Effects of water and heat on seedling emergence

Abstract

The effects of soil temperature and soil moisture content on germination and seedling emergence as known from literature are reviewed. To relate temperature and emergence the heat sum concept can be used. This approach is frequently applied in practice to schedule plantings, to predict maturity, to select crop varieties appropriate to different areas, etc. In such type of studies mostly the environmental air temperature is used. In the case of emergence of seeds it is advisable, however, to register the temperature in the direct environment of the seed, c.q. to measure soil temperature at sowing depth. The results of investigations on the influence of soil moisture content on germination and emergence as reported from laboratory experiments in literature, generally differ widely, mainly because of differences in applied experimental conditions as for example the use of a solution instead of soil as a germination medium. The combined effect of soil temperature and moisture content on seedling emergence was studied with four different kinds of vegetable seeds in field experiments in a clay and a sandy loam profile, both with a shallow and a deep groundwater table. Various sowing dates were applied. From temperature measurements it appeared that the mean daily temperatures of the plots with the higher groundwater tables were 1 to 2 °C lower than the temperature of the plots with the deeper groundwater tables. With the same groundwater depth, clay proved to be warmer than sandy loam. It could be concluded that on the investigated soils the difference in groundwater level was playing a more important role than the difference in type of profile. The maxima and minima in the top soil were higher and the amplitudes decreased with depth faster in soils which had a deep groundwater table. The decrease was more marked in the clay than in the sandy loam soil. It appeared that emergence was highly correlated with rainfall, and that because of more favourable hydrological properties of the soil seeds emerged earlier in sandy loam than in clay. It was found that on all sowing dates the sandy loam plots with the shallow groundwater table showed the highest emergence rate as well as the highest total emergence percentage. The mean heat sums required for 50% emergence were lower on sandy loam than on clay, and the heat sums of the shallow groundwater plots were lower than those of the deep groundwater plots. The minimum temperatures for emergence of the various seeds were calculated using only those treatments in which no limitation of water could be expected. The effect of soil moisture on emergence could be evaluated by calculating the heat sums of all treatments, taking into account the minimum temperature for emergence. As indicator for the minimum moisture content required for emergence, the first five days after sowing were used, for which the average soil moisture content was calculated from sampling data and precipitation records.