Sri Lanka can benefit from study into soil health improvement: Academic don 

BY Ruwan Laknath Jayakody 

Since there is limited literature available on soil health improvement in farming systems on tropical soils, any comprehensive study on soil health improvement under tropical conditions will be beneficial for bridging existing knowledge gaps.

This observation was made by Dr. M.G.T.S. Amarasekara (attached to the Rajarata University’s Agriculture Faculty’s Agricultural Engineering and Soil Science Department), in an editorial note on “Managing soil health towards sustainable agriculture” which was published in the Sri Lankan Journal of Agriculture and Ecosystems 3 (2) in December 2021.

Soil health has been defined by J.W. Doran and M.R. Zeiss in “Soil health and sustainability: Managing the biotic component of soil quality” as the capacity of a soil to function as a vital living system within the boundaries of ecosystems and the use of land, in order to sustain plant and animal production, maintain or enhance water and air quality, and promote plant and animal health. It is, Amarasekara explained, a key factor to be improved in order to move towards productive and environmentally sound farming systems, and hence, knowledge of soil functions such as decomposition, the cycling of nutrients and the dynamics of microbial populations, and their contributions to plant growth is vital so as to design soil health management practices. 

Soil health and soil quality are two terms used to indicate the condition of a given soil; however, they may be differentiated in terms of the time scale with “soil health” referring to the condition of soil in a short period and “soil quality” referring to the same over a longer period, per D.F. Acton and L.J. Gregorich’s “Understanding soil health”.

Decomposition and nutrient cycling processes in the soil are generally governed, Amarasekara elaborates, by soil microbes. 

1. Lal’s “Restoring soil quality to mitigate soil degradation” noted therefore that unhealthy soil characteristics such as conditions of salinisation, acidification, compaction, crusting and water logging may adversely affect the soil biota biodiversity, thus reducing the quality of healthy soils. 

On the other hand, A. Orgiazzi, R.D. Bardgett, E. Barrios, V. Behan-Pelletier, M.J.I. Briones, J.L. Chotte, G.B.D. Deyn, P. Eggleton, N. Fierer and T. Fraser’s “Global soil biodiversity atlas” explained that the population density of soil microbes depends on factors such as the climate, vegetation, soil organic carbon and soil pH (a scale used to specify the acidity or basicity of an aqueous solution). 

Soil aggregates, as explained in M. Drazkiewicz’s “Distribution of microorganisms in soil aggregates: Effect of aggregate size” play a significant role by providing the physical environment for soil microorganisms. The soil type is, as found in K. Qin, X. Dong, J. Jifon and D. Leskovar’s “Rhizosphere (the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root micro biome) microbial biomass is affected by the soil type, organic and water inputs in a bell pepper system”, an important variable which determines the microbial population. Soil organisms such as bacterial and fungal species are involved in the process of decomposing complex organic substances and the release of nutrients. Also, microbes involve many nutrient cycling processes in the soil system (for example, nitrogen in the soil undergoes many transformations through microbial activities, with the process of converting organic nitrogen to plant available ammonium being called mineralisation and the process of a specific group of bacteria converting ammonium to nitrate being called nitrification). 

Macro-organisms are also important for restoring the soil quality. Amarasekara pointed out that the conversion of conventional farming to organic farming with mulching, cover cropping and adding crop residues can increase the activity of soil macro-organisms while management practices such as the use of inappropriate implements, the overuse of agro chemicals and the burning of vegetation, result in a reduction of the soil biota, both in terms of the biomass and the diversity, thus altering the population of soil organisms. Therefore, Amarasekara added that identifying the threats and interventions to soil microbial functions is critical for both soil health management and agricultural sustainability. 

Moreover, soil organic matter (SOM), Amarasekara noted, improves soil health (varies from 1-8% in agricultural soils depending on the climate and management practices). Soil organic matter as noted in “Environmental and economic costs of soil erosion and conservation benefits” by D. Pimentel, C. Harvey, P. Resosudarmo, K. Sinclair, D. Kurz, M. Mcnair, S. Crist, L. Shpritz, L. Fitton, R. Saffouri and R. Blair, serves as a source of nutrient for plant growth, soil aggregation enhancement, soil moisture retention, increased infiltration and the reduction of the risk of soil erosion. It also provides food for living organisms in the soil, thus improving soil biodiversity. The depletion of the SOM pool in agricultural soils is an issue which indicates the risk of soil degradation. Therefore, Amarasekara observed that it is essential to manage the SOM pool above the threshold level (1-1.5%) in order to reverse the process of soil degradation. 

Soil carbon storage is a vital ecosystem service as it sequesters additional atmospheric carbon dioxide (CO2) into soil organic carbon (SOC). Since the SOC pool can be depleted when nutrients are limited for the growth of soil microbes, integrated nutrient management would be, per Amarasekara, a viable strategy to increase SOC in agricultural fields.

Soil degradation leads to the decline in soil quality parametres with significant reduction in ecosystem functions. The physical degradation of soil generally affects structural attributes including pore geometry and continuity, the formation of crust on the soil surface, the lowering of the water infiltration, the increasing of the surface runoff, and soil erosion, per Pimentel et al. Soil chemical degradation is reflected, per Amarasekara, by nutrient depletion, acidification, salinisation, the lowering of the cation exchange capacity (CEC – a measure of the soil’s ability to hold positively charged ions which is a very important soil property influencing the stability of the soil structure, the availability of nutrients, the soil pH and the soil’s reaction to fertilisers and other ameliorants), increased toxicities, and contamination by hazardous wastes. The biological degradation of soil is, for Amarasekara, characterised by the loss in soil biodiversity, the depletion of the SOC pool, and the increased emission of greenhouse gas from the soils into the atmosphere. For Lal, the combined effect of all three types of degradation processes leads to the disruption in ecosystem functions such as elemental cycling, water infiltration and purification, the altering of the hydrological cycle, and a decline in the net biome productivity.

Soil resilience is the capacity of the soil to recover its quality in response to any natural or anthropogenic alteration or as R. Lal’s “Degradation and resilience of soils” describes it, a kind of ability of a soil to regain its quality by mitigating adverse external influence. In this regard, J.K. Syers’s “Managing soils for long term productivity” emphasises that managing the quality and quantity of the SOC pool is one of the crucial factors in order to strengthen soil resilience and to reduce the risk of soil degradation.