Impediments to highly productive soils:
Addressing soil heavy metals and salinity.

Background

Several heavy metals and salts are beneficial to crops and stock when present in trace amounts, hence the use of  ‘trace elements’ when included in fertilisers.

While many crops tolerate a broad range of soil pH, including acidic soils, not many crops tolerate metal toxicity when heavy metals become more available as the soil becomes more acidic. For example, under acidic soil conditions, aluminium (Al), manganese (Mn), iron (Fe) rich soils may be highly toxic to plants[i]. Cadmium (Cd) is a non-essential element and is a significant pollutant due to its high toxicity and the solubility in water. Or crop essential heavy metals such as zinc (Zn), Mn, and copper (Cu) can become toxic when their concentrations are higher than acceptable levels[ii]. They can impact plant growth due to disrupting photosynthesis, nutrient uptake, phytohormones and amino acid synthesis and other biochemical processes within the plant[iii].

Soils with elevated magnesium (Mg) levels will be negatively impacted during flood or saturation events, as these soils will hold or retain water, and in combination with high sodium (Na) and/or potassium (K) levels, will block soil micropores resulting in oxygen deficiency. Such conditions are harmful to plants and beneficial soil biology, but ideal for soil borne diseases such as phytophthora, pythium, rhizoctonia and fusarium[iv].

Salinity and sodicity defines the level of salt or more particularly the level Na in soils. Excessive Na in soils can lead to accumulation in cell walls leading to osmotic stress within the plant cells. This affects photosynthesis mainly through a reduction in leaf area, chlorophyll content, and stomatal conductance[v]. Soil salinity also impacts a plants ability to access other nutrients within soils leading to nutrient (N, Ca, K, Fe, Zn) deficiency. Soil salinity significantly reduces a plants phosphorus (P) uptake[vi].

Sources of heavy metals and salinity

Heavy metals are nonbiodegradable and can be a natural part of the soil or can accumulate in soils from different sources. For example, improper disposal of the industrial waste or sewage, or long-term applications of pesticide and fertilisers containing heavy metals.
In some cases, flooding or more particularly erosion will remove topsoil and organic matter which act as a beneficial buffer to heavy metals and salts.

Salinity can be caused by a number of factors. Irrigation with water containing salts or heavy metals, even at low levels, will result in a build up over time of these elements in soils.

Toxicity of heavy metals and salinity can be increased as a result of high rainfalls, flooding or leaching of water through soils which ‘washes’ out beneficial cations such as Calcium (Ca), Mg and K of the topsoil and is a particular concern in sandy soils.

As heavy metals are nonbiodegradable (will not break down to non or less toxic by-products) their presence needs to be managed as part of a fertility program.

Reducing the toxicity of heavy metals and salinity/sodicity

As mentioned, soil pH greatly influences the availability of heavy metals, therefore managing soil pH is a key strategy in reducing the effects of heavy metals in soils.

Liming acidic soils will reduce the mobility and toxicity of heavy metals. While agricultural lime is the go-to product, consideration should also be given to Phosphate Rock products as a source of P, as these also have a liming effect, and negate the use of some if not all synthetic P fertiliser use (which are typically acidifying).

In soils with low cation or poor cation balance, excessive nitrogen (nitrates) applications, can negatively influence soil pH[vii]. As a result, managing nitrogen fertiliser applications is a part of managing the impacts of heavy metals in such soils.

Soil microorganisms play a vital role in reducing the harmful effect of heavy metals and salinity through biosorption and oxidation processes; the synthesis of bioactive compounds and formation of organo-chemical complexes[viii].

Amino acids, and particularly proline benefit crops under metal and salinity stress. These molecules have three major functions, metal binding, antioxidant defence, and signalling. Zinc toxicity can be eliminated through the formation of Zn-asparagine complexes. Amino acid asparagine is an effective biochelant (biological binder) of Cd, Pb, and Zn[ix].

Soil humates and fulvates, produced through the breakdown of organic carbon, are an effective biochelant of sodium, with products like BioAg’s HydraHume® and Soil & Seed® applied to increase humates in soils high in sodium.

Building soil microbial diversity through supply of soil inoculants or ameliorants is an effective way of enhancing soil fertility and combatting issues of soil toxicity.

No matter if it is issues that have appeared with recent flooding or issues that have been present for some time, heavy metals toxicity and salinity/sodicity can be alleviated or rectified by building soil fertility and nutrient equilibrium, improving organic/soil carbon content, and soil health. An annual investment in soil maintenance can provide returns through the reduced use of treatments needed to combat immediate crop issues.