Section 6: Special Considerations When Using Subirrigation and Recirulating Irrigation Systems
When using subirrigation and recirculating irrigation systems, it is important to remember that the method of irrigation impacts many other aspects of production.
When using subirrigation, leaching is either minimal or does not occur. Mineral elements that would normally leach out with overhead watering do not leach out to a significant extent with subirrigation. Because mineral elements are not lost through leaching, it is often recommended that fertilizer inputs be cut by 30% to 50% of the normally recommended rates used for overhead irrigation systems.
It is very important to monitor the E.C. of the substrate when using subirrigation systems since fertilizer salts can build up (since leaching does not occur). Because of evaporation from the substrate surface, the fertilizer salts will tend to move into the upper inch of the substrate. The E.C. in this portion of the substrate can be much higher than the E.C. of the lower portions of the substrate. Because of the elevated E.C. level, this region is often devoid of roots. A high E.C. in the upper portion of the substrate is not normally a problem. However, if subirrigation ceases and overhead irrigation is used, the salts in the upper substrate can leach down into the lower portions of the substrate and cause a rapid, and sometimes harmful, increase in the E.C. of the lower portions of the substrate. Also, when conducting substrate tests when subirrigation has been used, the upper ½ - 1 inch of substrate is usually discarded since this area will have an elevated E.C. level and not be representative of the substrate as a whole.
As discussed earlier in this unit, when using recirculating systems, the spread of plant pathogens (i.e. Pythium, Phytophthora, Erwinia, etc.) is a concern. If the irrigation system is used properly, there is minimal water movement from the substrate back into the irrigation solution and thus minimal opportunity for the spread of pathogens. However, if benches or floors are flooded for too long to too deep (which in reality often happens), the lower zones of the substrate become saturated and there can be movement of pathogenic organisms from the substrate back into the irrigation solution. Proper watering technique is important. Proper watering will also prevent the movement of fertilizer salts from the substrate into the irrigation solution. In most cases the E.C. of the irrigation solution is monitored to insure that fertilizer salts do not build up (or being depleted from it) in the irrigation solution, and the recirculated water of fertilizer solution is disinfested to prevent the spread of disease-causing organisms.
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