Water for the wheat crop

During the six month I spent in Africa, I put the whole vegetable garden under wheat and mustard. The bed we are following - where the sweetcorn had been grown - was sown to wheat on 25 April. The graph above gives a snapshot of the soil water regime at a depth of 30 cm from 1 July until harvest date. The topsoil remained wet up to the middle of September, as shown by the blue line (i.e. suction remained under 30 kPa). There were only two times during the season where the topsoil starts to dry – mid October and late November. But in both cases the rain soon returned. In fact it was a very wet season, with the crop receiving over 600 mm (shown on right hand axis).

The horizontal red line shows the period when the wetting front detector at 40 cm depth contained water. In this case the floats were removed from the detector and replaced by a simple electrode which could be logged (A friend downloaded the logger while I was away so I could see when water was collected and later sucked out of the detectors by the drying soil).

The pattern is quite obvious. When there is a lot of rain (the black cumulative rainfall line jumps up), the soil suction falls towards zero (the blue line). During the wettest periods the red line appears, showing the detector contained water. There are seven periods when the detector collected water – some for long periods and some for short. For example the detectors contained water for 109 hours in mid August after receiving 38 mm of rain over 3 days. But in mid October, after 57 mm rain, the detectors only held water for 25 hours. The reason is that the larger plants could suck water out of the soil much faster in the warmer October weather.

Over the six month period the detector at 40 cm depth contained water for a total period of 45 days. These are the periods when water is moving downwards quite quickly, from the topsoil into the heavy clay below. We expect much of the nitrate would be moving down into the subsoil as well.