Bugeye
Well-Known Member
Soil water retention isn't exactly a hot topic, but since my search here on the topic did not yield results, I am posting this information.
I am toying with adding slow drip reservoirs for my 65 gallon smart pots and wasn't really sure how much water they would hold when filled with a recycled super soil mix (approx. 20% top soil, 20% compost/worm casings, 20% perlite, 20% coir, 20% organic amendments ).
I researched water holding capacity of soil and learned some interesting terms. Here are some basic definitions that I have taken from the following website: http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447039&topicorder=10&maxto=10
Saturation is the soil water content when all pores are filled with water. The water content in the soil at saturation is equal to the percent porosity.Field capacity is the soil water content after the soil has been saturated and allowed to drain freely for about 24 to 48 hours. Free drainage occurs because of the force of gravity pulling on the water. When water stops draining, we know that the remaining water is held in the soil with a force greater than that of gravity.Permanent wilting point is the soil water content when plants have extracted all the water they can. At the permanent wilting point, a plant will wilt and not recover. Unavailable water is the soil water content that is strongly attached to soil particles and aggregates, and cannot be extracted by plants. This water is held as films coating soil particles. These terms illustrate soil from its wettest condition to its driest condition.
Several terms are used to describe the water held between these different water contents. Gravitational water refers to the amount of water held by the soil between saturation and field capacity. Water holding capacity refers to the amount of water held between field capacity and wilting point. Plant available water is that portion of the water holding capacity that can be absorbed by a plant. As a general rule, plant available water is considered to be 50 percent of the water holding capacity.
So the problem with all the research I found across various websites is that it did not give me an answer to my question. I could find saturation points for loam, sand, and clay mixes, but expressed as inches of rain per foot of soil. So a quick experiment to get a ball park answer to my question.
I filled a gallon container with my super soil mix and then spread it out in a tray to get bone dry over the course of a few days in very low humidity. Then refilled the container (which has drainage holes) and bottom watered from a one gallon jug. Came back a few times later and top watered being careful to catch all run off. Measured all my run off and left over water in the jug at about 12 cups. This means that about 4 cups were retained by the gallon of soil at saturation, which is about 25% of the soil volume.
I'm not sure I'll measure for the field capacity of my soil because it isn't all that important to what I needed to know. I now know that my project will not need reservoirs larger than 16 gallons per 65 gallon smart pot. Realistically, I doubt they need to be more than 10 gallons because they will never get as dry as the soil in my experiment.
I am toying with adding slow drip reservoirs for my 65 gallon smart pots and wasn't really sure how much water they would hold when filled with a recycled super soil mix (approx. 20% top soil, 20% compost/worm casings, 20% perlite, 20% coir, 20% organic amendments ).
I researched water holding capacity of soil and learned some interesting terms. Here are some basic definitions that I have taken from the following website: http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447039&topicorder=10&maxto=10
Saturation is the soil water content when all pores are filled with water. The water content in the soil at saturation is equal to the percent porosity.Field capacity is the soil water content after the soil has been saturated and allowed to drain freely for about 24 to 48 hours. Free drainage occurs because of the force of gravity pulling on the water. When water stops draining, we know that the remaining water is held in the soil with a force greater than that of gravity.Permanent wilting point is the soil water content when plants have extracted all the water they can. At the permanent wilting point, a plant will wilt and not recover. Unavailable water is the soil water content that is strongly attached to soil particles and aggregates, and cannot be extracted by plants. This water is held as films coating soil particles. These terms illustrate soil from its wettest condition to its driest condition.
Several terms are used to describe the water held between these different water contents. Gravitational water refers to the amount of water held by the soil between saturation and field capacity. Water holding capacity refers to the amount of water held between field capacity and wilting point. Plant available water is that portion of the water holding capacity that can be absorbed by a plant. As a general rule, plant available water is considered to be 50 percent of the water holding capacity.
So the problem with all the research I found across various websites is that it did not give me an answer to my question. I could find saturation points for loam, sand, and clay mixes, but expressed as inches of rain per foot of soil. So a quick experiment to get a ball park answer to my question.
I filled a gallon container with my super soil mix and then spread it out in a tray to get bone dry over the course of a few days in very low humidity. Then refilled the container (which has drainage holes) and bottom watered from a one gallon jug. Came back a few times later and top watered being careful to catch all run off. Measured all my run off and left over water in the jug at about 12 cups. This means that about 4 cups were retained by the gallon of soil at saturation, which is about 25% of the soil volume.
I'm not sure I'll measure for the field capacity of my soil because it isn't all that important to what I needed to know. I now know that my project will not need reservoirs larger than 16 gallons per 65 gallon smart pot. Realistically, I doubt they need to be more than 10 gallons because they will never get as dry as the soil in my experiment.
