Early strawberry production in commercial NGS hydroponic multilayer troughs showed a correct performance under a continuous irrigation regime, despite the fact that they are designed to work with irrigation pulses, and not for a continuous flow of water. The earliness of the productions was evaluated by studying the evolution of parameters such as the number of flower buds, number of flowers, number of fruitsand accumulated weight of fruits over time. Those treatments presenting higher values in these variables in the first stages can be considered better for early production of strawberries. The evolution of time in these variables was different between bare roots and rockwool, which explains the significant interaction between substrate type and time . Both the number of flower buds and the number of flowers started from a situation in which they were equalled and, after 28 days, there was a greater increase for bare root plants, while the values for those of rockwool decreased . Once 63–69 days had passed, this situation was reversed, in such a way that at the end of the study the system that presented the greatest number of flower buds and flowers was rockwool. It is possible that these changes were related to the variations in the parameters of water quality in that period, motivated by a faster growth of the plants and by an increase in the water replacement rate that reduced the nitrate content, the pH and the electrical conductivity values.
These are crucial factors in strawberry production. It should be remarked that strawberry is very sensitive to high pH values and to salinity in the soil or growing media. In addition, it is well-known that high nitrogen concentration in the growing media enhances vegetative over reproductive growth and reduces the precocity of fruit production. However,4x8ft rolling benches the number of flowers and fruits increased faster for bare root than for rockwool plants. The number of fruits in strawberry is known to increase with higher ratios of ammonium to nitrate in growing media . An improved aeration in rockwool, which is a media with a very high porosity, may enhance nitrification in the rhizosphere, and consequently this may decrease the ammonium to nitrate ratio. In addition, rockwool promotes a basic reaction and the high content in Ca may promote the precipitation of insoluble Ca phosphates, which may decrease P availability relative to bare root system . This may affect initial root development and growth precocity. The high porosity of rockwool may lead to lower thermal inertia than bare roots and thus to faster heating of root zone during day. This effect on average temperatures in the root growing media may also have effects on the initial development and precocity. In this sense, Duchein et al. pointed out that the ratio of the absorption rates of nutrients to the transpiration rate decreased during the warmer months for a rose crop grown in rockwool, which seems to indicate that substrate temperature affected nutrient absorption more than water absorption. It is possible that the latter explains why the precocity that the plants in rockwool initially showed for the accumulated fruit weight ended up being equal at the end of the test with that of the plants with bare roots.
Furthermore, a higher number of fruits for the bare root system amplified the sink effect for fruit development, with an increased competition between fruits for photoassimilates that may affect their ripening speed and the total weight that was harvested. In addition, the potential supply of additional K from rockwool may also have a positive effect on the increased fresh fruit weight at the beginning of the harvest season. As a consequence of all those interactions, at the end of the trial no significant differences were found in the total production between bare root and rockwool, confirming in this regard the results obtained by Palencia et al. , who indicated as more important the effects of the strawberry variety than those of the type of substrate used. However, the effect of the substrate type was significantly affecting the precocity of flowering and the initial number of flowers. This may be of interest for early production, which is crucial for ensuring benefits. Taking this into account, the recommendation for this case would be the choice of the bare root cultivation system, since it entails lower production costs and involves fewer potential environmental problems derived from the elimination of waste from the same . The results obtained in our study may be compared with those of the trials for commercial strawberry varieties carried out in the province of Huelva during the same season , where the yields and quality of the fruits were evaluated for the most widely used varieties in this area . In that study, in farms with conventional cultivation systems , extra-early productions of 60–150 g plant− 1 were obtained for Primoris cultivar, while for soilless cultivation systems productions of 50–120 g plant− 1 were reported, with highest values corresponding to the earliest planting date and the lowest values to the latest . In our study, the mean values for fruit weights produced up to February 28 were 31.9 and 33.6 g plant− 1 for rockwool and bare root substrates, respectively. Although the obtained values were lower than those reported for this variety, it must be taken into account that the planting date had a significant effect on the reported yields and, in our study, it took place one month later .
