In recent decades, the water shortage and growing population are challenging the supply of economical and suitable food. Therefore, globally greater attention has been given to greenhouse production systems of soilless culture. Investment in horticulture is usually expensive and risky for greenhouse producers. Therefore, monitoring of changes physical and hydraulic properties and stability of growth media due to root growth effects and drying and wetting cycles is important. In this study, fourteen growth media/mixtures were prepared from individual substrates with different volumetric ratios. In order to eva‎luate the alteration of growth media over the time (i.e., during consecutive irrigation events) in the greenhouse, ten drying and wetting cycles were applied on the mixtures in the lab. The water retention curve, saturated hydraulic conductivity (Ks) and oxygen diffusion (Dp/D0) of the growth media were determined before and after the drying and wetting cycles to characterize the alteration of pore size distribution. Besides, the subsidence, decrease of mass and decomposition of the growth media were determined over the time. Different physical and hydraulic indices of the growth media including easily available water (EAW), air after irrigation (AIR), water buffering capacity (WBC) and water holding capacity (WHC) were determined. The alteration of quality of growth media during drying and wetting cycles was eva‎luated by comparing/scoring five properties of EAW, WBC, AIR, Ks and Dp/D0. The BD, TP, EAW, Ks and WHC of the majority of growth media were in the optimal ranges and for some mixtures even better than Cocopeat-Perlite. The Dp/D0 vs. air-filled porosity (AFP) curves indicated that although the AFP values were greater than 10%v/v for some growth media, the Dp/D0 values were not in the optimum range. Thus, the AFP of 10%v/v may not solely be a suitable criterion for the eva‎luation of aeration in the growth media. The Dp/D0 values of some growth media such as RW(0.1)-Perlite, RW(0.2)-Perlite, and Cocopeat-Perlite were in the optimum range. Overall scoring and eva‎luation showed that ten mixtures were classified as good and four as medium. The highest total scores were calculated for Cocopeat-Perlite and WSB500-Rh-V. Drying and wetting cycles could affect the growth media through several processes such as decomposition of organic compounds, displacement and rearrangement of particles, fragmentation of particles, shrinkage, hardening and subsidence. The growth media with a high percent of organic substrates were unstable as compaired with those containing a high proportion of inorganic substrates. In general, the drying and wetting cycles increased the frequency of micropores in the growth media. The drying and wetting cycles positively affected EAW, WHC, AIR and water release percent of most growth media. However, the oxygen diffusion measurements showed that the pore tortuosity of growth media increased after drying and wetting cycles. Therefore, increments in pore tortuosity and micropores over the time decreased the Ks and Dp/D0 in the majority of growth media. This would be an important limitation for the use of growth media in consecutive greenhouse plantings. Upon drying and wetting cycles, the oxygen diffusion was unchanged in RW(0.1)-Perlite but increased in Cocopeat-Perlite. Overall, the total scores were increased, decreased or unchanged for different mixtures after the drying and wetting cycles. Two growth media of RW(0.1)-Perlite and Rh-PlmB300-V, composed of inorganic or mainly inorganic substrates, had the highest stability during drying and wetting cycles. It seems that such mixtures might be suitable substitutes for commercial greenhouse growing media (e.g., Cocopeat-Perlite).

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حسين شريعتمداري

محمدعلي حاج عباسي، مسعود تدين نژاد