Healthy soil, potential breakthrough research

Healthy soil differs from dirt in the myriad of life forms it contains.  Soil organisms (the biological properties of a soil) aerate, decompose plant material, assist nutrient absorption, build soil structure, reduce water requirements and play a vital role in the nitrogen cycle.
A proper balance of nutrients and pH obviously play a significant role in healthy plant growth.  Biological activity can also affect the pH of the soil and subsequent impacts on the utilisation of of nutrients.
The biological properties of a soil decompose the organic matter in the form of litter falling on the soil surface and subsequently incorporates it into the soil profile.  Nutrients are released as by-products, including the mineralization of nitrogen and nitrogen fixation (making nitrogen available to plants).  The soil’s physical properties, including moisture content, temperature, aeration (bulk density) pH and nutrient content are necessary for a diverse soil organism population and a high level of activity.  Limiting any of these factors limits soil organism activity, organic matter cycling and subsequently reduces soil fertility.
The undisturbed forest soil has organic layers in it’s profile.  These organic layers comprise leaf litter at the surface, a decomposition layer and a humus layer. These large, high-molecular-weight molecules are made up of carbon, nitrogen, soil minerals and soil aggregates. The resultant humus is a stable, inseparable part of the soil matrix that can remain intact for hundreds of years.
When OM is incorporated into the surface soil it forms the topsoil (A horizon) and this horizon has the most optimum conditions for plant growth.
Urban soils are generally highly disturbed soil environments that do not mimic the soil conditions of natural forest soils.  Human activity is the predominant agent in the formation of urban soils. Urbanisation also contributes unique amendments and contaminants to the urban soil.
Disturbing soil profiles, mixing and filling, and contamination lead to variability. This can also lead to compaction; restricted aeration and water drainage, crusting and bare, often hydrophobic surfaces.  Importantly, interrupted nutrient cycling and modified soil organism activity result in unhealthy (low vigour) growth and predisposes plants to other environmental and biological stresses.
Tree decline is as a result of several interacting factors, such as prolonged drought, competition with adjacent trees, pest and disease infestation and generally depleted soil environments.  Poor soil structure is a significant predisposing stress factor for plants.  Poor soil structure affects water infiltration and permeability, water holding capacity, aeration status and loss of pore space.  If plants do not have access to sufficient water and oxygen they will decline.
Healthy soil requires organic matter (OM); it aids in aggregation (soil structure), cation exchange (storehouse of nutrients) and water holding capacity. But most importantly, it sustains biological life in the soil; the greater amount and diversity of biological life there is in a soil the greater potential for healthier plant growth.  Soil health is the basis for plant health and by extension, healthy plants can help develop healthy humans.
Tree Logic has commenced a trial to incorporate humus into urban soils. Ninety established Plane (Platanus spp.) street trees exhibiting decline symptoms, were treated using liquid humus, Seasol® and urea.  A number of trees in parks and reserves within an eastern Melbourne municipality were also treated.
A sample of the trees to be treated along with controls in close proximity that included healthy Planes and Planes displaying similar malaise were established prior to the commencement of treatments. The samples looked at current season shoot tip extension and leaf size. In addition, leaf samples were fluoresced and data captured on short and long term stress indicators. This data will be compared against future samples to determine the efficacy of the treatments over a 2 year period.
Injecting liquid humus into urban soils will start to build up the organic content of the soils which will aid water holding capacity, cation exchange capacity, and reduce leaching. Organic matter can also buffer effects of any pesticides and stabilise pH.  Treatments are carried out in autumn and spring (before bud burst). Humified carbon differs physically, chemically and biologically from the labile pool of organic carbon that typically forms in agricultural soils. Labile organic carbon arises principally from biomass inputs (such as leaf litter) which are readily decomposed.
In conjunction with the liquid humus the soils can also be injected with liquid nitrogen. In degraded urban soils the major nutritional limitation on plant growth is lack of nitrogen (Pulford, 1991). Without an adequate pool of organic nitrogen, which microbial activity breaks down into forms (NH4+ & NO3-), mineral nitrogen is quickly depleted in soils and nitrogen deficiencies develop. Low organic matter content is typical of urban soils consequently nitrogen is the nutrient most limiting to growth (Roberts, Jackson, Smith, 2006).
The other consideration is the on-going nature of the applications, that is, to be successful a number of applications will be required. Products, application timing and rates and costs could be developed upon request.


About Stephen

Stephen is the Manager Consulting and a Director of Tree Logic

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