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Assessment of Soil Aggregate Stability Methodologies in Calcareous Silt Loams

Jenifer L. Yost1,*, Kevin Kruger2, David L. Bjorneberg3, Robert S. Dungan3, April B. Leytem3, Amber D. Moore4, Linda R. Schott2

Published in Journal of the ASABE 67(4): 879-887 (doi: 10.13031/ja.15650). 2024 American Society of Agricultural and Biological Engineers.

1 Grassland, Soil, and Water Research Laboratory, USDA ARS, Temple, Texas, USA.

2 Department of Soil and Water Systems, College of Agricultural and Life Sciences, University of Idaho, Twin Falls, Idaho, USA.

3 Northwest Irrigation and Soils Research Laboratory, USDA ARS, Kimberly, Idaho, USA.

4 Department of Crop and Soil Science, Oregon State University, Corvallis, Oregon, USA.

* Correspondence: jenifer.yost@usda.gov

The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution NonCommercial NoDerivatives 4.0 International License https://creative commons.org/licenses/by nc nd/4.0/

Submitted for review on 28 April 2023 as manuscript number NRES 15650; approved for publication as a Research Article and as part of the Soil Erosion Research Symposium Collection by Community Editor Dr. Kyle Mankin of the Natural Resources & Environmental Systems Community of ASABE on 1 April 2024.

Mention of trade names or commercial products in the publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. The USDA is an equal opportunity provider and employer.

Citation: Yost, J. L., Kruger, K., Bjorneberg, D., Dungan, R. S., Leytem, A., Moore, A., & Schott, L. (2024). Assessment of soil aggregate stability methodologies in calcareous silt loams. J. ASABE, 67(4), 879-887. https://doi.org/10.13031/ja.15650

Highlights

Abstract. Idaho’s Magic Valley is a highly productive agricultural region in the United States due to irrigation. The soils in this region are prone to crusting, have low organic matter, and are high in calcium carbonates, making them susceptible to erosion and water runoff. Soils need to be better managed to enhance aggregate stability to enable increased infiltration of irrigation water and decreased soil erosion in nearby waterways. However, to make management recommendations, the identification of appropriate measurements of aggregate stability needs to be identified, and few relevant studies exist. Thus, the overall goal of this project was to identify appropriate methods for the assessment of soil aggregate stability in the study region. The study sites were located in Southern Idaho and set up using common cropping rotations and agricultural management practices for the region using a variety of nutrient sources, tillage types, and cover cropping. Three methods were used to measure soil aggregate stability: wet sieving, simulated rainfall using a Cornell Sprinkle Infiltrometer (sprinkler height: 30, 90, and 150 cm), and the SLAKES mobile application. No differences in soil aggregate stability were found when the wet sieving or SLAKES methods were used at the three study sites, either due to the method or a general lack of differences between treatments. When using the Cornell Sprinkle Infiltrometer, no treatment differences were found at LT-Manure or GRACEnet; however, differences were observed between treatments at the Cover Crop study site at sprinkler heights of 30 and 90 cm. At a sprinkler height of 30 cm, the average mean weight diameter was the highest when winter forage crops (WFC) and solid dairy manure (SDM) were applied (3.73 mm) and the lowest in the control (3.12 mm). At a sprinkler height of 90 cm, the average mean weight diameter was the highest when WFC and SDM were applied (3.54 mm) and the lowest in the WFC Only treatment (2.46 mm). These results not only have implications for which methods are best for assessing progress but also for what management practices can be utilized to decrease soil erosion from irrigated cropland. This study highlighted that under these soils and management practices, simply increasing soil carbon does not increase aggregate stability, especially when root crops are included in the crop rotation with intensive tillage. In arid and semi-arid regions, the Cornell Sprinkle Infiltrometer may be a more sensitive method of measuring soil aggregate stability compared to traditional wet sieving.

Keywords.Agricultural management practices, Calcareous soils, Mean weight diameter, Slake test, Soil aggregate stability.

Most of Idaho’s Magic Valley is highly productive because of its proximity to the Upper Snake River watershed (Ippolito et al., 2017). The soils in the region are prone to physical crusting, are high in calcium carbonates, and have low soil nutrients and available water holding capacity (Yost et al., 2023). Due to the semi-arid climate, these soils are heavily irrigated, leading to issues such as soil erosion, surface sealing, nutrient deficiencies, and compaction. These issues can impact seedling emergence, rooting depth and density, absorption of water and nutrients, and crop quality and yield, leading to higher production costs (Sojka et al., 2007). More than 40% of irrigated crop production fields in the United States use sprinkler irrigation, reducing the risk of runoff and erosion as well as the amount of water being applied throughout the growing season (Ippolito et al., 2017). Many producers in the Magic Valley, however, still use surface irrigation (i.e., furrow), which typically leads to nutrient runoff and erosion problems (Bjorneberg et al., 2020). With continuing water quality concerns in the Upper Snake River watershed and decreased irrigation water storage in recent years, additional reductions in water runoff and soil erosion should be sought.

