Potassium leaching in cropping systems with reference to Western Australia

Fariya Abubakari

The use of potassium (K) fertilizers in Western Australia (WA) has progressively increased since 1990, but on average the low rates of K applied as fertilizer are less than removal of K by crops and as a result most cropping systems in WA have negative K balances. Moreover, to date there are no reliable estimates of additional K losses by leaching of K on these soils and their response to soil properties (clay content, pH, CEC, organic matter), K fertilizer rate and rainfall in this environment even though the potential for K leaching is well recognized. Therefore, this research conducted field work, glasshouse and laboratory experiments on several sandy soil types from the southwest of WA to: (i) determine the main factors influencing K leaching, (ii) quantify K leaching losses on selected soils, (iii) assess factors that decrease K leaching losses and increase K utilization, and (iv) determine whether a well-calibrated process model such as Hydrus 1D module can accurately predict K leaching losses in WA soils. Preliminary laboratory analysis examined the K+ supplying capacity of 13 soils with sand and sandy loam texture with a range of pH, CEC, clay and organic matter values. The activity ratio (AReK) increased with corresponding increase in extractable K+ concentration in the Arrino sand, Mingenew sand, Goomalling loamy sand, York sand-loamy sand and West Brookton sandy loam soils. At the highest level of applied K+ (5 cmol/kg), the adsorption of K+ in the soils varied from 2.8% to 10.5% indicating very low to low fixation of added K+. The labile K (KL) ranged between 0.11 and 0.21 cmol/kg. The PBCK significantly and positively correlated with soil CEC, pH and clay content but not with soil organic matter. On the same 13 soils had very low K release with values that ranged from 15.3 to 105.1 mg/kg in surface layers and from 22.4 to 86.7 mg/kg in subsurface soils. From Ca-saturated soils, the release of K was even lower (5.9 to 53.1 mg/kg from surface soils and 7.9 to 37.6 mg/kg from subsurface soils), suggesting limited release from nonexchangeable forms particularly in sandy soils (less than 10 mg/kg). A column leaching experiment (with wheat plants growing in the soil) was conducted in the glasshouse. The cumulative amount of K leached in the Goomalling loamy sand soils after 9 leaching events was 70, 152 and 252 mg/column in the control, medium ((110 mg K/column)) and high K (220 mg K/column) treated columns. The K leaching loss for events 1-9 as a % of K fertilizer added was 38.7% and 50.4% in the medium and high K treated columns, respectively, in the Goomalling loamy sand soils. By contrast, the K leaching loss for events 1-9 as a % of K fertilizer added was 28.4 and 29.6% in the medium and high K treated columns, respectively, in the West Brookton sandy loam soils. The K uptake by wheat from soil columns as a % of K fertilizer added for high and medium K treatments was 28.0% and 17.7% from the Goomalling loamy sand, respectively, while it was 44.6% and 33.8% in the West Brookton sandy loam soils, respectively. In a similar column leaching study without wheat plants, the cumulative K leaching loss for events 1-9 as a % of K fertilizer added was 72% and 77% in the medium and high K treated columns, respectively, in the Goomalling loamy sand soils and 60% and 67% in the medium and high K treated columns, respectively, in the West Brookton sandy loam soils. A glasshouse column experiment was conducted to test the hypothesis that increasing the pH of acid soils will decrease the rate and amount of K leaching by increasing cation exchange capacity. The results revealed that K loss as a % of K fertilizer added was 77% and 72% in the high (68 mg K/column) and medium (34 mg K/column) K treated columns, respectively in the Goomalling loamy sand soil at pH 4.5. However, raising soil pH to 5.5 and 6.5 was able to reduced K loss to 58% and 57% at high (68 mg K/column), respectively, and 53% and 28% at pH 5.5 and 6.5 of medium (34 mg K/column) K treated columns, respectively in the Goomalling loamy sand soil. The results further revealed that the K loss as a % of K fertilizer added was 67% and 60% in the high (68 mg K/column) and medium (34 mg K/column) K treated columns, respectively in the West Brookton sandy loam at pH 4.8. At pH 5.5 and 6.5, K loss was reduced to 51% and 38% at 68 mg K/column applied K, respectively, while it was 49% and 32% at pH 5.5 and 6.5 of medium (34 mg K/column) K treated columns, respectively in the West Brookton sandy loam. A field experiment was conducted to determine the rate and depth of leaching of K applied as muriate of potash on a deep sandy duplex profile. This study revealed that in the control plots, there was an increase in exchangeable K under the row and interrow by 9 and 25.7 mg K kg-1 in the 40-60 cm soil layer during the initial 5 weeks which suggests leaching to the subsoil. At 4 weeks after K application in the K treated plots, exchangeable K+ increased in the 40-60 cm soil layer by 87.1 and 45.2 mg K kg-1 increment relative to the same depth at 1 week before K application in the interrow