Soilless Culture : Run to waste or Recirculating System

The volume of nutrient solution applied to soilless cultures must be greater than the volume of water evaporated by the crop, to avoid accumulation of nutrients (salts) in the media. This is an essential characteristic of all soilless culture systems regardless of whether the media is rockwool, coir, bark, pumice, perlite, sawdust,gravel or whatever.

Run-to- waste growing system apply the nutrient solution and allow the excess to run to waste, such systems are sometimes called open systems. The waste pollutes the environment with un-used fertilisers and nutrients in the run-off solution.

The run-off can be collected and recirculated as part of the applied nutrient solution in subsequent irrigation, systems doing this are described as recirculating systems or closed nutrient systems.

Any grower using a run to waste system is short changing himself and needlessly polluting the environment for everyone.

A typical run-to waste system in New Zealand

• Recirculating systems conserve water, typically about 30% less water is used than in run-to-waste systems, and thus reduces growers production costs.
• Even more valuable is the saving in fertilisers, which could easily be 30% less for recirculation than for run-to-waste.
• Recirculating systems offer improved management of crop nutrition, as regular solution analysis can provide detailed information on crop nutrient uptake.
• The incidence of root disease may be lower in recirculating systems than in run-to-waste systems. Less raw water is used when recirculating and hence the load of pathogens applied with the water is less, but more importantly the recirculating solution contains a very active microbial population, lacking in raw water, which can compete with and exercise some control over pathogens.
• A recirculating system using good quality water supply should only need to dump the solution once per year, and the volume dumped could be as low as 5 cubic metres for each 1,000 square metres  of greenhouse, with such small volumes having very little impact on the environment.
• More frequent discharges would be necessary if poor quality water (for example with high sodium content) was used, but few growers in New Zealand have to use low quality water as in most places annual rainfall is greater than annual crop water use in greenhouses, and stored rainfall provides a high quality water source.

This system uses  rockwool slabs with recirculation of all run-off, but planter bags or horizontal bags work equally well in this system.

There are simple low cost cost systems for collecting clean run-off from soilles growing systems. The collected run-off does have to be pumped into a storage tank where it can be mixed with more fresh water. Nutrient stock solution can be added to the storage tank or added to the dilute nutrient solution as it is being pumped from the tank to the trickle irrigation system. Run-to-waste systems often have injectors adding nutrient stock solution to the waterat fixed dilution rates, but when recirculating these injectors need to be automatically controlled so that nutrient solutions of the proper conductivity are delivered tothe irrigation system regardless of the varying conductivity of the solution being drawn from the storage tank. This automatic control is the biggest cost item in making a conversion from run-to-waste to recirculating solution.