Soil has always been used as nursery and pot substrate in horticultural industry in Kenya, but it has problems related to aeration, drainage, non-uniformity and chemical suitability. Therefore, development of alternative potting substrates with optimal physical characteristics is necessary. The objective of this study was to evaluate physical suitability of forest soil (FS), compost (C), pine bark (PB), and rice husks (RH) as alternative amendments of potting substrates. Ten different substrate formulations containing pine bark (PB), rice husks (RH), forest soil (FS) and compost (CS) [T1 (100% FS), T2 (75% CS: 25% PB), T3 (50% CS: 50% PB), T4 (25% CS: 75% PB), T5 (75% CS: 25% RH), T6 (50% CS: 50% RH), T7 (25% CS: 75% RH), T8 (100% CS), T9 (100% PB) and T10 (100% RH)] were prepared and tested for aeration porosity (AP), total porosity (TP), bulk density (BD) and water holding capacity (WHC). Results showed that WHC, BD, AP and TP of the substrate were significantly affected (P£0.05) by the addition of the amendments. With exception of 25% CS: 75% PB, 25% CS: 75% RH, 100% PB and 100% RH, all the substrate formulations had acceptable water holding capacities. Addition of both RH and PB resulted in significantly (P£0.05) lower BD values, and higher AP and TP of the substrate formulations. This work shows that substrate formulations containing 75% CS: 25% PB, 50% CS: 50% PB, 75% CS: 25% RH and 50% CS: 50% RH had physical qualities within the optimal ranges and are recommended as potting substrates. In conclusion, PB, RH and CS are potentially cheaper alternative potting substrate media with superior or similar chemical qualities to soil and represent better utilization of agricultural and industrial waste materials.