Friday, May 20, 2011
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.