An indicator that is often used to indirectly measure soil erosion is soil aggregate stability (Nciizah and Wakindiki, 2014). Soil aggregate stability measures how resistant soil aggregates (formed by soil organic matter, roots, and mycorrhizal hyphae) are against disruptive forces (i.e., water/wind erosion, tillage, irrigation) (Almajmaie et al., 2017; Yost et al., 2016). Soils with high soil aggregate stability tend to have reduced erodibility and increased soil organic matter, infiltration, root development, and seedling emergence; however, the indicator is sensitive to agricultural management practices (Rieke et al., 2022). In Southern Idaho, common management practices include applying dairy manure to the soil, double cropping with winter forage crops, and implementing conservation tillage practices (Yost et al., 2023). For example, it has been found that soil aggregate stability was highest when manure was applied to the soil compared to inorganic fertilizer or no amendments (Gautam et al., 2021). Blanco-Canqui et al. (2013) found increases in soil aggregate stability when winter or spring cover crops were used compared to leaving the field fallow; however, mixed results have been reported (Blanco-Canqui and Ruis, 2020). Likewise, reduced tillage tends to have higher aggregate stability compared to conventional tillage (Paul et al., 2013). Previous studies in Southern Idaho have primarily focused on the effects of irrigation system type and design on aggregate stability rather than the effects of management practices such as tillage and manure application (Koehn et al., 2014; Lehrsch and Brown, 1995).

Many studies have measured soil aggregate stability; however, few have evaluated multiple methodologies. Common methods used to measure soil aggregate stability include traditional wet sieving (Nimmo and Perkins, 2002; Yoder, 1936), ultrasonic dispersion (Imeson and Vis, 1984), and simulated rainfall via the Cornell Sprinkle Infiltrometer (Schindelbeck et al., 2016). These methods are often time-consuming, require specialized equipment and laboratory space, and are costly (Almajmaie et al., 2017). The SLAKES smartphone application is a newer method used to measure soil aggregate stability that requires minimal equipment, laboratory space, and training (Flynn et al., 2020). Preliminary results show that the SLAKES test is sensitive to management practice and could be a quick and easy method for producers to use to assess soil aggregate stability in their fields (Flynn et al., 2020; Rieke et al., 2022).

Previous soil aggregate stability research in the calcareous silt loam soils of Idaho’s Magic Valley focused on freeze-thaw cycles, tensile strength, friability, and temporal changes (Koehn et al., 2014; Lehrsch, 1998; Lehrsch and Brown, 1995), however, none of the research has focused on methodologies or sensitivity to management practices. In this agriculturally important region, which has similarities to other regions around the globe, it is prudent to manage soils to enhance and protect aggregate stability to enable increased infiltration of irrigation water and decreased soil erosion in nearby waterways. However, to make management recommendations, the identification of appropriate measurements of aggregate stability needs to be identified. Thus, the goal of the present study was to evaluate soil aggregate stability using three different methodologies (wet sieving, simulated rainfall via the Cornell Sprinkle Infiltrometer, and the SLAKES smartphone application) based on common cropping systems in calcareous silt loams soils found in the Magic Valley. The objectives of this research were to: (1) evaluate different soil aggregate stability methodologies in calcareous silt loam soils, (2) determine what agricultural management practices influence variation in soil aggregate stability, and (3) identify which method is the most sensitive to agricultural management practices used in the Magic Valley.

Materials and Methods

Study Site Description

The three study sites used for the present study were located within Idaho’s Magic Valley at the USDA-ARS Northwest Irrigation and Soils Research Laboratory in Kimberly, Idaho, USA (42° 32’ 48” N, 114° 20’ 44” W). The soils at the study sites were characterized as loess deposits overlying basalt and were classified as Portneuf silt loam (coarse-silty, mixed, mesic Durixerollic Calciorthids) or Rad silt loam (coarse-silty, mixed, mesic Durixerollic Camborthids). The pre-treatment soil texture for each of the three sites is found in table 1. Within the study region, the average annual air temperature was 8.7°C and the average annual precipitation was 284 mm.

