The effects of seedbed preparation and its timing on soil strength parameters of a compacted loamy soil and yields of spring barley

The effects of seedbed preparation and its timing on soil strength parameters of a compacted loamy soil and yields of spring barley Loraine ten Damme Agro Environmental Management June 1, 2017

The effects of seedbed preparation and its timing on soil strength parameters of a compacted loamy soil and yields of spring barley Loraine ten Damme Student registration number: au547856 Agro Environmental Management Thesis 30 ECTS June 1, 2017 Main supervisor: Senior researcher Mathieu Lamandé Co supervisor: Senior researcher Lars J. Munkholm Co supervisor: PhD student Peter Bilson Obour Department of Agroecology Soil Physics and Hydropedology The images (on the front cover and in chapters) were taken by Peter Bilson Obour

Preface and acknowledgements Almost certain about my enthusiasm for soil related research I signed up for the summer course of 2016 on Advanced Soil Physics. Here, I met a lot of other people who thought that soil is sexy. My enthusiasm for soil grew, and although I was only starting to learn about it, I knew: my thesis will be about soil. Unfortunately, soil is not just soil and my interests were, and still are, broad. I could have been happy doing research on a lot of subjects, but I had troubles picking one. Fortunately, when I approached senior researcher Mathieu Lamandé, he was so kind to outline several studies and let me decide. I found my main supervisor, choose the study I liked, and became related to my co supervisor Peter Bilson Obour, his PhD research, and my co supervisor Lars J. Munkholm. I am very grateful for the situation I have had: an interesting study, available soil and crop data, and patient supervisors who provided constructive feedback. They knew much more than I did, but I was never afraid to ask dumb questions. An office in Foulum was a great help and escape, always provided with kind people and a comforting, different environment from both home and university. I want to give my special thanks to my main (co)supervisors Mathieu and Peter, for letting me study this topic and writing this thesis. Essential was the data of the soil compaction experiment carried out by the Norwegian University of Life Sciences (NMBU); thank you Norway. I appreciate the time Peter took to explain and show me the experiments and analyses he had been doing. This really provided me a clear idea of the research, moreover increased my knowledge and enthusiasm for (practical) research. I will thank both Mathieu and Peter again, for introducing me to Terranimo and R; the former easy to run but the latter I had happily avoided until then. It took several attempts, but Peter helped me obtain the right statistics myself. To Peter I owe a thousand thanks, as he, regardless of day and time, contacted me to discuss my questions. Once more I would like to thank Mathieu, Peter, and Lars, for the time, feedback, and guidance I received. Loraine ten Damme Aarhus, June 1, 2017 1

Summary In Europe, one of the major threats to agricultural soil quality is compaction. It is however possible to improve a degraded soil structure in the top soil layer after compaction through tillage. Secondary tillage is usually performed to break down large soil fragments, mainly to prepare seedbeds. A favourable seedbed is one in which nearly all viable seeds germinate and seedlings emerge. The timing of tillage is of major influence on the risks of soil degradation, the ease of tillage, and the desired result. The main objectives of this study were (1) to test the ability of secondary tillage to mitigate detrimental effects of compaction on the quality of seedbed preparation and (2) to investigate the importance of the timing of secondary tillage operations with regard to the first objective. A literature review formed the theoretical background of the study and focused on soil compaction (the causes and implications on soil properties), seedbed requirements and tillage, and soil strength (soil workability, tensile strength, and friability). A compaction experiment was established in Ås, Norway since 2014, and similar treatments were conducted three years in a row. The treatments consisted of early, optimal, and late sowing time (compaction and/or harrowing and sowing), conducted at matric potentials of 30 hpa (early) and 1,000 hpa (both optimal and late). Compaction was done with a MF 4225 tractor, ~4.5 Mg, wheel by wheel. The risks of compaction were evaluated with Terranimo. Soil sampling was done in 2016. Small soil cores (~100 cm 3 ) from both ~1 5 cm and ~5 10 cm depth were used to obtain the bulk density and soil water retention data. Bulk soil samples from 1 5 and 5 15 cm depth were used to create natural and remoulded aggregates to analyse for the soil strength. The current study focussed on the tensile strength, specific rupture energy, Young s modulus, friability, and workability from the natural 16 8 mm and the remoulded aggregates. Soil strength index and friability index were also determined. The current study focussed on. Large soil cores (~580 cm 3 ) were sampled at 5 15 cm depth and drained to 100 hpa and 300 hpa to evaluate soil fragmentation in a soil drop test. The effects of the treatments on the dry matter yield of spring barley was analysed for 2014 2016. The results showed that the timing of the treatments was often the most important factor influencing soil strength and seedbed quality. Regarding spring barley grain yield, compaction was the most influencing factor. When the soil was wet (at early sowing), the risk of soil compaction was severe. At such wet conditions, secondary tillage could not mitigate the effects of sowing on seedbed quality, whether the soil was compacted or not. In 2014 and 2015 yields were reduced in 2

