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It is a pity email does not allow for graphs... The point of measuring soil moisture volume rather than soil suction is that is it is easier, reliable and repeatable, especially at high suctions - the assumption is that there is a relationship to soil suction as as seen by the plant, but at the practical level you do not need to know what it is. It seems to me (from a civil engineering/geomechanics background) that the use of flow matrix and 'coefficients of permeability' is at best a design tool. Coefficients of permeability change by orders of magnitude (factors of 10), and in any case flow in real soils can be totally dominated by flow in cracks, etc. Water can 'appear' to flow vertically through essentially impermeable clay only by travelling down cracks - once the cracks are full, almost zero penetration. For example, the request by A. Jake Gusman on Irrigate-L for calculation methods for penetration of pesticides might be of interest at design time, but a grower actually paying for (and hoping to use the benefits of) fertilisers really needs to monitor what *actually* happens. I was recently shown the results of a simple trial in sugar cane of buried drip, on the four major soil types used in the area. They had used about 6 buried TDR sensors at different radial distances from the dripper. It took only a few weeks of measurements to develop a simple chart for growers in the area showing maximum pumping time for each different soil type before through drainage occured. From memory, one soil type had vertical upwards flow from the dripper !!! They didn't need to calculate coefficients of permeability - they knew what actually occured By measuring soil moisture by volume with a non-destructive instrument - neutron probe, capacitance, tdr - or soil suction more cheaply with tensiometers - hundreds of growers are routinely able to identify the 'inflection' points as the soil profile goes from over-full to full, and from 'normal' daily water use to stress dwu at the refill. It is cheap and easy. They feel they are working blind if they don't measure their soil moisture - for them it is like a periscope into the soil profile, the part of the crop they have never been able to see. But then comes the real potential - once they measure soil moisture to schedule irrigations, they are able to correlate that with fruit sugar content, vegetative growth rate, ... and go on to control the quality and yield. What is depressing is the approach of (some) researchers and advisors - that a mathematical model can describe the way real plants grow. Soils and soil profiles really are not made up of 'sand', 'loam' or 'clay'. A recent paper presented at the IAA conference in Adelaide on irrigation scheduling said the the grower will be given a schedule of 'based on soil type and crop', but this 'may need to be adjusted depending on weather'. Last week a colleague was show a case in Spain of a field of tomatoes irrigated automatically, with schedules calculated using a (very expensive) weather station. The final yield was about half that of the next field that was irrigated the old way. It appeared the problem was that a clay layer meant that the tomatoes were completely water logged at depth, but because the drip tape was buried too deeply the surface roots were dry. However, it appeared that the model (based on the weather station) said all ok. (Just a spade would have shown the problem) Cotton is a relatively new crop in Australia - it may be significant that almost the entire irrigated crop is scheduled based on direct soil moisture measurements. On the question of automating the points of inflection - it is not quite as straightforward as finding inflections on a TDR trace. The full point inflection of soil moisture (not soil suction) can be in *either* direction. A clay soil cannot be taken very far above full, but the depletion rate may fall as the crop is water logged. That is, the full point will be identified as the depletion rate rises from zero(water-logged) to 'normal' crop use) However, a sandy soil can be taken above the full point, and through-drainage will appear as a high soil depletion rate - it is only when the rate falls to 'crop rate' has the soil reached the full point. I will try a graph a la Bruce... CLAY ********-----------Full * * * * * *----------normal crop use * * SAND * **-----drainage * * -*--*----------Full * * * * * *----------normal crop use * * I gather that possibly the most important soil moisture measurement is *below* root extraction depth - changes at this point give a more reliabe indicator of full (and refill as well) The inflection at refill can be fairly easy to identify in some crops (making allowance for weather and state of the crop), but in other crops, such as sugar cane, it is not a clear cut point. In addition, if crop quality is to be controlled by manipulating the deficit then the refill needs to be adjusted during the growing season. ---- Trevor Finch Research Services New England 8/16 Nicholson St, Balmain NSW 2041 Australia email: rsne@mpx.com.au tel: +61 (2) 9810 3563 fax: +61 (2) 9810 3323 ----