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Cliff Hignett writes a very useful contribution to the sowacs list: ===8<============== I read the comments on gypsum blocks with interest. My interest arises from the fact that I have been working with a number of soil water sensing 'experts' from different continents to try and identify the shortcomings and benefits of all the available water sensing technologies with a view to advising the UN on 'the best technology'. We reached the conclusion several years ago that there is no such thing as 'the best' technology. All have strong points and weak points. The only generalisation that we have arrived at is that the neutron moisture meter (NMM) is the most univerally useful device we have so far. Its large measurement volume means that fewer NMM access tubes are needed to reach a given level of precision than any other available instrument - by a WIDE margin. (Yes I know it is 'radioactive' and no good for the top 20 cm of soil and can't be 'logged' - but except for these conditions, it leaves all other technologies for dead) This brings me back to gypsum blocks. What I consider is a general failure of most modern sensing technologies in the field has lead me to take another look at the gypsum block technology (GB). I found very quickly that it has one substantial advantage over ALL the technologies (except the NMM) - it equilibrates with a large soil volume - even if installation is a bit sloppy. This means that it has the potential to average out the micro variability that is the nemesis of all field experimenters. Add to this that a simple GB (ie two wires in a block of 'plaster of Paris') is the cheapest sensor element around and you have a case to take a good second look at this technology. As with all sensing technology - in some situations it works well, in others it fails - sometimes badly. 1. GB technology works best in an irrigation situation where the soil is regularly returned to field capacity - ie where hysteresis is not a problem. 2. GB's measure soil water suction - not water content - if you wish to infer soil water content then you had better be sure that there is no hysteresis and no variability in texture - or your results will be rubbish 3. A simple GB , with reasonable manufacturing quality control, operates successfully between 200kPa and 600kPa at around 20% precision. If you are prepared to accept a bigger error margin then you can use them 100kPa to 800kPa. 4. GB technology using a sand sized matrix (such as the 'watermark'sensor from USA) can be reliably used between 50kPa and 200kPa - I have calibrated large numbers of Watermark sensors out of the box and they can have 50% error HOWEVER most of this error is caused by a few non standard devices - a simple calibration can discard these and the remainder (about 70%) are good to 10% precision. 5. It is important to match the technology type to the soil type - a local firm (www.mea.com.au) has coined the term GBheavy (a conventional simple GB) for heavy textured soils and GBLite (a watermark sensor) for light textures soils - say a loam or sandier. 6. GB's measure suction NOT soil water content - it seems to me that this is an advantage as ultimately we usually need to know the plant water status - ie soil water suction - not soil water content 7. GB's do have hysteresis - a lot of it - if your soil is likely to wet and dry in random ranges then DONT use gypsum blocks - it is just too difficult. 8. Yes, GB's do dissolve in acid soils . At pH6 they will last 3 years . At pH 4 you might get 6 months from them. (in an acid environment, a bit of lime in the mixture may extend their life) Cliff Hignett cliff@soilwater.com.au Bruce -owner of sowacs wrote: >Arie Nadler writes to the sowacs list: >===8<============== >Dear Robbin Josef, > >Not many topics can trigger my response so fast. > >Using gypsum blocks is like praying for water when lost in a dry, hot day in >a desert: > >The act of praying relieves you temporarily of the pains but does not help >in practical solution. > >Gypsum blocks are not linear in their response to different levels of water >content, therefore you have to calibrate them. > >It is difficult to find two similar ones so you have to calibrate all of >them, and remember who is who. > >But due to gypsum dissolution, salt precipitation, OM plugging and a few >more factors, the calibration (you worked so hard to get) changes gradually >at an unknown rate. > >Lack of inexpensive, direct, simple and reliable methods for measuring WC >still does not justifies the gypsum blocks adventure. > >My tip: use tensiometers. > > >Arie Nadler >Dept. of Soil Physics >Institute of Soil, Water, and the Environment >A.R.O., Volcani Center, Min. of Agriculture >POB 6 Bet Dagan, Israel, 50250 >Tel: 972-3-9683865 >Fax: 972-3-9604017 >Home: 972-8-9404161 >Email: vwnad@volcani.agri.gov.il > >----- Original Message ----- >From: "Bruce -owner of sowacs" <bruce@sowacs.com> >Sent: Wednesday, December 17, 2003 11:04 AM >Subject: SOWACS: home made gypsum blocks - is it possible? > > > >>Robbin J. Kavunkal writes to the sowacs list: >>===8<============== >>Dear Friends, >>I'm doing a project on auto irrigation. Can anyone >>please tell me if gypsum blocks, for measurement of >>soil moisture ,can be made by me at home? If so how >>can i make them? >> An answer to my query will surely help me a lot to >>continue with my project. >>Thank you. >>Robbin Joseph >> ===8<============== NOTE: To get off this list, send an email to list@sowacs.com with the subject line: unsubscribe_sowacs For full instructions see this page: <http://www.sowacs.com/subscribe/index.html>