and row sampling, respectively, which suggests significant leaching of applied K fertilizer to the subsoil. At 4-8 weeks after K application in the K treated plots, the exchangeable K+ concentration levels in both surface and deeper soil layers gradually declined to the same exchangeable K+ concentration levels at 1 week before K application. Cumulative rainfall was 200.4 mm during the 4 weeks after K application while only a further 10 mm occurred from 4 to 8 weeks after K application. Finally, a laboratory column experiment was carried out with 22 soils to determine the effect of soil pH, clay content, CEC and rates of K application on K leaching. When K was applied at a medium rate (4.2 mg K/column) to soils with low pH (4.02 to 4.9), the K leaching loss was 38.8% to 48.1% of K addition in the Whitby loamy sand-sandy loam soils and 33.6% to 47% of K addition in the Greenhill sandy loam soils as compared to high K leaching loss 69.1% to 69.6% of K addition in the Goomalling sand soils. At high rate (11.2 mg K/column) of K application, the K leaching loss in the Whitby loamy sand-sandy loam soils ranged from 2.4 to 7.6 mg /column (54.3 to 63.8% of K addition), Greenhill sandy loam soils ranged between 8.2 to 8.7 mg/column (54.2 to 63.8% of K addition) and Goomalling sand soils ranged at 9.1 to 11.7 mg/column (72.2 to 72.6% of K addition) at lower pH values (4.02 to 4.9). Increasing soil pH to 5.5 and 6.5 delayed the K leaching peak up to 4.75 pore volumes (PV) among all the studied soils at high rate of K application and 4 to 4.5 PV at medium rate of K application. The study also demonstrated that soils with higher clay contents (16–17%) and low CEC (0.77–0.94 cmol/kg) at pH 4.6 and 4.5, or those with higher clay contents (14–19%) and low CEC (2.44–4.05 cmol/kg) at pH 4.4 and 4.6 at medium rate (4.2 mg K/column) of K application are not susceptible to K leaching. Similarly, there is no risk of K leaching in loamy sand soils with 11% clay content and with low to high CEC (4.09 to 11.13 cmol/kg) at pH 5.1 and 5.2 at medium rate (4.2 mg K/column) of K application. However, greatest risk of K leaching was observed at the highest rate (11.2 mg K/column) of K application which was evidenced in all the studied soils. Thus, clay content, pH, rate of K applied, and CEC strongly influences the degree of K leaching in this study. Using the above laboratory column leaching experiment results, the Hydrus 1D model was tested for simulating the water and K transport. The Hydrus 1D showed better prediction of K leaching losses at medium rate (4.2 mg K/column) of K application in contrast to high rate (11.2 mg K/column) of K application. Also, the Hydrus 1D accurately predicted K leaching from soils of higher pH (5.5 and 6.5), CEC (2.18 to 11.1 cmol/kg) and OC (0.35 to 2.64%). However, the accuracy of the model was reduced at low values of pH (4.02 to 4.9), CEC (0.11 to 0.77 cmol/kg) and %OC (0.38 to 1.38%). In conclusion, the study reveals that Hydrus 1D model is capable to predict K leaching for soils with increased in CEC (2.18 – 11.1 cmol/kg), %OC (0.35 to 2.64%) and at medium K treatment. In summary, typical loamy sand to sandy loam soils of southwest WA have limited K sorption and very low K release, which implies that these soils have very low exchangeable and nonexchangeable K. The K leaching losses were most pronounced in soils with very low clay content (4 to 7%) and CEC (0.61 to 0.89 cmol/kg) at pH 4.5. However, K leaching was pronounced even in soil with 13% clay and with CEC of 1.22 cmol/kg at pH 4.8. With higher clay contents (14–19%), K applied was less susceptible to K leaching notwithstanding low CEC (0.77–4.05 cmol/kg) and pH 4.4 - 4.6. Similarly, there is lower risk of K leaching in sandy loam soils with 11% clay content and with higher CEC (4.09 to 11.1 cmol/kg) at pH 5.1 -5.2. However, greatest risk of K leaching was at high rate of K application in these soils. The main factors influencing K leaching identified in these studies include rates of K applied, clay content, pH, rainfall (leachate volume) and K uptake. The current version of Hydrus 1D model is capable to predict K leaching for soils with pH 5.5 and above, higher CEC (2.18 – 11.1 cmol/kg), %OC (0.35 to 2.64%) and at medium K treatment. However, the inability of the Hydrus 1D model to accurately predict K leaching of the southwest WA soils, at higher K and with nil K application, and in soils with negligible K buffering capacity may stem from both the assumptions of the model (e.g., uniform flow and equilibrium adsorption) and the unique properties (low CEC, rapid infiltration, low clay) of the studied soils. Based on the findings of these studies, the combined effects of K uptake by plants and K leaching losses will increase the prevalence of soil K deficiency in these soils if K supply is not replenished with K fertilization. Also, the findings of this studies further show that raising soil pH to 5.5—the ideal pH for south-west WA soils—can significantly reduce K leaching loss on acid sand, sandy loam and loamy sand soils.

Potassium leaching in cropping systems with reference to Western Australia

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