Hydromulching – A Global Road Technology perspective

Hydromulching – Australia is a country of weather and climatic extremes. Climate change, man made or otherwise, seems to also be driving further extreme weather conditions being experienced over the past few years. Just recently, Australia’s hottest and driest year on record was in 2019 a clear indication of the dire conditions in the country. The ripple effect of such conditions is also accelerated by drought which affects soil moisture content and hence leads to damaged and depleted soils which are susceptible to soil, wind and water erosion. Bushfires in the recent times have also increase susceptibility to soil erosion, loss of nutrients and post-fire exposure to wind and rain erosion as a result of the loss of vegetative cover. Hydromulching is a technique that is used to reduce the effects of soil, water and wind erosion through provision of temporary soil cover. Ground cover is lost as a result of westerly winds that normally blow across the country with environmental casualties such as loss of topsoil and dust generation amongst many others. Given the harsh conditions there is definitely need for sustainable revegetation to cater for the rapidly changing environment marred by water scarcity, drought and various climate types. The article seeks to evaluate mulching in general leading to hydromulching and where it is utilized from a GRT perspective. 

Organic matter and topsoil

Soil erosion plays a pivotal role in the operations of ecosystems and disturbances such as bushfires can accelerate soil and fertility losses. Eradication of the vegetative and litter layer by fire escalates overland flow due to reduction of rainfall interception and resistance to flow with sediment losses increasing splash erosion by rainfall. Fire induced heating of the soil can reduce aggregate stability, decrease porosity and increase soil water repellency and these changes can decrease infiltration and increase soil erodibility. By definition, mulching is referred to as the agronomic practice of placing mulch on the soil surface for soil and water conservation and to favor plant growth. Mulching reduces both the overland flow generation rates and velocity by increasing roughness and it cuts the sediment and nutrient concentrations in runoff. It also improves infiltration capacity whilst increasing water intake and storage. Reduction in topsoil temperature is achieved through mulching which favors optimal germination and root development whilst decreasing evaporation. Mulching in general, is effective against erosion because it reduces runoff and erosion rates through two mechanisms. Firstly, it increases interception storage capacity, which then reduces the amount of rain available for producing runoff, it reduces runoff velocity, and it increases soil moisture. Secondly, mulch protects the soil surface against kinetic energy of rainfall drops and decreases the hydrodynamic power of flowing water. 

Hydromulching – an introduction

A variant of industrial mulching is known as hydromulching. It consists of a slurry of water as a carrier for the spraying of seed, tracking dye, some form of wood fiber mulch, and often a tackifier which is the chemical compound used to increase the stickiness of the slurry on the seedbed. These areas include low to moderate slopes and highly erosive soils more especially those adjacent to surface water features which partially or are completed denuded of vegetation. It can be easily applied through spraying onto slopes, rough seedbeds and uneven surfaces. Some of its advantages include strong binding to the soil surface by action of the soil-binding agent; therefore it is particularly useful for steep slopes and strongly modified areas such as quarries, construction sites and cut and fill slopes along roads. The structure and micromorphological features of hydromulching shows that hydroxyl generates intermolecular hydrogen bonds between carboxymethylcellulose sodium (CMC) and polyvinyl alcohol (PVA) with hydrogen bonds formed between the two molecules enhancing their compatibility therefore increasing effectiveness of hydromulching and shaping a dense mesh structure in the soil. Soil consists of an extremely complex system of porous solid particles and intergranular porosity. The liquid film injected into the soil can move through the pores between soil particles after contacting the porous soil particles the film forming properties of the hydromulching close the voids between soil particles. 

Hydromulching goals

The purpose of hydromulching is to foster revegetation in high risk areas by immediately creating temporary cover for the soil in the process reducing raindrop impact and binding together loose soil and ash to protect from overland flows and wind whilst increasing soil water retention. Combining this with seed, it is an efficient way of quick revegetation. Hydromulching can increase soil’s saturated hydraulic conductivity, water use efficiency, microbial and enzyme activity and production and temperature. Runoff and soil erosion are reduced owing to hydromulch’s ability to increase interception storage and protect the soil surface. The limitations of hydromulch come when it is utilized on long slopes where it is less effective because runoff water begins to concentrate and channelize as it flows down the hillside, carrying sediment and mulch with it. Hydromulch also costs dearly compared to other mulching variants such as straw mulching but the tradeoff to cost is the ability to be implemented for steep slopes and in areas that are difficult to access with preference for its use in value at risk areas such as at reservoirs, cultural or natural heritage sites and industrial plants. 

Benefits and limitations

Hydromulch is known to have positive effects on soil moisture conservation and the inhibition of evaporation. Growth mediums in hydromulching perform the role of holding and sustaining moisture long enough to support quicker vegetative growth whilst minimizing the effects of erosion. Primarily, hydromulching is restricted to areas that accessible with roads, bridges, homes because good access of roads is key for the applicator with application rates varying from 800 to 2000 pounds of mulch fiber per acre. Different hydromulch materials can be used as either permanent or semi-permanent protective cover over the soil surface. For all revegetation needs GRT offers seed germination and strike rate through their biocatalyst technology in GRT Nature Plus which allows for greater penetration, mobility and increased uptake of nutrients by plants and is an ideal product to give excellent growth, root development and sustainable vegetation health. Overall, Australian conditions requires hydromulching components which are suitable for the type of soil and climate present in Australia. 

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REFERENCES 

Jian, G., Guanhgua, Y., Ningning, M., Zhan-xiang, S., Xue, L., Shijie, W., and Xiaole, Z. 2018. Microscopic mechanisms of soil moisture related to hydromulching. Journal of Water and Climate Change. 

Prats, S.A., Malvar, M.C., Viera, D.C.S., MacDonald, L., and Keizer, J.J. 2013. Land Degradation & Development. 

Texas A&M Forest Service. 2012. Wildfire Recovery – Soil Erosion Control Practice Guide. 1-16.