Wool and Water
If one should happen to look at wool pellet labels from different mills, all kinds of claims are made about their wool pellet’s water holding capacity. We were confused by all the statements as wool is pretty much wool. We needed to know the truth to ensure our labeling was correct.
To the literature we went!
Across the globe there are hundreds of studies about wool used for clothing. These studies found wool fibers can adsorb between 30-35% of their dry weight in water without feeling wet. I’ve bolded the words “without feeling wet” as wool can hold a lot more water than this but it will feel wet.
It’s also important to remember that 35% is not the same thing as 35 times. I believe that is where much of the confusion in labeling occurs.
So how much water can wool or wool pellets hold?
Wool clothing research led us to wool mat research. There are multiple articles discussing woolen mats for use on roadside reclamation projects or used for weed suppression in the garden. An article written by Ament, Pokorny and Jennings states “Scoured weed-seed free wool can store up to 400% of its weight in water (Upton 2003). Wool becomes saturated at 33% of its weight of moisture-free fibers (D’Arcy 1990). That is, when scoured wool absorbs water greater than 33% of its weight, the moisture is more readily available for plant growth and adsorption.In addition, sheep wool contains up to 17% nitrogen and can act as a slow release fertilizer for plant growth and development (Herfort 2010).Research from Europe testing the use of woolen fabrics for establishing vegetation on green roofs resulted in over three times more plant canopy cover when wool was used in mats compared to traditional coconut fibers mats (Herfort 2010).”
400% is 4 times wool’s weight in water! Yea, for wool mats.
What about wool pellets? Surprisingly, I found only 2 studies both done in Europe. The second study noted below is written in German and my two years of high school German were not sufficient for me to interpret. I only found a paragraph reference to that study in English and am relying on that author’s interpretation below.
The first study is from a research group out of Romania who looked at different sizes of wool pellets. They found their pellets were able to absorb 227% of their weight with water or 2.27 times wool’s weight in water.
The study from Germany found their “novel pelleted organic fertilizer, based on low grade coarse greasy wool, exhibited the characteristics of an organic NK fertilizer, with an effect duration of up to 10 months. It also proved soil loosening ability by swelling effect, as it absorbed water up to 3.5 times of its weight (IASP, 2008).”
So what about Woolly Belly Pellets?
To the kitchen we went!
We weighed 55 grams of wool pellets and added 165 grams of water or 3 times its weight in water. After 10 minutes all of the water was absorbed. The pellets felt wet but could have taken on more water. There was no water left in container. When we added 4 times the amount of water a little bit of water remained in the bottom of the bowl. Our pellets absorb at least 3 times their weight in water but less than 4 times.
Why all of the different numbers? It could be the humidity or water vapors the wool we used already had bound in it. It could be the temperature. It could be the different types of wool being used. There are hundreds of different types of sheep in the world and each breed’s wool can differ in length, crimp and micron count or the diameter of the wool fiber. All of these differences could impact some of the absorption of the water. We have found shredding and pelleting darker colored fleeces always seem to be more “wet” than white fleeces. Has it been more humid when we have worked with darker fleeces or is there something with the color or breed of sheep which produced this darker colored wool that makes it feel more damp? When we find some time, we will experiment! We do not have access to a humidity and temperature controlled environment. Maybe if we did, we would find our wool does hold 4 times its weight in water.
If you would like to get into the weeds a bit more, check out the article “Water Vapor Sorption and Desorption by Wool” by C. Popescu and FJ Wortmann, Jan 2002. They discuss multiple ways water and wool work together. They write, “…there is still a debate on the way the water molecules are bonded within the fibre. Some authors describe water as being in various stages, viz.: strongly bonded, loosely bonded and network water… Others consider that water is physically sorbed and it is found in the capillaries of the fibre.”
These researchers investigated water’s desorption rate to figure out exactly what is happening with the sorption process. Their results show the wonderful relationship between wool and water.
Wool is a fiber that is always working with the environment around itself. Water molecules are also mobile and they contain energy! They will form an adsorbed film on/in the wool. When wool adsorbs water it releases heat (an exothermic reaction occurs) but as more and more water is adsorbed there is a decrease with the heat of adsoption because of the energetic equivalence of all of the water molecules.
It is truly a dance of connections between the water and the wool and we can use that to help us water less!
So much is happening between water and wool. As gardeners, using an all natural, renewable and sustainable substance such as Woolly Belly Pellets which hold at least 3 times their weight in water is what we need!