Growth of Subterranean Clover in a Range Soil as Affected by Microclimate and Phosphorus Availability. II. Laboratory and Phytotron Studies
1977
Osman, A. | Raguse, C. A. | Sumner, D. C.
Growth of subterranean clover (Trifolium subterraneum L) seedlings on a soil known to be P-deficient was studied under controlled-environment conditions representative of the winter growth period in California's Mediterranean climate annual rangeland. Ambient air temperature was 20C maximum (4 hours) and 4.4C minimum (12 hours), employing natural photoperiods and light intensities from November to March. Phosphorus treatments were approximately 0, 22, 45, 90, and 180 kg/ha. A P adsorption isotherm was determined for the Sobrante and Las Posas soils used. Inoculated seeds of ‘Woogenellup’ subterranean clover were seeded into pots containing 3.6 kg of soil. Plants were sampled at the 1.5, 3.0, and 4.5 stages of trifoliolate leaf development for plant weight and P content determinations. The pots were reseeded with the same cultlvar at 30 seeds/dm² and grown to the five-leaf stage. Shoots and roots were removed separately and used to determine nodule numbers, shoot and root weights, and P and nitrogen tissue content. The P adsorption isotherm predicted a fixation capacity of ca 80 and 120 kg/ha at equilibrium solution values of 0.1 and 0.2 ppm P respectively. Plant weights were significantly affected by P rates only at the 3.0 and 4.5 leaf stages. Total P per plant was a more consistent indicator of P availability than was percent P. Shoot:root ratios, nodules per plant, and percent crude protein in shoot and root increased with P availability. Fertilizer application rates that provide a minimum soil-solution level of 0.2 ppm P are indicated for optimal subterranean clover growth during Mediterranean climate winter conditions.
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