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The Effect of Seeding Depth, Soil Moisture Regime, and Crust Strength on Emergence of Rape Cultivars¹

Emergence of rape seedlings Brassica napus L. and B. campestris L.) on Typic Cryoboralf (Gray Wooded, Luvisolic) soils may be reduced by the formation of hard crusts. Twelve of these soils were sampled from the surface horizon (Ap) to examine the effect of soil type on emergence of rape and determine if particle-size distribution and organic matter or organic carbon percentage were related to soil crust hardness. Four split plot greenhouse experiments were conducted to examine the effect of seeding depth and moisture regime on seedling emergence of rape cultivars.

Multiple regression analyses showed that silt percentage (0.05-0.002 mm size) was positively related to crust strength measured by modulus of rupture and that organic matter and organic carbon percentage were negatively related to crust strength.

The seedling emergence of rape cvs. ‘Target’, ‘Zephyr’, ‘Span’, ‘Regent’, and ‘Candle’ was significantly higher at the shallow seeding depth of 1.5 cm than the 3.0 cm depth (18, 38, 49, 14, 16 vs. 7, 35, 42, 7, and 7% respectively, at 6 days after seeding). Emergence of rape seedlings campestris species, Span was 69% vs. 61% napus species, Zephyr 13 days after seeding. However, with the low glucosinolate Candle (campestris) and Regent (napus) on the average emergence was 56 vs. 74%, respectively, under a high moisture regime (watered to 100% field capacity). Under low moisture regime (50% field capacity) emergence was 15% for both species. Under the low moisture regime, there was no emergence of Regent on seven soils, whereas with Candle there was no emergence on four soils. These results indicated a different threshold soil condition under which these two cultivars would emerge and resulted in significant soil and crop by moisture interactions.

The greater percentage emergence of the smaller seeds of Span than the larger seeds of Zephyr was attributed to less energy required to push the smaller cotyledons of Span through the soil crust. Similarly, under dry soil conditions, the smaller seeds of Candle emerged on more soils than the larger seeds of Regent.

Seedling emergence of Candle and Regent grown on Typic Cryoboralf soils was significantly related by regression analyses to soil crust strength measured by penetrometer and by modulus of rupture under different seeding depth and moisture regime conditions.

Key Words: Modulus of rupture • Organic matter • Organic carbon • Penetrometer measurements

² Research scientist in charge of Soils Section.

Received for publication July 24, 1981.