Soil Temperature

Marijuana Horticulture

by Jorge Cervantes

Raising the soil temperature speeds the chemical process and can hasten nutrient uptake. Ideally, the soil temperature should range from 65-70F for the most chemical activity. Warm the soil with soil heating cables or a heating pad. Fasten heating cables to a board or table and set a heat conducting pad on top of the cables to distribute heat evenly. Set cutting and seedlings in shallow flats or growing trays on top of the heat conducting pad. The added heat speeds root growth when soil temperature is below 65F.

Soil heating cables cost much less than soil heating pads but must be installed, whereas the pads are ready to use. Most commercial nurseries carry cables, and hydroponic stores carry heating pads. When rooting clones, a heating pad or cables virtually ensure success and expedite root growth.

Cold soil slows water and nutrient uptake and stifles growth. Growers often overwater when the soil is too cold or the room cools unexpectedly, which further slows growth. Pots on cold concrete floors stay as cold as the concrete, which is always colder than the ambient temperature. Increase soil temperature by moving pots up off the floor a few inches. Set them on an insulating bard or piece of Styrofoam.

Soil temperatures that climb above 75F dehydrate roots, and at higher temperatures the roots actually cook! It is relatively easy to heat the soil in a pot. If the light or any heat source is too close to small pots, it can easily heat up the outside layer of soil where the majority of the feeder roots are located. Once destroyed, roots take one or two weeks to grow back. Two weeks accounts for one quarter of the flowering cycle!

The more feeder root hairs there are to absorb water and nutrients, the faster and stronger plants will grow. Once roots go beyond their comfort zone, they send stress signals to foliage and stomata via hormones to close and conserve moisture.

Oxygen is essential for clones that are growing roots. Water holds under one percent dissolved oxygen at 70F. Bump the temperature up to 85F and it holds less than 0.5 percent oxygen.

Root temperatures below 40F make water expand, which causes cell damage. Temperatures above 92F cause excessive vapor pressure within the roots, which can cause damage. At high temperatures roots send stress signals t shut the leaves down before damage can occur.