No abstract

No abstract

The variety of materials and mixes of materials used in horticulture as growing media is large. Peat products, wood products, composts, mineral fibres, mineral particles, synthetic foams, synthetic fibres and organic fibres; all these products are more or less suitable growing media.Selected physical aspects related to plant growth requirements, are used to discuss the suitability of growing media for plant growth. Aspects mentioned include bulk density, total pore volume, structural stability, root resistance, water retention, rewetting, hydrophobicity, water transport, and oxygen transport. Methods to measure these aspects and experience with the results are also discussed in this review.In conclusion, the physical properties used to assess the suitability of substrates for growth share a large dependence on pore architecture. This made it possible to relate the formulas for water retention, hydraulic conductivity and diffusivity to each other. It seems possible to extend this unification to resistance to rooting and rewetting. Bulk density, total pore space and structural stability exercise their influence on plant growth mainly through the key characteristics for growth assessment: resistance to rooting, water retention, rewetting and water and oxygen transport. Better definitions and possibly new methods to characterise rewetting, hydrophobicity and refreshment rate are advocated. When assessing the influence of water and oxygen transport, individual layers and gradients towards the roots are of importance because of the very fast transport rate changes with water or air filled pore space.

No abstract

A soil oxygen simulation model (OXSI) was tested and evaluated for evaluating growing media with respect to aeration. In the model, local oxygen concentrations are calculated from coefficients of diffusion and consumption (respiration), assuming equilibrium conditions. Apparent oxygen diffusion coefficients (D) were determined under laboratory conditions in 5 cm high samples at different water contents (-3.2, -10 and -20 cm pressure heads). D values were positively related to air-filled porosity (AFP). For fine-graded perlite D ranged from 9.10 -7 at AFP of 34% to 5.10 -9 m 2 s -1 at AFP of 19 %. Possibly due to absence of closed pores in rockwool, the AFP vs. D relation was different for rockwool compared to perlite: D for rockwool ranged from 2.10 -6 at AFP of 56% to 3.10 -9 m 2 s -1 at AFP of 3 %. A greenhouse experiment with cucumber was carried out to determine respiration and realised oxygen concentrations. The cucumbers were grown in 20 cm high, 3.5 litre containers filled with fine-graded perlite and supplied with high-frequency irrigation. AFP varied between 25 and 45%. At three heights and on four occasions during growth, oxygen concentration (% of volume) in the medium varied between 16.6 and 20% in the perlite. Root respiration of the cucumbers as determined by two independent methods (in vivo and in vitro) ranged from 1.4 to 5.4 10 -6 ml.ml -1 .s -1 . Using these respiration rates, OXSI calculated that no oxygen depletion may occur at D > 1 to 5 10 -7 m 2 s -1 , corresponding with an AFP of 30% for both perlite and rockwool. Anoxic condtions were calculated for D values of 10 -8 m 2 s -1 , corresponding with AFP below 10% for rockwool and 20% for perlite.