Run-to-waste
describes standard ‘soil culture’. That is, the excess nutrient, or
"run-off” is discarded instead of being re-circulated.
Media with high water holding capacity are
used (e.g. soil, coco fibre, Rockwool), and feeds are small and
infrequent.
The ‘run-off’ is either drained directly onto
the ground (Fig 5.7),
or it is collected (Fig
5.5b & 5.6).
Collecting the run-off allows feed volume and
frequency to be calculated more accurately. This practice is
preferred because it helps prevent either under-dosing or over-dosing
(over-dosing causes water-logging/ disease and wasted nutrient). It
also allows the waste nutrient to be disposed of more responsibly.
Feed volume and frequency is calculated so that the percentage run-off is
typically 10-20% (Table 6.1).
With either method, plain water flushes are
usually needed at frequent intervals through the same plumbing. This helps
minimize salt build up in the root zone, blocked drippers, etc.
System design is similar to satellite
systems. Plants are either typically seated in pots (Fig
5.5b & 5.7) or leach trays (Fig
5.6).
Advantages
to run to waste systems
+
pH and EC of the nutrient feed solution is stable.
+
Plants receive fresh nutrient at each feed.
+
The use of media having high water holding capacity minimizes the risk of
plant death in the event of nutrient pump failure.
+
In the event of root disease outbreak, because the nutrient is not
re-circulated, there is less risk of cross contamination between pots/
trays.
+
Can be an advantage for higher salinity waters.
Disadvantages to run to waste systems
+
Prone to salt build up at the root zone, and blockage of drippers. This
can be minimized by periodic flushing with water.
+
Unlike re-circulating systems, it can be difficult to control the
nutrient’s EC at the root zone.
+
High water holding capacity mediums are prone to water-logging and
therefore waterings must be carefully controlled. This can be especially
difficult when air temperatures fluctuate greatly.
+
Top-fed nutrient tends to channel downward through the medium where root
density is lowest (Fig 5.10).
+
Where this is practiced on a commercial scale, it is rapidly being
outlawed because the dumping of ‘run-off’ causes contamination of
groundwater.
