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For effective management of irrigation and fertilization, a complete understanding of the hydraulic properties of container media is essential. This study was conducted to test the applicability of an existing predictive model for calculating the unsaturated hydraulic conductivity K(h) of tuff (Scoria, granulated volcanic ash). Two texturally different types of tuff as well as five fractions (0-1, 1-2, 2-4, 4-8, and > 8 mm), obtained from the natural material by sieving, were investigated. A 0- to 1-mm fraction of quartz sand was also tested and compared to the corresponding fraction of tuff. Water retention curves 0(h) (main drying and primary wetting scanning curves) of the media were measured over a 0- to 120-cm suction range, which covers the range of horticultural interest. The saturated hydraulic conductivity K was measured after the determination of the range of validity of Darcy's law. The model parameters were determined by curve-fitting of the measured retention data, and the K(h) relationship was obtained by multiplying the calculated relative hydraulic conductivity curve K,(h). The model prediction of K(h) was validated following direct and indirect approaches. The results showed that a reliable prediction of the unsaturated hydraulic conductivity of coarsely textured container media consisting of tuff is possible using a model commonly used for regular soils.
Rose (Rosa sp.) plants (`Mercedes') were grown in yellow tuff (YT) (volcanic ash, scoria) and pumice from Italy (PI) and Greece (PG) for which physical and hydraulic characteristics were determined. The differences among the measured retention curves of these materials result in significant differences among their relative hydraulic conductivity functions. The hydraulic conductivity of YT is much higher than that of PI, which is higher than that of PG. The plants were subjected to optimal growth and nutrition conditions. Irrigation was controlled using electronic tensiometers, at suction values well within the range of easily available water: 13 cm for YT and 8 cm for the two pumice types. Nonetheless, yields were significantly higher in YT than in PI; yields were even lower in PG. We suggest that the limiting factor was the dynamic water availability to the plants, which is affected mainly by the unsaturated hydraulic conductivity. The relative hydraulic conductivity of YT at 13 cm is more than an order of magnitude higher than that of PI at 8 cm. The relative hydraulic conductivity of PG at 8 cm is two orders of magnitude lower than that of YT at 13 cm. It seems that the current concept of easily available water, based on a predetermined suction range, independent of the hydraulic characteristics of the media, is not an appropriate parameter for irrigation management in soilless culture. The unsaturated hydraulic conductivity, being a characteristic function of the medium and highly sensitive to moisture variation, indicates better the actual availability of water to the roots. Therefore, it should be used for irrigation control in containers filled with porous substrates.
Optimization of irrigation and fertilization regimes in greenhouses and other controlled environments requires accurate and frequent measurements of soil-water content. Recent studies on TDR use in gravely soils and in closed-container studies have indicated a potential use of this method in horticulture. In this study, TDR calibration curves were determined for tuff (granulated volcanic ash), vermiculite, perlite and a mix of two composted agricultural wastes (grape marc, separated cow manure). Widely used as horticultural substrates, mixes of these materials were tested as well. For all soil substitutes tested, measured calibration results are well described by linear equations throughout tested values of water content that cover the working range in horticulture. Ledieu's equation, widely used in soils, describes fairly well the measured results for perlite, but underestimates those obtained for organic media, vermiculite (because of the presence of bound water) and tuff (probably due to water in occluded pores). The differences obtained between the measured calibration equations and Ledieu's equation indicate that in order to avoid an erroneous irrigation management, calibration is necessary whenever a new soil substitute is used.
The effect of fall irrigation level in `Mauritius' and `Floridian' lychee (Litchi chinensis Sonn.) on soil and plant water status, flowering intensity, and yield the following year was studied in a field during 2 consecutive years. At the end of the second vegetative flush after harvest (1 Oct. 1994 and 10 Oct. 1995), four irrigation treatments were initiated: 0.5, 0.25, 0.125, and 0 Class A pan evaporation coefficients designated 100%, 50%, 25%, and 0%. The three lower irrigation levels effectively stopped shoot growth, suggesting the 50% treatment to be the threshold for shoot growth cessation in both years. For both years, flowering intensity and yield in the 100% treatment were lower than those following the other three treatments. Soil and plant water-stress indicators responded to the water-stress irrigation treatments. However soil water-potential values were highly variable relative to plant water potentials. Stem water potential differed more markedly between treatments than leaf water potential. Midday stem water potential appeared to be the best water-stress indicator for irrigation control. Midday stem water potential in both years was correlated with midday vapor-pressure deficit, suggesting that the threshold for irrigation control should take into account evaporative demand.