Wednesday, May 18, 2011

From drought to flood

This next sequence of blogs is all about nitrogen.

I left Australia to work in Africa in June last year, during the worst drought in living memory. I returned in January 2011 to the worst floods in a quarter of a century.

During the drought I was worried about salt in my soil because I had been using waste water. During the continuous wet weather my attention switched to soil nitrogen. Would it all be leached away?

Before I get into this new story, a quick recap: I started this blog in January 2010 with the following paragraph

“This blog accompanies my book “Out of the Scientist’s Garden”. The book is about how the world uses water in the business of feeding itself. It’s a serious book on a difficult topic, so I wrote in story form, through the lens of my fruit and vegetable garden. I kept the book free of tables and graphs because this type of information is for the specialist, and I was writing the book for everybody interested in water.”

Of course you can’t get away from numbers and graphs if you really want to understand water. Those of you who have followed this blog will see that it is full of such information. I have been running a series of ‘live’ experiments and building up the data sets week by week as the experiments unfold. This makes all the data much easier to follow.

The blog started with a sweetcorn crop grown using water from the washing machine. During the long drought and accompanying water restrictions, many people gave up on gardening all together. We maintained a fully irrigated and productive food garden whilst using less water than the average per capita consumption for our city. To keep this up we had to exploit every opportunity, and the challenge of waste water was one of those opportunities.

Water from the washing machine contains salt and plants don’t like salt. The first of the ‘live’ experiments was about monitoring how the salt was building up in the soil. One of the surprises from this experiments (and all good experiments should have surprises) was how much nitrate was in my soil, even before any compost, manure or fertiliser was added.

Plants need nitrogen as it is the essential component of enzymes, particularly the photosynthetic enzyme packed into the leaves. Most of the nitrogen the corn plant uses comes in the form of nitrate dissolved in the soil water. You will recall that wetting front detectors were placed at 20 and 40 cm depth in the soil. With a few assumptions I’m going to chart the level nitrate in the soil from the very start (Dec 09 to now May 11).

The assumptions are
1) I take the water sample from the detector at 40 cm depth to be the average value over the top 50 cm of soil
2) I assume the soil is 35% volumetric water content when the soil water sample is captured
3) I use the ‘RQEasy’ meter to read the nitrate level off a colour test strip
4) The value of nitrate in mg/L is converted into kg/ha.

Here is the corn crop that would have taken up approximately 200 kg of nitrogen on a per hectare basis. We only added the equivalent of 20 kg. The rest came from the soil. You can see all the details by scrolling back to the very beginning. In following blogs we will look what happens to the soil supply of nitrate over the next 3 crops.

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