After lots of rain, this time I do not expect to find a lot of salt!
Thursday, February 25, 2010
After all the rain during week 8, I decided on a small irrigation for week 9 - only 11 mm or 0.36 of potential water loss. This was just enough to activate the detector at 20 cm depth. I irrigated because my soil sensor at 30 cm depth showed the soil was already getting dry - but I wish I had a water sensor in the sub soil.
Wednesday, February 17, 2010
I thought 125 mm of rain would wash all the salt away. Not so - the salt is still there, but as I've said each week it is not as high as I expected it to be. It was obvious during the rain that water had trouble infiltrating into the soil. Water ran off the beds into the furrows. It took two days and 100 mm of rain before the deeper detector collected a sample. In other parts of the garden, detectors at the same depth responded with half that amount of rain. Clearly the salt in my washing powder is changing the soil structure.
Wednesday, February 10, 2010
I applied another 10 kg/ha of nitrogen fertiliser this week in the wastewater, exactly the same as I did in week 4. But this time there was no jump in nitrate in the 20 cm detector. Since it rained a couple of days before I irrigated, I may have pushed the stored rain water into the detector, with the nitrate still sitting in the soil above it.
I've changed the order of the posts around this week. First we looked at the crop, now the irrigation and rain, and then we go on to see what is happening in the soil. It rained during week - 18 mm! That was not enough to activate the detector at 20 cm depth so I also applied 24 mm of washing machine water, which was just over 80% of the potential corn water use for the week.
Wednesday, February 3, 2010
The fourth week with no rain. Last week the total potential water use was calculated at 43 mm by the Canberra City meteoroligical station. I applied 22 mm of irrigation, so that gives me an 'irrigation ratio' of about 0.5. The crop is close to full canopy now, so could probably have used the full 43 mm. I under-irrigated on purpose because I want the crop to use water deeper in the soil. I know there is water at depth and I know there is nitrate a depth and I also know the crop is at a stage where a bit of water stress will not hurt too much. Moreover the bureau are forecasting rain, but I'm not sure I believe it anymore.
Despite the large irrigation last week, the top soil is drying out rapidly. Yes, I know I should measure water at more than one depth. But I also know that the clay subsoil is wet (I dug a hole to 60 cm before planting). The couple of weeks prior to flowering is the key time for the corn to pick up some of the deep water and nitrate. A bit of mild water stress will not worry the crop too much at this stage.
After the big irrigation on week 5, I collected water from the 20 and 40 cm detectors. There was plenty of nitrate at 40 cm depth (red bar). So this week I did a smaller irrigation because I do not want to wash the nitrate down even further. Not much nitrate left at 20 cm (green bar), but the crop will extract nitrate from the 'red bar'.