Broccoli Crop (2)

Things happen slowly in winter. This July we had 17 mornings below zero with a minimum of -8 degrees C. I’m pleased to see such low temperatures because it will kill any overwintering fruit fly larvae. On the other hand the plants are marking time and the microbes are barely turning over. The nitrate level has gone down by less than 2 kg per ha per week over the last two months. I’m starting to hang out for spring!

The third crop - broccoli

After the next crop is planted, the nitrate drops quite sharply. The blue line now represents the third crop in the sequence – broccoli. Some of the decrease in nitrate is due to crop uptake. However the initial decline is a bit too sudden to be explained by uptake alone. I had to irrigate to get the new crop started and rain following irrigation inevitably leads to some leaching.

The second fallow

Despite all the wet weather, the nitrate comes bouncing back after the wheat harvest. Over 32 days the nitrate jumps from 23 to 109 kg N per ha. In other words the soil organic matter is mineralising at a rate that supplies 19 kg N per ha per week, even faster than we saw after the sweetcorn crop a year ago.

My gardens have had regular applications of compost over the years, which of course is a good thing. But you can’t turn the microbes off, and so in warm wet soil they are having a field day. The implication of this is that the soluble nitrate must now be managed by very careful irrigation. Even a bed managed ‘organically’ can be swimming in nutrients in a highly soluble form.

Nitrate during the what crop

The crop was close enough to 5 t/ha. Each ton of grain harvested removes about 20 kg N per ha. Let’s say the harvest index was 0.4 (grain / grain plus stubble), then the standing stubble would come in at 7.5 t/ha. This stubble would contain around 0.4% N, making 30 kg. So the total N uptake would be 100 kg (grain) + 30 kg (stubble) = 130 kg N.

The change in soil nitrate between sowing and harvest was 149 kg – 20 kg = 129 kg (the gold line in the graph above). This is almost the same value as the 130 kg N uptake calculated above – but the match is fortuitous. There would have been mineralisation of organic matter after sowing, losses through leaching, and other factors in play.

A bumper 'crop'

We returned to Australia on 31 December and I harvested the wheat a week later. It came in at 4.95 tonnes per hectare – a very respectable yield! All I did was sow, weed once and then harvest six months later.

Of course it was an extraordinarily favourable season, although I was surprised to get such a high yield with no added fertiliser, especially after very little had been applied to the previous crop. In addition there were 45 days with high potential leaching (see red lines on last post). Unfortunately I have no idea how deep the roots went.

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.

Nitrate in the fallow

We are going to move fairly quickly through 18 months of soil nitrate monitoring. The green line shows the drop in soil nitrate during the 12 weeks the sweet corn crop was in the ground. We started with 221 kg/ha and this fell to 9 kg/ha at harvest. I hope most of this nitrate was picked up by the crop, but no doubt some escaped below the root zone.

Lesson #1 is don’t necessary believe the recommendations to apply fertiliser at planting. It’s prudent to measure first.

The graph above shows what happened during the 77 days of fallow between the end of the corn crop and the subsequent wheat crop. The soil was still warm during March and April, and there were regular rainfall events. On 23 May the wetting front again reached 40 cm depth and the nitrate had risen from 6 kg/ha to 149 kg/ha. This equates to 143 kg/ha or 13 kg of nitrogen ‘appearing’ in the soil each week. The source of nitrate is the mineralisation of organic matter. In other words soil microbes feeding on the soil organic matter break it back down into plant nutrients. I only measured the nitrate, but the complete suite of plant nutrients would be there too.