P. J. Murray and R. O. Clements
Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon, EX20 2SB, UK.
Methods and materials
White clover (Trifolium repens L.) has the reputation of being difficult to establish. There may be several reasons for this, eg. pests, diseases, environmental conditions including soil physical factors. However, interactions between these factors make it difficult to determine which are the dominant influences (Frame and Newbould, 1986). The present series of controlled experiments investigated the effects of three physical factors (soil compaction, soil moisture and depth of sowing), in isolation from each other, on the emergence of white clover.
In all three experiments white clover cv. Olwen, was used from the same seed lot. A proprietary compost was used as the sowing medium, five replicate pots of 125mm diameter each sown with 20 seeds were used in each treatment and the % of seedlings emerging was recorded daily.
Experiment 1. Depth of sowing. Seeds were sown at 5 depths (0, 5, 10, 30 and 60mm) using a card template to set the depth from the top of the pot to the sowing level. The pots were watered as necessary and kept in a glasshouse at 25°C for the duration of the experiment.
Experiment 2. Soil moisture. Five levels of soil moisture were established by adding 0, 20, 30, 50 or 100ml of water to dry compost in 100mm diameter flowerpot saucers. A quantity of the variously watered composts were tested for their moisture contents by oven drying at 80°C for 24 hours. The relative moisture contents were 46, 56, 60, 63 and 68% respectively. After sowing the saucers were placed in sealed polythene bags and placed in an incubator at 25°C with a 14h/8h light/dark regime.
Experiment 3. Soil compaction. Potting compost at field capacity contained in 100mm diameter pots was subjected to 3 pressures (300, 600 and 900KPa) using a modified lever arm oedometer (Scholefield et al., 1985). Two further treatments were also used: firm hand pressure on the compost and compost uncompacted at all. The seeds were sown on the soil surface, covered with a further 10mm of compost and the same pressures were applied as before, to consolidate the soil surfaces.
In experiment 1, maximum, and quickest emergence was for seed sown at a depth of 5mm. Seeds sown at 10mm emerged significantly more quickly (P< 0.05) than those on the soil surface or at 30mm. However after 8 days there was no significant difference in emergence between these depths. Few of the seeds planted 60mm below the surface emerged (Figure 1.).
Optimum moisture content of the soil (experiment 2.) for emergence was 56% - 60%. Emergence was reduced by around 10% at moisture contents of 46% or 63%. Few seeds planted in the wettest soil (68%) emerged (Figure 2.).
Increasing soil compaction reduced seedling emergence, with pots given firm hand pressure having the highest level of emergence. Seeds planted in soil which had not been compacted had lower emergence levels than seeds planted in hand compacted soil (Figure 3.).
Reduction of emergence in sub-optimal conditions is probably attributable to a number of factors. Seeds of white clover are small and consequently have few reserves to sustain a developing seedling. Thus any condition which makes it difficult for the cotyledons of developing seedlings to reach the surface quickly, eg. soil compaction, depth of sowing, will have an adverse effect on the subsequent emergence and development of the plant. The phenomenon of slowed emergence together with moisture stress increases the exposure of the developing seedling to a range of diseases (Skipp and Christensen, 1982) which are potentially injurious to the plant. It is essential therefore that, when sowing white clover, optimal abiotic conditions, which were investigated comprehensively in the present work are created.
Figure 1. Effects of sowing depth on emergence of white clover seedlings
Figure 2 Effects of soil moisture on emergence of white clover seedlings
Figure 3. Effects of soil compaction on the emergence of white clover seedlings
This work was done as part of a commission by the Ministry of Agriculture Fisheries and Food.
FRAME, J. and NEWBOULD, P. (1986) Agronomy of white clover. Advances in Agronomy. 40, 1-88.
SCHOLEFIELD, D.S., PATTO, P.M. and HALL, D.M. (1985) Laboratory research on the compressibility of four topsoils from grassland. Soil and Tillage Research. 6, 1-16.
SKIPP, R.A. and CHRISTENSEN, M.J. (1982) Invasion of white clover roots by fungi and other soil microorganisms. New Zealand Journal of Agricultural Research. 25, 97-101.