The establishment of research protocol for horticultural substrates must include the measurements of physical and hydraulic properties. Both empirical and mechanistic approaches must be utilized in protocol development. The steps in establishing a protocol should be: 1) the development of standard definitions for properties and terms, 2) development of a mechanistic framework based on modeling, and 3) development of procedures for data collection. Pore space diagnostics and available water determinations must be reexamined to more appropriately describe these terms. The concept of hydraulic status is introduced as one diagnostic protocol for physical and hydraulic properties.

Addition of a polyacrylamide hydrogel to pine bark and pine bark + sand substrates had no effect on total porosity, regardless of incorporation rate. Container capacity was increased with increasing rate of hydrogel in both substrates. Air space in pine bark was slightly increased at the lowest rate but was reduced with higher incorporation rates. Air space in pine bark + sand was reduced with all hydrogel additions. The dry weigh', of hydrogel cubes recovered from both substrates was similar to amounts predicted. This result indicates that blending hydrogel granules into the substrates was uniform and did not contribute to variability in hydrogel studies. After allowing dry hydrogel granules to expand freely in distilled water for 24 hours, hydrogel granules expanded 317 and 372 times their dry weights at the lowest and highest rates, respectively. Reduction of expansion (in water) at higher rates may have been due to physical restriction of expansion. Conversely, recovered hydrogel cubes from substrates watered to drainage (-10% excess) for 6 weeks absorbed 25 to 55 times their dry weight while in the container. Subsequent rehydration of extracted gels in distilled water was greater for hydrogel cubes from the pine bark + sand medium (104 to 130) than in pine bark alone (51 to 88). Because of anomalies in hydraulic conductivity and pressure plate contact, three techniques were used to study unavailable water content in gels expanded in distilled water. Hydrogel cubes placed in direct contact with the pressure plate released ≈95% of their water at pressures ≤ 1.5 MPa. Effectiveness of ployacrylamide gels in coarse-structured substrates is influenced by physical restrictions to expansion in the substrate and hydraulic conductivity between the hydrogel cubes and the surrounding substrate.

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Pore sizes have traditionally been divided into macropores and micropores with the division between the two being arbitrary. Since most mixes used in container production are ≥ 80% pores by volume, a more detailed pore-fraction analysis seems warranted. Taking into account hydraulic properties and irrigation parameters, pore-size distribution curves were separated into four ranges. Macropores were selected as pore sizes > 416 µ. Pore sizes within the macropore range cannot hold water under tension induced by gravity when allowed to drain after saturation. Mesopores were selected as being in the pore size range of ≤ 416 to ≥ 10 µ. Micropores were categorized into the pore-size range of 0.2 to 10 µ. This would be equivalent to volumes of water held between 30 kPa and 1.5 MPa. The water in these pores may be viewed as a type of water stress "buffer" not commonly used under normal irrigations but extracted by plant roots when suctions exceed 30 kPa. Ultramicropores hold water at suctions > 1.5 MPa and would be found in pores with effective pore diameters < 0.2 µ. This water would be considered unavailable to plants. Data derived from this analysis were in good agreement with traditional measures of pore space and particle size distributions for peat-based, bark-based, and soilbased substrates.

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