The Canadian Society for Bioengineering La Société Canadienne de Génie The Canadian society for engineering in agricultural, food, Agroalimentaire et de Bioingénierie environmental, and biological systems. La société canadienne de génie agroalimentaire, de la bioingénierie et de l’environnement Paper No. CSBE13-071 Effect of soil compaction on early soybean plant growth in sandy loam Manitoba soil La, A., J. Brown, Yixuan. Chen, P. Kumaravelayutham, G. Munch, M. Nandanvar, M.C. Panditharatne, T. Peranantham, U. Sharma, M. Sadek, Y. Chen Department of Biosystems Engineering, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2 Written for presentation at the CSBE/SCGAB 2013 Annual Conference University of Saskatchewan, Saskatoon, Saskatchewan 7-10 July 2013 ABSTRACT Sandy loam Manitoba soil was subjected to different levels of soil compaction to determine its effect on early growth of soybean plants. Soybean seeds were planted within finite containers of soil samples and were subjected to low, medium and high compaction treatments as well as a no compaction control treatment. Factors affecting plant growth such as room temperature, relative humidity, initial moisture, initial bulk density, and soil surface strength were measured experimentally. There was a significant difference in soil surface strength (P<0.0001), but not in bulk density across compaction treatments (P=0.2000). The increased soil surface strength of the high compaction treatment, which had a mean compaction effort of 17.25 kJ m-3 (SD=4.15), corresponded with a soil compaction layer that was more difficult to overcome, resulting in 25% of seedlings not emerging in the two-week observation period. The delay in seedling emergence for low, medium, and high compaction seedlings caused significant differences between the plant heights of the control and the compacted soil samples. However, there was not a significant difference in leaf count per container across compaction treatments. A limitation of this study was the use of finite containers, which resulted in compaction that differed from that present in field conditions. Papers presented before CSBE/SCGAB meetings are considered the property of the Society. In general, the Society reserves the right of first publication of such papers, in complete form; however, CSBE/SCGAB has no objections to publication, in condensed form, with credit to the Society and the author, in other publications prior to use in Society publications. Permission to publish a paper in full may be requested from the CSBE/SCGAB Secretary, 2028 Calico Crescent, Orleans, ON, K4A 4L7 or contact [email protected]. The Society is not responsible for statements or opinions advanced in papers or discussions at its meetings. INTRODUCTION Soil compaction is often the result of utilizing agricultural equipment, such as press wheels and other heavy equipment, for crop production. Although such equipment is necessary in modern agricultural practices, the resulting soil compaction can be detrimental due to the subsequent negative effects on seedling emergence and early growth of crops. Both Hyatt et al. (2007) and Musaka and Khumbala (2007) concluded that increasing soil compaction within finite containers caused subsequent decrease in soybean and coffee plant seedling emergence, respectively. Hyatt et al. (2007) also determined that greater compaction of soil led to increasing amounts of dead seeds (for seeds of low and medium vigor) and increased presence of germinated seedlings that were unable to surpass the hardened compacted crust. Musaka and Khumbula (2007) found that coffee plants grown in compacted soils had decreased stem height, stem girth, and root biomass compared to their counterparts grown in non-compacted soils due to decrease in air-filled porosity. In contrast, a field study showed that intra-row soil compaction increased the cone index value, which is used to quantify soil strength, as well as the percentage of emergence (PE) of red lentil seedlings due to increased soil-seed contact in tilled soils (Altikat and Celik 2011). Overall, soil compaction conditions required for enhanced percentage of seedling emergence varies for different crops. In common agricultural practices, a press wheel is often used after seeding to cover and press the seed furrow to increase the soil-seed contact (Ying et al. 2004). Finlay et al. (1994) and Ying et al. (2004) performed field studies to determine the effect of using press wheels on wheat seedling emergence. Finlay et al. (1994) concluded that 100 N of force applied by the press wheel did not affect seedling germination or the total length of roots per