Table 1. Average percent sand, silt, and clay content at the three study sites prior to applying dairy manure, fertilizer, and/or winter forage crops to the fields.[a]
Study
Site		TreatmentSand
(%)		Silt
(%)		Clay
(%)
LT-ManureCont.187210
(2012) IF197011
18A19748
18B186715
36A187310
36B20719
52A197210
52B176914
Cover CropCont.17758
(2015)WFC Only18748
SDM Only227222
WFC + SDMNDNDND
GRACEnetControl20719
(2012)U20728
SU21709
C + U20728
FM21709
SM23697

    [a] Cont = control, IF = inorganic fertilizer, 18A = 18 Mg manure ha-1 (annual), 18B = 18 Mg manure ha-1 (biennial), 36A = 36 Mg manure ha-1 (annual), 36B = 36 Mg manure ha-1 (biennial), 52A = 52 Mg manure ha-1 (annual), 52B = 52 Mg manure ha-1 (biennial), WFC = winter forage crop, SDM = solid dairy manure, U = urea, SU = SuperU, C = compost, FM = fall manure, SM = spring manure, and ND = no data.

LT-Manure

The Long-Term Manure study site, also known as LT-Manure, was initiated in the fall of 2012 with a four-year crop rotation of wheat-potato-barley-sugarbeet. The study was set up as a randomized complete block design with four replicates and eight treatments, and the plots were 18 m by 12 m. The treatments were: (1) no amendments (Cont), (2) spring application of synthetic fertilizer (Fert), annual application of drystack dairy manure applied at rates of (3) 18 Mg ha-1 (18A), (4) 36 Mg ha-1 (36A), and (5) 52 Mg ha-1 (52A), and biennial application of drystack dairy manure applied at rates of (6) 18 Mg ha-1 (18B), (7) 36 Mg ha-1 (36B), and (8) 52 Mg ha-1 (52B) on a dry weight basis. Tillage included moldboard plowing (15 cm depth) before potato and sugarbeet crops and disking (10 cm depth) before wheat and barley crops. The entire field was irrigated uniformly with a solid-set sprinkler system during the irrigation season. More information about the LT-Manure study site is reported in Leytem et al. (2019).

Cover Crop

The Cover Crop study site was initiated in the fall of 2015 as a continuous corn cropping system. The study was set up as a split block design with four replicates, and the plots were 12 m by 12 m. Each block was split into two strips with tillage (strip till vs. disk/chisel plow) randomly assigned within each block. Within each tillage strip, four treatments were randomly assigned and consisted of (1) no dairy manure or cover crop (Cont), (2) winter forage crop only (WFC Only), (3) solid dairy manure only (SDM Only), and (4) winter forage crop + solid dairy manure (WFC + SDM). Solid dairy manure was applied with a manure spreader at an average rate of 52 Mg ha-1 (dry weight) in the fall after silage corn harvest and incorporated by disking (10 cm depth) or left on the surface. The winter forage crop used in this study was triticale (x Triticosecale Wittmack), and it was planted soon after silage corn harvest and harvested prior to silage corn planting. The entire field was irrigated uniformly with a solid-set sprinkler system during the irrigation season. More information about the Cover Crop study site is reported in Yost et al. (2023).

GRACEnet

The Greenhouse Gas Reduction through Agricultural Carbon Enhancement Network study site, also known as GRACEnet, was initiated in the fall of 2012 following the dairy forage rotation that is common in the Magic Valley of corn-barley-alfalfa-alfalfa-alfalfa. The study was set up as a randomized complete block design with four replicates and six treatments, and the plots were 21 m by 23 m. The treatments were as follows: (1) no manure or synthetic fertilizer (Cont), (2) spring applied urea (U), (3) spring applied super-urea (SU), (4) 33 Mg ha-1 (dry weight) of fall applied composted dairy manure + spring applied urea (C + U), (5) 52 Mg ha-1 (dry weight) of fall applied dairy manure (FM), and (6) 52 Mg ha-1 (dry weight) spring applied dairy manure (SM). Drystack manure, composted manure, and synthetic fertilizer were incorporated into the top 15 cm after application. The entire field was irrigated uniformly with a lateral move irrigation system during the irrigation season. More information about the GRACEnet study site is reported in Dungan et al. (2021).

Soil Aggregate Stability

Sample Collection

At all study sites, soil samples (~ 1000 g) were collected from the top 15 cm using a drain spade from each plot during the 2020 or 2021 growing season in early August. At the time of sampling, LT-Manure was under sugarbeet, Cover Crop was under silage corn, and GRACEnet was under second year alfalfa. The soil samples were gently placed into aluminum tins in the field and air-dried at room temperature in the laboratory for a minimum of four days to ensure adequate dryness. Subsamples were collected from each tin and oven dried at 105°C to determine the initial soil moisture content of each subsample after air drying but just prior to aggregate stability assessment. Soil aggregate stability was measured using three different methodologies from the samples collected from each study site. Two replicates were analyzed from each plot and averaged by mass fraction.

