irrigation may be needed and is generally quite beneficial on soils with low available water capacity.
Total Soil Water Storage Capacity
The total soil water storage capacity refers to when all the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded. A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels will have the lowest total porosity of around 30 to 40% by volume. Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40 to 60%. Restricted drainage conditions can cause the soil to attain its total porosity water content, at which time free water is observed and perched water tables develop (in layered soils) or the apparent water table is found near the surface.
When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is severely restricted. Most agricultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen diffusion to the roots. Without air-filled pores, the concentration of carbon dioxide and other gases like ethylene increase, producing toxic conditions and limiting plant growth. Root cells switch to anaerobic respiration, which is much less efficient than aerobic respiration in converting glucose molecules to ATP (adenosine triphosphate, the chemical energy within cells for metabolism and cell division).
As anaerobic (reduced) conditions develop in the soil, nitrification ceases and denitrification is enhanced. Corn plants will quickly yellow in response to this saturated soil state as nitrogen becomes limiting, and the plant tries to adjust by producing more adventitious roots. Prolonged anaerobic conditions in the soil starts to reduce manganese, iron (causing phosphorus to be more soluble), sulfur (producing hydrogen sulfide), and eventually methane gases. Hydrophytic (wetland type) plants are adapted to saturated soils because they are able to obtain oxygen through other forms of plant structure adaptations (i.e. pneumataphores, lenticels, aerenchyma).
Drainable Porosity   
The drainable porosity is the pore volume of water that is removed (or added) when the water table is lowered (or raised) in response to gravity and in the absence of evaporation. Consider a soil that is saturated with the water table at the surface. If this soil has a subsurface drainage pipe (tile) buried several feet down and it is discharging to the atmosphere at some lower elevation, the drainable porosity water content will be released to the tile drain until the water table is lowered to the depth of the drain.    
Any nutrients or pesticides dissolved or suspended in this readily drainable pore space will also be carried along with this water, either flowing to the tile drain or continuing downward to the water table via deep percolation if no drainage restriction exists. In large pores, nutrients that might otherwise adsorb to the soil particles (ammonium or phosphate) will bypass the soil because of limited time for contact and chemical reactions to occur with the soil surface area. Soils with a wide range of different pore sizes (sandy loams) or soils with mostly small sized pores are better at filtering nutrients and pesticides as they leach through the soil profile. 
The combined aspect of low available water holding capacity and high drainable porosity for sandy soils causes these soils to have a high leaching potential. It will not take much rain or irrigation (or application of liquid manure) to replenish the available soil water and to raise the soil water content to a drainable state. Applying the proper amount (depth) of irrigation to these soils will both conserve water and enhance irrigation and nutrient use efficiency.
Macroporosity/Preferential Flow
Macropores refers to those soil pores through which water flows primarily in response to gravity. Macropores occur in coarse sands and gravels, soil structural cracks, or may form as the result of worm holes, other small burrowing microorganisms, decaying roots, and some tillage operations. Since water can be infiltrated quickly and flows rapidly downward in macropores, it is also termed preferential flow.
The significance of macropores and preferential flow is that nutrients and other dissolved and suspended substances can be rapidly transported down past the root zone without substantial filtration or other biochemical remediating interactions. Although the magnitude of macroporosity in soils is generally small, when only a small concentration amount of a nutrient, pesticide or other contaminant creates great risk to water quality, the environmental threat may still be significant. Macroporosity is generally beneficial to air and water exchange, soil health, and to providing more optimum conditions for plant growth, but it has also lead to water quality impacts when dissolved and suspended materials are transported to tile drain outlets or groundwater.
Since water flow into and through macropores is most prevalent when soils are already in their wettest state, avoidance of applying potential contaminants during this time and prior to rainfall is one method of minimizing unwanted impacts. Applying nutrients or other materials at lower rates will also reduce concentrations of contaminants occurring in preferential flow.
Source: http://nrcca.cals.cornell.edu/soil/CA2/index.php

 

 
The source website is correct. It is a much larger course with a focus on agriculture vs. horticulture. I took what I felt where the most pertinent to horticulture and soil building.
 

 
From another site. Many thanks to the author, this synopsis of Dr. Koranski's work made it all click for me regarding irrigation. Not such a simple subject. We watch plenty of new growers saturate the soil and kill their plants.
 
I use a combination of sub irrigation and the old "lift the pot" method of determining moisture levels while always keeping this in the back of my mind.
 
Consider your soil in this style, originally adopted by Dr. David Koranski;

Moisture Levels 1 = Dry , 5 = Wet;

5. Substrate is totally saturated with water, if a small portion of soil is placed on a clean surface or a sheet of paper, it leaves a clearly visible wet spot, even when no pressure is applied.

4. The substrate is moist but doesn't contain any "free" water. It appears wet and droplets can be squeezed out without very much effort. It does not however, leave any wet spots when placed on a clean surface without pressure.

3. The media will change colour from dark or black to light and tan-ish, containing visibly less moisture. Water droplets can still be squeezed out of the soil, but a considerable amount of force is required to do so.

2. The media is tan or light in colour and does not contain any visible moisture. No water can be squeezed from the soil with any amount of pressure. The crop is under water stress, but does not wilt just yet. This is the absolute driest we should take most crops to without fear of permanent damage.