the compacted treatments, and in the late sowed treatments. In 2016 however, neither compaction nor sowing time affected the yields. The effects of timing of the treatments on tillage and grain yield is as important to consider in both compacted and non compacted soils (dry conditions are favoured), and the effect of compaction on grain yield is important to consider regardless of the timing of sowing (non compacted conditions are favoured). 3

Table of Contents Preface and acknowledgements... 1 Summary... 2 1. Introduction... 6 1.1 Objectives, research questions... 6 1.2 Hypothesis... 7 2. Theoretical background... 8 2.1 Consequences of soil compaction... 8 2.2 Susceptibility of soils to compaction... 8 2.3 Tillage and seedbed preparation... 9 2.4 Timing of tillage... 9 2.5 Parameters of soil physical conditions... 10 3. Materials and methods... 13 3.1 Experimental field... 13 3.2 Experimental design... 13 3.3 Yields... 14 3.4 Soil sampling... 14 3.5 Soil preparation... 14 3.6 Laboratory measurements... 15 3.6.1 Tensile strength... 15 3.6.2 Soil fragmentation... 16 3.7 Calculations... 16 3.8 Evaluation of soil compaction risk... 18 3.9 Statistical analysis... 18 4. Results and discussion... 19 4.1 Vertical stress in the soil profile... 19 4

4.2 Effect of compaction and sowing time on bulk density... 19 4.3 The effects of compaction and sowing time on aggregate strength... 21 4.3.1 Airdried natural aggregates 16 8 mm... 21 4.3.2 Remoulded aggregates... 22 4.4 Friability index and strength index... 24 4.4 Soil fragmentation... 26 4.5 Effects of compaction and sowing time on the yield of spring barley... 27 5. Conclusion... 28 5.1 Implications and perspectives... 29 Bibliography... 30 5

1. Introduction Soil degradation is a worldwide problem caused by different physical, chemical, and biological processes (Blum, 2008), yet hard to quantify: estimations range from 15% 80% of the agricultural land (Gomiero, 2016). In Europe, one of the most frequent threats to agricultural soil quality is soil compaction (Houšková and Montanarella, 2008). Tillage after compaction can improve a degraded soil structure in the top soil layer, and plays a crucial role in especially arable farming. Secondary tillage is usually performed to breakdown large soil fragments, mainly to prepare seedbeds. A seedbed is favourable when nearly all viable seeds germinate and seedlings emerge. As a rule of thumb, aggregates in an optimum seedbed are no smaller than 0.5 1 mm and no bigger than 5 6 mm (Braunack and Dexter, 1989b). The timing of tillage is of major influence on both the ease of tillage and the desired result. At optimum soil conditions, tillage produces the seedbeds while reducing the need of subsequent field operations and the risk of soil and environmental damage. The optimum soil conditions exist over limited periods, the windows of opportunity, in which the soil is both trafficable and workable. 1.1 Objectives, research questions The main objectives of this study were (1) to test the ability of secondary tillage to mitigate detrimental effects of compaction on the quality of seedbed preparation and (2) to investigate the importance of the timing of secondary tillage operations with regard to the first objective. To reach these objectives, a literature study was carried out and soil samples from a compaction experiment were analysed. The following research questions were addressed: A) What are the effects of topsoil compaction, timing of field operations, and their interaction on the strength of aggregates of the soil under study? B) What are the effects of topsoil compaction, timing of field operations, and their interaction on the soil fragmentation of the soil under study? C) What are the effects of soil compaction, timing of field operations, and their interaction on yields of spring barley of the fields under study? 6

