In the protected cultivation of ornamental crops, the use of artificial substrates continues to grow worldwide1. Each growing media has its own specific characteristics. Production protocols therefore vary according to the substrate used. Among the preferred materials are rockwool and coir – two substrates that illustrate perfectly the need for a different management approach, especially when making an analysis for fertilizer recommendations.
by Anabel Evans
Switching from one substrate to another is a choice for growers. The price of a substrate is commonly the number one reason to investigate alternative options. Fer Weerheijm, managing director Dutch Plantin, says, “On some horticultural nurseries, e.g. in Korea and Mexico, we are also seeing that the choice for coir substrates has been based on the extra cost savings realized with coir compared to rockwool when disposing of the material after production is finished.” Replacing rockwool with coir, however, means more than the elimination of hazardous disposal problems. The growing protocol for each of these substrates is different.
Weerheijm explains, “Coir mixes have both a high water holding capacity and air-filled porosity; if the right coir mixes are used they can, therefore, never be too wet. At the same time it is not necessary to have an irrigation regime where 30-40% of the water and fertilizer is drained away. Since irrigation systems where drain water is re-circulated are very limited, this over-drain procedure (common to rockwool) not only incurs redundant fertilizer costs, it is also an environmental threat.”
The combination of water holding capacity and a natural tolerance of salts is the main reason why over-drain can be moderated with coir based hydroponics (see Figure 1). Weerheijm likes to use the comparison of a yacht and a jet-ski saying, “Steering plant growth using coir products is like sailing a yacht, while rockwool is comparable to driving a jet-ski (in other words the influence of changing climate or irrigation is felt immediately and effects the situation of the roots).”
Substrate, not drain water analysis
The buffering ability of coir substrates also means that the procedures for EC and pH measuring and control should be based on a substrate sample analysis rather than drain water. Drain water provides a spot measurement while a substrate sample can be used as a forecast for fertilizer recommendations over a two to four week period. Gert-Jan Krook, international sales manager Blgg, an agricultural laboratory, says, “The services we provide based on a substrate analysis do require growers to take samples from different locations in the greenhouse, rather than at one point as in the case of drain water. The frequency of the analysis is every three weeks for coir, instead of two weeks in the case of rockwool drain water. Regular analyses are a valuable security measure for growers; a tool to monitor actual against planned growing protocols. Data over time is also used to fine-tune growing systems in individual nurseries. To carry out the analysis, growers are required to send 200 ml of water from their water source (it is recommended to indicate if the water is sourced from a well to additionally measure total iron), regular substrate samples and, on occasion, a drip control sample (to ensure that the fertilizer mix in the tank is correct).” (see Figure 2)
Meten is Weten translates to Measuring is Knowing – a phrase taken to heart in sophisticated Dutch nurseries – but Krook can confirm why more (all) growers should regularly control the EC, pH and nutrients in hydroponics: “Cost saving is the standpoint of the Dutch horticultural sector if we look at the uptake in regular laboratory analysis. The alternative is excessive over-watering (to safeguard against inadequate fertilizer programs or variation in irrigation coverage in the greenhouse) but 40% drain means 40% of the fertilizer (when not re-circulated) is lost to the environment. Apart from the question mark over the environmental responsibility of this practice, can the grower afford to have such a high level of wasted fertilizer inputs? An advisory-based target, which is linked to the Blgg laboratories service, is to achieve a 20-30% drain for our clients. Re-circulation on a broader basis is a longer term ambition. For three years now the analysis of drain water has included plant pathogens to eliminate the risks associated with using re-circulated water (RISCover). We also believe that data collated from laboratory analyses should (in the long term) make it possible to gear the growing protocols for growing media more and more by cultivation method/ variety rather than crop.”
1 Digital Magazine www.floracultureinternational.com: March 2007, Soil-less production of cut flowers continues to increase (pg 14) and Growing media: developments and grower choice (pg 17); September 2007, Coir value relies on quality supply (pg 20); March 2008, When lifespan is prolonged by ageing! (pg 14).
The substrate sample for analysis can be wet or dry since the procedure at Blgg uses standard volume samples, all with the same density. (Note: An initial water sample from the nursery’s water source should not contain air; a completely filled bottle must be submitted.)
Figure 1. Ion exchange.
Fertigation of substrates involves an interaction between “difficult-available water” and “easy-available water” supply to the roots. Once the media is saturated the ions in the difficult-available water will slowly become available to the roots as the nutrients are taken up by the plant as an exchange of ions across a concentration difference occurs.
Figure 2. Example of fertilizer recommendations.
The Blgg analysis results are tailored to individual clients so as not to interfere with the services of plant material, substrate or fertilizer suppliers. The advice (Point 7 in example) is straightforward and based on the fertilizers available (information supplied by the grower) on a per tank basis. Remember! The advice is time sensitive with re-sampling required every three to four weeks for coir based substrates.
Analysis procedure: sample is mixed with distilled water, shaken for 20 minutes and filtrated. Two tests are made per sample; if they vary significantly a new analysis is conducted.
Fertilizer mix, Ca containing fertilizers in the A-tank; P and S containing fertilizers in the B-tank. Blgg recommends Fe chelates in the A-tank, the other micro nutrients in the B-tank (check Fertilizer Mix