seedling. The press wheel accelerated the rate of emergence by decreasing the distance between the seed and the surface but its use did not result in statistically significant differences in the overall quantity of seedlings to emerge (Finlay et al. 1994). Ying et al. (2004) showed that the absence of press guard and/or a gauge wheel led to delayed seedling emergence as well as reduced yield on dry to normal soil conditions; however, utilizing a press wheel on wet, zero-tillage soil caused a statistically significant decrease in the speed of emerging wheat seedlings due to compaction of the wet soil above the seed (Ying et al. 2004). Ying et al. (2004) also performed the same study within an indoor soil bin containing sandy loam Manitoba soil. The results of the study showed that the presence and absence of the press wheel during seeding did not affect the rate of seedling emergence. It is possible that the degree of compaction provided by the press wheel was insufficient to affect seedling emergence in the sandy loam Manitoba soil; however, heavier agricultural equipment is capable of causing higher levels of compaction and thus affect seedling emergence and early growth. The objective of this study was to investigate the effect of varying degrees of soil compaction on soybean seedling emergence and early growth in sandy loam Manitoba soil using laboratory-based compaction tests. 2 MATERIALS AND METHODS Soil Preparation The soil type utilized in this experiment was sandy loam soil (70% sand, 16% silt, and 14% clay) that originated from the same indoor soil bin studied in Ying et al. (2004). For initial soil preparation, the soil bed was loosened and the soil was shifted to produce two walls in the soil bed. Water was poured between the two walls to create a damming effect and prevent water leakage. Water was allowed to infiltrate the soil, which was covered with a plastic sheet for one week prior to sample preparation to prevent the soil from drying out. Sample Preparation Sixteen individual soil samples were prepared such that there were four different treatments of soil compaction with four replicates. Coffee can containers were utilized for storing the soil samples during compaction and observation. Three drain holes were drilled into the bottom of each coffee can container to ensure adequate drainage if required. Container dimensions were measured to determine the volume of soil for individual soil samples. Soil was inserted into the containers until the soil height was equivalent to the height of the container minus 80 mm. The longest diameters of 48 soybean seeds were measured with an electronic caliper and each seed was inspected to ensure no signs of physical damage. For each soil sample, three seeds were manually placed on the soil surface and were spaced 38 mm in a triangular shape (Hyatt et al. 2007). After seeding, a 40-mm layer of soil was added over the seeds. Soil Compaction Four different treatments of soil compaction consisting of no compaction, low compaction, medium compaction, and high compaction were applied to soil samples. Compaction was achieved by utilizing a proctor device (Hyatt et al. 2007). The proctor device was positioned on top of the soil surface and its hammer was lifted to the maximum height of 0.45 m before being dropped. The no compaction treatment (control) served as a means to determine if the low, medium and high compaction treatments had a significant effect on seedling growth parameters. The low, medium, and high compaction treatments consisted of one, two, and three hammer drops, respectively. The total drop-height was equivalent to the number of drops multiplied by the height of one drop (0.45 m). Excess soil that was not compacted was removed from the containers. Compaction effort (CE), which is the amount of energy applied over a unit volume of soil, was calculated similarly to Hyatt et al. (2007): 𝐸 = 𝑚  ×𝑔×𝐿 (1) ! ! 𝐶𝐸 = ×   (2) !""" ! where m = mass of the weight dropped on soil samples (kg), g = gravitational constant (m/s2), L = total length that the mass was dropped (m), CE = compaction effort (kJ m-3), V = volume of soil before compaction (m3). 