1.2 Hypothesis The hypotheses of this study were: 1) Soil compaction increases the tensile strength of soil aggregates; 2) Soil compaction and wet sowing reduces soil fragmentation at a given matric potential. 7

2. Theoretical background 2.1 Consequences of soil compaction Soil compaction is a form of degradation in agricultural fields difficult to observe directly. Compaction alters soil properties that govern its basic functioning. Implications on general soil behavioural properties and on crop production are extensively studied by among others Alblas et al. (1994), Smith et al. (1997), Munkholm et al. (2002), van Dijck & van Asch (2002), and Arvidsson & Håkansson (2014). Compaction increases bulk density, deforms the soil structure, which affects soil physical functions such as strength and water and air transport. It thereby impacts the suitability for field operations and crop production (Rounsevell, 1993; Edwards et al., 2016). Yields can only respond to external stimulus as climate, breed, economic demand, and crop protection where soil conditions do not restrict plant growth (Greenland and Dennis, 1997). Compaction can also increase the risks of soil erosion, flooding, leaching of pollutants and nutrients and emissions of greenhouse gasses (O sullivan and Vinten, 1999). 2.2 Susceptibility of soils to compaction Compaction can occur at different soil depths: in the topsoil (down to the maximum depth of tillage), and in the subsoil (below the plough layer). It is generally caused by traffic, but can be caused by animal trampling or for example freezing and thawing. Compaction only occurs when the stress on the soil exceeds the soil strength. Soil deformation is a result of the surface stress applied to a soil, the distribution of stress through the soil, and the stress strain behaviour of the soil (Keller and Lamandé, 2010). Lamandé et al. (2007) showed that stress in the topsoil primarily is determined by contact stress, whereas stress in the subsoil is primarily a result of the load. Important aspects to consider with regard to compaction induced by traffic, are the load (generally expressed as wheel or axel load), tyre specifications (such as the number, type, dimension, and inflation pressure), and the soil strength (clay content, bulk density, and matric potential) (Keller and Arvidsson, 2004). One will generally find an increasing risk of compaction with increasing load, increasing tire inflation, reducing soil contact area, decreasing clay content, decreasing bulk density, and increasing soil water content (Alblas et al., 1994; Jones, Spoor and Thomasson, 2003; van den Akker, Arvidsson and Horn, 2003; Keller and Arvidsson, 2004). Especially the soil water content at the time of stress exposition is critical to the magnitude of compaction (Jones, Spoor and Thomasson, 2003). For example, traffic with a high axel load on a soil with low clay content and/or high moisture content presents a considerable risk of soil 8

deformation. Subsoil compaction is generally considered a threat to long term sustainability, whereas topsoil compaction threatens the immediate farm economy and environment (van Dijck and van Asch, 2002; Alakukku et al., 2003; van den Akker and Hoogland, 2011). The soil structure is a key factor in the emergence and development of a crop, and thus important to optimise. It is nevertheless possible to improve a degraded soil structure after compaction in the top soil layer through tillage. 2.3 Tillage and seedbed preparation Tillage plays a crucial role in particularly arable farming. It is used to incorporate organic materials into soil and to control weeds, and in the preparation of seedbeds for crop establishment, it is performed to improve soil structures. Tillage comprises primary and secondary tillage. Primary tillage involves digging, stirring and turning over the soil, such as during ploughing. Secondary tillage is usually performed to break down large soil fragments that are produced during primary tillage, mainly to prepare favourable seedbeds. A majority of large aggregates in a seedbed control erosion, but are less valuable in terms of crop establishment (Dexter and Birkas, 2004). A favourable seedbed is one in which nearly all viable seeds germinate and seedlings emerge. As a rule of thumb, aggregates in an optimum seedbed are no smaller than 0.5 1 mm and no bigger than 5 6 mm (Braunack and Dexter, 1989a). Optimum aggregate size varies however with specific crops, soil type, and climatic conditions. For small grains, good seedbeds consist for over 50% of aggregates smaller than 5 mm (Håkansson, Myrbeck and Etana, 2002). Braunack & Dexter (1989b) have reviewed the optimum aggregate size ranges for different crops for cereals 1 2 and 2 4 mm. 2.4 Timing of tillage The timing of tillage is of major influence on both the ease of tillage and the desired result. At optimum soil conditions, tillage obtains the desired seedbed conditions while reducing the need of subsequent operations, thus reducing the required energy input and redundant operation costs. A soil is in such conditions at the range of optimum water content : within the upper (or wet) tillage limit, and the lower (or dry) tillage limit (Dexter and Bird, 2001). The optimal water content for tillage has by the authors been defined as the water content at which tillage produces the greatest proportion of small aggregates. When tillage is performed outside of the range, soil quality and other environmental issues are at state (Arvidsson, Keller and Gustafsson, 2004). 9

