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Article

Investigating the Effects of Soil Type and Potassium Fertiliser Timing on Potassium Leaching: A Five-Soil Lysimeter Study

by
Thomas P. McCarthy
1,2,
John B. Murphy
1 and
Patrick J. Forrestal
1,*
1
Soils, Land Use and Environment Department, Teagasc, Johnstown Castle, Co., Y35 TC97 Wexford, Ireland
2
DairyNZ Ltd., 605 Ruakura Road, Hamilton 3240, New Zealand
*
Author to whom correspondence should be addressed.
Soil Syst. 2025, 9(4), 110; https://doi.org/10.3390/soilsystems9040110
Submission received: 18 July 2025 / Revised: 29 September 2025 / Accepted: 5 October 2025 / Published: 13 October 2025
(This article belongs to the Topic Soil Health and Nutrient Management for Crop Productivity)
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Abstract

Potassium (K) is essential for grassland productivity, but soil K leaching can reduce fertiliser use efficiency, increasing environmental losses. International evidence suggests soil type and K fertiliser timing influence K leaching, yet limited data exist for Ireland’s diverse soil types. This study investigated the effects of K fertiliser timing (autumn, winter, and spring) and soil type on K leaching using a controlled lysimeter facility with five representative Irish soils sown with perennial ryegrass. Potassium fertiliser (125 kg K ha−1) was applied in October, December, or February, with leachate collected from October to April. Soil type affected cumulative K leaching (1.4–9.8 kg ha−1; p ≤ 0.001), with the greatest losses observed in sandy soils. Potassium and nitrogen uptake in spring-harvested grass were also influenced by soil type (p ≤ 0.05), with strong positive correlation between the two nutrients (R2 = 0.78; p ≤ 0.001). Temporally, significant interactions (p ≤ 0.05) between K application timing and sampling date were found for K leachate in three of the five soils tested. Autumn and winter applications tended to increase cumulative leaching risk, especially on coarser-textured soils such as the Oakpark soil (p ≤ 0.05). The study indicates that applying K in early spring will tend to reduce leaching K losses, particularly on sandy soils.

Graphical Abstract

1. Introduction

Globally, the efficient use of mineral or artificial fertilisers is coming under scrutiny [1]. Potassium (K) fertiliser is no exception, as the mining of K fertilisers is a carbon-intensive process [2,3]. The average emissions associated with potassium chloride (KCl) fertiliser production is estimated at 0.6 tonnes CO2e per tonne [4]. The movement of K beyond the root zone via leaching results in K losses from the soil–plant system [5,6,7], and understanding how K management influences leaching in different soils is increasingly relevant as K fertilisers become more expensive [8] Compared to nitrogen (N) and phosphorous (P), K leaching from agricultural soils has received relatively little research investigation over recent decades. This is largely because it is not directly linked with eutrophication of waterbodies [5,9,10]. However, certain invertebrate aquatic species are sensitive to K toxicity [11].
Adequate K nutrition is essential in crop production [12,13], including grasslands [13,14,15,16], where it performs irreplaceable functions in regulating photosynthesis [17,18]. Specifically, the accumulation of carbon dioxide, phloem loading, and photosynthate translocation around the plant [19,20], as well as enzyme activation [21]. Soils have an inherent capacity to supply K to crops, with the release of non-exchangeable K, i.e., K bound either tightly to or within clay lattice structures, into the exchangeable soil K pool for plant K uptake [21,22,23]. However, the release rate of non-exchangeable K to exchangeable K pools can often be inadequate to meet plant K demand [24,25], particularly in managed ryegrass swards exhibiting rapid growth and high K demand during periods such as spring [26]. As a result, fertilisers, including KCl and organic manures such as dairy cattle slurry, are utilised to supplement the availability of soil K to crops [13,27]. Increases in dry matter yield (DMY) and increased nutritional feed quality for livestock are observed from the addition of artificial fertiliser K (AFK) in Irish grasslands [28,29] by enhancing K uptake and plant metabolic processes such as protein synthesis [30]. Artificial fertiliser K addition is particularly pertinent on grass swards used for feed conservation (silage) where large offtakes of K are removed in the harvested biomass [31,32]. In grassland swards used for livestock grazing, AFK recommendations are more moderate compared to where silage is cut and removed due to the recycling of K under grazing. Indeed, 90 to 92% of K ingested by ruminants is recycled back to the soil in discrete highly concentrated deposits of urine and dung [33,34]. In Ireland, a maximum AFK rate of 95 kg ha−1 is recommended for grazed grassland to mitigate luxury sward K uptake [35]. Luxury K uptake in grass species results in depressed uptake of magnesium (Mg) and calcium (Ca). Dietary deficiencies for Mg and Ca in grazing ruminants are linked to the onset of hypomagnesaemia (grass tetany) [36] and hypocalcaemia (milk fever) due to abnormally low blood plasma levels of Mg and Ca, respectively [37]. On swards used for silage production, the K recommendations aim to offset offtakes at an approximate replenishment rate of 30 g K kg DM−1. Furthermore, traditional AFK advice in Ireland to increase STK fertility on grazed swards is to apply AFK in the autumn/early winter period to reduce the susceptibility for luxury sward K uptake in the subsequent grazing season.
Queries, however, on the fate of autumn-applied AFKhave been raised; a previous study from the United Kingdom demonstrated that applying AFK in autumn resulted in lower K use efficiency in the following season relative to a springtime application, reflected in lower K concentrations in the grass [15]. This was likely due to K leaching over the winter period, as demonstrated in other studies [5,6,38,39]. The risk of K leaching has been attributed to excessive K in the soil solution in numerous studies, e.g., [4,26,39,40,41]. In a study by [4], K leaching losses were notably higher in a clay soil compared to sandy or loam soils, likely due to preferential water flow. On the other hand, many of the previous studies on grassland K leaching dynamics examined one soil type, e.g., a silty clay loam [26], whereas [10] studied a loamy sand soil. In a study by [9], the leaching risk of K was attributed to the rate of transfer of exchangeable K to non-exchangeable soil pools, a process mediated by the soil’s capacity to buffer K, as described by [42,43]. There exists a rich diversity in the chemical and physical composition of various mineral soil types across the Island of Ireland [35,44]. Considering the highly soil-specific nature of processes attributing to K leaching, a study quantifying K leaching under a range of representative Irish soils and using commonly practiced K fertilisation regimes for Irish grasslands is pertinent.
The objective of this study was to evaluate the effect of different application timings of AFK on K leaching from grassland soils. The study incorporated five contrasting soils that represent a range of representative Irish soil types. Potassium fertiliser (KCl) was applied at three different timings, autumn (Kt1), mid-winter (Kt2), and early-spring (Kt3) to a soil lysimeter facility sown with perennial ryegrass swards. Detailed measurements of K leaching were taken to determine the influence of (i) application timing and (ii) soil type on K leaching. The following hypotheses were tested: (i) Timing of AFK application will affect the concentration and quantity of K leached. (ii) The effect of AFK application timing on K leaching will vary by soil type.

