Plants like water but it is easy to give too much. This can wash nutrients out of the root-zone. The most difficult nutrient to manage is nitrogen. Most of the nitrogen that is available for plants to take up through their roots is in the nitrate form. Nitrate is a negatively charged molecule, and does not bind to soil particles (clay particles have a negative charge). Therefore nitrate tends to move with the water. If too much water is applied, and the excess drains out of the bottom of the root-zone, then nitrate will be carried down with the draining water and wasted. See the video.
I’ve sent many samples to the lab for testing. I’ve used ion selective electrodes. But at the end of the day the humble nitrate test strip is the easiest way to monitor the nitrogen availability in your soil.
This video was inspired from a recent workshop with farmers in Morogoro, Tanzania. The farmers were growing tomatoes and taking weekly measurements of their soil nitrate levels. In most fields, nitrate levels started high (purple test strips) but quickly dropped to lower levels (pink or white test strips). Some of farmers quickly worked out that they were applying to much water, as the Chameleon usually read blue at all depths, even before irrigation. Other farmer observed that their nitrate stayed longer in the soil, and they believed this was because they used animal manure. The farmers wanted to know why the different nutrients sources gave then different results.
You have probably seen this illustration before about how a wetting front moves through soil. The aim of the cut out funnel is to show how a Wetting Front Detector works. This time I am illustrating how solutes move in soil. You will see how the water already in the soil (green) is pushed down by the water added in the next irrigation (red). This is very important in understanding leaching of nitrate and salts.