Methodologies for Measuring Aggregate Stability

Wet Sieving

Traditional wet sieving was used to determine soil aggregate stability using the method described by Nimmo and Perkins (2002) and the mechanical wet sieving apparatus described by Yoder (1936). Briefly, approximately 100 g of field air-dried soil sample from each plot was weighed, prewetted to field capacity by capillary rise, and placed into a 4°C refrigerator for 24 hours to reach a homogeneous state. The prewetted soil samples were then passed through an 8 mm sieve and organic matter longer than 1 cm (such as roots and plant material) was removed. From there, the prewetted soil samples (< 8 mm diameter) were evenly spread on top of a series of nested sieves (4000, 2000, 250, and 53 µm) and submerged into water (the top sieve is just covered with water on the upstroke of the apparatus) for five minutes. After the initial pre-soak, the wet sieve apparatus mechanically oscillated vertically at 30 oscillations min-1 for 10 minutes, and organic matter larger than 2 mm was carefully removed from the water. The soil aggregates collected on each sieve were transferred to a metal tin, oven-dried at 105°C for 48 hours, and then weighed. Subsamples from each soil aggregate size were dispersed with sodium hexametaphosphate to account for sand content using a 53 µm sieve and subtracted from the weight of each soil aggregate size.

Cornell Sprinkle Infiltrometer

The second method used to determine soil aggregate stability was rainfall simulation using the Cornell Sprinkle Infiltrometer, as described by Schindelbeck et al. (2016). Briefly, approximately 25 g of air-dried soil crumbs (< 8 mm diameter) were evenly spread on top of a series of nested sieves (2000 and 250 µm). The Cornell Sprinkle Infiltrometer was calibrated to release 2 cm of tap water over a 10 min time period with the air-entry tube set at 4 cm. The Cornell Sprinkler Infiltrometer was suspended at heights of 30, 90, and 150 cm to evaluate multiple water droplet velocities (2.00, 4.03, and 4.97 m s-1, respectively) and therefore multiple total kinetic energies delivered to the soil samples. Similar to wet sieving, the soil aggregates collected on each sieve were transferred to a metal tin, oven-dried at 105°C for 48 hours, weighed, and corrected for sand content.

Slake Test

The last method used to measure soil aggregate stability was the smartphone application called SLAKES, as described by Flynn et al. (2020). The SLAKES method is simple, requires minimum training and laboratory equipment, is efficient, and is low in cost. In brief, the SLAKES application was downloaded onto an iPhone 12 with a 12 MP rear camera. The smartphone was placed on a ring stand and suspended approximately 21 cm above a petri dish. The petri dish was placed on top of a piece of white paper to provide a contrast between the air-dried soil crumbs and the laboratory bench. In the laboratory, standard overhead LED lights were used, and the smartphone was positioned to prevent shadows caused by the overhead lights.

Three air-dried soil crumbs were selected for measurement from each plot and were approximately 2 to 15 mm in diameter, with preference given to symmetrical soil crumbs that were approximately 10 to 15 mm in diameter. The air-dried soil crumbs were first placed inside an empty petri dish to capture a reference image. After 10 mL of deionized water was slowly added to the petri dish to minimize disturbance, the SLAKES application would collect images for 10 minutes. Based on the Gompertz function, a slaking index value (coefficient a) was calculated. Soil aggregates were considered stable if coefficient a was less than 3 and unstable if coefficient a was higher than 7 (Flynn et al., 2020).

Calculations and Data Analysis

To compare soil aggregate stability between the wet sieving and Cornell Sprinkle Infiltrometer methods, the mean weight diameter (= 250 µm) was calculated using the following equation:

(1)

where

MWD = mean weight diameter (mm)

= mean diameter of aggregates over each sieve size (mm)

Wi = weight of the aggregates in that size range as a fraction of total dry weight

n = number of sieves.

Differences in mean weight diameter were analyzed using a one-way analysis of variance (ANOVA) and the Tukey test to determine whether they showed the same patterns of response to treatment and method using JMP v16. Likewise, an ANOVA was used to determine if there were treatment differences in coefficient a. A multivariate correlation was performed to compare soil aggregate stability methods.