2. Materials and Methods

2.1. Description of the Soil Lysimeter Facility and the Soil Characteristics

The lysimeter facility used for this study is located at Teagasc Johnstown Castle Research Centre, Co. Wexford, Ireland (51°17′32″ N 6°29′59″ W) (51 m above sea level). A total of 45 undisturbed monolith lysimeters (0.58 m diameter, 0.26 m2, 1 m deep) were collected from five contrasting Irish soils, as described by [45]. For each soil type, there are nine experimental units (lysimeters). The soils represent a range of productive agricultural Irish soils with varying properties, including variation in soil texture, organic matter content, and drainage (Table 1). The soils were classified according to the World Reference Base Classifications [46] and the ‘Irish Soil Information System’ (http://gis.teagasc.ie/soils/) (accessed 17 February 2025). Soil textural classification used by [45] were essentially those of the Soil Survey Manual, U.S.D.A. Handbook No. 18, Washington, D.C., 1951, described by [47].
The Oak Park soil (soil a) is a gravelly brown earth of coarse sandy loam texture derived from calcareous, fluvio-glacial gravels, composed mainly of limestone (Table 1). The Oak Park soil has a relatively low moisture-holding capacity and is prone to drought during dry periods [45]. The Elton (soil b) is a well-drained grey-brown podzolic of gravelly loam overlying gravelly clay loam texture, derived from predominantly limestone glacial drift [45]. The Clonroche soil (soil c) is a well-drained loam to clay loam overlying a shaly loam-textured brown earth [45]. The Castlecomer soil (soil d) is a poorly drained clay loam derived from dense, tenacious, non-calcareous solifluction drift. The impermeable nature and the fine texture and structure of the soil contribute to poor drainage [45]. The Rathangan soil (soil e) is a poorly drained loam overlying a clay loam texture, derived from Cambrian shale-quartzite-Irish Sea drift [45].

2.2. Meteorological Data

The local climate is humid temperate oceanic, with a long potential growing season (circa 10 months). Weather data (soil temperature at 10 cm depth and rainfall) were recorded every 30 min using the Meteorological station at the Johnstown Castle Research Farm (Campbell Scientific Ltd., Loughborough, UK) for the duration of the experiment; October 2020 to April 2021. Likewise, weather data were also collated for each year between 2010 and 2019 to calculate the 10-year average (Figure 1).

2.3. Experimental Design

The experiment was conducted at the lysimeter facility on existing perennial ryegrass swards (Lolium perenne L.) from 28 October 2020 to 30 April 2021. There was a completely random assignment of treatments within the experimental units of each of the five soils described above. There were two factors in this experiment: (i) soil type (n = 5) as described above and (ii) timing of AFK application—hereinafter referred to as ‘K timing’ (Kt) (n = 3). The three levels of Kt were (i) autumn (Kt1); applied on 28 October 2020, (ii) mid-winter (Kt2); applied on 17 December 2020, and (iii) early-spring (Kt3); applied on 18 February 2021. In all cases, AFK was applied as KCl (50% K) fertiliser at single rate of 125 kg K ha−1.

2.4. Sward Maintenance

Prior to the commencement of the experiment on 21 October 2020, the grass on the lysimeters was cut to 40 mm height using a hand-held cutter (Bosh Isio Shape and Edge Hedge and Grass Trimmer, Leinfelden-Echterdingen, Germany), as described by [48,49]. The lysimeters were cut again on 29 March 2021.

2.5. Sward Yield and Grass Mineral Analysis

At harvesting, a 100 g freshweight sample was taken from each lysimeter and dried at 70 °C in force draft oven for >72 h for determination of dry matter (DM). The DMY was expressed as kg ha−1 of DM. Subsamples of the dried grass from each plot were ground to <2 mm (Christy and Norris 5-inch hammer mill, Christy Turner, Suffolk, UK) in preparation for mineral analysis, as outlined by [48]. Elemental K, Mg, and Ca concentrations in grass DM were measured using the ‘hot acid’ extraction method, where the ground sample was firstly digested with 69% nitric acid (HNO3) and microwaved (35 min at 200 °C; 1080 watts) in a CEM MARS6 microwave digester (CEM Corporation, Matthews, NX, USA). The nutrient concentrations were subsequently determined using an Agilent 5100 synchronous vertical dual view inductively coupled plasma optical emission spectrometer (Agilent 5100 ICP-OES, Agilent Technologies, Incorporated Santa Clara, CA, USA) as outlined by [49,50]. Nitrogen and organic carbon (C) content of the grass were determined using a C&N analyser (Leco Corp, St. Joseph, MI, USA).

