Organic Greenhouse Production: Part Two

We are now into late November and have been enjoying 16 varieties of leafy organic greens. In case this is the first time you have glanced at Maximum Yield, this article is part II from a previous article detailing the organic production of baby greens under poly in South Western B.C. The first installment discussed the construction of the greenhouse, raised beds, organic soilless mix preparation, and the brewing of an aerobic organic tea solution (for fertigation).

Some might ask: “well, how’s it growin?” Firstly, I must say that organic greenhouse production in raised beds is by far one of the easiest and least management intensive methods of production I have practiced.

Moisture management is very simple in this greenhouse. I almost never need to water! The average temperature within the greenhouse is near 15°C, and since we are growing leafy greens, the watering requirements are low to begin with. Condensation is not usually welcome in greenhouses, but is a fact of life. With certain coverings and certain installation techniques it is possible to reduce the amount and of condensation within the structure helping to minimize “drip”. Since the greenhouse is covered with single poly, it is fairly airtight. Just for comparison an average glass house loses it’s internal volume of air about every hour, while in a double poly house it usually takes about five hours. This means that as well as trapping heat in the winter, it also traps moisture which condenses on the surface of the greenhouse covering as it is cooler than the surrounding air. The result is sort of like a terrarium. The moisture rises from the beds, condenses on the covering and drips back down into the beds. This has greatly reduced the watering requirements, but is not necessarily desirable.

Firstly, the film of moisture on the greenhouse covering reduces light transmission. In B.C., winter light levels are painfully low to begin with so this is not especially welcome. If greens were not the crop cultivated the moisture dripping from the covering into the beds could create foliar diseases in crops. Luckily, for our purposes this has not proven to be a major problem, although growth rates are probably a little lower due to reduced transpiration from the leaves in a humid environment. The solution would be to install a dehumidifier or provide a timed hourly air exchange, regardless of temperatures. Installing a dehumidistat in a greenhouse exhaust system is not practical during cooler months. The exhaust fan would run near constant, removing all the heat from the greenhouse.

Good air circulation can greatly reduce the level of condensation. However, blowing air swiftly over as much of your covering as possible is not the best idea either. Heat transfer from inside to outside would be increased effectively reducing heating efficiency.

I am also guessing that a lot of moisture is rising from the ground. By installing a sheet of heavy vinyl or polyethylene as a ground covering before adding gravel to the foundation may help to reduce humidity levels.

Usually the only time I have added any moisture to the beds during cooler months has been to apply the organic fertilizer tea. It is added to the reservoir and then applied to the beds. In fact to reduce the amount of moisture added to the beds, I have applied the fertilizer tea undiluted with no apparent nutrient “burn” or imbalance created. I think it’s pretty fair to say that you could not do this with most synthetic fertilizers.

If you are used to working with synthetic fertilizers, switching to organic production methods requires a change of ideas.

It seems through practical experience and analysis that levels of available nutrients in the soil solution are relatively low, while the potential for nutrient levels is very high. For the scope of this article we will address nitrogen, although each nutrient has it’s own cycle. The soil test indicated that there is only 23.75 ppm of Ammonical Nitrogen, and 17 ppm of Nitrate Nitrogen. By all standards this should create a deficiency of nitrogen in an actively growing crop. However, as the soil system is “alive” it seems to be capable of maintaining it’s own nutrient levels near optimal, provided there is a source of food and good conditions for beneficial microbes. Note for example, microbes thrive in lower nutrient conditions, while plants tend to do a little better with higher nutrient conditions. So a total of near 40 ppm plant available Nitrogen is probably near the bare minimum for plants and near maximum for microbes. However, there is likely a high rate of potential nitrogen. Nitrogen, like other nutrients, can be stored in the soil in forms unavailable to the plant but resistant to loss from factors such as leeching and microbial digestion. As available nitrogen levels are lowered by the crop (and some microbes) microbial life in the soil will go to work on the unavailable stored nitrogen converting it to plant available forms. The microbial populations associated with the conversion of the unavailable forms will then reduce in numbers as the nutrient level becomes higher than optimal for their existence. As plants use up the available forms, the cycle continues.

