In hydroponic cultivation, monitoring and quantification of nutrients is of paramount importance. Precision agriculture has an urgent need for measuring fertilization and plant nutrient uptake. Reliable, robust and accurate sensors for measuring nitrogen (N), phosphorus (P) and potassium (K) are regarded as critical in this process. It is vital to understand nutrients’ interference; thusly, a Hoagland fertilizer solution-based orthogonal experimental design was deployed. Concentration ranges were varied in a target analyte-independent style, as follows: [N] = [103.17–554.85] ppm; [P] = [15.06–515.35] ppm; [K] = [113.78–516.45] ppm, by dilution from individual stock solutions. Quantitative results for N and K, and qualitative results for P were obtained.

Pot grown herbs are often cultivated as certified organic products, and there is an increasing demand for organically certified ornamental plants. Supplying the required nutrients using organic fertilizers is a challenge with respect to matching the mineralization and thus the availability of dissolved nutrients in the growing medium with plant demand. In experiments, sweet basil and Pelargonium × hortorum were cultivated using two different organic fertilizer strategies and controlled-release mineral nutrients as control treatment. The two organic strategies were, i) blood meal + Baralith® Enslow (a plant-based organic fertilizer), and ii) poultry manure. The availability of dissolved nitrogen was monitored during the crop cycle by under-pressure lysimeter sampling. Plant development parameters were measured along with chlorophyll fluorescence and chlorophyll concentration of leaves. For both organic treatments, nitrate-N availability was low at the beginning of the experiment, whereas ammonium-N was high. During the experiment, ammonium availability decreased at the same time as nitrate availability increased after a few weeks and then declined again by the end of the experiment. The blood meal + Enslow treatment caused poor germination and slow growth in basil. Plant height and fresh weight was also affected by this treatment for basil but not for Pelargonium. Chlorophyll concentration was affected by treatment, with also visually detectable paler leaves in the treatment with poultry manure. There were no differences in chlorophyll fluorescence (F v /F m) between treatments, indicating that plants were not stressed in any of the treatments.

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In greenhouse organic horticulture there is a great challenge in supplying the crop with adequate amounts of nutrients at the right stage of crop development. This has been identified as one of the main factors compromising yields in organic systems as compared with conventional hydroponic systems based on the use of synthetic fertilisers. In organic systems, the supply of nutrients is reliant on microbial degradation of organic complexes, a process that is dependent on factors such as temperature, soil water content and pH. Different organic fertilisers will also have different characteristics with respect to mineralisation of nutrients. In order to evaluate different strategies for organic fertilisation in long-term greenhouse crops such as high-wire tomato crops, an experiment with three different treatments was performed. The different strategies evaluated were one based on blood meal, kalimagnesia and the commercial product Baralith Enslow (composed of clay and ground lucerne), one with poultry manure and kalimagnesia, and one with solid biogas digestate and kalimagnesia. A five-month tomato crop was grown. Lysimeter samples were taken from the growing media biweekly for monitoring of plant available nutrients. The results suggested that nitrogen was likely to have been the limiting factor for plant growth, however, the biogas digestate delivered mineralised nitrogen throughout the experiment.

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