2.6. Measurement of Sward K Uptake

Potassium in grass DM was determined for grass samples taken in March 2021. The elemental composition of the grass sample from laboratory analysis and DMYs were used to calculate sward nutrient uptake using the following equation:
Sward K uptake (kg K ha−1) = [Sward yield kg DM ha−1 × grass sample K%]

2.7. Measurement of Leachate and Analysis

The lysimeters surrounded an open trench that contained opaque plastic drums (23 L volume). Each lysimeter was drained under gravity via plastic tubing into their respective drums. The mass of leachate (kg) at each sample date was recorded using a top-end balance (less the mass of the plastic drum). To convert kg to litres, a water density of 1 g cm3−1 at 4 °C was assumed. A subsample (~50 mL) was taken for nutrient content analysis. Concentrations of K in these samples were determined using ICP-OES.
The calculation of K leached (kg ha−1) is as follows:
(Mass leachate (kg) × Leachate Kconc (mg L−1) ÷ 1,000,000) × ha conversion factor
where
1,000,000 = Conversion factor from mg to kg
Ha area conversion factor = 38,461 (no. of lysimeters ha−1), i.e., 0.26 m2 × 38,461 = 10,000 m2
Temporal K leaching refers simply to the concentration of K in leachate (mg L−1) at each discreet sampling date. The temporal trend in leachate load (kg ha−1) at each sample date was the total amount of K leached (kg ha−1) up to that sample date. For example, the K leachate load at sample date 3 was the total sum of K leached (kg ha−1) on sample dates 1, 2 and 3, and so on.
The total cumulative quantity of K leached over the experiment was the sum of the K leached at each sampling date for the duration of the experiment.

2.8. Soil Analysis

All lysimeters were soil-sampled in July 2019, when the lysimeters were tilled and the grass sward was established, and again in April 2021 using a soil corer (15 mm diameter) to approximately 10 cm depth as described by [44]. The lysimeters were not sampled immediately before the experiment began to avoid creation of preferential flow pathways where the cores were removed. The samples were dried at 40 °C for >5 days in a force-draft oven immediately post-sampling and then sieved to pass through a 2 mm sieve to remove debris. A full suite of analysis was previously performed on the soil samples in July 2019 [48]. Exchangeable K and P concentrations were determined following extraction using Morgan’s solution (Na acetate + acetic acid, pH 4.8). Extracted solutions were filtered and subsequently analysed for K concentration by atomic emission at 766.5 nm using a Sherwood Flame Photometer (Sherwood Scientific Ltd., Cambridge, UK) as outlined by [38]. Soil P content was determined by ICP-OES analysis of the extract as outlined above. Cation exchange capacity (CEC) was determined using the ammonium acetate (NH4AOc; buffered to pH 7) methodology using a mechanical vacuum extractor (Sampletek, MAVCO Industries, Lincoln, NE, USA). The method is an adaption of that described by [51,52].
In the years prior to this experiment (2017 to 2020), fertiliser P and K were applied at the prescribed fertiliser maintenance rate for grassland [35] in order to maintain soil P and K levels within the agronomic optimum range as described by [45,46].

2.9. Calculating Available Water Holding Capacity (AWHC)

AWHC% was estimated using a pedotransfer function (PTF) as θFC − θPWP, according to [53], where θFC was the water content at field capacity and θPWP was the water content at permanent wilting point. This is a robust PTF for calculating plant available water holding capacity that underwent both internal and external validation. Internal validation was made against the North American Project to Evaluate Soil Health Measurements (NAPESHM) data (used for model creation). For external validation, the functions were applied to 1797 soil samples from the National Cooperative Soil Survey Characterization (NCSS) database and compared to the widely used functions proposed by [54].
The calculation of AWHC was by the following equation:
θFC = 37.217 − 0.140 × Clay_(%) − 0.304 × Sand_(%) − 0.222 × SOC_(%) + 0.051 × (Sand_(%) × SOC_(%)) + 0.085 × (Clay_(%) × SOC_(%)) + 0.002 × (Clay_(%) × Sand_(%))
θPWP = 7.222 + 0.296 × Clay_(%) − 0.074 × Sand_(%) − 0.309 × SOC_(%) + 0.022 × (Sand_(%) × SOC_(%)) + 0.022 × (Clay_(%) × SOC_(%))
AWHC% = θFC − θPWP
AWHC% to AWHC mm = AWHC% × 10

2.10. Data Processing and Statistical Analysis

Data were analysed using SAS 9.4 (SAS Institute, Cary, NC, USA). All raw data were collated to carry out basic descriptive statistics and check normality, homogeneity of variances, and sphericity to ascertain appropriate statistical tests. Where parameters failed to meet the assumptions of normality using a Shapiro-Wilk’s test with PROC UNIVARIATE in SAS, e.g., temporal parameters K concentration in leachate (mg L−1) and K leaching (kg ha−1), a log transformation was applied to the data to stabilise the variance and to approximate a normal distribution.

2.11. Temporal Parameters

Repeated-measures analysis of variance (ANOVA) was conducted on the temporal data within each soil type using PROC GLIMMIX procedure in SAS 9.4 to test the effects of (i) K timing, (ii) sample date, and (iii) the interactions between K timing and sample date on K concentration in leachate (mg L−1). In this analysis, K timing was the fixed effect, the K concentration in leachate was the dependent variable, and leachate sampling dates was the repeated measures factor in the repeated measures analysis (Table 2). This analysis was also conducted to test these effects on dependent variable K leachate load (kg ha−1; Table 2).

2.12. Cumulative and Total Parameters

An ANOVA analysis using PROC GLIMIX was also used to test the effect of (i) K timing, (ii) soil type, and (iii) their interaction on the dependent variables of grass DMY, sward K, and N uptake in grass DM in March 2021 and STK in April 2021. Differences between treatments were determined using the F-protected least significant difference (LSD) test at a 95% confidence level. A pooled standard deviation (PSD) was carried out on the data to determine the weighted average of standard deviations for each response variable listed above among the three K timings, both within each soil type and across all soils (Table 3). A linear correlation model was used to test for correlation between the following dependent variables: (i) K uptake in grass DM and (ii) N uptake in grass DM, using the PROC GLM procedure in SAS.