3 Soil properties Soil from various locations in the prepared soil bed was shovelled into tin foil containers. The mass of the soil and foil containers were measured prior to and after the samples were oven-dried for 24 hours at 105°C. The initial moisture content and bulk density of the soil was then calculated. For low, medium, and high compaction treatments, three dummy soil samples were assembled for determining the soil surface strength. A pocket penetrometer was inserted into each dummy soil sample and provided offset and reading values, measured in kg. The soil surface strength was equivalent to the reading value minus the offset value. The pocket penetrometer was used to measure the soil surface strength at three locations of each dummy soil sample to provide a total of nine measurements per compaction treatment. Seedling emergence Initially, soil containers were covered with cloths to prevent excessive moisture loss from evaporation. When seedlings began to emerge, the cloths were removed to prevent cloths from inhibiting growth. Samples were positioned under ultraviolet light in a laboratory with unrestrained room temperature and relative humidity. Replicates of compaction treatments were observed for a period of two weeks for change in weight, seedling emergence, plant growth in height, and leaf growth. Each observation was preceded with a measurement of temperature and relative humidity. Other interesting phenomena that occurred were also noted. Statistical Analysis Based on a model for completely randomized design (CRD), an Analysis of Variance (ANOVA) was performed on compaction effort, bulk density, soil surface strength, plant height, and leaf count. Scheffe’s test was used to determine if there was a significant difference between treatment means of these factors at a significance level of 0.05. The statistical analysis was executed using SAS version 9.1.3. RESULTS AND DISCUSSION The mean of the seed diameters used in the study was 8.15 mm with a standard deviation of 0.43. The average moisture content of soil prior to seeding was 27.2% with a standard deviation of 0.2. Factors affecting early plant growth Daily temperature and relative humidity Hyatt et al. (2007) previously found that ambient temperatures that are less than 25°C reduced soybean seedling emergence, in which many seedlings exposed to temperatures of 15 and 20°C were able to germinate but were unable to emerge from the soil bed. Temperature also increased the effect due to compaction on seedling emergence (Hyatt et al. 2007). In the present study, with the exception of the first three days of the observation period, the temperature of the room was less than 25°C (Table 1). It is possible that this may have adversely affected the number of seedlings to emerge as well as the rate of emergence but the findings of this study cannot definitively determine if this was the case. The relative humidity of the laboratory room may have also affected seedling growth. Garg and Prakesh (2000) stated that the optimum relative humidity range for plant growth falls between 30 and 70%. The relative humidity of the room was suboptimal for plant growth and most likely led to moisture loss from the soil from evaporation. The initial soil moisture content of 27.2% was also the 4 only moisture available to the seedlings. Therefore, the limitation in moisture was a possible factor in deterring overall seedling emergence and growth. However, the impact of limited moisture in conjunction with seedling emergence and growth requires further investigation for definitive conclusions. Table 1. Temperature and relative humidity of the laboratory during observation period. Temperature (°C) Relative Humidity (%) Day N Mean SD Mean SD 0 4* 24.2 0.0 17.1 N/A 1 3* 25.7 0.0 16.2 N/A 2 2* 24.9 0.0 16.9 N/A 4 4 22.4 0.1 17.9 0.4 5 3 21.5 0.0 16.2 1.3 6 4 22.4 0.2 16.9 0.6 7 3 22.4 0.8 18.7 0.4 8 4 22.3 0.2 16.0 3.9 9 2 21.5 0.1 10.5 0.0 10 1 21.2 N/A 10.5 N/A 11 4 22.1 0.4 10.9 0.8 12 2 22.2 0.0 13.7 0.0 *One measurement for relative humidity Compaction effort The CE of the control treatment (CE = 0 kJ m-3) was not significantly different from the low compaction treatment (P=0.0672) but was significantly different from the CE of the medium (P=0.007) and high compaction treatments (P<0.0001) (Fig. 1). The CE of the low compaction treatment was not significantly different from the CE of the medium compaction treatment (P=0.0864) but was significantly different from the CE of the high compaction treatment (P=0.0005). The CE of the medium compaction treatment was not significantly different from the CE of the high compaction treatment (P=0.0506). 