How can soil be healthy?

The characteristics of the exudates changes according to the age of the tree, season, chemical inputs, water availability and tree nutritional status. Therefore, the rhizosphere is a constantly changing environment in which the relative number of each member group adjusts according to the nature of the exudates and the environmental conditions. For example, when a plant wilts due to water stress, more amino acids are released by the roots to the rhizosphere. Likewise, more organic acids are released into the rhizosphere in response to iron (Fe) or P deficiency, which then solubilise Fe, P, Zn and other metals, thereby enhancing their bioavailability (McLaughlin et al., 2000). Thus, the addition of external soil microbiological systems has little direct effect on root and plant health or plant nutrition. We can then, instead of referring to "healthy soil", rather pursue healthy roots through well-managed soil.

In established orchards, maintenance of an adequate amount of organic carbon (C) in the soil, ensures that microbial activity and diversity in the soil outside the rhizosphere is sustained. The simplest and most logical way to increase the soil biological activity and diversity beyond the root zone is therefore simply to increase the soil organic C (preferably Walkley-Black analysed) – it was proven that there is a direct correlation between soil C content and microbial biomass and activity (Witter et al., 1993; Leita et al., 1999; Bhogal et al., 2009), as shown in Figure 1. In addition, soil organic material also increases the water holding capacity and nutrient retention ability of soil.

An increase in soil C from a very low level, and subsequent increase in soil biological activity, will also enhance overall soil physical conditions, i.e., reduce susceptibility to compaction and increase soil aeration. This benefits root health and growth, and consequently the rhizosphere microbial population, which fosters tree health. The maintenance of an appropriate (not necessarily high) soil organic C level is, therefore, good common sense.

Soils that are higher in clay content generally retain more organic C than sandy soils. As a result, well-managed clayey soils have higher biological activity than sandy soils. But structure, water holding capacity, nutrient retention and biological activity of sandy soils can be increased when inputs of organic material are greater than losses, but the converse is also true. In situ plant biomass production is a viable input, whereas amendments added to soils are more costly. And since losses occur when organic matter decomposes, the cost and effort to retain a certain level of soil organic C needs to be weighed up against the benefits. It often does not justify the cost and effort.

Producers who want to establish the soil C levels must note that laboratory analysis must be interpreted correctly. Total soil carbon (TC) cannot be equated with soil organic carbon that is oxidisable and therefore, utilised by the soil micro-organisms (i.e., Walkley-Black analysed carbon). Any TC analysis also includes other carbon sources, like the carbon in lime (CaCO3), biochar and even plastic or coal (ash) residues. All last-mentioned forms of C are inert and do not originate from biological organisms. Therefore, total carbon is not a certain indicator of the soil's microbiologically available carbon. It is suggested that producers who want to monitor and manage their soil carbon, distinguish between the different types of carbon found in the soil, as illustrated in Figure 2.

Many soils contain free lime, and sometimes residues of biochar or wood ash. Hence, a Leco analysed carbon value (total combustion) is not always helpful. Rather, it's advisable that either the total C (TC) as well as the inorganic C (TIC) are analysed to obtain the total organic C (i.e., TC-TIC = TOC), or that the Walkley-Black carbon (i.e., oxidisable C) is quantified. For reference, appropriate soil oxidisable organic carbon levels (as analysed by the Walkley-Black method) for South African conditions are indicated in Table 1.

Producers who are concerned about soil health, should focus on responsible soil management practices, i.e., correcting and maintaining the soil chemical and nutritional status, limit soil compaction and avoid over-irrigation. Where physical and chemical conditions are optimal for the intended crop, balanced and beneficial soil biology is established. Maintenance of soil C is usually also a given in well-managed orchards where the soil is not cultivated regularly, and the produced plant biomass (e.g., dropped leaves, prunings and inter-row vegetation) is not removed. A well-managed soil is a "healthy soil".

References
Bhogal, A., F.A. Nicholson, and B.J. Chambers. 2009. "Organic carbon additions: effects on soil bio-physical and physico-chemical properties." European Journal of Soil Science 60, 276-286.

Isonio, E. 2021. What is healthy soil? FAO coins the official term. RE Soil Foundation. https://resoilfoundation.org/en/environment/healthy-soil-official-definition/

Leita, L., M. De Nobili, C. Mondini, G. Muhlbachova, L. Marchiol, G. Bragato, and M. Contin. 1999. "Influence of inorganic and organic fertilization on soil microbial biomass, metabolic quotient and heavy metal bioavailability." Biol. Fertil. Soils 28, 371-376.

McLaughlin, M.J., R.E. Hamon, R.G. McLaren, T.W. Speir, and S.L. Rogers. 2000. "Review: A bioavailability-based rationale for controlling metal and metalloid contamination of agricultural land in Australia and New Zealand." Aust. J. Soil Res. 38, 1037-1086.

Schütte, S., R. Schulze, and G. Paterson. 2019. "Identification and mapping of soils rich in organic carbon in South Africa as a climate change mitigation option (2019)." Department of Environmental Affairs, Pretoria, South Africa.

Witter, E., A.M. Martensson, and F.V. Garcia. 1993. "Size of the soil microbial biomass in a long-term field experiment as affected by different N-fertilizers and manures." Soil Biol. Biochem. 25(6), 659-669.