Assist. Prof. Rabah Salim Shareef Gumaa – Environment Department

The availability of the nutrients is either directly or indirectly related to geological features, water availability, soil pH and cation exchange capacity, in which these elements play essential roles in plant physiological functions . For example, magnesium (Mg) is an important component of chlorophyll as it contributes to photosynthesis and enzyme activation while calcium (Ca) maintains membrane stability, increases drought and heat resistance in desert plants In addition, iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu) are also essential for enzyme activities. Soil pH can impact the growth of a plant based on its influence on the availability of essential plant nutrients and also on the concentration of toxic elements in the plants.

Pass work show that the increase of soil pH value has led to slight raise in  the soil alkaline hydrolyzable nitrogen (N) - available phosphorus (P) and available potassium (K) have gradually decreased, and exchangeable Mg has declined marginally. Puga et al. (2015) report that the addition of biochar to the soil has led to the increase in the pH (about 0.3 units). This has caused the N, P, K, and sulfur (S) concentrations increased in shoots of Jack bean and Mucuna aterrima with an increasing rate of biochar. Besides, Novak et al. (2009) report that biochar can produce alkaline pH, which will eventually increase the soil pH and decrease the soil acidity. Thus, some researchers indicate biochar that eases the uptake of nutrients from the soil basically has increased the nutrient contents in a plant . In a long term, the application of biochar increases the availability of plant nutrient.

The relative abundance of P is highly dependent on soil pH. The experiment was considered as the optimum soil pH in terms of the uptake of P in maize. In the experiment, plants were grown in pots filled with silt loam with a known P content. The soil was either acidified or limed.  The soil pH for optimum amount of P availability was at pH 4.7, pH 5.7 and 6.5 respectively. The P uptake was low at soil pH above 6.5 and at pH 4.5, the P availability decreased rapidly. This caused the root and growth of plants were retarded severely.

Macronutrients like Ca and Mg are more available within a pH range of 6 to 8. Many studies show that the deficiency of Ca is common on acidic soils. Ca and Mg deficiencies are due to leaching while aluminum (Al) and Fe bind P in acidic soils. Furthermore, Pandey (2015) states that the acidic soils are usually excessive in soluble of Al and Mn and deficient in P, Ca, Mg and molbydiom (Mo), that may cause their reduced uptake and lead to nutrient imbalances in plants.

The increase of soil pH is due to biochar application in the polluted soil, which leads to the increase of P, K, Ca, and Mg concentrations in rapeseed shoots as compared to unamended control. In addition, Jampeetong et al. (2013) explains that the Ca concentrations in leaves of Job’s Tears are the highest at pH 6.5 and 8.5.

Effect of calcium on soil pH

Heavy precipitation usually leaches very high amounts of plant nutrients, particularly calcium and magnesium from soil. These losses will further reduce the soil fertility. For example, highly weathered soils of Thailand have caused low production of crop due to heavy precipitation. Soil texture, soil reaction (pH) and original nutrient status are important factors affecting the leaching behavior of nutrient cations (K⁺, Ca⁺² and Mg⁺²) in tropical soils.

indicate that the high root zone pH also decreases Ca and boron (B) concentrations, but the examined concentrations of Ca in the nutrient solution have little effect on the composition of the examined elements that include Fe, P, K, Cu, B, Mn, and Zn. Researchers have found that the poor growth of conifer trees reported for calcareous soils is likely due to impaired root growth and the effects of gas exchange, which is possibly caused by the reduced water uptake at high pH and elevated Ca levels. Marschner (2012) states that calcareous soils are characterized by ample of Ca supply and high soil pH, usually at the range of 7.5-8.5.

In another study, Fan et al. (2014) confirmed that the application of Ca in water treatment residue to Cu contaminated acidic sandy soils reduces Cu loading on the surface of the runoff water and improves soil conditions by raising soil pH and Ca concentration, and reducing Cu stress. These, in turn, improve citrus nutrition and subsequently increase the fruit quality and yields. The results of the analysis have revealed that the cement dust, which contains high calcium has impacted the soils with high soil pH, Ca content, total bases, base saturation and pH dependent cation exchange capacity (CEC). Liu et al. (2012) state moderate CaO₂ application increases celery shoots yield and reduces celery arsenic (As) uptake. The results of this experiment show that the pH could be changed in soil after CaO₂ was applied to the soil, in which the soil pH significantly increased with the increasing of CaO₂ application rates. The highest pH 8.5 was achieved in the treatment of 10 g.kg^-1 CaO₂ being about 3 units higher than the controlled one. Also, the concentration and proportion of arsenic in the plot decreased with the increasing of CaO₂ application from 0 to 2.5 g. kg^-1 , and ascended with the increasing dosage thereafter (from 2.5 to 10 g. kg^-1 CaO₂). Chunh, (2012) reports that decrease in bacterial diversity of acidobacteria and gram-positive taxa, which are dominant in soil bacterial communities is due to limed soil compared to non-limed soil. The decrease in metabolic activity observed in the lime ammonium nitrate (LAN) and chicken manure treatment may be due to a high nitrogen input in the soil, causing a shift of microbial communities towards nitrifying bacteria and a reduction in other microbial organisms.

 Jiang et al. (2012) confirm that soil pH increases markedly after the addition of biochar. These changes in soil properties are advantageous for heavy metal immobilization in the soil. The acid soluble Cu and lead (Pb) decreased by 19.7–100 % and 18.8–77.0%, respectively, as the amount of biochar added increased.