5 25 ) 3 m20 g/ k t (15 r o f f E10 n o ti c 5 a p m o 0 C Low Medium High Compaction Compaction Compaction Compaction Level Figure 1. Compaction effort of low, medium, and high compaction treatments on soil samples. Bulk Density The compaction treatments did not significantly change the bulk density of soil samples (P=0.2000) (Fig. 2). The bulk density of the soil samples was related to the soil’s air-filled porosity. An insignificant change in bulk density suggests that the air-filled porosity was not affected by the compaction treatments performed in this study. Musaka and Khumbala (2007) concluded that the decreased air-filled porosity in compacted soil samples led to decreased oxygen levels required for the uptake of nutrients from soil, which affected the stem biomass of seedlings. From the results of the current study, it is unlikely that the compaction treatments led to the anoxic conditions described in Musaka and Khumbala (2007) and the differences in among seedling emergence, plant height, and leaf count across compaction treatments were most likely not due to insufficient oxygen in the soil. 1.8 ) 3 m c 1.35 g/ ( y sit 0.9 n e D k 0.45 ul B 0 No Low Medium High Compaction Compaction Compaction Compaction Compaction Level Figure 2. Initial bulk density of soil samples subjected to no, low, medium, and high compaction treatments. 6 Soil Surface Strength There was a significant difference between the soil surface strengths across low, medium and high compaction treatments (P<0.0001) (Fig. 3). Soybean seedlings must overcome a soil compaction layer once germinated in order to emerge from the soil bed (Hyatt et al. 2007). Increased soil surface strength, due to higher compaction effort, corresponded with a stronger soil compaction layer that was more difficult to emerge from. While a growing hypocotyl would adapt if it encountered a hardened crust of soil, it would ultimately remain under the soil surface if it is unable to surpass the soil compaction layer (Rathore et al. 1981 in Hyatt et al. 2007). In the case of Hyatt et al. (2007), at least 20% of seedlings germinated did not emerge when the soil was compacted with CE = 18.3 kJ m-3 (Fig. 4); less emergence occurred for low and medium vigor seedlings due to this reason. Assuming the soybean seedlings in the current study were of low or medium vigor, the results of the current study correspond to Hyatt et al. (2007) as 25% of the seedlings did not emerge from soil samples subjected to the high compaction treatment (mean = 17.25 kJ m-3, SD = 4.15). It was also possible that the soil compaction led to dead seeds and abnormal seedlings, as seen in Hyatt et al. (2007). 8   ) g k h ( 6   t g n e 4   r t S e 2   c a f r u 0   S oil ComLpoawc tion CoMmepdaiucmtio n comHpiagcht ion S Compaction Treatment Figure 3. Soil surface strength of dummy soil samples subjected to low, medium, and high compaction treatments. An interesting phenomenon that occurred was triangular, ‘Y-shaped’ cracking appearing in the soil before seedling emergence (Fig. 4). It was previously found that seed spacing of 38 mm increased emergence of soybean seedlings as the combined effort of the three seedlings allowed for the seedlings to surpass the compacted soil layer and subsequently emerge (Hyatt et al. 2007). A seed spacing of 38 mm was used in the current study, in which the spacing allowed for the effort of the three seedlings to combine and thus allow the seedlings to break through the soil compaction layer and overcome the soil surface strength. 7 Figure 4. Triangular cracking on soil surface from emerging soybean seedlings Early Plant Growth Indicators Soybean seedling emergence In a field study by Altikat and Celik (2011), the best emergence of red lentil seedlings and soil conditions resulted from a combination of reduced tillage using a vertical axis rotary tiller and 60 kJ m-3 intra-row compaction. This contradicted the results found in this study (Fig. 5), in which the compaction effort of the high compaction treatment (mean = 17.25 kJ m-3, SD = 4.15) decreased the emergence of soybean seedlings. Reasons for this difference may be due to different plant species as well as the fact that the other study was performed in an outdoor field. As discussed in Hyatt et al. (2007), when soil was subjected to compaction from the top within a finite container, the walls of the container acted to resist soil expansion and movement while also causing soil compaction in the horizontal direction; in contrast, when compaction was applied on top of the soil within a field, the soil had a seemingly infinite space to expand and compaction only occurred in one direction. In the current study, although the applied compaction effort of the high compaction treatment was significantly less than that applied in the field experiment (Altikat and Celik 2011), the additional compaction caused by the container walls may have been significant enough to cause seedlings to be unable to emerge. Comparatively, the optimum compaction in the field study was the adequate force to apply in conjunction with the reduced tillage method to increase the soil-seed contact. As a result, it is possible that the laboratory model used in this experiment does not accurately portray the complexities of soil compaction and seedling emergence in field conditions and further investigation in a field setting is required. 