3. Results

3.1. Meteorological Data Results

The mean monthly soil temperature over ten years at 0–10 cm depth for the typical main autumn–winter–spring leaching period from October to April (2010 to 2019), ranged from 11.7 °C in October to 5.8 °C in January. During the experimental period from October 2020 to April 2021, the highest soil temperature was 10.9 °C in October, while the lowest was 4.4 °C in January (Figure 1a). The 10-year average rainfall ranged from as high as 115 mm in October to as low as 60 mm in April. During the experimental period, the highest precipitation was in January (144 mm), while April was the driest month (12 mm) (Figure 1b). The monthly precipitation was above the 10-year average in October, November, December, January, and February but below the 10-year average in March and April (Figure 1b).

3.2. Temporal K Leaching

A significant interaction between K timing and sample date was observed in the Castlecomer soil (p ≤ 0.001) for K concentration observed in leachate (Table 2). K timing significantly affected K concentrations in leachate in the Rathangan (p ≤ 0.05) soil. Individually, sampling date significantly affected K concentrations in leachate in all soils apart from the Clonroche soil (Table 2). For the temporal trends in K leachate load, there were significant interactions between K timing and sample date in the Oakpark, Castlecomer, and Rathangan soils (p ≤ 0.05; Table 2). Sample date influenced K leachate load (p ≤ 0.001) in the Elton and Clonroche soils, while K timing had no influence on these soils (Table 2).

3.3. Potassium Concentrations in Leachate

The very well-drained Oakpark soil exhibited higher K concentrations in the leachate for Kt2 compared to Kt1 and Kt3, yet there was no significant interaction with sample date (Table 2). There was no temporal effect of K timing on K concentrations in leachate for the well-drained Elton soil (Figure 2b; Table 2). In the well-drained Clonroche soil, K timing did not significantly affect K concentrations in leachate (Figure 2c). For the poorly drained Castlecomer soil, there was an interaction (p ≤ 0.001) between K timing and sample date for the K concentrations in leachate. Specifically, Kt1 exhibited significantly (p ≤ 0.01) greater K concentrations following Kt1 compared with Kt2 and Kt3 treatments in February 2021 (Figure 2d). In the poorly drained Rathangan soil, there was no interaction between K timing and sampling date for K concentrations in leachate (Table 2; Figure 2e), although there was a trend towards greater K concentrations in leachate following Kt1 compared with Kt2 and Kt3.

3.4. K Leachate Load

In the Oakpark soil, there was a significant (p ≤ 0.05) K timing and sample date interaction on K leachate load (Table 2), where the highest K leaching resulted from Kt2. There was no significant effect of K timing on K leachate load from the Elton or Clonroche soils (Figure 3b,c, respectively). For the Clonroche soil, there was a tendency for higher K leaching loads following Kt1, followed by Kt2 and Kt3, but this was not significant (Figure 3c). In the Castlecomer soil, there was a significant interaction between K timing and sample date on K leachate load (p ≤ 0.05) (Figure 3d), where Kt1 exhibited higher leachate loads than Kt2 and Kt3. In February 2021, Kt2 exhibited higher (p ≤ 0.05) K leachate loads than Kt3. There was also a significant interaction between K timing and sample date for the Rathangan soil (Table 2). Temporally, in the Rathangan soil, Kt1 exhibited greater K leaching than Kt2 and Kt3 on three dates in November 2020 (p ≤ 0.05); Kt1 was greater than Kt3 (p ≤ 0.001) in December 2020, on two dates in January (p ≤ 0.05), and also in February and March 2021 (p ≤ 0.05) (Figure 3e).

3.5. Cumulative K Leached

There was a significant effect of soil type (p ≤ 0.001) on the total cumulative quantity of K leached over the experiment from 30 October 2020 to 30 April 2021 (Table 3). Oakpark (soil a) exhibited the highest (p ≤ 0.05) cumulative K leached at 9.8 kg K ha−1 averaged across the three K timings, and ranging from 4.5 to 18.4 kg K ha−1 (Table 3). The Elton soil (soil b) exhibited the lowest (p ≤ 0.05) levels of K leached overall at 1.4 kg K ha−1, ranging from 0.5 to 3.0 kg K ha−1. There were no significant differences between soils Castlecomer, Clonroche, and Rathangan, at 3.3, 4.5, and 2.0 kg K ha−1, respectively, averaged across the three K timings (Figure 4).
There was no effect of K timing observed for cumulative K leached overall across the five soils. However, within each soil, there was significantly higher (p ≤ 0.01) cumulative K leached from Kt1 compared to Kt3 for the Clonroche soil. There was also significantly higher (p ≤ 0.05) cumulative K leached from Kt1 compared with Kt2 and Kt3 for the Rathangan soil (Table 3).

3.6. Grassland Production and Nutrient Uptake

Grass DMY in March 2021 was not significantly affected by soil type or K timing. There was a significant effect of soil type on K uptake in grass DM in March 2021 (p ≤ 0.05). The Rathangan soil (soil e) exhibited the highest quantity of K uptake in the grass, while no other significant differences between the other soils were observed. A similar trend was observed for N uptake in grass DM in March 2021; the Rathangan soil exhibited the greatest (p ≤ 0.01) quantity of N uptake, with no other observed differences between the remaining soils. There was no significant effect of K timing on these parameters. For grass swards harvested in March 2021, there was a strong correlation (R2 = 0.78; p ≤ 0.001) between the uptake of K and uptake of N in grass DM (Table 3).

3.7. Soil K Fertility Levels

There was a significant effect of soil type (p ≤ 0.001) on STK levels in April 2021; however, there was no observed effect of K timing. The Clonroche and Oakpark soils had the highest (p ≤ 0.05) STK levels in April 2021. There were no significant differences observed between soils, Castlecomer, Elton, or Rathangan, for STK in April 2021 (Table 3).

4. Discussion

The present study has demonstrated the influence of soil type on both the temporal and cumulative patterns for K leaching losses over a drainage season under grassland. Both the magnitude and variability in K leaching observed across the soils used in this study highlight the importance of tailoring K fertilisation strategies to soil type in order to mitigate K loss potential and improve the use efficiency of K fertilisation in grassland systems.

