Pot soil air composition in conditions of high soil moisture and its influence on chrysanthemum growth

Chrysanthemums were grown in 15.2 cm standard pots in a heavy mix of clay loam soil + sphagnum peat moss (2:1). A fine texture mix was used to accentuate undesirable gas profiles in;he soil. Soil air was analyzed at five depths in the soil profile. In one set of tests, water was applied to the top of the pot at a matrix potential in the center of the soil profile of -5 kPa. The average gas concentrations in soil air in the top acid bottom fifths of soil were for O-2-20.0 and 14.5%, for CO2-0.8 and 2.4%, and for C2H4-0 and 0.08 mu l dm(-3). Smooth concentration gradients of each gas occurred from top to bottom of the soil profile. The composition of soil air changed greatly during the drying cycle. At soil moisture tensions of -0.7, -2.5, and -5 kPa in the center of the soil profile, the gas concentrations in the lowest fifth of soil were for O-2-9.6, 15.3, and 20.3%, and for CO2-4.5, 3.5, and 0.6%, respectively. Thus, soil atmospheric conditions for plant growth were poorest immediately after watering and continuously improved up to the time of watering. When pots of chrysanthemum were watered by capillary action from mats, the average concentration of gases in soil air in the lowest fifth of soil were 5.8% O-2, 3.6% CO2, and 0.38 mu l dm(-3) C2H4. This gas profile was less desirable for growth than the profile found in top-watered pots. Unlike the situation in top-watered pots, the gas concentrations in mat-watered pots were stable. Roots in top-watered pots were restricted to the upper two thirds of the soil ball, and were distributed evenly in the inner part of the soil and at the periphery. Roots of mat-watered plants developed further down the vertical periphery of the pot than roots of top-watered plants, but they did not grow inside the ball. Chrysanthemum plants were grown through a hole in the side wall of each of five 3.9 cm tall by 15.2 cm diameter plastic rings stacked vertically and separated by stainless steel screens that allowed for passage of water but not roots. Water was applied to the top of these cylinder stacks. The largest plants developed in the top ring with progressively smaller plants at lower depths. Plants in the lower two rings developed interveinal chlorosis and did not reach commercial size. (C) 1998 Elsevier Science B.V.