8 100 ) % ( e c 80 n e g r e 60 m No Compaction E of 40 Low Compaction e g a Medium nt 20 Compaction e c High Compaction r e P 0 0 2 4 6 8 10 12 14 Days after seeding (days) Figure 4. Percentage of soybean seedling emergence over two-week observation period. Plant Height Observation days with less than eight observations out of a total of twelve were omitted from Table 2. The ANOVA and Scheffe’s tests were performed on days six, eight, and eleven; plant growth had begun prior to these days and observation of all plants had occurred. For days six, eight, and eleven, the plant height of control plants were significantly different from the plant heights of low compaction plants (P =0.0002, P =0.0034, and P =0.0325), medium 6 8 11 compaction plants (P <0.0001 and P =0.003), and high compaction plants (P <0.0001). Plant 6,8 11 6.8,11 heights of the control treatment were likely greatest throughout the span of the experiment because all of the seedlings were able to emerge by day seven, thus providing them with a head start to grow. On day six, there was no significant difference between plant heights of low and medium compaction plants (P =0.3513), low and high compaction plants (P =0.4382), and medium and high 6 6 compaction plants (P =0.9998). However, on days eight and eleven, there was a significant 6 difference in plant height between low and high compaction plants (P = 0.0107, P =0.0037) 8 11 although there was no significant difference between plant heights of low and medium compaction plants (P =0.1427, P =0.3574), and medium and high compaction plants (P =0.7293, P =0.2368). 8 11 8 11 All of the low compaction treatment seedlings had emerged from the soil surface by day eight but the remaining high compaction treatment seedlings ceased emerging on the same day. The significant difference in plant height between low and high compaction treatments was due to the fact that emerged low compaction seedlings were able to grow in height, while some of the emerged high compaction seedlings were unable to grow and were likely abnormal seedlings. As discussed earlier, it was unlikely that there were anoxic conditions existing in the soil due to compaction treatments. The differences in plant height between the control and compaction treatments were most likely caused by the delay in soybean seedling emergence due to increased soil surface strength. The presence of seedlings that had not emerged in soil samples (plant height = 0 mm) subjected to the medium and high compaction treatments also decreased the mean plant height of the compaction treatments. 9 Table 2. Plant height of soybean seedlings subjected to no, low, medium, and high compaction treatments over a two-week observation period Plant Height (mm) Medium No Compaction Low Compaction High Compaction Compaction Day n Mean SD Mean SD Mean* SD Mean* SD 0 12 0 0 0 0 0 0 0 0 1 9 0 0 0 0 0 0 0 0 4 12 1 2 0 0 0 0 0 0 5 9 23 18 6 9 0 0 0 0 6 12 51 29 14 20 1 2 2 4 7 9 76 37 41 31 12 18 0 0 8 12 106 36 57 32 26 26 11 26 11 12 164 46 103 47 67 54 24 44 *Means were calculated with n-1 observations due to missing data. Leaf Count The ANOVA and Scheffe’s tests were performed on leaf count per container rather than leaf count per plant because leaf counts were not separated by plant (Fig. 6). The ANOVA tests on days six and eight indicated that at least one of the treatments affected the leaf count per container differently than the others (P =0.0360 and P =0.0118) , while ANOVA test on day eleven 6 8 indicates that all the treatments have the same effect on leaf count per container (P =0.1227). 11 However, the Scheffe’s tests indicated that there was no significant difference in leaf count per container between treatments on day six. On day eight, there was a significant difference in leaf count within containers between the control treatment and the high compaction treatment (P =0.0153), while there was no significant difference between other treatments. The inability of 8 the statistical analysis to find a significant difference across treatments was due to the high variability in leaf count per container across treatments. Additionally, leaf development occurred quickly after seedlings emerged from soil. Because seedlings began to emerge around the same time across treatments (albeit, at different rates), it was not surprising to see that there was not significant differences in leaf count per container across treatments. 10