Results and Discussion

Wet Sieving

In general, mean weight diameter using the traditional wet sieving method ranged from 1.11 to 3.62 mm at LT-Manure (average: 2.32 mm), 1.30 to 4.28 mm at Cover Crop (average: 2.77 mm), and 1.37 to 4.06 mm at GRACEnet (average: 2.64 mm) (figs. 1-3A). In the present study, there were no differences in mean weight diameter between the nutrient addition treatments (inorganic fertilizer and dairy manure) using traditional wet sieving; however, mixed results have been found in the literature. Similar to LT-Manure and GRACEnet, Domingo-Olive et al. (2016) and Nyiraneza et al. (2009) did not find a difference in mean weight diameter between the control, inorganic fertilizer, and dairy manure treatments. This is likely due to previous conventional tillage and intensive harvest practices depending on the cash crop, suggesting that manure and carbon additions alone do not change soil physical properties. Zhang et al. (2021) studied the effects of long-term nutrient application and found that mean weight diameter was significantly higher when manure and inorganic fertilizer were applied for 13 years compared to soils that did not receive manure. This is likely due to an increase in soil organic carbon and microbial activity in the topsoil.

In another long-term study, mean weight diameter in the top 15 cm was significantly higher when farmyard manure was applied for 41 years compared to the control (0.65 and 0.43 mm, respectively) and was highest when inorganic fertilizer and farmyard manure were both applied (0.78 mm) (Tripathi et al., 2014). Both of these studies suggest that long-term nutrient applications can increase mean weight diameter; however, no differences were observed at LT-Manure or GRACEnet after multiple years of nutrient application. This may be due to the high silt content in these soils, which makes it difficult to increase soil aggregate stability. At LT-Manure, no significant differences were found in mean weight diameter among the treatments, likely due to intensive tillage and harvest practices. For example, soil samples were collected at LT-Manure during the sugarbeet growing season in 2020. Prior to planting, manure and inorganic fertilizer were applied to the plots in the fall and immediately disked into the soil. Moldboard plowing also occurred before planting, destroying any soil aggregates that may have been in the topsoil. At GRACEnet, treatment differences were likely not observed due to conventional tillage prior to planting alfalfa in the fall of 2019 and collecting soil aggregates during the second year of alfalfa.

Similar to the nutrient application treatments, winter forage crops did not improve mean weight diameter when compared to the control at the Cover Crop study using traditional wet sieving (fig. 2A). The average mean weight diameter was 2.48 mm for the control, 2.72 mm for WFC Only, and 2.94 mm for WFC + SDM. Jokela et al. (2009) looked at the effects of cover crops on soil quality indicators in silt loam soils in Wisconsin and found that kura clover-corn had a higher mean weight diameter compared to the control from 5 to 15 cm (0.52 and 0.37 mm, respectively) and 15 to 30 cm (0.31 and 0.24 mm, respectively). With the exception of the top 5 cm, the other cover crops used in the study (Italian rye and winter rye) were not significantly different than the control (Jokela et al., 2009). Similarly, Rieke et al. (2022) did not find a significant increase in mean weight diameter when cover crops were used. In a 12-year study, Blanco-Canqui and Jasa (2019) found that mean weight diameter was significantly higher under grass cover crop (2.25 mm) compared to the control and legume cover crop (approximately 1.75 mm) in the top 7.5 cm, likely due to the increase in belowground biomass. In their review on the effect of cover crops on wet aggregate stability, Blanco-Canqui and Ruis (2020) reported increases even when cover crops were incorporated into tilled systems. Because winter forage crops were only planted for four years prior to sampling at the Cover Crop study, this suggests that we may see differences between the WFC Only and control treatments within the next eight years.

Figure 1. Average mean weight diameter (mm) by wet sieving and Cornell Sprinkle Infiltrometer methods (1A) and SLAKES method coefficient a (1B) by treatment at the LT-Manure study site in the summer of 2020. Bars represent mean plus standard error. Columns within methods (1A; uppercase), by method (1A; lowercase), or by treatment (1B; uppercase) not connected by the same letter are significantly different (p < 0.05). Cont = control; IF = inorganic fertilizer; 18A = 18 Mg manure ha-1 (annual); 18B = 18 Mg manure ha-1 (biennial); 36A = 36 Mg manure ha-1 (annual); 36B = 36 Mg manure ha-1 (biennial); 52A = 52 Mg manure ha-1 (annual); 52B = 52 Mg manure ha-1 (biennial).
Figure 2. Average mean weight diameter (mm) by wet sieving and Cornell Sprinkle Infiltrometer methods (2A) and SLAKES method coefficient a (2B) by treatment at the Cover Crop study site in the summer of 2020. Bars represent mean plus standard error. Columns within methods (2A; uppercase), by method (2A; lowercase), or by treatment (2B; uppercase) not connected by the same letter are significantly different (p < 0.05). Cont = control; WFC = winter forage crop; SDM = solid dairy manure.

