Thursday, December 16, 2010

What we learned from the Zimpeto couve crop

This is the final post on the Zimpeto couve crop. So what did we learn?

It is possible, with the measurement techniques we had available, to grow a big couve crop with very little water, and in our case, no added nutrients.

We could do this by watching the watermark sensors to check the soil did not get too dry, and at the same time ensuring that the 500 mm detectors were only activated occasionally, to check that the soil did not get too wet.

Our irrigation strategy was very influenced by the nitrate readings. The very high readings around day 13 meant that we needed to be careful not to leach the nitrate out of the profile.

For example, we measured above 500 mg nitrate/L at 300 mm depth at the start of the experiment. At a soil water content of 25%, 500 mg nitrate/L adds up to around 85 kg N per ha in the top 300 mm of soil. We add another 10 kg in the irrigation water. This would be a large proportion of the total nitrogen fertiliser the crop requires.

How well could we have done if we just had the detectors and not the nitrate, conductivity and watermark sensors?

We can draw some lessons from this experience which suit this soil type and irrigation system as follows:
When we do not activate the detectors at 300 mm depth, the plants might be getting just enough water, or more likely not quite enough. When we activate the 500 mm detectors, the soil is very wet.

We could come up with the following guidelines:

1) When the plants are small, irrigate frequently but do NOT activate a 300 mm detector. This will ensure the nutrients are not leached.

2) As the plants start to grow quickly, activate the 300 mm detector once or twice per week

3) When the crop is at a yield-sensitive growth stage (for vegetables this is usually flowering time, or ‘hearting’ time for leafy crops) activate the 500 mm detectors a couple of times.
There is one remaining lesson. We must minimise the leaching of nutrients, but we must also manage the salt. By the end of the season the wet side had an extra 624 kg Salt per ha in the roozone. The dry side had more (660 kg of salt / ha), applied in the irrigation water.
A few larer irrigation events towards the end of the season, when the soil nutirent content tends to be low, will ensure salt levels do not build up too much in this soil. By deep I mean activating the 500 mm detectors. The detectors placed at 700 mm were too deep to be useful.

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