Notes from Windward: #69


Snowy Peppers, Duck Ponds and Nitrifying Bacteria


     Back in September when we transitioned the duck-ponics grow beds from summer crops over to fall/winter crops, in the vein of experimenting and determining the limits to our systems, I transplanted some pepper plants from the garden into the grow beds.

     First I wanted to see if the water stressed pepper plants from the main garden would respond to the abundance of water in the duck-ponics system. Also, the water in the duck pond that is pumped up to water the plants in the grow tanks will theoretically stay around 45-50°F because it is insulated by the thermal mass of the earth, even when outside air temperatures drop below freezing. Since peppers are a heat loving plant and are not frost tolerant, they are a good plant to use to test this notion that the duck-ponics system can keep the growing environment above freezing temperatures longer.
a live pepper in mid-November

     Well, its the middle of November, we have had several light snows, night-time temperatures have dropped down into the twenties, the peppers and tomatoes growing in the main garden succumbed to the cold in early October, temperatures inside Vermadise have even dropped below freezing and the peppers transplanted into duck-ponics are still alive and green. They have stopped flowering and new fruit growth/production has been pretty minimal since they were transplanted, so I think it was probably too late in the growing season to try to revive the plants from their droughty life in the garden.

     In spite of the lack of fruit production, I think the live pepper plants are evidence enough that the duck-ponics system can keep the growing environment tolerable for frost sensitive plants even when it is snowing outside. Soon we will add a protective layer of plastic over the grow tanks (creating a mini-greenhouse) to help keep the plants' environment warmer as the temperatures steadily drop. But the lack of any covering over the plants demonstrates even more the ability of the earth's thermal mass and water's high specific heat capacity to keep the plants warm even when fully exposed to outside temperatures.

     While the temperatures are staying warm in the grown tanks, the relatively slow overall plant growth rate and the yellowish coloring on some of the kale and chard leaves led me to do a little more research into the nitrogen situation. I can identify two potential causes of nitrogen deficiency in the system. The first is that now that the rains have come and there is water throughout the forest, the ducks don't spend as much time (and thus do not leave their nitrogen rich deposits) in the duck pond.

     The second possibility has more to do with the biochemistry of converting duck and fish manure into nitrogen compounds the plants can utilize. The ducks and fish create an ammonia (NH3) rich effluent. Plants can utilize NH3 and its close cousin ammonium (NH4+), however they more readily utilize nitrogen in the form of nitrates (NO3-).

     The nitrifying bacteria that convert ammonia into nitrites (Nitrosomonas) and nitrites into nitrates (Nitrobacter ) live on the pea gravel that forms the soil-less substrate in the duck-ponics grow tank. Apparently the optimum temperatures for these bacteria are between 70-85° F. So while these nitrifying bacteria may still function at temperatures of 45-50°F, they function much more slowly, converting less duck/fish effluent into nitrogen compounds readily taken up by the plants' roots, potentially leading to overall low nitrogen availability for the plants.

Notes From Windward - Index - Vol. 69