DTPA-Extractable Zinc in Rice Soils and Its Availability to Rice

Zinc (Zn) deficiency is a fairly wide spread agronomic constraint in many of the world rice production regions. Information on soil Zn distribution is essential for understanding its chemical reactions and bioavailability. In this backdrop, we tried to find out the relationship between physicchemical properties and DTPA-extractable Zn (available Zn) content of rice soils. For this eighty four (84) surface soil samples (0-20cm) were collected from three villages (viz. Saharapali, Nuagarh and Adgaon) of Bargarh district under the Hirakud Command Area of Odisha. Analytical observations revealed that the soils were slightly acidic in reaction with moderately high content of soil organic carbon. Considering the critical limit of available Zn in soils to be 0.6 mg kg-1, all soils of the study area were found to be well supplied with DTPA-extractable Zn. Soil organic carbon (SOC) content maintained positive and significant correlation with available zinc content in soils. However, soil pH showed significant but negative correlation with available zinc content of soils.


INTRODUCTION
Zinc is an impor tant trace element which is essential for the healthy crop growth and metabolism 1 .Zinc is also vital for terrestrial life since it is required either as a structural component or reaction site in numerous proteins.It is required by rice in small quantity, but its deficiency has an adverse effect on healthy crop growth and yield may reduce up to 30 percent 2 .Zinc deficiency in soils and plants is of major concern since thirty per cent of world soils are Zn deficient 3 .In India 48.1 % of the soils are found to be Zn deficient 4 though the figure for the soils of Odisha were reported to be low 5 .Zinc deficiency is one of the most widespread nutrient disorders in lowland rice after nitrogen and phosphorus.Such deficiency arises from adoption of modern technology with intensive cultivation and growing of high yielding varieties and also shifting towards the use of high analysis NPK fertilizers with inadequate use of organics or no application of Zn fertilizers.Though it is required by plants in very trace amounts, its availability in soils for plants is a complex mechanism.Availability of Zn in soils for plants is a function of soil properties.Soil properties such as soil pH, redox potential, organic matter and moisture content exert large impact on adsorption-desorption and dissolution-precipitation reactions.Thus, soil properties regulate the amount of Zn dissolved in soil solution 6 .Soil pH is the master variable affecting nutrient availability and the nutrient zinc is not far behind in this process 7 .Increasing soil pH, especially above 6.5, results in decreased extractability and plant availability of soil Zn.Soil Zn is usually more available in soils with greater organic matter content 8 and a relatively higher proportion of clay 9,6 .Exchange positions are important in maintaining the Zn level sufficient for wet land rice 10 and in this regard the cation exchange capacity of oils seems to play dominant role.The rise in soil pH increases the adsorption of Zn onto soil constituents such as metal oxides and clay minerals which is linked to significant decline in Zn concentrations in soil solution and plant tissues 11,12 .Hirakud command area under the central western table land agro-climatic zones is the major paddy growing region of Odisha, popularly known as rice bowl of Odisha.The soils of this area are continuously under submerged condition which enables rice plants to absorb Zn mostly from solubilisation of bound Zn in the rhizosphere because the availability in the soils is very limited due to flooded condition 13 .
Available Zn content in the paddy growing soils of Bargarh district of Odisha was reported to be high 5 though discrete patches of Zn deficient areas were identified in soils under the Hirakud command area.Distribution of available Zn content in the paddy growing soils of the study area has not been studied extensively so far.Knowledge on distribution of available zinc content in wetland rice soils and various soil properties influencing its availability might prove to be the best approach for obtaining reliable information about the status of zinc.Therefore, we took an attempt to study the DTPA-extractable Zn (available Zn) content and also its relationship with physic-chemical properties of soils in some villages of Bargarh Block under the Hirakud command area.

Site Description
The study area comprising of Saharapali, Nuagarh and Adgaon villages in Bargarh district, Odisha, lies within the lower Hirakud Command area which falls under the Western Central table land Agro-climatic zones of Odisha.It has an elevation of about 189.3m above the msl.Most of the soils developed on Archean rocks.The soil types in these areas are mixed red and black type.Climate of these areas is characterized by dry hot summer, monsoon rains and cold winter receiving an average annual rainfall of 1527mm and experiencing mean annual temperature of 30-35 0 C.

Soil sample collection and analysis
Surface soil samples (0-20cm) were collected during kharif (2015) before puddling of fields.Eighty four (84) composite soil samples were taken randomly from rice fields of the study areas.Moist soil samples were collected in the moisture box with the help of core sampler to estimate the bulk density following the standard protocol.The soil samples were further air dried, grinded and passed through the 2mm sieve for analysis of other soil chemical parameters.Oxidisable organic carbon of the soils was analyzed by the method of Walkley and Black 14 .Soil pH was measured in soil: water:: 1:2.5 suspension 15 .Electrical conductivity of the soils was determined by following standard protocol 15 .Cation exchange capacity of soils was estimated by following standard method 15 .Available zinc content of soils was measured in atomic absorption spectrophotometer (AAS) after extracting the soils with 0.005M DTPA (Diethylene triaminepenta acetic acid) in 1:2 ratio (Soil: Extractant) following the standard procedure 16 .

