Monday, December 13, 2010

How good were we really?

This was a good crop, grown with very little water. The first detector samples at 30 cm depth showed that we had lots of nitrate in the soil, left over from the chicken manure applied to previous crops. So we managed with just the nutrients already in this sandy soil. But how good were we really?

For those who follow this blog you will know that I think across five strands of enquiry to answer this question:

1. LOCAL KNOWLEDGE: what do the people I’m working with already know? What do they do and why, and how did they arrive at these practices?
2. THERMODYNAMICS: Measure the water going on - by flow meter, irrigation run time, collecting water in cups (or whatever) and compare with the theoretical amount that can be evaporated - from a weather station, a model, an evaporation pan (or whatever). It does not matter if the method is rough – as long as it is consistent.
3. SOIL WATER STATUS: I go for the simplest measurement protocol and, once I have some local experience, target only a few depths that can give me most of what I need to know (usually Watermark sensors at two depths)
4. WETTING DEPTH: Wetting front detectors that give a visual indication to the irrigator of how deep the irrigation water has penetrated into the root zone
5. SOIL SOLUTION: Measure the electrical conductivity and nitrate concentration from the water samples collected from the wetting front detectors

The reason for five strands of enquiry is that they are independent, but complementary lines of evidence. Frequently, if you look at one strand alone, you think you are doing OK (say soil water status: strand 3). Then you look at nitrate leaching or salt accumulation (strand 5), and it gives a rather different picture. So then you need additional information, like the ratio between water applied and potential evapotranspiration so you can troubleshoot (strand 2).

Wetting front detectors are useful at showing how deep the water is going, and the salt and nutrients that are being carried with the water. But they are not perfect. For example, infiltrating water is captured by the funnel, but when the soil in the funnel is saturated, and the soil outside the funnel is slightly drier, then water can be ‘wicked’ out of the funnel as fast as it arrives. This happens especially in fine sandy soils with no structure, and also when the wetting front detectors are placed quite deep. The water is passing the detector quite slowly, and although some water is accumulating inside, it is wicked out again before the float can pop up.

This was a perfect site to test the ‘problem’ of deep detectors, and to see how much water and nutrient could go past.

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