High quality irrigation water (Table 1) is a prerequisite for both soil and soilless culture. In soilless culture, in particular, high alkalinity, salinity, or pH, create problems. Such water may require treatment before use, or if possible, another water source should be located.
pH
Rootzone
pH affects nutrient availability. In acid soils, calcium, phosphorus,
magnesium and molybdenum are the nutrients most likely to be deficient.
Optimum pH for most crops is about 6.0, but field tomatoes are considered
to be moderately tolerant of soil acidity (pH 6.8-5.5). Tomatoes in soilless
culture appear to be able to tolerate even lower pH values. Maximal yields
were obtained at pH 4.5-5.0 and increasing the pH to 7.0 decreased the
yield of tomatoes by 25% (Adams, 1999).
Salinity
In the soil,
as water is depleted, salinity rises rapidly. Soil salinity is measured
by the electrical conductivity, in units of mmho/cm, dS/m, or mS/cm. These
units are equivalent and are approximately equal to 640 mg/l salt. High
salinity reduces plant uptake of both water and nutrients. Hot, dry conditions
coupled with high salinity can cause severe wilting and permanent damage
(Fig. 1). High salinity can also result
in toxic concentrations of ions in plants. Soil-grown tomatoes are moderately
sensitive to soil salinity, compared to other vegetables, with a maximum
threshold for yield loss of 2.5 mS/cm and a 10% yield decrease per mS
cm-1 above this threshold (Maas,
1984).
In soilless culture, salinity may be increased above recommended levels to improve fruit quality. This can be done in several ways, the simplest of which is to increase the concentration of all nutrients in the fertilizer solution. However this is relatively expensive, and can increase discharge of N and P to the environment. An alternative to raising concentrations of all nutrients is to add sodium chloride to the fertilizer water, and at the same time, carefully balance the nutrient solution, and hold the water supply constant . In nutrient film technique (NFT), NaCl at concentrations of 500 g l-1 Na and 750 g l-1 Cl gave maximal yields for early tomatoes (Adams, 1999). High salinity can also be used in rockwool culture as frequent irrigation flushes the root zone and prevents salt accumulation. Over the range of 3-17 mS cm-1, salinity did not seem to cause blossom-end rot (BER) in soilless culture, although salinity is a prime cause of BER when combined with other stress, such as high air temperature (Adams, 1999). This subject is discussed further in the section of this chapter dealing with BER.