During earlier project trials, RDFs have been tested in agricultural crops and showed high potential for mineral fertiliser replacement.
As quite some mineral fertiliser is used in greenhouses, they represent the second biggest market for possible RDF uptake.
Therefore, as greenhouse require pure products, some of the most pure RDFs, namely ammonium nitrate from ammonia stripping and ammonium sulphate from chemical air washers, were then selected for greenhouse trials.
At Inagro, these two RDFs were tested on greenhouse lettuce (Lactuca sativa var capitata) grown in soil. The lettuce was grown in containers filled with a mixture of greenhouse soil and perlite. Perlite was added to keep the soil light to ensure a good water and air supply to the roots, since greenhouse soil tends to clog together when irrigated. Irrigation was done via drip irrigation, where each plant was provided with a separate drip nozzle.
During this trial the effect of the RDFs in different doses was evaluated. Both the products were given at 100%, 70% and 40% of the advised nitrogen fertilisation amount, compared to a control mineral fertiliser treatment, each in four repetitions.
At harvest all four crops of each trial object are harvested and for each crop yield (fresh weight and marketable weight) and quality (uniformity, canopy cover, crop filling, leaf colour, tip burn and basal rot) were evaluated.
The yield and quality of the lettuce grown on the RDFs was good when the fertiliser advice was followed. The crops grown on ammonium sulphate (100%) were a little bit more compact in growth.
It is important to take into consideration that the used fertilisers need to be as pure as possible. Too much ballast elements can cause problems in later crop cycles as they can accumulate in the soil and are not flushed by rainfall or irrigation, in comparison to use in the field.
In greenhouses in Belgium there is a high demand of nitrogen and potassium but a low demand of phosphate. This might give some restrictions to the types of RDF that are suited.