Above the range, tillage can induce structural damage like smearing. Below the range, excessive energy is required (Droogers, Fermont and Bouma, 1996). Optimum soil conditions occur over periods of time, the windows of opportunity, in which the soil is both trafficable and workable. Either of these can limit the readiness of a soil (Edwards et al., 2016). The optimum conditions for tillage are restricted by soil properties, climatic conditions, and by field management (crop, machinery, and operation) (Edwards et al., 2016). Increased bulk density, resulting from soil compaction, reduces the upper and lower tillage limits, and the range of water content for tillage, whereas increasing clay content results in higher limits and a smaller range (Dexter & Bird 2001). Soil trafficability refers to the capacity of a soil to support traffic while avoiding compaction, whereas soil workability refers to the condition (or mechanical strength) of the soil in which field operations result in the desired effect, (e.g. tillage operations produce desirable seedbeds), without causing structural damage (Rounsevell, 1993; Edwards et al., 2016). The most critical factor on soil workability is the soil water content (Dexter and Birkas, 2004). Friability is another important parameter for especially agricultural soils and represents the tendency of a mass of soil to breakdown into smaller soil masses of particular size ranges under applied stress (Utomo and Dexter, 1981). 2.5 Parameters of soil physical conditions Soil can be evaluated for its conditions, its readiness and its seedbed suitability using various parameters. Soil conditions are often evaluated using soil bulk density and tensile strength of soil aggregates, in comparison with another or a reference soil. A soils readiness for tillage can be assessed using workability and friability, parameters which can be calculated from the tensile strength of soil aggregates. Seedbed quality can be described via the aggregate distribution, and the soil strength index. Changes in bulk density indicate the deformation or resilience of a soil, and bulk density is therefore often used to indicate the degree of compaction. Tensile strength, commonly defined as the maximum stress applied before failure, is a sensitive indicator of a soil s condition, and can be measured on single aggregates by simple crushing tests; a method described by Dexter & Kroesbergen (1985). Measurements for tensile strength can also be used to identify the Young s modulus. Presented as the slope of the elasticity region on the stress strain curve, it indicates the elasticity of the aggregates before failure. Higher values indicate a larger displacement under 10

