Comparative Survey of Organic Matter, Biological and Acid-causing Treatments Effects on Availability of Nutrient Elements in a Calcareous Soil

Calcareous soils are described as soils containing quantities of calcium carbonate which have an enormously effect on the soil properties (physical, consisting of soil water relations and soil crusting, or chemical consisting of the availability of plant nutrients) and plant growth. Calcareous soils arise clearly in arid and semi-arid areas due to rare precipitation and little leaching. It has been evaluated that these soils contain over one-third of the world's surface zone and their CaCO3 content ranged from a few to 95%. Calcareous soils faced many challenges such as shortage of organic matter, low structure stability, low water holding capacity, low CEC, high pH, surface crusting and cracking and great infiltration rate which cause loss of essential plant nutrients via leaching or deep percolation. Another problem in calcareous soils is low availability of plant nutrients particularly phosphorous and micronutrients specially zinc, iron and manganese, and a nutritional imbalance between elements such as potassium, magnesium and calcium. Although a calcareous soil is dominated by free lime, it could also incorporate large quantities of iron, aluminum, and manganese. These metals provide more strong sorption sites for phosphorus and are mostly more significant in controlling phosphorus solubility in calcareous soils than calcium carbonate itself. Under such severe conditions, desired yield levels are difficult to attain. Calcareous soils lack the organic matter required for optimal crop yield. Therefore, numerous studies have made efforts to increase the availability of nutrients in the soils through different treatments. Common methods for dealing with these deficiencies, is the use of chemical fertilizers that have the risk of environmental pollution in addition to the high cost and low efficiency. Oxidation of sulfur leads to sulfuric acid formation which can decrease the soil pH and increase dissolution of insoluble soil minerals and release of essential plant nutrients. Furthermore, the addition of organic amendments improves the soil chemical and physical properties, initiates nutrient cycling, and provides a functioning environment for vegetation. The objective of this research was to increase solubility of nutrient elements in a calcareous soil considering nine treatments (i.e., control (Blank), Soil + Humic Acid (HA), Soil + Sulfuric Acid (H2SO4 ), Soil + Thiobacillus (T), Soil + Sulphur (Sº), Soil + Sulphur + Thiobacillus (Sº +T), Soil + Vermicompost (VC), Soil + Vermicompost + Thiobacillus (VC+T) and Soil + Sulphur + Vermicompost + Thiobacillus (Sº +VC+T)). The experimental design was factorial arrangement in randomized complete block, with all the treatments replicated three times. All the treatments were incubated under the laboratory condition for 90 days in 25 ± 2 °C and 70% of water holding capacity by distilled water. During the incubation period, the moisture of the samples was kept at 70% FC by daily addition of deionized water based on weight loss. At the end of incubation period the pH value, electrical conductivity (EC), available form of macro elements (K, P and N) and micro elements (Zn, Mn, Fe and Cu) were determined in all treatments by standard methods. The results showed that, the soil pH value significantly decreased in Sº+T and Sº+VC+T treatments, in compared to the blank. While, the EC of these treatments significantly increased with respect to the blank. The results also showed that most of the treatments have been able to increase the solubility of the nutrients. However, the effect of Sº +VC+T treatment on increasing the availability of studied soil nutrients and decreasing pH value was more significant than the other treatments. Analysis of  the results obtained from this study using classical statistic methods showed that applying a single treatment cannot remove all obstacles to increase nutrient availability in calcareous soils. This may be attributed to high buffering capacity of calcareous soils and complexity of factors which control mineral solubility and nutrient availability. While, treatments that simultaneously provide soil organic matter and lower pH (such as Sº+VC+T) can significantly remove barriers to increase nutrient uptake in these soils. As a result, the simultaneous application of organic fertilizers, elemental sulfur and Thiobacillus bacteria can be a promising approach to increase the solubility of nutrients in calcareous soils and to increase the quantitative and qualitative growth of plants in these soils.