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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Leonard Ornstein writes: ===8<============== Just a few comments on the very useful summary by Bob Sojka: (inserted below) Len Ornstein At 4:52 AM +0000 3/7/02, owner-sowacs@aqua.ccwr.ac.za wrote: > >Richard G. Allen writes: >===8<============== >Dear Gerben, Len, et al. > >I was intrigued by your comments and conversation on >polyacrylamides, and ran your discussion past Dr. Bob Sojka >of the USDA-ARS at Kimberly, Idaho, since Bob has worked >rather extensively in using PAM in irrigation erosion control. > >Bob enjoyed reading the discussion and provided some >comments back to me. With his permission, I am submitting >these to the group, in case they are useful or of interest. Bob >has offered some ideas on the cause of the reduction of >potential within the porous cup over time and some ideas on >testing for the cause. Feel free to send comments to me and >I will forward them to Bob, or you can correspond directly with >Bob at <sojka@nwisrl.ars.usda.gov> > >Best regards. > >Rick Allen >Univ. Idaho > >----------------- >(from Bob Sojka) > >Rick, Interesting. My understanding is that crosslinked PAM >(homogeneously mixed with soil) changes the water release >curve of the soil volume it is mixed with, biasing it toward >more water held at the high potential (wet) end... ie the PAM >water is easy for plants to extract. Thus the appropriate >measure of availability is probably water per unit potential. It can >be expressed in a user friendly mode as volumetric water >availble at any of a number of given potential thresholds. Agreed; water available per unit potential gives a direct measure of utility as a useful moisture reservoir for the crop. > Neither the crosslinked or non-crosslinked versions of the >molecule are very mobile-- really almost not at all. With respect to the non-crosslinked molecules; their mobility is largely a function of there size (molecular weight). The smaller molecules can be quite mobile. A second issue is whether they are pure polyacrylamide, or contain a few acrylic acid groups. It is the latter that provide binding sites for cations, like sodium and calcium, and bind almost irreversibly to some mineral components in the soil, thus making them immobile, even at low molecular weight. > My guess >is cup-related reading effects in tensiometers are due to >viscosity and pore blockage affecting response time of the >tensiometers. Work we've done here has shown that for non- >crosslinked PAMs, very small amounts of PAM can affect soil >hydraulic conductivity at concentrations as low as .5 ppm. At >higher concentrations the effects are much greater. Non- >crosslinked PAMs are highly surface-active and irreversibly >adsorbed to soil particle surfaces upon drying. Their affinity >is high enough that they have nearly no migration even if not >completely dried. Again, note that this behavior describes the properties of PAM which contains acrylic acid impurities, which is most commercial PAM. >Also, the water soluble non-crosslinked >molecules used for erosion control and most flocculation >processes are too large to cross cellular membranes. > >Crosslinked PAMs are essentially gels when hydrated. >Again, they don't migrate. Some contain sodium or potassium >on their exchange sites, and the extent and nature of the >sites can vary from compound to compound. Specific ion >effects (e.g. Na) could be more of a concern in some >situations than osmotic effects. Once more; mainly the effects of acrylic acid impurities. > From what I know of the >literature, only extremely sensitive plants would be adversely >affected by the gel's retention of water. It is conceivable that >if the gel completely coated roots it could form a barrier to >movement of water, where the conductivity was the problem, >and not the water retention per unit potential. > >Of course all this begs the simplest questions... If you have >doubts, set up some simple comparisons and get some data. >The track record of gel PAMs (ie crosslinked high water >absorbing PAMs, including starch co- polymers of PAM such >as "super-slurper") is pretty good in the limited applications >where they are cost effective. It is the cost that is the real >problem. Even at a 1000 to 1 water retention by weight, it is >a costly way to increase water retention on a field basis for all >but specialty applications. Farmers have done the math over >and over again, and it is why there have only been a few >limited applications that have been commercially successful >in the past thirty years, mostly in greenhouse, or high value >nursery crop type situations. Even banding of crosslinked >PAMs for seed germination enhancement via enhanced >water retention can be fairly costly. > >Finding new ways of using crosslinked materials by thinking >out of the box (ie the same old approach of affecting bulk soil >water retention via homogeneous mixing into the soil matrix) >is where the action is. Canal sealing, forming subsoil leaching >barriers, water harvesting barriers etc. As you know, the non- >crosslinked materials are having a hey day in preventing >erosion in irrigated settings at rates of 1 kg per ha. About half >a million ha use the approach in the US now, and the >application is spreading to other approaches and to other >countries. Infiltration enhancement via the stabilization of soil >surface structure and prevention of conductivity-reducing soil >surface seals (using ultra-low application/concentration >rates)-- or by intentional sealing via viscosity effects at higher >concentrations (opposite effects depending on the application >strategy) is a current wave of new interest. > >Managing infiltration uniformity with PAM is likely one of the >major new big directions for the research, especially under >sprinklers... again we are talking non-crosslinked, linear, >water soluble PAMs. The crosslinked PAMs and PAM-starch >co-polymers have been around since the mid 70s, and most >of these water retention questions have already been looked >at. > >A major challenge for new comers to this area is keeping the >effects and literature of the two classes of PAMs (Crosslinked >vs non- crosslinked) straight. Agreed! > I think one of the current gurus >of crosslinked PAMs for ag is Carlyle Thompson of KSU. He >might be able to answer some of these questions. > >For non-crosslinked PAMs see our website: > >http://kimberly.ars.usda.gov/pamPage.shtml > > Cheers, > Bob > >===8<=========== ===8<===========