December 2021 - Keeping the Soil Warm and Moist! Kind of…

Despite my love for citrus fruit, I can’t rationalize expending oodles of energy to create the kind of environment they prefer. The greenhouse relies on passive solar energy for heating except in the winter when outside temperatures drop below -2°C, or during prolonged cloudy weather. During these cold spells, thermal water tanks in the greenhouse are heated with a high-efficiency wood gasifier. The warmth from the tanks is circulated with horizontal air flow (HAV) to ensure the air temperature maintains at least 1°C.

As a result of these conservative practices, in December and January the greenhouse soil temperature drops to around 6°C. This is well below citrus’ 12°C limit for root elongation and their optimal soil temperature of 26°C. (see reference)

We are recording soil temperature throughout the year to help us in our assessments of plant function and health, and the activity of soil biology.

Figure 1 The Garden Greenhouse Soil Temperature (C°) at December 28, 2021

Figure 1 shows the soil temperature (°C) throughout the greenhouse December 28, 2021.

We record soil temperature with a Hanna Checktemp 1 Digital Thermometer - HI98509.

Soil Moisture

The trees throughout the greenhouse vary in size and vigour. Because each tree has its own ‘pocket’ of amended soil surrounded by a compacted C horizon with little evidence of pedogenic activity, we are recording soil moisture levels at each tree.

Figure 2 - The Garden - Soil Moisture Map - December 28, 2021

This map represents soil moisture values for each tree using an Extech MO750 Soil Moisture Meter with Heavy-Duty 8" (20cm) Probe. The probe is inserted 8” deep at each tree in its pocket of amended soil and the moisture level is recorded. The meter measures up to a 50% moisture level. As this map indicates, we are operating at low moisture levels. The main reason for this is the limited water supply on the farm. We have 70,000 gallons (Imp) of rainwater catchment which we use with restraint through the summer as often four months will pass with no substantial rainfall.

Figure 3 - The Garden - Soil moisture level - Location A1

Figure 3 shows moisture levels for a Meyer lemon tree at location A3 over a period of three months. Of note is the spike in moisture June 27, 2021. This was the third day of the British Columbia ‘Heat Dome’ when extreme temperatures rose to above 45°C in the greenhouse over a period of seven days. Interestingly, there was no change in the watering regime.

Most commercial growers control the greenhouse environment to match their crop’s optimal growing conditions. This results in a maximum yield of high quality crops. I think what adds interest to this study is that we aren’t providing optimal conditions. In the winter we only heat to keep the trees above 1°C (32°F) and in the summer we control water use to maintain our limited water supply. This offers us the opportunity, through leaf sap analysis, to monitor the crop's response to these less-than-ideal conditions. We hope this leads to understanding which management practices increase plant resilience and the production of high-quality fruit.