Fertilization
Principles
In applying fertilizer
to either a soil-grown or soilless media grown plant, the goal is to match
the nutrient uptake of the crop as closely as possible to the amount provided
as fertilizer. This will prevent both salt buildup and nutrient deficiency.
In addition, it will prevent excessive fertilizer runoff. Historically,
growers have added more N and P than the plants could use immediately
to avoid depletion in the rootzone. This was done by preplant incorporation
of fertilizers in field crops or by maintaining high levels of nutrient
solution in soilless culture (>200 ppm N, >30 ppm P). In both field
and greenhouse production, growers are now under pressure to reduce their
inputs of nitrogen and phosphorus, in particular, as these are the most
important contaminants in ground and surface water systems.
Current monitoring and nutrient delivery capabilities enable growers to implement "just-in-time" and "just-enough" fertilization practices. For example, in field production, development of guidelines for petiole sap testing guidelines (Table 2), for tissue nutrient levels (Table 3) and weekly fertigation guidelines (Tables 4 and 8) allow fertilizer additions to be made when they are needed by the crop rather than attempting to supply all nutrients preplant or in one or two large additions during the growing season.
In soilless culture, runoff can be reduced by replacing the open systems used in rockwool and perlite culture, which require 20-30% excess water application with recirculating systems, such as those used in nutrient flow technique (NFT). In recirculating systems, EC, pH, and nutrient levels are constantly monitored and additions made to the solution as necessary. The solution cannot be reused indefinitely, however, even with nutrient replacement and pH control, because not all ions are taken up in equal proportion. Those not taken up, accumulate over time in the nutrient solution. This is why EC alone cannot be used to determine when the solution is depleted. Although disinfection using ultraviolet light or ozone allows for longer periods of recirculation, the solution is eventually discarded and another made up fresh because of concerns about disease transmission, solution imbalances, and root exudates. By the year 2000, all greenhouse production in the Netherlands is expected to be in closed, recirculating soilless systems in order to reduce runoff (De Kreij, 1995).