Statistical Analysis
The data were subjected to multiple correlation study with the help of SPSS software version 20.0 for establishing the relationship between available Zn content in soils and physic-chemical properties of soils.

Physic-chemical properties of soils
The results of soil analysis pertaining some salient properties under study are presented in table.1,2& 3. The mean values of the studied soil parameters indicated that the soils were slightly acidic in reaction with slightly high in SOC content and there was no accumulation of appreciable quantities of salt in all the study sites indicating that the soils were non-saline in nature.The pH of the soils ranged from 4.89 to 6.52 with average value 5.96, 5.19 to 6.75 with average value 6.09 and 4.99 to 6.86 with average value 5.88 in Saharapali, Nuagarh and Adgaon village respectively.Although the soil pH was slightly acidic in reaction it was well suited for growing rice in these areas.Electrical conductivity of the soils ranged from 0.12-1.20 (mean 0.34), 0.12-1.90(0.78) and 0.19-1.90(0.80) in Saharapali, Nuagarh and Adgaon respectively.Such magnitude of salt content was reported in paddy growing soils of Karnataka 17 .Soil organic carbon (SOC) is an important attributes influencing the soil physical, chemical and biological properties.Highest amount of SOC content (gkg -1 ) was observed in soils of Saharapali village (average 6.10) followed by Adgaon (5.68) and Nuagarh (5.67) village.The higher content of SOC could be due to in situ incorporation of rice stubbles and addition of organic manures 17 .Bulk density in soils of Saharapali, Nuagarh and Adgaon were found to be 1.26-1.65 (mean 1.42), 1.26-1.75(1.49) and 1.34-1.68(1.53) respectively.Occurrence of such high bulk density in these soils could be due to the intensive cultivation despite of having significant amount of organic matter.Puddling of soils intensively for growing rice under submerged condition leads to formation of dense soil layer with high bulk density.The mean cation exchange capacity (CEC) of soils of Nuagarh (14.95) was found to be higher compare to Adgaon (14.42) and Saharapali (12.38).Such occurrence of CEC in these soils could be due to the presence of various functional groups in the decomposed organic matter and also due to the finer fractions of

DTPA-extractable Zn content in soils:
Availability of zinc in submerged rice soil is a complex phenomenon.As soil submergence due to rice cultivation is likely to influence the solubility of native Zn in soils either favorably or adversely it controls the Zn nutrition of rice.The total concentration of zinc in soils depends on the type of parent material and soil mineralogy, especially the concentration of quartz, which tends to dilute the concentration of most elements.Only a small fraction of total zinc is exchangeable or soluble 19 .The soils were well supplied with DTPA-extractable Zn (available Zn).Considering 0.6 mg kg -1 as the critical limit of available zinc (DTPA-extractable) in soils, all the soils under the three villages (Saharapali, Nuagarh and Adgaon) were found to be sufficient.Similar findings reported by some workers 5 .The available Zn content ranged from 0.82 mg kg -1 to 1.62 mg kg -1 with average 1.08 mg kg -1 in Saharapali village, 0.72 mg kg -1 to 1.42 mg kg -1 with an average of 0.95 mg kg -1 in Nuagarh village and 0.70 mg kg -1 to 1.52 mg kg -1 with an average of 1.04 mg kg -1 in Adgaon village.Among the three villages, the highest amount of available Zn was reported from Saharapali village.Similar magnitude of available Zn (DTPA-extractable) content was reported from a rice based long-term experiment under the central western table land agro-climatic zones of Odisha 20 .Such a high content of available zinc might be due to higher amount of SOC as indicated by their strong relationship (r= 0.857**) and also due to the finer fractions of soils leading to increase surface area for ion exchange and hence contributed to higher amount of DTPA-extractable zinc in soils 21 .

Relationship between DTPA-extractable Zn content and other physic-chemical properties of soils
Knowledge regarding the status of available Zn and its relationship with soil properties  22,23,24 .
# EC-Electrical Conductivity, BD-Bulk Density, SOC-Soil Organic Carbon, CEC-Cation Exchange Capacity Organic matter is helpful in improving the soil structure and aeration, hence protecting the precipitation of Zn into unavailable forms.Therefore, availability of Zn improves significantly with increase in organic matter.The DTPA-extractable Zn was significantly and negatively correlated with pH and positively correlated with SOC 25 .An inverse relationship between soil pH and Zn availability was reported 26 .Soil organic carbon content showed significant but negative correlation with soil pH.
Crop management (like native Zn content of soil, soil pH, organic matter, submergence, partial pressure of CO 2 , organic acids) and environmental factors combinely influence the availability of Zn to rice crops.However, SOC content and pH are the main soil characteristics which control the availability of Zn in soils.It is apparent from the study that the soils are adequately supplied with DTPA-extractable Zn.Higher amount of DTPA-extractable Zn were obtained in soils with high SOC content and low pH values.To increase the yield potential of paddy growing areas, farmers are advocated to use organic matter or crop residues along with application of chemical Zn fertilizers.Thus, maintaining the soil adequate in available Zn content for plant nutrition.

Table 4 : Relationship between available Zn and soil properties
* Correlation is significant at the 0.01 level (2-tailed) * Correlation is significant at the 0.05 level(2-tailed) *