Phytotronics WATER PRO, Руководство пользователя

Страница: 9 / 18

9
To go the next step with the available information, the targets for each day that the crop
will be under irrigation at this specific station can be calculated. Do the following: Take
the NEW TARGET and divide by the number of hours between the two irrigations. This
gives the VPD per hour. Multiply the number of hours by the VPD per hour to develop a
target for each day. Hint: If developing a model to do irrigation, the targets should go
down as the plant grows. Targets going down mean that the amount of irrigation is going
up.
UNDERSTANDING VPD (VAPOR PRESSURE DEFICIT)
There has been a significant amount of study by environmental and horticultural experts
concerning the basic principles of VPD and its usefulness in determining the watering
needs of plants. Unlike other methods used to determine watering needs, VPD has turned
out to be the most accurate method to date. VPD looks at all the environmental conditions
surrounding the plant. The MICRO GROW VPD Sensor measures the air temperature,
relative humidity, and the temperature of the plant leaf. These instantaneous sensor values
determine the VPD of the plants in the greenhouse at regular intervals. As the
environmental conditions change so does the VPD. By adding the VPD intervals together,
a pattern or target emerges where we know the plant must have water. After watering the
plant, the process starts all over again.
Just what is VPD and how does it relate to plants? To understand VPD we must understand
the following:
1. How water moves through a plant.
2. A plant’s structure.
3. How environmental factors affect a plant’s water use.
Plant Water Movement
The amount of water a plant needs is controlled by what takes place in the leaf, not in the
soil. When plants are watered, the roots take up water which moves through the stem to the
leaf. There, it evaporates into the air in a process known as “transpiration”. As water
evaporates from the leaf, leaf tissues start to dry, which results in the leaf pulling more
water in through the stem from the roots. This water vapor movement out of the leaf is
critical because the faster the water evaporates from the leaf; the more water is taken from
the soil and pulled through the plant to satisfy its needs. It’s the changes that take place in
the leaf, not in the soil, that determines the plant’s watering requirements.
Plant Structure
Water evaporates from plant cells to the air inside the leaf. This air is always saturated at
100 percent relative humidity. Vapor Pressure Deficit is a measure of the difference in the
amount of water (humidity) in the air inside the leaf and the amount of water in the air
around the leaf. The greater this difference, the faster the plants lose water.
Plants and the Environment
One example to understand the basics of VPD is as follows: Take a basin of water and a