Abstract: K fixation of soils is an important phenomenon affecting the status of soil K and its availability to crops. Four typifying pedons representing acid alfisols in eastern India were studied for their K fixation characteristics. Available K of soils increased with the rate of added K in all the soil series. Percentage of K fixed increased upto 800 mg kg-1 and maximum fixation levels were observed at 100 mg kg-1. Maximum K requirement per unit increase in available K of soils was observed in Hatiapathar series, which connect well with the low recovery rate of K in these soils. Significant correlation obtained for K recovery, K fixation and K requirement with pH, clay percent and CEC may indicate that these properties play dominant role in the soil K dynamics.
Introduction
Potassium, one of the major macro elements and an essential component of the complex biochemistry of plant growth play an important role in soil-plant relations. Alfisols developed under tropical environment in Eastern India often show low K status.
Coarse texture, high rainfall, low organic carbon with low pH is the basic characteristics of these soils, which are responsible for low K status in this zone. Besides this, increasing agricultural activity in these soils produced added pressure on the native K-status. Fixation of added fertilizer potassium is important in the dynamics of soil K and it affects the availability of K to crops. Investigation on some Indian soils show that crop K needs were mostly met from non exchangeable fraction of soil K in the absence of optimum K supply in some of the crops/and cropping systems (Raychaudhuri and Sanyal, 1999). Fixation studies provide information on the reaction rates of added K between different phases of soil K and the fate of added fertilizer K, which results in efficient management of K. Further K fixation and its retention in slowly available form is beneficial specially in light textured sandy soils under high rainfall condition (Srinivasa Rao et al., 2000). The present study aims to investigate the potassium fixation capacity of some acid alfisols of eastern India.
Materials and Methods
The study area represents a transitional zone of tropical dry sub-humid type of climate with mean annual temperature of 24.6°C and mean annual rainfall of 1588 mm (Velayutham et al., 1999). The area thus exhibits hyperthermic temperature regime and ustic moisture regime.
Based on reconnaissance soil survey four typical pedons representing the dominant soils developed on granite-gneiss parent material were collected from different locations in the Gondowana landscape of Eastern India. The physical and chemical characteristics of the soil samples (<2 mm.) were analysed for the determination of particle size distribution, pH, organic carbon, CEC and available potash following standard procedures (Black, 1965). Two gram of processed soil samples were incubated with 2 mL of K solution varying in K levels i.e., 0, 50, 100, 200, 400 and 800 mg kg-1 soils for 72 h. After incubation, 8 mL of 1N ammonium acetate solution was added and shaken for 5 min (Hanway and Heidel, 1952). Potassium in NH4OAc extract was estimated by flame photometer and expressed in mg kg-1. Fixed K was computed using the formula:
Kfixed = Kapplied – (Extractable Ktreated – extractable Kcontrol) as suggested by Srinivasa Rao et al. (2000).
Results and Discussion
Physicochemical Properties
The soils were slightly acidic to neutral in reaction with pH varying from
5.1 to 6.7 (Table 1). In all the cases pH(KCl)
was lower than pH(H2O) indicating that all the soils under natural
pH conditions carried a net negative charge and contained considerable amount
of acidity (Brady, 1984; Bleeker and Sageman, 1990). The organic carbon content
in the surface soils ranged from 3.6 to 8.1 g kg-1, which decreased
down the profile indicating the maturity of the profile, developed on very stable
landform (Sahu et al., 1990). Particle size distribution showed that
sand percentage was much higher (39.2 to 82.2%) than the silt or clay fractions.
These fractions are inert and are of no consequence in further weathering.
| Table 1: | Physicochemical properties of some alfisols |
The clay percentage of soil ranges from 11.8 to 38.7%, increasing down the profile with some exceptions in Hazaribag soil series. The clay percent in the B-horizons is 1.2 times more than the overlying layers of all the pedons, confirming the presence of an argillic horizon. The CEC of the surface soils varies from 2.4 to 19.0 cmol (P+) kg-1 which increases in the endopedon apparently due to increase in clay percent.
