Vegetable growers have always had to work with a wide variety of inputs (and the list seems to be growing). The goal of course, is to massage the inputs in such a way that the end results will show lower per-unit growing costs, improved quality, increased yields of marketable produce, and the ripening will occur during the period of time that the grower calculates to be his/her best selling opportunity.
A quick review of some of the most important inputs related to the soil and climatic conditions might be helpful in this discussion. Within broad climatic areas, the interaction of soil, plant cover, use of plastic coverings, topography, wind control, level of moisture and nutrients in the soil, and proximity to large bodies of water will lead to the creation of microclimates. Most factors affecting plant growth can be modified to create improved conditions but some cannot.
The type of soil of a particular field (whether sandy, silty, or clay in varying degrees) cannot be altered and even the organic content is difficult to change. However, choosing the type of plants to grow, when to plant, etc. are all manageable factors.
While we cannot influence the amount of solar radiation that reaches the crop environment, how well a grower can utilize and retain solar-thermal energy will have a marked effect on the success of the crop being grown.
When solar radiation is absorbed by the soil surface, it is transformed to heat energy and produces a temperature increase in the soil. Soil acts as the ultimate medium of storage of solar heat. A daytime cloud cover will intercept and diffuse some of the solar radiation before it reaches the earth’s surface. Also, the earth will always re-radiate some of the solar energy it receives. At night, heat is lost because there is no incoming radiation but a cloud cover at night may act as a barrier to this heat loss.
Also we know that sandy soils do not absorb or transmit heat very deeply particularly when soil moisture is low. Loam soils have greater conductivity and transmit and retain heat more effectively. However, when soils are wet, they absorb heat slowly and much of the heat energy is used to evaporate the excess water which in turn creates a cooling effect on the soil.
The goal for working the soil in the spring should be aimed at creating a relatively aerated layer of soil to create a degree of soil drying. While the drier surface soil will be lower in conductivity, maintaining enough humidity deeper in the soil will allow for good thermal conductivity. Maintaining proper soil moisture is important in encouraging soil warming in the spring.
Hills and valleys have a major impact on the earliness of soil warming. Valley locations collect cooler temperatures and are susceptible to frosty nights. Southern slopes will always warm first. Trenches oriented in an east-west direction will be warmer than north-south trenches.
Protecting the soil and early season crops from the wind is very important. The wind increases evaporative cooling so a wind-protected spot will be warmer during the day. However, on a frosty night, protected areas often get colder. Of great importance is the protection of young plant material from the abrasive action of the wind especially when combined with soil and refuse particles. Insect pollination will be more effective when plants are protected from the wind.
In the use of film mulch, it is very important to have the film mulch tight to the soil surface and well secured on the sides. An embossed film will be easier to stretch and keep in proper place due to its much greater strength. If the film mulch flutters, damage to the transplants could occur. The heating effect of the plastic film mulch as well as its assist in retaining soil temperature is a great aid in stimulating soil bacterial growth and the transference of nutrients within the soil. Stimulating the absorption of phosphorous and potassium are essential to developing hardy, fast growing young transplants.
Raised beds with film plastic stretched over it and trickle/drip irrigation tubing under the plastic offers the best of all worlds. The film plastic heats the soil and helps retain the heat. It prevents weed competition and directs carbon dioxide gases under the plastic to exit right where the growing plants are since this is the only place where holes exist.
Higher carbon dioxide levels increase plant growth. Where soil fumigants have been used, the film plastic retains the gases so they can do their work, The trickle/drip irrigation can be used to maintain soil moisture levels at optimum and will transport the nutrients to the plants as needed. Small fertilizer applications on a very frequent basis have given outstanding results.
If raised beds are to be made, it is essential to wait until the upper six inches of soil has dried out sufficiently to be well aerated but not so dry that it won’t form properly. The optimum field conditions for bedding and laying plastic may not last very long so it is necessary to be ready and give priority to this job.
Vegetable growers who have developed the microclimates described have had to put more initial dollars into growing their crops but the increased yield have usually been so large that they still end up with a lower per unit cost. The lower cost per unit combined with improved quality and earlier ripening make this a system worth considering.
This article is reprinted from TRICKLE DOWN... April 1994