Dominic Standing and Ran Pauker
Green Point Botanical Garden
Mulch is widely known to be beneficial in the garden: it helps conserve water, reduces weeds, protects against temperature extremes, stops the formation of soil crusts1 and helps improve the soil. In terms of water conservation the following observation dramatically illustrates just how important mulch can be.
Here, at Kibbutz Nir Oz Green Point botanical garden we received 205 mm rain in just three days (December, 2013). Three days after the rains stopped we took some measurements of soil wetting depth in one of our experimental plot. On a flat, unvegetated and unmulched area of sandy loess soil that does not receive run-off we used a 2.4m augur to determine the wetting front in the soil profile. We removed 10 cm samples and visually inspected them. After 85 cm depth the wetting front was reached and the soil below that was dry (Figure 1). In an adjacent, mulched plot we removed the surface layer of mulch from under an Acacia redolens 'desert carpet' plant and repeated the drilling. We found that the soil was still damp after 240cm, i.e. the entire length of our augur (Figure 2)! In another nearby site (all three drillings were done in the a 10 x10 m plot) we removed the surface litter from underneath a Pistacia atlantica tree and repeated the drilling. The result was the same as for the Acacia redolens 'desert carpet' (Figure 3)
Without mulch we lost more than 70% of the water to runoff!
However, applying mulch comes with a cost. We have found that a 20 cm depth of mulch
requires 20 mm rain/irrigation to thoroughly it (Figure 4). This water does not enter the soil and is 'lost' to the plants. If there is a long dry spell between rain and irrigation and the mulch dries out then an additional 20 mm of rain or irrigation is needed (note that this does not apply to drip irrigation systems). We have called this the 'wetting tax'. Despite this we still strongly recommend the use of shallow mulch as in Figure 3 in gardens. This is because of the longterm benefits of mulching in maintaining soil moisture levels, i.e. the additional water used in each sprinkler irrigation event is more than compensated for by the increased time needed between irrigation cycles. Mulching is effective at depths over 2 cm and obviously, the thicker the mulch, the higher the 'wetting tax'. For most gardens 10 – 20 cm depth is recommended.
Soil crusts can be physcial or biological. Physical crusts form by the action of water droplets hitting the soil surface and causing dispersion of aggregates and separation of the fine and coarse material. The fine material, such as clays, remain at the surface and form the crust. The soil and water chemistry also plays a part in this process 9Agassi et al., 1981). In dry loess soil initial water penetration can be 60 mm per hour. As the crust forms this rate can drop to 2 mm per hour and it can take as little as 30 mm of rain for the crust to form (Agassi et al., 1981)! Biological soil crusts form through the growth of a community of soil flora (bacteria, archaea, fungi, algae, mosses, lichens, cyanobacteria) on uncovered soil. The crust is composed of these organisms, their byproducts and soil particles. The composition of the crust dictates infiltration rate and can either be higher or lower than bare soil.
Reference
Agassi, M., Shainberg, I. and Morin, J. (1981). Effect of electrolyte concentration and soil sodicity on infiltration rate and crust formation. Soil Sci. Soc. Am. J. 45:848 – 851.
