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Critical Ca Levels and Ca/Mg Ratios in Trifolium subterraneum L. Grown on Serpentine Soil¹

Serpentine soils are known to have a low Ca/Mg ratio and to respond to applied Ca in many instances. To assist in assaying the Ca status of these soils we determined the effect of growth stage, plant part, and defoliation treatment on the critical Ca concentrations and Ca/Mg ratios in subclover growing in potted serpentine soil of the Henneke series, a lithic Argixeroll.

One of six rates (0, 200, 400, 800, 1,600, and 3,200 ppm) of Ca as CaSO₄ and uniform rates of P, K, S, and Mo were applied to each pot and the soil was mixed uniformly and planted to ‘Geraldton’ subclover (Trifolium subterraneum L.). On one set, leaves were removed 61, 85, and 114 days after planting. On a second set, leaves were removed on days 85 and 114, and the third set grew without clipping until day 114.

Critical Ca percentages in the three youngest leaves ranged from 0.55 to 0.69%, with no consistent trend due to growth stage or number of times the plant had been defoliated. In old leaves, critical Ca levels ranged from 0.81 to 1.47y0 and were higher for ll4-day-old plants than for 85-day-old plants. The more often the plant was defoliated the lower the critical Ca level in old leaves. There was no significant difference between young and old stem tissue in critical Ca levels: 0.30 to 0.38% Ca. Defoliation did not induce a consistent change.

Critical Ca/Mg ratios in young leaves increased from 0.42 to 0.60 between plant ages of 61 and 85 days, whereas no significant change occurred from day 85 to 114. With plants defoliated for the third time at day 114 the critical Ca/Mg ratio was 0.36, although at the first and second clippings their critical ratios were 0.60. Calcium/ Mg ratios were similar in young and old leaves. In young stems the critical Ca/Mg ratios on day 114 for the first, second, and third clippings were respectively 0.48, 13.39, and 0.21. The same values for old stem tissue were 0.38, 0.27, and 0.18.

It was concluded that percent Ca and Ca/Mg ratios in new and old stems were insensitive as measures of Ca status since these values changed less in stem than in leaves over a wide range of Ca levels. Percent Ca in young leaves was the best measure of Ca status because critical values varied less due to stage of growth or clipping treatment than did percent Ca in old leaves or Ca/Mg ratios in young or old leaves.

Key Words: Magnesium in subclover • Defoliation • effect on critical Ca and Ca/Mg • Maturity • effect on critical Ca and/Mg • Plant part • effect on critical Ca and Ca/Mg

² Agronomist and staff research associate, Dep. of Agronomy and Range Science; staff research associate, Univ. of California, Hopland Field Station, Hopland, CA 95449.

Received for publication February 6, 1976.