I met a grower down in the Oxnard area that grows over 600 acres of blueberries in substrate (pots) and is very successful selling his organic product to Driscoll.
One of his primary challenges is growing during the winter. The pots cool off each night much more quickly than soils and thus, the next day the plants take much longer to start metabolizing, photosynthesizing and pushing fruit. Meanwhile, the shorter days mean there’s less time available for the plant to get it together and do what it’s supposed to do!
I started to think about how we alter the temperature dynamics of the soil. How do we impact soil temperature and what are the ramifications?
There are studies linking carbohydrate and nitrogen reserves in grapevines with vegetative growth and fruiting responses, suggesting that monitoring stored reserves at dormancy could provide an advance indication of yield and growth potential for the following season. (As a side note, this is why post-harvest applications are extremely important!)
We all know that when we make a compost pile, the microbes go into a feeding frenzy which causes the pile to heat up. This is an extreme example of a supercharged active site, but with healthy, biologically active soil, you will have more microbes active and so generate warmer temperatures than with low-level, biologically inactive soils.. Generally, soil warming promotes utilization of non-structural carbohydrates in the roots and increases shoot growth rates and flower number as nutrients and carbohydrates in reserve are mobilized. In contrast, soil cooling can increase the abscission of flowers and decrease fruit set.
Regular composting can alter the temperature dynamics of the soil but so can pruning practices such as shading and opening up the leaves (canopies) and vegetation, as well as cover crops and straw mulches.
A team of researchers led by Holzapfel investigated specific effects of varying soil temperatures on wine grapevine response. This study was conducted on mature, field-grown grapevines V. vinifera L. ‘Cabernet Sauvignon’ over two growing seasons, to examine the carbohydrate reserve dynamics and growth responses to soil temperature as modified by plastic sheet and straw mulches. Here’s what they found:
- The carbohydrate root reserve concentrations declined from budbreak to flowering by more than half, with the increases in soil temperature considerably elevating the mobilization of the root reserves.
- The implemented soil temperature regimes had a minor influence on canopy growth and reproductive development. However, warmer soil temperature induced an advancement in berry ripening in terms of reaching brix targets earlier.
We are so reliant on ambient temperature to drive fruit, vegetable or nut ripening, but there’s another component, and it makes perfect sense. It’s called soil management. Building soil health has a plethora of potential benefits!