In the Cover Crop study, the strip till plots had a slightly higher mean weight diameter than the conventionally tilled plots (2.98 and 2.75 mm, respectively); however, it was not significantly different (data not shown). In a long-term study in Tennessee, Nouri et al. (2019) found differences between the tillage treatments after 34 years, with no-till having a higher mean weight diameter than conventional tillage in the top 15 cm (3.08 and 2.61 mm, respectively). Similar results were found by Li et al. (2020) and Sheehy et al. (2015), however, Sheehy et al. (2015) only saw significant differences between conventional tillage and no-till at two of the four study sites. At Cover Crop, it is possible that more time is needed to see differences in mean weight diameter between the tillage types (conventional and reduced tillage). Based on the results of the present study, wet sieving is not the best method to use to measure soil aggregate stability in the calcareous silt loams soils in Idaho’s Magic Valley.

Cornell Sprinkle Infiltrometer

Treatment differences were only observed for mean weight diameter by the Cornell Sprinkle Infiltrometer at Cover Crop (fig. 2A). At Cover Crop, mean weight diameter ranged from 2.62 to 4.35 mm at a sprinkler height of 30 cm, 1.72 to 4.10 mm at a sprinkler height of 90 cm, and 0.78 to 3.35 mm at a sprinkler height of 150 cm. Mean weight diameter decreased as the sprinkler height increased (average: 3.55 mm at 30 cm, 3.02 mm at 90 cm, and 2.12 mm at 150 cm) (fig. 2A), likely due to the increase in water droplet energy. When the soil surface is exposed to rainfall or irrigation, water droplets cause soil crumbs to break into smaller particles, which increases with droplet size and input energy (Almajmaie et al., 2017). However, this was not the case at LT-Manure or GRACEnet. The mean weight diameter ranged from 3.13 to 4.25 mm at a sprinkler height of 30 cm, 2.43 to 4.44 mm at a sprinkler height of 90 cm, and 2.26 to 4.15 mm at a sprinkler height of 150 cm at LT-Manure (fig. 1A). At GRACEnet, mean weight diameter ranged from 3.15 to 4.47 mm at a sprinkler height of 30 cm, 3.18 to 4.62 mm at a sprinkler height of 90 cm, and 2.77 to 4.33 mm at a sprinkler height of 150 cm (fig. 3A). Unlike Cover Crop, no treatment differences were observed; however, mean weight diameter was lower at a sprinkler height of 150 cm (average: 3.51 mm) compared to 30 cm (average: 4.06 mm) at GRACEnet.

Figure 3. Average mean weight diameter (mm) by wet sieving and Cornell Sprinkle Infiltrometer methods (3A) and SLAKES method coefficient a (3B) by treatment at the GRACEnet study site in the summer of 2021. Bars represent mean plus standard error. Columns within methods (3A; uppercase), by method (3A; lowercase), or by treatment (3B; uppercase) not connected by the same letter are significantly different (p < 0.05). Cont = control; U = urea; SU = SuperU; C = compost; FM = fall manure; SM = spring manure.

Few studies have used the Cornell Sprinkle Infiltrometer to measure soil aggregate stability. Antosh et al. (2020) investigated the effects of various winter cover crop species on mean weight diameter (sprinkler height: 60 cm) in a calcareous loam and clay loam in semi-arid New Mexico. In general, mean weight diameter increased over a three-year period with each winter cover crop treatment, and mean weight diameter was highest in the mustard-rye-vetch treatment at the end of the study. The mean weight diameter was lowest when the field was left fallow in the winter. A similar range of mean weight diameter was observed between the present study and the observations made by Antosh et al. (2020). Omer et al. (2018) evaluated the variations in mean weight stability by season in arid cropping systems. Mean weight diameter was the lowest in winter and spring (40% retention) and highest in summer (78% retention); however, no differences were found by crop management (Omer et al., 2018).

Similar to this study, Almajmaie et al. (2017) measured soil aggregate stability at two different moisture levels (dry [air-dried] and moist [field capacity]) using multiple methods, including the Cornell Sprinkle Infiltrometer (sprinkler height: 184 mm). It was found that only 40% of dry soil aggregates were retained on a 2-mm sieve when using the Cornell Sprinkle Infiltrometer, while 90% of moist soil aggregates were retained (Almajmaie et al., 2017). While these results have implications in a laboratory setting when samples can be prewetted, field conditions in arid or semi-arid regions, like Idaho’s Magic Valley, may have minimal soil moisture prior to irrigation events, depending on the irrigation system being used.