1. The substrate is totally dehydrated, crop is experiencing severe or serious water stress and turgor pressure is lost resulting in wilt. Sensitive crops will often be permanently damaged by these conditions.

When soil reaches level ~2.5 irrigate and hydrate it to the upper end of level 3 to the lower end of level 4.

Apply your irrigation in several smaller "passes" instead of just straight dumping it on the soil. For example; If you are applying 1 litre per container, apply 1 cup to each container and repeat 4 times to allow correct dissipation and permeation without displacing an undue portion of oxygen. This allows you to also make a distinction between a complete hydration and a touch up.
 
HTH

 

 
You mean besides running (5) 35 gallon no-till geoplanter SIPs myself?
 
http://www.insideurbangreen.org/sub-irrigation-history/
 
The rise of water in small bore tubes or soil pours is called capillary action. To explain capillary action, one must consider cohesion, adhesion, and surface tension. Cohesive forces result from attraction between like molecules. Water molecules exhibit cohesion because they form hydrogen bonds with each other. Adhesive forces result from attraction between unlike molecules. Water adheres to a solid surface if the surface molecules form hydrogen bonds with water. Such a surface is called hydrophilic because it gets wet easy. Adhesive forces between water and the solid surface or greater than cohesive forces between water molecules. Water will bead on a hydrophobic surface and run off for cohesion is stronger than adhesion. For example, raw wood wets readily, but a coat of paint causes wood to shed water. The net cohesive force on molecules in a body of water is zero, because the cohesive forces cannot act above the surface, and molecules of the surface layer are subjected to an inward cohesive force from the molecules below the surface. The surface molecules act as a skin over the surface to provide surface tension, which permits insects and spiders to walk over the surface of water, and needles and razor blades to float when gently placed on the water surface. The strength of the surface film decreases with increasing temperature, and increases when electrolytes are added to water. Soap and most other organic substances decrease surface tension when dissolved in water.
 
Water rises to considerable heights in a thin glass tube or in clay or fine-silt soils. Capillary action is the combined effect of surface tension, adhesion, and cohesion. In a thin tube, water adheres to its walls and spreads upward over as much surface as possible. Water moving up the wall is attached to the surface film, and molecules in the surface film are joined by cohesion to the molecules below.as adhesion drags the surface film upward, it pulls a column of water up the tube against the force of gravity. Of course the column of water below the surface film is under tension because the water pressure is less than the atmospheric pressure.capillary rise is inversely proportional to the tube diameter or soil for size. Space exists among soil particles because they do not fit together perfectly. This space can function in as much the same manner as the thin glass tube, and permits water to rice and a dry soil.capillarity and soil increases in the following order: sand < silt < silty clay < clay. Of course if capillary tube walls or soil for size are hydrophobic, capillary action can be down word instead of upward.
 
Source: Hydrology and water supply pond aquaculture by Kyung H. Hoo
 
To further assist in the molecular reaction is the plant itself. 
 
Cohesion-tension theory
The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity) through the xylem of plants. It was proposed in 1894 by John Joly and Henry Horatio Dixon. Despite numerous objections, this is the most widely accepted theory for the transport of water through a plant's vascular system based on the classical research of Dixon-Joly (1894), Askenasy (1895), and Dixon (1914,1924).
Water is a polar molecule. When two water molecules approach one another, the slightly negatively charged oxygen atom of one forms a hydrogen bond with a slightly positively charged hydrogen atom in the other. This attractive force, along with other intermolecular forces, is one of the principal factors responsible for the occurrence of surface tension in liquid water. It also allows plants to draw water from the root through the xylem to the leaf.
Water is constantly lost through transpiration from the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and tension. Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves forces water to move into them.
Transpiration in leaves creates tension (differential pressure) in the cell walls of mesophyllcells. Because of this tension, water is being pulled up from the roots into the leaves, helped by cohesion (the pull between individual water molecules, due to hydrogen bonds) and adhesion (the stickiness between water molecules and the hydrophilic cell walls of plants). This mechanism of water flow works because of water potential (water flows from high to low potential), and the rules of simple diffusion.
Over the past century, there has been a great deal of research regarding the mechanism of xylem sap transport; today, most plant scientists continue to agree that the cohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.
Source: http://en.m.wikipedia.org/wiki/Xylem

HTH

 

 
Haha! Finally got some free time from work. I've seen a formula to use with tensiometers, but without one IDK. Given the research above we can assume that peat based soil holds 70-80% of Total soil water storage capacity. I know when I fill my SIPs it's more like 50% but I'm not going for total saturation.
 

 
I'd be right there with you waktoo. I guess the real question for me would be where to start measuring. Is your soil completely hydrophobic (permanent wilting point)?

 

 
sbgrowery, I'm not sure why your using nutes with a new transplant. My plants frequently look droopy before lights out, they know their schedule. It could be that the peat wasn't hydrated properly prior to mixing soil. Did you let the soil dry out while cycling? Are you running your pots from wet to dry? If the peat mix is hydrophobic you can bring it back by slowly hydrating over several passes. If you have aloe vera a 1/4 cup per gallon h20 probably wouldn't hurt. I'm glad your coco is draining better.

 

 
I'm glad you got it solved. Regarding a dry period
 
From post #4
 
"When soil reaches level ~2.5 irrigate and hydrate it to the upper end of level 3 to the lower end of level 4."
 
No dry period.