4.1. Temporal K Leaching and Soil Type Effects

Significant interactions between K timing and sample date were observed in the Oakpark, Castlecomer, and Rathangan soils for temporal trends in K leachate load (Figure 3; Table 2), while this interaction was also observed for leachate K concentrations in the Castlecomer soil (Figure 2). It is clear from the coarse-textured Oakpark soil, that K applications in autumn to mid-winter created a clear trend in increasing K susceptibility, both temporally (Figure 3a) and in the cumulative findings in Figure 4. Indeed, the risk susceptibility to K leaching losses in coarse-textured sandy soils is well documented [9,10,13,26,55] yet AFK recommendations for increasing soil K fertility in temperate grassland regions, particularly for higher rates under grazed grasslands, are often to apply in autumn to mitigate hypomagnesaemic risk. This rationale is to lower the potential K concentrations in herbage in the subsequent spring, as discussed elsewhere [14,26]. The present study, however, highlights the potential for leaching losses of applied K when applied in the autumn or winter period, indicating an inefficient use of mineral K resources [2], with implications for profitability at farm level [8]. It is interesting to note that the Castlecomer and Rathangan soils, which are finer-textured, with poorer drainage and higher clay content, also exhibited greater K leachate loads following autumn K applications relative to mid-winter or spring. The interaction between K timing and sampling date for these soils indicates that these soils retained the autumn-applied K relatively well initially post application; however, with progression through the drainage season, the autumn-applied K exhibited the highest K leaching loss potential on these soils, observed by the sustained higher K leaching loads from Kt1 from January to April (Figure 3d,e), respectively. Indeed, this highlights the soil specificity of K leaching processes, as discussed by [9,10,13], and displays that K leaching processes are nuanced by inherent soil characteristics. A potential limitation to this analysis is acknowledged in that using cumulative K leachate load as the response variable in repeated-measures analysis introduces statistical limitations, particularly due to the non-independence of observations over time. While this approach highlights total nutrient loss over time, it may obscure treatment effects. Future work may benefit from analysing interval loads independently over time similar to the K concentrations in this study to better capture temporal dynamics.
Previous studies have highlighted the role of soil clay content and CEC in mediating K retention and leaching susceptibility over time. In the present study, the Oakpark soil (a) demonstrated the highest K leaching losses overall (Figure 4), particularly under a mid-winter K application (Kt2), where prolonged periods of high leachate K loads were observed (p ≤ 0.001). The finding is in agreement with [10], who found that late-season applications increased K leaching in a lysimeter study with a gleyic podzol soil compared to within-season K applications. The Oakpark soil in this study exhibited the lowest % clay composition, CEC, OM, and AWHC of the five soils tested while having the highest drainage potential (Table 1). The greatest quantities of cumulative K leached from this soil demonstrates the susceptibility of these soils to K leaching losses, particularly when applied during periods of high rainfall and through-drainage, as was the case during December 2020 during this study, at 164 mm, 27% above the 10-year average (Figure 1b). This finding is also in agreement with [47,48], who observed high K leaching susceptibility for sandy soils.
It is acknowledged that prediction of AWHC using the PTF highlighted by [53] may be a limitation relative to measured values. However, the PTF is suitable for mineral, non-calcareous soils used in this study [52,54] Although the PTF has limited applicability in soils with >35% clay, soils in this study were below this threshold (Table 1). The PTF performed well and was highly correlated to soil clay content (R2 = 0.88; p ≤ 0.001), indicating alignment. Additionally, this approach avoided destructive lysimeter sampling, where disturbance and depletion of the soils is undesirable.

4.2. Effect of K Timing on K Leaching

The importance of K timing in mediating K leaching risk is provided in the temporal interactions between K timing and sample date for K leachate load in three of the five soils tested (Table 2). For example, the Rathangan and Castlecomer soils exhibited greater leaching following the autumn-applied fertiliser K (Kt1), particularly over the winter and early spring (Figure 3d,e). In comparison, the Oakpark soil exhibited the greatest K leaching following mid-winter (Kt2) application of AFK and sustained elevated K leachate loads, particularly from December through to April (Figure 3a). While the K timing effect was insignificant for this soil, across the five soils tested, Oakpark exhibited the highest K concentrations in leachate (Figure 2a). This again is likely due to the high drainage potential and lower K buffering capacity of this soil, which exacerbates K leaching loss [9]. This is due to the lower ability of the soil to retain K on exchangeable K sites on the cation exchange complex, as discussed by [6,23]. The findings of the current study support this interpretation, given the comparatively low CEC (10.6 cmol+ kg soil−1) observed for this soil, which would be associated with the lower range for CEC across Irish soils, and therefore likely to have a low K retention capacity [6,23]. On the other hand, the Elton soil exhibited no K timing response for temporal K leaching (Figure 2c and Figure 3c). While the chemical characteristics of this soil (clay = 17% and CEC = 15.6 cmol+ kg soil−1; Table 1) suggest a moderate K buffering capacity, it may also be due to past management of the soil. Previous STK analysis from July 2019 showed that the average STK level for the Elton soil was in the agronomic optimum range for Irish soils [35] at 144 mg L−1. It is likely that this level did not change significantly over the intervening period due to maintenance AFK rates being applied, which may account for the lack of a K timing effect on K leaching for this soil. Another explanation for the lack of K timing effect on K leaching may be that time is needed for AFK to interact with the soil in order for K to become mobile, as described by [5]. In the context of the timeframe of the present study, this may have resulted in no detection of a K timing effect on K leaching for this soil, especially where strong equilibrium exists between the soil solution, exchangeable and non-exchangeable K pools, as described by [41,56,57]. Nevertheless, irrespective of soil chemical characteristics and their influence in mediating K leaching, there was a tendency for greater quantities of cumulative K to leach from the Oakpark, Clonroche, and Rathangan soils following autumn fertiliser K applications compared to mid-winter or early spring applications, although soil type was the dominant factor influencing the total quantity leached over the study period (Figure 4, Table 3). Irrespective of soil type, the study findings support delaying grassland fertiliser K applications from autumn to early spring (mid-February onwards as tested in this study) to mitigate K losses and improve use efficiency in temperate oceanic regions such as Ireland, where elevated precipitation over the winter period exacerbate K leaching losses.

