Three eco-friendly approaches were investigated for their effects onimproving K-uptake by zucchini plants; hence, increasing shoot growth and fruit productivity. These approaches were: (1) amending soil with biochar (0 and 5g kg(-1)), (2) spraying plants with Amphora extract (0 and lg L-1) and (3) substituting inorganic-K fertilizers partially with organic ones. In this concern, five levels of compost (0, 1.5, 3, 4.5 and 6 g kg(-1)) were applied to a poor fertile sandy soil (98.5% sand) in complementation with inorganic NPK-fertilizers to bring their concentrations, even in the control treatment, to the recommended levels. This investigation was conducted under the greenhouse conditions during the summer season of 2020. Results obtained herein reveal that application of either of biochar or Amphora extract exhibited positive effects on soil available-K (20 and 6.5% respectively), Kconcentrations in fruits and total K-uptake by the aboveground plant parts. Furthermore, these two amendments raised considerably the dry weights of both zucchini shoots and fruits. On the other hand, compost could partially substituteinorganic-K fertilizers when it was applied at a rate not exceeding 3gkg(-1),recording significant improvements in all the investigated parameters. Nevertheless application of high compost doses reduced significantly fruit yield. Generally, there existed a highly significant linear relation between the total K-uptake by plants and fruit dry weights. Accordingly, treatments that improved considerably K-uptake by plants(the combination between foliar application of amphora extract + 5 g kg(-1) biochar + 0-3 gkg(-1) compost)recorded the highest improvements in productivity of zucchini plants.

Compost is commonly utilized to improve properties of infertile sandy soils, despite its high biodegradability which may increase greenhouse gases emissions. It is possible to combine compost with biochar, which degrades at a slower rate, forming a “co-composted biochar” product. This mixture could enhance plant growth parameters beyond those attained for using each component, individually. To investigate this assumption, zucchini was selected as a test plant to be grown, under greenhouse conditions, on a sandy soil that received biochar, compost or co-composted biochar (from rice straw or sugarcane bagasse (SB)) for a duration of 15 days. This timeframe was deemed sufficient to achieve a relatively stable degradation rate for compost. Application of organic materials increased both fresh and dry weights of zucchini plants, particularly when co-composted biochar of SB was used. Specifically, plant fresh weights increased by 1.24–1.71 folds when using this additive versus the control group. Additionally, availability of nitrogen, phosphorus, and potassium in soil and their uptake by plants significantly increased owing to application of all additives, with superiority for the co-composted biochar of SB. Enhancements in plant fresh weights were strongly correlated with increasing availability and uptake of phosphorus by plants. In conclusion, organic amendments have a substantial positive impact on enhancing the nutritional status and growth of zucchini, even during the early vegetative growth stage (within the first 15 days after planting). The greatest improvements were observed when co-composted biochar of SB was used and this confirm the main hypothesis of the study.