Nutrient availability is also affected by temperature. During the coldest periods, the temperature on the heating thermostat is lowered for heating fuel economy as it will cycle a little less frequently. The soil temperature will drop slightly to 13°C (55°F). At temperatures this low many nutrients, especially phosphorous become less or unavailable. There has been no apparent sign of deficiencies. However, with the exception of a few varieties, baby greens are not a nutrient intensive crop. Although microbial activity is slowed during cooler conditions, I suspect that many nutrients remain available due to symbiotic relations in the soil between the plants roots and microbes. For example, phosphate (PO4) is the form of phosphorous commonly available to plants, although other forms of phosphorous may be stored in the soil. As temperature decreases the availability of PO4 from the soil solution, organisms that live on and in plant roots can digest the unavailable forms in the soil, and exude plant available phosphorous and feed it directly into the plant roots. So, even when soil conditions are less than optimal, some nutrients may remain available in part to microbial activity. However, mass flow (the movement of phosphate across the root membrane) is the primary method of phosphorous absorption and is the most capable of delivering phosphorous when it is in high demand.

The soilless mix and organic amendments break down into humus and finally into to humic/fulvic acids. There are thousands of studies that demonstrate the benefits of humus and humic acids in soils. Firstly, fulvic acids (a light molecular weight fraction of humic acids) greatly increase the availability of many nutrients such as calcium and iron. Not only is their absorption increased, but the plant expends less energy in nutrient transport. Fulvic acids also allow nutrient absorption through a broader range of growing conditions, including pH fluctuations and mildly saline conditions. Humic acids, having a heavier molecular weight are closely associated with water transport and may also encourage root development and assist microbial life.

In examining the nutrient testing results of our soilless mix, it seems that immediately following the tea application, available nutrient levels were actually lowered while the overall concentration of the soil solution increased. I would guess that this may be a demonstration of the idea that a healthy soil solution is self regulating. As the solution was applied it appears to have leached some of the available nutrients away, and possibly triggered the microbial population to convert excess nutrients into unavailable forms (not tested for) that are stored and converted as required. It is also interesting to note that those elements (Potassium, Calcium, Magnesium, Iron, etc) more closely associated with colloids and soil CEC (cation exchange capacity) seem to be present in relatively higher quantities than those elements more closely associated with microbial activity. However, this may be in part the result of greater nutrient solubility in one component of the organic tea brew versus others. Iron and potassium are abundant in most sources of dried kelp intended for agriculture. However, since most kelps come from salt water sources the sodium level in the soilless mix appears to have doubled.

As for heating, the area is sheltered by trees so it helps to reduce the rate of heat loss through the greenhouse covering due to reduced winds. When it gets below-5°C and the wind is blowing hard, a 25 LB propane tank lasts no more than 36 hours with a temperature set point at 12°C. However, during milder overcast conditions a single 25 LB propane tank can last over five days. When the weather is milder the heating set point is maintained at 15°C as it cycles less frequently and is economical in increasing production rates. During cooler weather the heat set point is maintained at 10°C to conserve heating fuel. Incidentally, there is usually more sun available during cooler conditions so day time temperatures are actually warmer, while nights are cooler due to a reduction in fuel consumption. It has also become apparent that red hues exhibited by some varieties of greens are enhanced with cooler temperatures. If all rows were planted at the same time it may be worth the increase in aesthetic appeal to lower temperatures slightly a few days prior to harvest.

To date, we harvest about one row of fresh organic greens every week. The flavour, texture, and colour are unbeatable. Obviously, it doesn’t get much fresher than this. The best part is having piece of mind about where it came from and what it was grown with, which in this case is sunshine and TLC.