4.3. Grassland Production and Sward Nutrient Uptake

Despite the variances in K leaching losses, grass DMY in March 2021 was not affected by soil type or by K timing. However, the uptake of K and N in grass DM in this harvest was significantly affected by soil type, with K uptake being significantly higher for the Rathangan soil (p ≤ 0.05), with a similar trend observed for N uptake. The strong positive correlation between K and N uptake (R2 = 0.78, p ≤ 0.001) is indicative of the synergistic relationship between K and N for plant uptake, where K is intrinsically linked with the uptake of nitrate (NO3−-N) [51,52,53]. There is a tendency for plant species to assimilate potassium cations (K+) and anionic nitrate (NO3−-N) in corresponding quantities to maintain overall net neutrality [18]. This finding is noteworthy as it demonstrates that an adequate K supply to grasslands may help mitigate NO3−-N losses during the winter drainage period, thereby mitigating the quantity of N available for leaching to groundwater. There was no significant effect of K timing on K uptake in grass DM; this finding varies from the findings of [15,58], who both observed lower K concentrations in grass DM in spring grass from fertiliser K applied in autumn compared to spring applications. In the current study, having no effect of K timing on K recovered in grass DM is likely due to the relatively early spring harvest, which occurred on 29 March 2021, reflected in the low DM yields observed (averaged across all treatments, 1304 kg DM ha−1; Table 3). Extending the harvest date until April or May to allow increased sward DM production would increase sward K demand and potentially further elucidate the effect of K timing on K uptake.

4.4. Effects on Soil Test Potassium Level

By the end of the experiment in April 2021, the significant effect of soil type (p ≤ 0.001) on STK levels saw the Clonroche soil having the highest STK levels along with the Oakpark soil. For the Clonroche soil, which averaged 343 mg L−1 across the three K timings, this was likely due to the comparatively low levels of cumulative K leached, averaging 2 kg ha−1 across the three K timings over the experimental period (Table 3), and likely to the STK levels at the beginning of the experimental period, which were not directly measured in this study. It is also indicative of the soil’s K buffering capacity to retain applied AFK [42]. On the other hand, it is interesting that the Oakpark soil had the second highest STK levels in April 2021, averaging 318 mg L−1 across the three K timings. While this soil exhibited the highest cumulative levels of K leaching (Figure 4), it also demonstrated significantly higher (p ≤ 0.05) STK levels than the Rathangan and Elton soils at the end of the experiment. This finding is consistent with [26], who observed rapid increases in the exchangeable-K pool following AFK application in coarse-textured sandy soils with low K buffering capacity. Such soils typically exhibit limited phase transfer of K to non-exchangeable pools [3,59] resulting in a build-up of exchangeable K, as measured in the present study by Morgan’s soil K test. This accumulation can elevate soluble K to levels that increase the risk of leaching losses from the soil system. This interpretation aligns with [9,10], who both linked K leaching susceptibility to the concentration of exchangeable K, a relationship that is also supported by the Oakpark soil in the present study (Figure 4). Furthermore, there was a weakly correlated significant relationship (R2 = 0.2; p < 0.01) between the STK levels observed in April 2021 and cumulative K leached.4.5. Implications for Grassland K Management.
Overall, soil type played a dominant role in determining cumulative K leaching over the study period, uptake in grass DM, and STK levels. Autumn and mid-winter K applications are particularly inefficient on coarse-textured sandy soils, and the temporal patterns of K leaching in this study highlight the importance of avoiding excessive AFK applications on these soils, particularly during or ahead of the main drainage season to mitigate K leaching losses [5,10]. This finding is also in agreement with [60], who highlighted the potential for excessive K fertilisation on sandy soil to exacerbate K leaching loss, and suggested that K fertilisation strategies on sandy soils should prioritise crop needs and the soil’s capacity to release non-exchangeable K, rather than simply maintaining high soil test levels.

5. Conclusions

This study has shown that soil type was the main factor influencing K leaching loss susceptibility over the main drainage winter to early spring period across five representative Irish soils. The interactions observed between the timing of fertiliser K application (autumn, mid-winter, early-spring) and sampling dates across the drainage season in certain soils highlight the variability in K leaching risk potential across different soils, with cumulative losses ranging from 1.1 to 5.5 kg ha−1 on poorly drained soils and from 0.5 to 18.4 kg ha−1 on free-draining sandy or loam-textured soils. Specifically, more coarsely textured sandy soils with lower K buffering and higher drainage potential are prone to greater magnitudes of K leaching from autumn or mid-winter fertiliser K applications. The accumulation of soluble ‘exchangeable‘ soil K by the end of the study period (determined by Morgan’s soil K extraction) following fertiliser application was a driver of leaching (R2 = 0.2; p < 0.01), especially in coarse-textured soils. In contrast, finer-textured soils showed reduced K leaching losses and were less sensitive to application timing. It can be inferred that in coarse-textured Irish grassland soils with low K buffering capacity, applied K remains largely in the exchangeable K pool; thus, it is at most risk to leaching. In mineral grassland soils in Ireland, such as those tested in this study, the fertiliser K programme should therefore consider the soil’s capacity to buffer K. Conversely, finer-textured soils with higher K buffering capacity have a greater ability to mitigate K leaching loss potential, and K timing was less critical, as evident in this study from the Elton and Rathangan soils in particular. The findings of this study are important from the perspective of increasing sustainable use of finite mineral fertiliser in Irish grassland soils by mitigating loss potential through more targeted and efficient application strategies. Using the soils tested in this experiment as a case study, it is seen how applying K fertilisers to grassland soils in autumn to build STK levels in temperate regions such as Ireland comes with the potential for K leaching loss over the winter period. An industry-relevant conclusion is that spring applications are preferable from a loss limitation perspective, better aligning K supply with the growth requirements of the grass sward, and are less likely to coincide with high precipitation that can increase leaching losses of soluble K. When K is applied, fertiliser programmes should take account of the potential for luxury K uptake in herbage and associated animal disorders such as hypomagnesaemia and hypocalcaemia.
Further research should expand on the findings of the current study by comparing different sources of K fertilisation (e.g., organic sources such as cattle slurry and manure) with chemical fertilisers and evaluating their relative effects on leaching risk across representative Irish soils during the drainage season [5]. Future studies should also quantify changes in STK from the beginning to the end of the experiment as a function of K leaching across soil types, to better understand K movement within the soil system. Additionally, short-term studies such as this may be limited by weather anomalies, particularly the higher-than-average precipitation observed during the study period, which can influence leaching dynamics. Extending the study over a longer duration would help capture cumulative and delayed responses to K application timing and improve the representativeness of findings in relation to K recovery in grass dry matter, K leaching, and phase transfers of K within the soil system.

