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Potassium Status and Release Characteristics of Twelve Floodplain Soils of Bangladesh

Khan H. R.
Department of Soil Science, University of Dhaka-1000, Bangladesh

Chowdhury, M. S.
Department of Soil Science, University of Dhaka-1000, Bangladesh

Elahi, S. F.
Department of Soil Science, University of Dhaka-1000, Bangladesh

M. S. Hussain
Department of Soil Science, University of Dhaka-1000, Bangladesh

Adachi, T.
Faculty of Agriculture, Okayama University, Okayama-7000, Japan

Abstract/ Executive Summary:

An introductory investigation on soil conditions in relation to K status and release characteristics of twelve soils reveal that the physical and chemical properties of the studied soils varied considerably with the variation of soil parent material, hydrology and climatic conditions. The texture of the soils varied from sandy to clay, organic matter content was rather low (6.9-19.8g kg^-1). Except for Sara soil, the pH (1:1, water) of the soils ranged between 4.7 and 5.3, indicating that the soils are acidic. Sara soil contained about 60 g kg^-1 CaC0₃ and had an alkaline pH (8.2). Among the twelve soils, seven soils had exchangeable K⁺ of more than 0.20 cmol (+) kg^-1, which can be considered as the critical level of K for these soils. The exchangeable K⁺/exchangeable Mg²⁺ ratio of the soils varied from 0.05 to 0.14. The structural K was about 93 to 99% of the total K, while exchangeable (0.26-1.02%) and non-exchangeable 07-8.33%) K (1.07-8.33%) K comprised a small percentage of the total K. Non-exchangeable K in the soils determined by 0.7M H₂SO₄, and 1M HNO₃ methods varied from 0.11 to 3.26 and 0.44 to 3.94 cmol (+) kg^-1, respectively. Rate of release of K from the soils was very high in the first extraction by both the 1M CH₃COONH₄, (1.02-3.07mmol kg^-1) and 0.3M NaCI (0.90-2.47 mmol kg^-1) solutions. With 1M CH₃COONH₄ the release of was drastically reduced on the second day and virtually stopped on the third day of extraction. With 0.3M NaCI extraction, the rate of release of K was reduced gradually and even on the 11th day of extraction the release of K was considerable. Only K released with 0.3M NaCI showed a significant positive (r=0.70) correlation with the non-exchangeable K.

Keywords: Acidic soils, Critical level of soil K, Exchangeable and non-exchangeable K, Exchangeable K+ /exchangeable Mg2+ ratio, Release of soil K

Location:

Start Date: 00-00-1990

End Date: 00-00-1991

Program Area: Crop-Soil-Water Management

Commodity: Soil fertility

Objectives:

Objective of this study was to investigate on the physico-chemical conditions of the soils with respect to K chemistry, and on the K release characteristics of the soils as assessed by using different analytical techniques.

Methodology:

Climate: On the basis of the distribution of temperature over the year, the study area has two distinct seasons - the cool and the warm. Mean annual temperature (MAT) of the area is 25.3^oC. The mean winter (December to February) and mean summer (April to July) temperatures are 19.0 and 28.8^oC, respectively. The summer maxima of the study area ranges from 33 to 36^oC in April and the minimum temperature of 6^oC is reached in January. Temperature regime can, therefore, be classed as hyperthermic (MAT= >22.5^oC). The mean annual rainfall over the entire country is about 2300 mm). The distribution of total annual rainfall in the study areas vary considerably and ranges between 1500 and 3000 mm. In the recent years, drought have been so severe in April and May that rain fed agriculture has been severely affected and signs of decertification has become apparent. The study area can be classified into 4 categories on the basis of drought condition. The mean annual evaporation of the study area is about 1280 mm. Total annual evaporation is smaller than total annual rainfall but the evaporation exceeds rainfall during the dry seasons. Therefore, the existence of seasonal moisture surplus and deficit is prominent. Soil samples: A laboratory experiment was conducted using 12 floodplain soils obtained from semi-arid regions of Bangladesh during 1990-91. Bulk soil samples (0-15 cm depth) of Belabo, Domar, Gangachara, Kaunia, Baliadangi, Fara bari, Laskara, Amnura, Lauta, Sara, Ekdala and Uttargaon series were collected for these studies. Sampling occurred in areas in which no K fertilizer had been applied lor at least one year. The samples were air-dried and gently crushed to pass a 2.0mm sieve and sub-samples were obtained by passing through 1.0 and 0.25mm sieves as required. After treatment wi th 30% H₂O₂, particle size distribution was determined by the pipette method. Moisture at field capacity was determined by gravimetric method after oven drying at 105^oC). Organic matter content was determined by wet combustion with K₂Cr₂O₇. Soil pH was measured for 1 : 1 suspension of air-dried soils in water, 1M KCl and 0.01M CaCh₂ using a Corning pH meter Model 7. Cation exchange capacity (CEC) was determined by saturation with 1M CH₃COONH₄ (pH 7.0), ethanol washing, NH₄⁺ displacement with acididified 10% NaCl solution, and subsequent analysis by steam (Kjeldahl method) distillation). Exchangeable Na⁺, Ca²⁺ and Mg²⁺ were extracted with 1M CH₃COONH₄, at pH 7.0. The modified BaCl₂-trieth-anolamine method was used for the determination of exchangeable H⁺. Several availability indexes were used to evaluate the K status of these soils: exchangeable K by 1M CH₃COONH₄ buffered at pH 7.0 non-exchangeable K by 0.7M H₂SO₄ and hot 1M HNO₃ and total K by HF digestion. Structural K contents were estimated by subtracting the 0.7M H₂SO₄, and 1M HNO₃, extractable K from total K. The rate of release of K from soils were determined by using two (method I and II) methods. In method 1, triplicate 2.5 g portions of each soil (<2mm sieved) were placed in 50 cm³ centrifuge tubes and 25 ml of 1M CH₃COONH₄ (pH 7.0) was added. The stoppered tubes were shaken for 24 hours and centrifuged. The supernatents liquids were analyzed for K. ln method II, triplicate 5g portions of each soil were suspended in 50 ml of 0.3M NaCI in centrifuge tubes and supernatents were collected. The processes (Methods I and II) were repeated with the same soil for about 30 days till a plateau of K was reached.

Source of Information: Soil Phys. Cond. Plant Growth, Jpn, 68, 15-24, 1993

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Achievement/Findings:

The results of the present investigation on soil conditions relating to K status and release mechanism conclude that the parent material, hydrology and climatic conditions have great influence on K-chemistry of the soils. Most of the studied soils were acidic, had moderate physical properties and the dominant texture was loam. The 1M CH₃COON₄ extractable K of 0.20 cmol (+) kg^-1 was thought as the critical level of K for these soils. The exchangeable K⁺ /Mg²⁺ ratio of the soils varied from 0.05 to 0.14, suggesting that K availability might be hindered at the lower K⁺ /Mg²⁺ ratio soils. Therefore, K⁺/Mg²⁺ ratio of the soils should be considered before predicting K availability. Structural K was about 93 to 99% of the total K, which indicated that the reserve of K in the soils was very high that could be supplying K over a period of time to plants and could thus account for the lack of crop response to applied K. The hot 1M HNO₃ method was found to extract a little higher amount of non-exchangeable K than 0.7M H₂SO₄. The rate of release of K differed very much depending on the extractant used. The 0.3M NaCI extractant would be more suitable than 1M CH₃COONH₄ in order to predict the K release characteristics of the soils. Domar soil released K even on the 30th day of extraction with 0.3M NaC1. This behavior of K release by 0.3M NaCI extractant needs further investigation.

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