Soils deferred widely in available K content. Irrespective of the extractant, available K was highest in Hazaribag soil series (mean 812 mg kg-1) followed by Hatiapathar (mean 523 mg kg-1), Pusaro (mean 263 mg kg-1) and Maran (mean 174 mg kg-1) soil series. Among the three chemical methods, 1N HNO3 method released maximum amount of potassium. This can be described as dissolution of the more mineral K by the reagent (1N HNO3) as compared to others (Sharma and Swami, 2000). Similar results were obtained by Srinivasa Rao and Takkar (1997).
Effect of Added K on Available K
Available K and increase in available K over control at different levels
of added K indicated that (Table 2) irrespective of soil depth,
the availability of K increased with increase in the K level from 0 to 800 mg
kg-1. Maran series shows the maximum increase in the available K
over control at 800 mg kg-1 of added K followed by Pusaro, Hagaribag
and Hatiapathar soil series. The pattern of availability of K over control with
addition of different level of K indicated that, availability of K was in some
way related to the distribution pattern of clay down the profile. Ramanathan
and Krishnamoorthy (1982) pointed out that the variation in status of available
K content depends upon nature and amount of clay in the soil.
| Table 2: | Available K and increase in available K over control (mg kg-1) at different level of added K |
| *Figures in parentheses are increase in available K over control | |
Fixation of Added K
The amount of K fixed in soils increases with the increase in the amount
of added K from 0 to 800 mg kg-1 (Table 3). Overall
K fixation down the soil profile did not follow any trend, however it increased
down the profile in case of Hatiapathar and Hagaribag soil series while decreasing
trend was observed in case of Pusaro and Maran soil series. Variation in nature
and amounts of clay mineral distribution may have set up different trend of
K fixation in the soils. Though the amount of K fixed increased upto 800 mg
kg-1 added K, the maximum percent fixation levels were observed at
100 mg kg-1 of K in all the soil series except Hatiapathar series
where it was observed at 50 mg kg-1. Beyond these levels of added
K, the percent K fixation decreased in all the soils. Higher K fixation levels
were observed in surface soils as compared to soils of lower horizon, which
may be attributed to the presence of higher organic matter in the surface soils.
Among all the soils, Hatiapathar series showed the highest percent of K fixation,
varying from 27 to 50% in the surface soils. This could be explained by the
presence of higher clay percent in the surface soils of Hatiapathar series.
Potassium Requirement per Unit Increase in Available K
Recovery of K per unit of added K were found to be higher for Maran series
(0.92 to 0.95) followed by Pusaro (0.89 to 0.91), Hagaribag (0.84 to 0.87) and
Hatiapathar (0.70 to 0.74) soil series (Table 4). Recovery
of potassium seems to be influenced by higher sand percentage and lower organic
carbon content. Based on slope values of regression equations connecting increase
in available K(Y) with added K (X), the unit fertilizer K requirements were
calculated per unit increase in the available K in the soils. Overall status
of available K was low in soils of Hatiapathar series, which showed maximum
requirement for added K while minimum requirement was observed for soils of
Maran series.
| Table 3: | Fixation of added K (mg kg-1) and percent K fixed at different level of added K by the different soil series |
| Table 4: | Potassium recovery rate and fixation rate from unit fertilizer K required in some alfisols |
| Table 5: | Correlation coefficient |
Relationship Between Soil Parameters with Soil K
Correlation coefficient of some soil physico-chemical properties viz pH,
organic carbon, clay and cation exchange capacity with K recovery rate, K fixation
rate and K requirement (Table 5) shows that K fixation rate
and required K rate are significantly and positively correlated with pH (0.655
and 0.692), clay percent (0.707 and 0.701) and cation exchange capacity (0.964
and 0.973). However K recovery rate was significantly and negatively correlated
with pH (-0.655), clay percent (-0.707) and cation exchange capacity (-0.964).
Acid alfisols of eastern India have varying K fixation capacity due to difference in their physico-chemical properties. Generation of detailed quantitative data set on K fixation and unit fertilizer K required per unit increase of available K in soil will provide an important basis for prescribing fertilizer K more precisely. Judicious application of K fertilizer will ultimately help in maintaining crop productivity without disturbing the soil health.