similar stress, hence a more effective use of applied stress to fracture aggregates (Munkholm and Kay, 2002). Workability is generally identified by the previous introduced range of water content: the optimal water content for tillage, and the upper and lower tillage limits. Within these threshold limits for workability, tillage has positive effects on soil structure (Droogers, Fermont and Bouma, 1996). The optimal water content is often identified as a fraction of the lower Atterberg limit (also: plastic limit, the moisture content where a thread breaks apart at a diameter of 3.2 mm ), but the factor appeared to vary from soil to soil (for example as reviewed by Dexter & Richard (2009), factors vary from 0.7 to 0.91). Dexter & Bird (2001) identified an inflection point on the soil water retention curve to predict the optimal water content and the upper and lower tillage limits. The inflection point was defined as the water content of a draining soil at which air is entering the most rapidly with increasing suction. Another method for qualifying workability was introduced by Arthur et al. (2014). Instead of using threshold values and a range, the authors introduced an index to define the ease to which a soil is tilled. This method requires knowledge of the friability and tensile strength of aggregates. Generally, workability decreases with increasing tensile strength (Arthur et al., 2014). The larger the index for workability, the better the conditions of the soil to support field operations obtaining the desired effect. Friability is another important parameter for agricultural soils and the ease of fulfilling the objective of secondary tillage. Flaws and microcracks divide soil into emerging sub volumes, which in turn become smaller aggregates when the soil breaks (Dexter, 2004). The dispersion of these flaws and cracks cause a clod to fragment under pressure or not, hence this defines an aggregate s strength. At maximum friability soil crumbles to the desired aggregate size distribution (Munkholm and Kay, 2002). Vice versa, a soil with low friability will break into aggregates of arbitrary sizes (Utomo and Dexter, 1981). A friability index of a soil can be estimated from the tensile strength of aggregates of different size fractions. At small indexes, the strength of larger and smaller aggregates does not differ, whereas higher friability index indicates a smaller strength for large aggregates than for smaller aggregates, i.e. large aggregates are fragmented easily whereas smaller aggregates are difficult to fragment (Utomo and Dexter, 1981). Assessing workability and friability using the tensile strength of aggregates is only one of a few methods, yet widespread because of its high quantitative character and sensitivity to soil management (Munkholm, 2011). Various other methods were reviewed by Munkholm (2011), among which via specific rupture energy, the soil drop test, and visual assessment. The specific 11

rupture energy represents the amount of energy needed to break a mass of soil. Moreover, a soils strength index can be used to assess the soil structure of natural soil aggregates. This index is assumes that a well structured soil is likely to have a smaller tensile strength, due to its developed internal structure (Getahun, Munkholm and Schjønning, 2016). High values of soil strength (approaching 1) indicate larger differences between the natural and remoulded aggregates, and implies a good structural conditions and a soil suitable as a seedbed (Watts and Dexter, 1998), consequently lower values indicate lower seedbed quality. 12

3. Materials and methods 3.1 Experimental field Soil samples were taken from an experimental field in Ås, southeast Norway. The region is typified by sub humid conditions with an average annual precipitation of 785 mm, and average annual temperature of 5.3 C (Kværnø, Haugen and Børresen, 2007). The soil is classified as typical Haplaquept according to the USDA Soil Taxonomy, also: sandy loam. Its main characteristics to a depth of 1 15 cm were 22% clay, 29% silt, 29% fine sand, 15% coarse sand, and 4.4% organic matter. 3.2 Experimental design The experiment was established since 2014. Similar treatments were conducted three years in a row, and designed as a randomised block design with two replicates. The blocks were allocated with a different timing of secondary tillage: early (A1) when the soil was expected wet; optimal (A2) when the soil was expected moist to dry, and; late (A3) when the soil was expected dry. The early treatments were conducted April 11, 2016, at a gravimetric soil water content of 34.7% at 1 5 cm depth, and 36.3% at 5 10 cm depth (equivalent to a matric potential of around 30 hpa). The optimal treatments were conducted April 25, 2016, at a gravimetric water content of 19.2% at 1 5 cm depth, and 24.2% at 5 10 cm depth. The late treatments were conducted May 9, 2016, at a gravimetric water content of 19.2% at 1 5 cm depth, and 27.3% at 5 10 cm depth. At both the optimal and late treatments, the soil water content was equivalent to a matric potential over 1,000 hpa. The plots had different compaction treatments, namely by single, two or three passes (B1, B2 and B3 respectively). Control plots (B0) were not compacted. Compaction was done with a MF 4225 tractor, ~4.5 Mg, wheel by wheel. The front axle had a load of 900 kg, and 11.2 R 25 tires with an inflation pressure of 1.5 kg/cm 2. The rear axle had a load of 3,500 kg, and 16.9 R 30 tires with an inflation pressure of 1.0 kg/cm 2. After compaction, all plots (i.e. both the compacted and control plots) were harrowed to ~5 cm depth. In the compacted treatments, there was thus a loosened soil in the upper most layer (~1 5 cm depth) and a compacted soil below. Seeding of spring barley (Hordeum vulgare L.) was done at ~3 cm depth using a combined seeding and fertilising seeder. It should be emphasised that compaction, harrowing and seeding were done simultaneously for each of the treatment. This aspect of timing is hereafter addressed as sowing (time). Prior to the compaction treatments, the experimental field was ploughed to ~20 cm depth the previous year, on September 30, 2015, with a reversible plough with two mouldboards. The 13