Author Contributions

T.P.M.: Methodology, Software, Validation, Formal Analysis, Data Curation, Writing—Original Draft, Visualisation, Writing—Review and Editing. J.B.M.: Investigation, Data Curation, Resources. P.J.F.: Conceptualisation, Writing—Review and Editing, Supervision, Visualisation, Formal Analysis, Project Administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available upon reasonable request to the authors via email.

Acknowledgments

This research was conducted as part of the Teagasc Climate Centre. The authors wish to acknowledge the input of the technical and laboratory staff at Teagasc Johnstown Castle Research Centre for data collection and sample analysis.

Conflicts of Interest

Author Thomas McCarthy is currently employed at DairyNZ Ltd. The research was carried out at Teagasc Johnstown Castle Research Centre while all authors were employed there. All authors declare that the research was conducted in the absence of any commer-cial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AFK Artificial fertiliser potassium
ANOVAAnalysis of variance
AWHCAvailable water holding capacity
CCarbon
CaCalcium
CECCation exchange capacity
CO2eCarbon dioxide equivalent
DMDry matter
DMYDry matter yield
HaHectare
K Potassium
KClPotassium chloride
KgKilogram
Kt1Autumn-applied fertiliser K
Kt2Mid-winter-applied fertiliser K
Kt3Early-spring-applied fertiliser K
Mg Magnesium
Mg L−1Milligram per litre
N Nitrogen
NaSodium
NH4AOcAmmonium acetate
NO3−−NNitrate nitrogen
OMOrganic matter
P Phosphorous
PSDPooled standard deviation
PTFPedotransfer function
SOCSoil organic carbon
STKSoil test K
UKUnited Kingdom
USAUnited States of America
USDAUnited States Department of Agriculture