The results from Almajmaie et al. (2017) highlight shortcomings in the methodologies used in existing literature regarding consistency and agreement on the exact methodology for the Cornell Sprinkle Infiltrometer. The methodology to measure soil aggregate stability using the Cornell Sprinkle Infiltrometer is based on the total kinetic energy delivered to the soil (Schindelbeck et al., 2016), but the total kinetic energy is dependent upon several variables such as water droplet size and rate as well as sprinkler height from the soil surface. While insightful for whether mean weight diameter was affected by management practices, the values of mean weight diameter reported in the studies by Almajmaie et al. (2017), Omer et al. (2018), and Antosh et al. (2020) are not directly comparable to the present study, nor are they directly comparable to one another. Therefore, consistency within methodologies should be sought.

Slake Test

No treatment differences were observed for coefficient a using the SLAKES smartphone application at the three study sites (figs. 1-3B). The average coefficient a ranged from 0.55 to 1.75 at LT-Manure (average: 1.13), 0.05 to 1.45 at Cover Crop (average: 0.60), and 0.00 to 1.00 at GRACEnet (average: 0.30). The results suggest that the soil crumbs were very stable at the three study sites, even though these calcareous silt loams in Idaho’s Magic Valley are prone to surface sealing and high erodibility regardless of treatment. To date, only two papers have used the SLAKES smartphone application to measure soil aggregate stability. Although the present study did not find treatment differences using this method, Rieke et al. (2022) and Flynn et al. (2020) both reported treatment differences. According to Rieke et al. (2022), the use of cover crops, decreased tillage, and residue retention all showed a positive response compared to the control from soils collected at 124 long-term experimental agricultural sites in North America. Flynn et al. (2020) found differences by management practice in the high clay soils of Central Texas, with conventional tillage having the highest coefficient a (lower aggregate stability) and no-till being the lowest (higher aggregate stability).

Although not significant due to the large variation in values, the average coefficient a at GRACEnet (fig. 3B) may have been affected by treatment. Unlike at LT-Manure and Cover Crop, tillage was performed approximately two years prior to collecting soil aggregate samples at GRACEnet. This pause in tillage may have allowed the belowground biomass from the alfalfa to increase soil aggregate stability based on the nutrients they received prior to planting alfalfa at the study site. Two replicates may not be enough when trying to see treatment differences when using the SLAKES smartphone application, and coefficient a may not be the best representation of soil aggregate stability. Similar to wet sieving, the SLAKES smartphone application does not appear to be sensitive enough to measure treatment differences in soil aggregate stability in the calcareous silt loams soils in Idaho’s Magic Valley.

Comparison of Aggregate Stability Methods

The calculation of mean weight diameter is based on a geometric mean, meaning that using different sieve sizes will result in different mean weight diameter. For wet sieving, it is standard procedure to calculate the mean weight diameter using three aggregate sizes (8–4 mm, 4–2 mm, and 2–0.25 mm) (Nimmo and Perkins, 2002). However, the Cornell Sprinkle Infiltrometer method uses only two aggregate sizes (8–2 mm and 2–0.25 mm). When mean weight diameter was calculated for wet sieving by combining the 8–4 mm and 4–2 mm aggregate sizes (i.e., two aggregate classes instead of three), the resulting mean weight diameter differed by an average of 5% (±0.13 mm; data not shown). Statistically, there were also no differences between treatments for any study when two aggregate classes were used instead of three for the mean weight diameter calculation.

The average mean weight diameter varied by method when comparing traditional wet sieving to three different sprinkler heights using the Cornell Sprinkle Infiltrometer (figs. 1-3A). At LT-Manure, mean weight diameter was highest with the Cornell Sprinkle Infiltrometer (3.63 mm) and lowest with wet sieving (2.32 mm); however, there were no differences by sprinkler height. At Cover Crop, mean weight diameter was highest using the Cornell Sprinkle Infiltrometer at a height of 30 cm (3.02 mm) and lowest at 150 cm (2.12 mm). Similarly, mean weight diameter was highest using the Cornell Sprinkle Infiltrometer at a sprinkler height of 30 cm at GRACEnet (4.06 mm) but lowest using wet sieving (2.64 mm). Unlike the results found in the present study, Almajmaie et al. (2017) found the opposite results, suggesting that percent soil aggregate stability was highest under wet sieving compared to the Cornell Sprinkle Infiltrometer at a sprinkler height of 184 cm. This is likely due to variations in the Cornell Sprinkle Infiltrometer protocol. In the present study, three sprinkler heights were used (30, 90, and 150 cm), while only one sprinkler height (184 cm) was used in Almajmaie et al. (2017).

