Hey I found this very interesting article on SGA website:
The Dark Crystal
Water crystals - this has got to be one of the hottest topics at the moment... and I'll give it to you straight.. I don't use them. Ever. I have never found it necessary, as my soil is so chock full of organic matter that it remains nice and damp all the time. There is a plethora of products on the market, proclaiming to "store" water in the soil.
I myself have a few concerns about some products on the market at the moment, particularly polymer based products. You know the stuff I'm talking about... the little "jelly" looking crystals that can absorb hundreds of times their weight in water and can last for years. The insides of baby nappies are essentially the exact same product that is utilised in many water crystals! They are a polymer based product (a petrochemical as we all know), and is, in fact a copolymer of polyacrylamide (a big name that spells bad news!). I have significant issues with the overuse of polyacrylamide in the garden, one because it is a petrochemical (not very sustainable, and slightly toxic), but it gets worse.
Putting it simply, polyacrylamide is a polymer formed from acrylamide subunits that can also be readily cross-linked. Handle this stuff with caution... and think carefully about using it in the garden, especially near edible plants. Some research indicates that polyacrylamide can degrade under normal environmental conditions, releasing acrylamide, a known nerve toxin. That's not really ideal, and I reckon it's unnecessary!
Polymer based products need water to perform, and some research has also suggested that polymer based water products may compete with the plant for water if allowed to dry out. Also, a whole pile of this stuff is manufactured in China and shipped to Australia, pretty unsustainable emissions-wise if you ask me. The other issue I have with polymer based watering products is that they are highly unpredictable, and the rate of moisture can be significantly altered by high temperatures and other factors, which pretty well defeats the purpose!
What I reckon people need to be aware of is how they place these products if they are going to use them, especially for the establishment of trees. We all know that roots "seek out water" and this is no exception with solid water or water storage crystals. If placed incorrectly, there is a real possibility of roots all heading off in one direction to where the water is, creating a "lopsided" root system, which isn't a problem early on, but will be as the tree grows. Lopsided roots + wind + damp soil = bad news.
There are some products available that are fairly natural, about 98% water with 2% natural vegetable gum to hold it all together. It's not affected by temperature, and won't over water as it ceases dispersing when the surrounding soil is wet. I think these are pretty good when attempting large scale plant establishment, especially trees within a landscape. Just be aware that some of these products are also manufactured overseas, which gives them some negative points in the whole sustainable thing!
It must be remembered that these are irrigation supplements, not total irrigation solutions. If you must use this stuff, don't rely on it to do the hard work for you, and for goodness sake follow the instructions on the container... overdoing it will lead to plants literally "popping out of the ground"! The best water storage aid? Good compost, good aged animal manures, and good soil management.
1. Wisconsin Department of Transportation. Madison, WI."Polyacrylamide as a Soil Stabilizer for Erosion Control." January 2001. Report No. WI 06-98.
2. ^ Smith, E. A., S. L. Prues, F. W. Oehme."Environmental Degradation of Polyacrylamides and its Effects of Environmental (Outdoor) Exposure." Ecotoxicology and Environmental Safety 37(1). June 1, 1997.