Interacting effects of subsoil acidity and water on the root behaviour and shoot growth of some genotypes of lucerne (Medicago sativa L.)

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Abstract
The effects of additions of calcium carbonate to a low-calcium, acidic subsoil, underlying a fertilized, limed surface soil, on the root and shoot growth of four lucerne genotypes were studied in deep (85 cm) soil columns. Separate surface and subsoil watering treatments were included after plants were established. A comparison showed that, at low rates of addition, calcium carbonate was more effective than magnesium carbonate in promoting root elongation, but more generally the increase in pH appeared to be the important factor. Surface droughting reduced shoot growth, but the response to subsoil lime continued as long as subsoil moisture was supplied, in accordance with the increased root penetration and ability for water uptake. Thus shoot growth under surface drought reflected (r = 0.973) the total length of deep roots (> 60 cm depth) over all genotypes and lime treatments. The genotypes, which included three from cv. Hunter River and one from cv. Siro Peruvian, differed in several respects. Our selections from Hunter River varied in root morphology, shoot:root ratios and type of response to subsoil lime. The Siro Peruvian genotype produced abundant deep, fine roots and good shoot growth under surface drought.