Although measuring soil aggregate stability with a Cornell Sprinkle Infiltrometer is relatively new, it may be a good method to use in arid and semi-arid irrigated cropping systems. Due to its sensitivity to management practices in one study (Cover Crop), the Cornell Sprinkle Infiltrometer should be used to assess the impacts of different irrigation systems on soil erosion and runoff. Other benefits of the Cornell Sprinkle Infiltrometer include shorter sample analysis times (no pre-wetting, fewer sieves) and an overall low cost since there are no mechanical parts. The drawbacks of the Cornell Sprinkle Infiltrometer are that once soil crumbs have passed onto the second sieve, they are no longer exposed to the same forces as the first sieve since the first sieve reduces water droplet energy. The large number of variable settings (sprinkler height, droplet size, and droplet rate) can be a drawback for optimizing consistency in reporting and interpreting results. Benefits of the wet sieving apparatus include the large body of reported results in the literature; however, it is time consuming for sample analysis, and initial equipment costs are the highest of all the methods tested. In the present study, the SLAKES app was not sensitive to management practices, indicating that these aggregates are stable even though they are prone to crusting. However, it seems to be a valuable tool in other areas.

Table 2. Correlations between mean weight diameter (MWD) and coefficient a at the three study sites in silt loam soils (coarse-silty, mixed, mesic Durixerollic Calciorthids; coarse-silty, mixed, mesic Durixerollic Camborthids) in Idaho’s Magic Valley. The numbers correspond to the correlation coefficient (r).
MethodMWD
(Wet
Sieve)		MWD (Stand
30 cm)		MWD (Stand
90 cm)		MWD (Stand
150 cm)
MWD (Stand 30 cm) 0.00
MWD (Stand 90 cm) 0.03 0.41
MWD (Stand 150 cm)-0.12 0.47 0.45
Coefficient a-0.31-0.14-0.02 0.18

The soil aggregate stability methods used in the present study were slightly to moderately correlated with each other, with Pearson’s correlation coefficient ranging from -0.31 to 0.47 (table 2). The strongest correlations were found when comparing the mean weight diameters between the different sprinkler heights using the Cornell Sprinkle Infiltrometer (r > 0.40). Although the other correlations were weak, mean weight diameter from wet sieving was moderately correlated to coefficient a from the SLAKES test (r = -0.31). Unlike the present study, Rieke et al. (2022) found moderate correlations between soil aggregate stability methods (r > 0.44), and this may be due to a diverse set of soils collected from over 100 research sites in North America. Similar to this study, Almajmaie et al. (2017) found a weak correlation between wet sieving and rainfall simulation when the samples were prewetted (R2 = 0.17); however, a strong correlation was found when the soil aggregates were not prewetted (R2 = 0.74). Overall, more research is needed to determine the relationship between these three methodologies.

Conclusions

This research evaluated soil aggregate stability using three different methodologies based on common cropping systems found in Idaho’s Magic Valley. Overall, soil aggregate stability was more sensitive to the Cornell Sprinkle Infiltrometer method compared to traditional wet sieving and the SLAKES app and may be influenced by nutrient application (inorganic fertilizer and solid dairy manure), cover cropping, and tillage. The Cornell Sprinkle Infiltrometer was sensitive to the treatments, making it the recommended method to assess typical erosion processes occurring in the irrigated cropping systems of Idaho’s Magic Valley. Although differences were not found by sprinkler height with Cornell Sprinkle Infiltrometer at LT-Manure, soil aggregate stability was highest at 30 cm of height and lowest at 150 cm of height at Cover Crop and GRACEnet. Based on the results of this study, the SLAKES smartphone application and traditional wet sieving methods are not recommended for measuring soil aggregate stability in calcareous silty loam soils due to a lack of treatment differences on wet aggregates stability. This study highlights that additional soil carbon does not always increase aggregate stability, especially with intensive tillage (disking and moldboard plow) and root crops included in the rotation. More research is needed to determine if other values from the SLAKES smartphone application are more sensitive to treatments than coefficient a. In addition, the Cornell Sprinkle Infiltrometer method needs to be modified to be more sensitive for low stability soils by assessing different rainfall rates, air entry potentials, and sprinkler heights.

Acknowledgments

The research was funded by USDA NIFA Project Number IDA01657 and USDA Cooperative Agreement Project Number 2054-13000-009-08S. The authors would like to thank Emerson Kemper and Breyer Meeks for collecting and processing the soil aggregate stability samples.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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