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Farmers have a wide choice of species that can be sown as cover crops during fallow periods to improve soil quality and reduce soil erosion and runoff. But which should they choose? Seed germination is a key stage in plant establishment, particularly when sowing occurs in summer, when temperatures are high and water availability low, and germination for 34 species of potential cover crops in four families was monitored in the laboratory at temperatures ranging from 4.5 to 43°C and at four water potentials (Figure 2.21). Optimal temperatures for germination of seeds varied from 21.3 to 37.2°C; maximum temperatures at which the species could germinate varied from 27.7 to 43.0°C; and base water potentials, the lowest water potential at which a seed can germinate, varied from –0.1 to –2.6 MPa. (Note that at a potential of 0 MPa soil is in a state of saturation, while at –1.5 MPa soil is at its permanent wilting point (see Section 3.3.2).)

Figure 2.21 Niches of cover crops in terms of temperature and base water potential. (a) Response curves to temperature for selected species of cover crops in terms of percentage of seeds that germinate. (b) Niches in one dimension for various species of cover crop. Base water potential is the lowest water potential at which a seed can germinate. At one extreme, Vicia faba is very sensitive to water potential for germination while, at the other, Secale cereale can germinate at very low water potentials.

Source: From Tribouillois et al. (2016).

Most of the cover crops were adapted to summer sowing with a high mean optimal temperature for germination, but some, such as Vicia sativa (Figure 2.21a), were more sensitive to high temperatures. Others, such as Secale cereale (Figure 2.21b), were more resistant to water deficit and germinated even when water potential was very low. Tribouillois et al. (2016) classified the cover crops into functional groups that are of value to farmers when choosing species appropriate for their particular conditions. Thus, functional group 1, which includes Guizotia abyssinica and Setaria italica, has a minimal temperature of 10ºC, a maximal temperature of 41.2ºC and a base water potential of –0.9 KPa. Functional group 4, on the other hand, which includes Brassica rapa and Secale cereale, has a minimal temperature of 0.4ºC, a maximal temperature of 38.6ºC and a base water potential of –2.4 KPa.