Frequency of application depends on rooting volume, irradiance, temperature, and vapor pressure deficit (VPD) as well as on stage of plant growth. Application of excess water in either the field or greenhouse has the potential to pollute surface and groundwater supplies by carrying nutrients and pesticides out of the field. This is an especially important issue in areas with high production density, such as some parts of California and Florida in the US and in the areas of Holland where the greenhouse industry is concentrated. Excess water can also reduce rootzone oxygen, especially when coupled with poor drainage. Insufficient water, on the other hand reduces plant growth and can cause permanent injury.
Water consumption
of tomato plants follows a typical sigmoid curve as they grow (Rudich
and Luchinsky, 1986), starting from low values in seedlings, then
increasing gradually until the start of flowering, and climbing to a maximum
during the peak of fruit ripening, at which time leaf area is also maximal.
From that point, water consumption remains constant until the onset of
ripening, after which time, in determinate cultivars, such as those usually
grown in the field, rates of water consumption decrease. This period is
characterized by slower leaf growth and leaf aging. Field irrigation systems
are sometimes automated to the extent that watering frequency is based
on soil moisture tension but more typically are adjusted based on the
grower's perception of plant needs.
Indeterminate cultivars are usually grown in the greenhouse. In these cultivars, water consumption patterns resemble those in determinate cultivars up to the stage of fruit ripening. For indeterminate cultivars, however, leaf, stem, and fruit production continues, and water consumption also continues at virtually the same level until such time as the crop is severely deleafed or topped. During this phase of growth, the level of water consumption is mainly determined by solar radiation, and most greenhouse computer control systems base watering frequency on received MJ solar radiation and stage of crop growth to avoid under or over-watering and excess fertilizer runoff.
Overwatering can reduce the amount of air available in the root zone in either the field or greenhouse, although it is more common in soilless culture because of the reduced drainage and smaller rooting volume. In either system, low oxygen levels (<3 gm l-1) reduce not only nutrient uptake, but growth and yield as well (Adams, 1999). In the field, this is most frequently a problem with poorly-draining soils or those with sub-surface hardpans. In soilless culture, low oxygen levels generally occur in hot weather because the available oxygen in the solution decreases as the root zone temperature rises. It is also more likely to occur with peat substrates because they have a higher water-holding capacity, i.e. drain less freely, than rockwool and perlite. In peat, overwatering can lead to iron deficiency, which must be corrected with a reduction in watering amount and an addition of iron to the feed (Adams, 1999).
Another concern in greenhouse irrigation is the concentration of the fertilizer solution. When air temperature increases, the water uptake rate increases more than nutrient uptake. Therefore, the concentration of the remaining nutrient solution in the root zone increases and salinity rises. If this effect is not controlled, both water and nutrient uptake may be reduced, resulting in wilting and slower growth. Generally this tendency is overcome by feeding a more concentrated solution in the winter, when both low root zone temperatures and low light may reduce nutrient uptake, and feeding a more dilute solution in the summer.
Relative humidity also affects water uptake. At high relative humidity, transpiration is reduced, which reduces water uptake, which in turn reduces calcium uptake, since calcium only moves in the xylem. Lack of calcium can result in BER, particularly when coupled with other stress factors. This issue is discussed further under BER.