Fertilization
and EC
I. Maintaining optimal levels of growth
High levels of nutrients are necessary to ensure high production levels.
In soil culture, nutrients are usually added to the soil preplant, as
in field production, and water is supplied daily by a drip irrigation
system. Additional fertilizers are injected into the watering lines as
needed or on a weekly basis. Daily irrigation in soilless culture supplies
both water and nutrients. This results in a relatively high salinity of
the nutrient solution. Nutrient solution recommendations for tomatoes
grown in rockwool, perlite or nutrient film technique in Florida are presented
in Table 3. Somewhat higher levels are
recommended in Canada (Table 4).
The issue of nutrition is discussed in more detail, including discussion
of physiological disorders, and recommended tissue nutrient levels in
Chapter X.
The EC (Electric Conductivity; mS cm-1) is an important measure of total solute concentration (salinity level) of the nutrient solution in the root environment. EC mainly influences plant functioning through its effect on plant water relations. High salinity in the root environment decreases the availability of water to the roots of the plant and therefore decreases water uptake and overall growth rate. The osmotic potential near the root, which depends on the nutrient concentration in the irrigation water, is used to control plant growth and development and fruit quality. In general practice, EC should be around 2 mS cm-1, depending on the production system, crop, crop status and the desired quality and yield of the fruit. EC values for typical nutrient solutions are shown in Tables 3 and 4.
II.
Balancing vegetative and generative
growth
Regulating the
balance between generative and vegetative growth using temperature manipulations
was discussed above, but nitrogen and the water supply are also to control
the balance between vegetative and reproductive growth. Reducing the crop
growth rate steers the plant towards generative growth, while increasing
the crop growth rate steers it towards vegetative growth. The water supply
can be regulated directly, by increasing or decreasing the amount of water
supplied, or indirectly, by adjusting the relative humidity in the greenhouse
and the electrical conductivity of the irrigation water. Light irrigation,
low relative humidity, and high electrical conductivity in the irrigation
water tend to make water less available to the plants and result in ‘hard’
plants and slow growth, steering the plant towards generative growth.
The nitrogen supply can also be regulated directly, by adjusting the nitrogen
fertilization, or indirectly, by varying the supply of other nutrients,
e.g., potassium. Maintaining a high potassium-to-nitrogen ratio in the
fertilizer feed is another technique used by some growers to reduce the rate of
growth, and steer plants towards generative development [37].
There is a fine line between reducing growth and inducing plant stress,
so it is probably better to err on the side of caution, and make only
small, incremental changes.