The existing scientific literature on the differences in early production and quality of strawberries in conventional field crops using plants in plugs or barerooted shows highly variable results . Thus, earlier and higher yields were obtained in Florida for plug plants than for bare roots , while in Queensland these differences were not found.Both firmness and soluble solids content were similar for the two types of substrates evaluated, again justifying the option for the bare root culture system. Regarding fruit quality parameters, differences were found with those described in the report by Medina Mínguez et al. . Thus, for example, average weights of fruits were lower, with values of 16.4 and 17.4 g for rockwool and bare root, respectively, compared to the 25–28 g described for conventional crops and the 28 g for soilless crops. The soluble solids contents were 6.9 and 8.4 ºBx for bare root and rockwool, respectively, slightly lower than those obtained for conventional and soilless crops with 7.5–8 and 9 ºBx, respectively. In contrast, firmness of the fruit was higher, with values of 571 and 638 g for bare root and rockwool, respectively,flood and drain table compared to the 350–400 and 380 g reported for conventional and soilless crops, respectively. These stated differences lead us to think that the fruits were probably harvested in this trial before their optimum ripening point. The substrate type did not significantly affect the SPAD values measured in old or new leaves, so it is assumed that there were no significant differences in the nutritional status of the plants. However, SPAD values are sensitive to the status of nutrients related to chlorophyll content, such as N, Mg, or Fe. It should be mentioned that potential differences in P availability between both cropping systems cannot be assessed with SPAD.
Nevertheless, it was observed an effect of the elapsed time, which could be related to the variations in the water quality parameters previously mentioned, in particular nitrate concentration, or to the larger size of the plants and, therefore, greater nutritional needs of the plants in the final stage of the study. Urban agriculture has the potential to significantly increase food security in cities . Increasing green areas in urban landscapes have been gaining popularity, and with new technologies, greening and food production have been taken to building roofs, facades and even indoors . In particular, soilless agriculture is highly attractive in urban settings because of the reduced weight load on building structures, inert substrate conditions, and overall control of plant nutrition and health , as well as because it provides an alternative to contaminated soils. Soilless production can also be a beneficial system to improve water savings since a more controlled environment can be ensured with more accurate irrigation systems as well as water recirculation depending on the installation . As shown by Appolloni et al. among 92 cases of urban agriculture identified from 2011 to 2019 a 46% produced with a soilless system. In addition to increasing food sovereignty, UA can promote biodiversity, CO2 capture and pollination but can also have negative effects, such as the extensive use of mineral and synthetic fertilizers . Soilless agriculture does not contemplate the addition of nutrients through the substrate but through a nutrient solution given with the irrigation system . Previous work on life cycle assessment of hydroponic production systems shows that,while these fertilizers secure direct nutrient uptake by the plant, their production, extraction, use and disposal are known to have adverse consequences for the surrounding natural ecosystems . Alternatives for the fertilization in soilless agriculture are gaining interest being aquaponic systems most know for the efficient use of fish debris as nutrient for crop production reducing the potential impact of the production system . However, aquaponic installations can entail a great initial investment and call for an additional production fish and therefore a greater need for maintenance and skill . The extraction of phosphate rock, the main source of phosphorus for fertilizer use, has become a necessity for modern agriculture and is an indispensable nutrient for plant growth and animal feed . However, phosphate rock deposits are limited due to the slow regeneration rate of their cycle compared to carbon or nitrogen, already generating supply shortages due to increasing prices and unequal distribution . In recent decades, estimations have been made regarding imminent depletion if extraction continues at the present rate , which can be drastically shortened by soil erosion caused by unsustainable production practices .
All the P extracted is mostly “lost” from agricultural lands and livestock management through surface and underground runoff . This one-way nutrient flow has increased fourfold since preindustrial times and contributes to great ecosystem damage, such as eutrophication, especially in freshwater environments . While this ever-growing thread demands better management of P sources, there is possible recovery from an ongoing loss of nutrients occurring daily in our wastewater treatment plants . These nutrients contained in wastewater sludge are disposed and managed mostly in complex processes due to the high content of heavy metals, pathogens and other compounds, making it a toxic residue . While direct application of sewage sludge to the soil is practiced in several countries, it’s application can entail a bad management of the soil, due to over application for P fertilization as well as the increase of pathogens and heavy metals into the soil, and the potential problematic of social acceptance of this practice due to the emitting odors . Countries like Sweden have seen a reduction of the unwanted toxic metals since the 1970 and have started to regard sewage sludge as a potential nutrient provider and soil amendment, still only 20% of the sludge is applied in arable land . In recent decades, research has been conducted on the shift from a removal to a recovery approach in urban water cycles in terms of nutrients, not only for their further use in other production sectors but also to prevent their environmental damage in their disposal . One of the byproducts in these sewage treatment plants is magnesium ammonium phosphate , a crystal commonly called MAP struvite or just struvite. Struvite is not a new material; its precipitation was first documented in Los Angeles , but it was approached as a great problem since its precipitation occurs spontaneously at a 1:1:1 molar ratio of magnesium , ammonium and phosphate and under suitable pH conditions . The purging of this uncontrolled struvite precipitation can be the cause of additional expenses due to damaged equipment that needs replacement or increased labor costs . Since then, technologies aimed at struvite removal through induced precipitation in WWTPs have unveiled a product with great fertilizer potential.