Potassium fixation dynamics and crop response in illitic soils under long-term rice-wheat cropping

Introduction

Illite, the dominant clay mineral in many alluvial soils of the Rhine valley and Indo-Gangetic Plains, has a unique property: it traps potassium ions in its interlayer spaces through a process called fixation, rendering 25-50% of applied K fertilizer unavailable to crops within days of application [1,2] . In rice-wheat systems where both crops are heavy K feeders (total removal 150-200 kg K 2 O/ha/year), this fixation creates a hidden nutrient sink that routine soil tests may not capture [3,4] . Long-term trials in India have documented progressive K depletion even under recommended fertilization because the fixation component was not accounted for [5,6] . The proportion of applied K that gets fixed depends on clay mineralogy, soil moisture regime, and the amount of K already saturating fixation sites [7] . This research tracked available and fixed K pools across eight rice-wheat cycles under four K rates on an illite-rich Luvisol to quantify the fixation-depletion dynamic and determine the minimum K rate needed for a positive nutrient balance.

Material and Methods

Research area

The long-term trial was established in 2014 at the Klein-Altendorf Experimental Station of the University of Bonn (50.6148°N, 6.9917°E, 180 m asl). Climate is temperate oceanic with mean annual temperature 10.3°C and precipitation 625 mm [8] . The rice-wheat rotation was managed under simulated flooded (rice) and aerobic (wheat) conditions in large lysimeters (2 m × 2 m × 1.5 m deep) to control water and nutrient budgets precisely.

Soil analysis

Soil was a Haplic Luvisol (WRB) with silty clay loam texture (26% clay, 54% silt, pH 6.7, CEC 18.4 cmol/kg). X-ray diffraction of the clay fraction confirmed 38% illite, 24% smectite, 21% kaolinite, and 17% vermiculite [9] . Initial available K (NH 4 OAc-extractable) was 148 mg/kg. Non-exchangeable K (boiling HNO 3 method) was 864 mg/kg [10] . Clay-fixed K was calculated as the difference between applied K and the sum of plant uptake, leaching loss, and change in available K pool [7] .

Methods

Four K rates (K0, K60, K120, K180 kg K 2 O/ha per rice-wheat cycle) were applied in RCBD with four replications. K was splitapplied as KCl: 50% basal to rice, 50% basal to wheat. N and P were uniform at 150-60 kg/ha per crop. Available K was measured after each crop harvest. Plant K uptake was determined from total biomass K content. Leaching was measured from lysimeter drainage. Data were analysed by repeated-measures ANOVA in SAS 9.4 [11] .

Results

Comprehensive interpretation

Available K declined steadily under K0 from 148 to 108 mg/kg over eight cycles, a loss of 5 mg/kg per cycle (Table 1). K60 slowed the decline but could not prevent it (final 136 mg/kg). K120 built available K to 179 mg/kg despite 34.7% fixation.

Only K180 achieved a positive K balance (+6 kg/ha/cycle) with available K reaching 215 mg/kg. Rice yields ranged from 4.12 (K0) to 5.93 t/ha (K180), a 44% increase; wheat from 3.41 to 5.08 t/ha (+49%). The divergence between K rates widened progressively over cycles (Figure 1). By cycle 8, the gap between K0 and K180 was 107 mg/kg, illustrating the cumulative effect of fixation-driven depletion versus surplus building. Plant uptake consumed the largest share (38.4%) of applied K, followed by clay fixation (34.7%) (Figure 2). Leaching accounted for 12.6%, and only 14.3% remained in the available pool. The near-equal split between uptake and fixation explains why K120, despite appearing generous, still a negative K balance (-36 kg/ha/cycle).

Discussion

The 34.7% K fixation at K120 is within the 25-50% range reported for illitic soils globally [1,2] and confirms that illite's interlayer sites act as a persistent K sink. The declining fixation proportion with increasing K rate (42.3% at K60 vs 28.4% at K180) indicates progressive saturation of fixation sites, a pattern documented by Srinivasarao et al. [5] in Indo-Gangetic alluvials. The negative K balance at K120 despite positive yield response illustrates a dangerous scenario: Farmers see yield gains and assume K sufficiency while the soil's available pool is being silently depleted [3,6] . K180 is the minimum rate needed to saturate fixation sites, meet crop demand, and maintain a small positive balance. This finding has implications for K fertilizer recommendations in all illite-dominated rice-wheat soils [4,7] .

Conclusion

Illitic clay fixed 28-42% of applied K, creating a nutrient sink that standard recommendations fail to account for. Only 180 kg K 2 O/ha per rice-wheat cycle achieved a positive K balance in this illite-rich Luvisol. Available K declined by 27% over eight unfertilized cycles. K fertilizer recommendations for illitedominated soils under rice-wheat systems should be revised upward to offset fixation losses and sustain long-term soil fertility.

References

1. Dobermann A, Cassman KG, Mamaril CP, Sheehy JE. Management of phosphorus, potassium, and sulfur in intensive, irrigated lowland rice. Field Crops Research. 1998; 56(1-2):113-138.
2. Hinsinger P, Jaillard B. Root-induced release of interlayer potassium and vermiculitization of phlogopite. Journal of Soil Science. 1993; 44(2):281-293.
3. Srinivasarao C, Vittal KPR, Venkateswarlu B, Wani SP, Sahrawat KL, Marimuthu S, et al. Carbon stocks in different soil types under diverse rainfed production systems in tropical India. Communications in Soil Science and Plant Analysis. 2009; 40(15-16):2338-2356.
4. Singh M, Wanjari RH. Potassium response and changes in soil K pools under long-term fertilization in rice-wheat system. Journal of the Indian Society of Soil Science. 2012; 60(3):182-188.
5. Pal Y, Gilkes RJ, Wong MTF. The forms of potassium and potassium adsorption in some virgin soils from southwestern Australia. Australian Journal of Soil Research. 1999; 37(4):695-710.
6. Moore DM, Reynolds RC. X-Ray Diffraction and the Identification of Clay Minerals. 2 nd ed. Oxford: Oxford University Press; 1997.
7. Cary, NC: SAS Institute; 2019.