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Soilsystems 09 00110 g001
Figure 1. (a). (i) Monthly soil temperature at 0–10 cm depth at the Johnstown Castle Research Centre (bars) from October 2020 to April 2021. (ii) Mean monthly soil temperature at 10 cm depth between 2010 and 2019 (dotted line). (b). (i) Monthly cumulative precipitation at the Johnstown Castle Research Centre (bars) from October 2020 to April 2021. (ii) Mean monthly precipitation between 2010 and 2019 (dotted line).
Figure 1. (a). (i) Monthly soil temperature at 0–10 cm depth at the Johnstown Castle Research Centre (bars) from October 2020 to April 2021. (ii) Mean monthly soil temperature at 10 cm depth between 2010 and 2019 (dotted line). (b). (i) Monthly cumulative precipitation at the Johnstown Castle Research Centre (bars) from October 2020 to April 2021. (ii) Mean monthly precipitation between 2010 and 2019 (dotted line).
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Soilsystems 09 00110 g002aSoilsystems 09 00110 g002b
Figure 2. The effect of K application timing on K concentrations (mg L−1) in leachate (October 2020 to April 2021) in each of the five soils in this lysimeter study. There was a K timing effect (p ≤ 0.05) in the Rathangan soil and K timing x sample date interaction (p ≤ 0.001) in the Castlecomer soil. **: Significant at p < 0.01. PSD: Pooled standard deviation of the mean.
Figure 2. The effect of K application timing on K concentrations (mg L−1) in leachate (October 2020 to April 2021) in each of the five soils in this lysimeter study. There was a K timing effect (p ≤ 0.05) in the Rathangan soil and K timing x sample date interaction (p ≤ 0.001) in the Castlecomer soil. **: Significant at p < 0.01. PSD: Pooled standard deviation of the mean.
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Soilsystems 09 00110 g003aSoilsystems 09 00110 g003b
Figure 3. Cumulative K leachate load (kg ha−1) (October 2020 to April 2021) by K application timing in each of the five soils in the grassland K lysimeter experiment. Significant interactions between K timing and sample date were observed in the Oakpark, Rathangan, and Castlecomer soils (p ≤ 0.05). *: Significant at p < 0.05; **: Significant at p < 0.01; PSD: Pooled standard deviation of the mean.
Figure 3. Cumulative K leachate load (kg ha−1) (October 2020 to April 2021) by K application timing in each of the five soils in the grassland K lysimeter experiment. Significant interactions between K timing and sample date were observed in the Oakpark, Rathangan, and Castlecomer soils (p ≤ 0.05). *: Significant at p < 0.05; **: Significant at p < 0.01; PSD: Pooled standard deviation of the mean.
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Figure 4. The effect of soil type on cumulative K leached between October 2020 and April 2021. There was a significant effect (p ≤ 0.01) of soil type on cumulative K leached. Groups with the same lettering are not significantly different.
Figure 4. The effect of soil type on cumulative K leached between October 2020 and April 2021. There was a significant effect (p ≤ 0.01) of soil type on cumulative K leached. Groups with the same lettering are not significantly different.
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Table 1. Description of soil classification, and physical and chemical characteristics of samples taken to a 10 cm depth in 2019 for the 5 soils used in the grassland K lysimeter experiment at the Johnstown Castle Research Centre.
Table 1. Description of soil classification, and physical and chemical characteristics of samples taken to a 10 cm depth in 2019 for the 5 soils used in the grassland K lysimeter experiment at the Johnstown Castle Research Centre.
Soil Name(a) Oakpark(b) Elton(c) Clonroche(d) Castlecomer(e) Rathangan
Soil TypeHaplic CambisolCutanic Luvisol Haplic CambisolAlbic Gleyic LixsolLuvic Stagnosol
Parent MaterialFluvioglacial gravelsGlacial driftGlacial driftFine loamy with siliceous stonesGlacial sea drift
DrainageVery wellWellWell PoorPoor
Soil TextureCoarse sandy loamGravelly LoamClay loamClay loamClay loam
Clay (%)16.31727.43324.8
Silt (%)17.93533.538.628
Sand (%)65.74839.128.447.3
CEC (cmol+ kg soil−1)10.615.613.014.537.8
pH (water)6.35.9665.9
Total C (%)3.53.63.64.63.5
Organic matter (g kg soil−1)69869210379
Total N (%)0.290.380.360.380.32
Total S (%)0.0250.0340.040.0340.035
Morgan’s soil test P (mg L−1)6.27.17.77.96.5
Morgan’s soil test K (mg L−1)132144121148127
Soil AWHC (%)24.826.929.133.527.8
Table 2. Effect of K application timing, sample date, and their interaction on (i) K concentration in leachate (mg L−1) and (ii) K leachate load (kg ha−1) within each soil, using a repeated-measures ANOVA for the grassland K lysimeter experiment at the Johnstown Castle Research Centre.
Table 2. Effect of K application timing, sample date, and their interaction on (i) K concentration in leachate (mg L−1) and (ii) K leachate load (kg ha−1) within each soil, using a repeated-measures ANOVA for the grassland K lysimeter experiment at the Johnstown Castle Research Centre.
K Concentration in Leachate (mg L−1) K Leachate Load (kg ha−1)
Oakpark
K timingNSNS
Sample date****
K timing × Sample dateNS*
Elton
K timingNSNS
Sample date******
K timing × Sample dateNSNS
Clonroche
K timingNSNS
Sample dateNS***
K timing × Sample dateNSNS
Castlecomer
K timingNSNS
Sample date******
K timing × Sample date****
Rathangan
K timing**
Sample date****
K timing × Sample dateNS*
*: p ≤ 0.05; ***: p ≤ 0.001; NS: no significance.
Table 3. The effect of (i) soil type, (ii) K timing, and (iii) interactions on (i) sward DMY, (ii) uptake of K and N in grass DM in March 2021, (iii) STK level in April 2021, and (iv) cumulative K leached between October 2020 and April 2021.
Table 3. The effect of (i) soil type, (ii) K timing, and (iii) interactions on (i) sward DMY, (ii) uptake of K and N in grass DM in March 2021, (iii) STK level in April 2021, and (iv) cumulative K leached between October 2020 and April 2021.
Grass DMY March 2021K Uptake in Grass DM March 2021N uptake in Grass DM March 2021STK April 2021Cumulative K Leached
Soil TypeK Application Datekg K ha−1kg K ha−1kg K ha−1Mg L−1kg K ha−1
OakparkKt1—28 October 20201382bac24bc25bc322bac4.2abcd
Kt2—17 December 20201205bc24bc23c316bdac18.4a
Kt3—18 February 20211183bc23bc22c316bdac6.7ab
EltonKt1—28 October 20201469bac34ba30bac229e0.5e
Kt2—17 December 20201322bac24bc25bc291ebdac0.6e
Kt3—18 February 20211345bac25bac27bac263ebdc3.0bcde
ClonrocheKt1—28 October 20201522bac30bac26bac344a3.6ab
Kt2—17 December 20201177bc22bc23c337a1.2abcd
Kt3—18 February 20211094c21c22c348ba1.1de
CastlecomerKt1—28 October 20201278bac25bac24bc289ebdac5.5abcd
Kt2—17 December 20201412bac30bac25bac270ebdc4.0abc
Kt3—18 February 20211276bac22bc25bac319bdac4.1abc
RathanganKt1—28 October 20201644ba34ba32bac243edc3.6abcd
Kt2—17 December 20201766a37a35a248edc1.2bcde
Kt3—18 February 20211538bac37a34ba252ed1.1cde
PSD3178.06.453.60.9
Effects
Soil Type NS*********
K timingNSNSNSNSNS
Soil Type × K timing interactionNSNSNSNSNS
*: Significant at p ≤ 0.05 level; **: significant at p ≤ 0.01 level; ***: p ≤ 0.001; NS: not significant; PSD: Pooled standard of the mean; groups with the same lettering (a, b, c, d, e) are not significantly different.
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McCarthy, T.P.; Murphy, J.B.; Forrestal, P.J. Investigating the Effects of Soil Type and Potassium Fertiliser Timing on Potassium Leaching: A Five-Soil Lysimeter Study. Soil Syst. 2025, 9, 110. https://doi.org/10.3390/soilsystems9040110

AMA Style

McCarthy TP, Murphy JB, Forrestal PJ. Investigating the Effects of Soil Type and Potassium Fertiliser Timing on Potassium Leaching: A Five-Soil Lysimeter Study. Soil Systems. 2025; 9(4):110. https://doi.org/10.3390/soilsystems9040110

Chicago/Turabian Style

McCarthy, Thomas P., John B. Murphy, and Patrick J. Forrestal. 2025. "Investigating the Effects of Soil Type and Potassium Fertiliser Timing on Potassium Leaching: A Five-Soil Lysimeter Study" Soil Systems 9, no. 4: 110. https://doi.org/10.3390/soilsystems9040110

APA Style

McCarthy, T. P., Murphy, J. B., & Forrestal, P. J. (2025). Investigating the Effects of Soil Type and Potassium Fertiliser Timing on Potassium Leaching: A Five-Soil Lysimeter Study. Soil Systems, 9(4), 110. https://doi.org/10.3390/soilsystems9040110

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