Recycling nutrient solution from day one of the cultivation
It’s time for the big clean up! Everyone is now focused on preparing for the new crop and for a clean start it is essential that the greenhouse is disinfected properly. Some useful tips how to structure this operation can be found in the Fact Sheet; Cleaning the greenhouse.
However in order to live up to society’s expectations for a more sustainable greenhouse sector we must now think responsibly in terms of how the waste solution from the clean up is treated. The waste solution will contain chemicals harmful to the environment and plant growth so it should be discharged in an environmentally friendly way [in line with local regulations] and above all should be prevented from entering surface water. If you clean the drain basins make sure these are properly rinsed and are free of chemicals prior to the start of the new crop. Keeping the drain basins free from chemicals used during clean up and ensuring that the irrigation system has been properly flushed with clean water will allow you to recycle nutrient solution from the very beginning of the new crop, in short minimizing fertilizer [N-NO3, P-PO4] emission to the environment as well as reducing input costs.
When the new stone wool slabs are placed into the greenhouse, onto hanging gutters or the ground they should be saturated with complete nutrient solution, the EC of which will be dependant on the crop and individual growing circumstances. For winter planted crops the slabs should remain fully saturated for at least 24 hours before the drain hole(s) are cut. For maximum drain efficiency Grodan recommends 1 drain hole per linear 133 cm slab length. This should be made at the lowest point in the slab and should be opened sufficiently to allow excess water to drain away easily. For more information on how to cut the drainage holes and some examples see the Factsheet; cutting the drainhole. When the drain hole(s) are cut approximately 2,0 litres of balanced nutrient solution will flow from a slab with dimensions of 100 x 15 x 7.5 cm (Tables 1 and 2).
Table 1: Nutrient samples from irrigation water first drain in mmol/l
Table 2: Nutrient samples from irrigation water first drain in μmol/l
Remember that no chemicals harmful to plant growth are released from Grodan slabs so it is perfectly safe to collect and recycle this solution, a fact which has already been communicated by LTO Groeiservice , NL in 2010. To ensure that you collect 100% of the initial drain volume the drain holes may be configured in a number of ways (Figure 1).
- Cutting the final drain hole directly: this will result in a large flow of water (Figure 1 yellow line) so it is advisable to start cutting the drain holes from slabs located at the lowest point on the gutter to prevent the drainage channels from overflowing. Remind your staff when they cross the central pathway to think again at which end of the row the lowest drainage point is situated.
- Draining the slab slowly: this is achieved by first making a small (1 cm) cut in the bottom of the slab to allow the water to flow out slowly before the final hole is cut (Figure 1 blue line). This allows the drain collection system to handle the large volume of water over a longer period of time and therefore prevents the channels from overflowing.
- Cutting the drain hole in two stages: this is achieved by puncturing a small hole halfway up the slab so that only the top 50% will drain (Figure 1 red line). In the graphic the second hole was cut after 20 minutes so the same cumulative drain volume is achieved as with the other methods. However in practice the second cut can be performed after 2 or 3 weeks. Therefore the remaining nutrient solution will be available for the crop allowing irrigation during Phase 2 to be minimized facilitating a generative irrigation strategy. The final cut being made in the bottom of the slab when the EC needs to be leveled. Obviously this strategy will also result in a lower cumulative drain volume due to uptake by the crop. However please note Grodan only recommends this strategy for tomato crops; if you are in any doubt please contact your local Grodan advisor
If you are hesitant about recycling from day 1 and would like further advice please speak to your local Grodan advisor. In the meantime the experiences of these growers offer food for thought:
- Adam Hubbeard, general manager at Pinnacles Acres in Leamington Canada, growing cucumber on Grotop Expert, saying; ‘with Grodan it’s no problem to recycle drain water from day one.’
- Throw it away? That’s a notion that almost never enters Martien Duijndam’s mind. “The fertilisers are much too valuable to simply throw them away with the drainage water,” says Duijndam. Duijndam is co-owner of the greenhouse horticulture company Themato in Berkel en Rodenrijs, in the Netherlands, which grows Roma tomatoes in more than 5.5 hectares of greenhouses.