crop was irrigated after the soil samples were taken. After harvest, the field was left fallow. The current study focussed on the single pass compaction (B1) which was compared to the control plots (B0). Soil sampling was done in the third year: 2016. 3.3 Yields The grain of spring barley was harvested with a research plot combine. Harvest took place August 25, 2014, August 18, 2015, and September 7, 2016. Samples of the grain harvested were dried to 105 C to assess the dry matter yield at 15% moisture content (w/w). Straw yield was not measured. 3.4 Soil sampling Soil sampling was carried out at a soil water content of about field capacity, on May 24 and May 25, 2016 (Images 1 and 2). For each of the plots large undisturbed soil cores (~580 cm 3 ), and small undisturbed soil cores (~100 cm 3 ) were taken from two sampling positions outside the wheel tracks using metal cylinders. The small soil cores were taken from both ~1 5 cm and ~5 10 cm depth and used to obtain the bulk density and soil water retention data. The large soil cores were taken from ~5 15 cm depth only, and used in the soil drop test. A total of 192 small soil cores and 96 large cores were sampled. Bulk soil samples were taken from each sampling position and both depths for soil texture analysis and soil aggregates tensile strength measurements. These samples were kept in zipped plastic bags and kept 2 C until laboratory analyses. Image 1. Soil core sampling Image 2. Bulk soil sampling 3.5 Soil preparation The undisturbed large soil cores were drained at 100, 300 and 1,000 hpa matric potentials and used in the soil drop test. The bulk soil samples were air dried by spreading the soil in a ventilated 14

room with a temperature of ~20 C (Image 3). Prior to air dying, the bulk soil samples were gently fractionated by hands, so that the soil broke along natural cracks. The air dried soil clods were then crushed mechanically using the roller method (as suggested by Hartge (1971)) before passed through a nest of sieves with openings of 16, 8, 4, 2 and 1 mm (Images 4 and 5) to obtain four different classes of soil aggregates, namely 16 8, 8 4, 4 2, and 2 1 mm in diameter. Some of the airdried soil was crushed and passed through a 2 mm sieve. The obtained soil (particles < 2 mm) were used to determine soil texture and to make remoulded soil aggregates (spherical, ~12 mm in diameter) at plot level. Moulding destroys the pre existing soil structure, and mimics the worse form of mechanical damage to soil. Image 3. Air drying of sampled soil Image 4. Mechanical crushing of airdried soil clods Image 5. Sieving nest to obtain different classes of natural aggregates 3.6 Laboratory measurements 3.6.1 Tensile strength The tensile strength was measured on the natural airdried aggregates (2 1, 4 2, 8 4 and 16 8 mm) and remoulded aggregates by an indirect test (Dexter and Kroesbergen, 1985) by crushing the individual aggregates between two parallel plates (Rogowski, 1964) using a tensile tester (INSTRON model 5969) (Images 6 8). Image 6. Natural aggregates Image 7. Remoulded aggregates Image 8. Tensile tester (INSTRON model 5969) 15

3.6.2 Soil fragmentation The soil drop test (Schjønning et al., 2002) was performed on twenty four of the undisturbed large soil cores drained at 100, 300 and 1,000 hpa matric potentials. In brief, a soil core was dropped from the metal cylinder from a height of 2 m (for illustration: Images 9 11), after which the soil fragments were collected and air dried, and sieved through a nest of sieves with openings of 32, 16, 8, 4, and 2 mm to obtain six different classes of aggregates, namely > 32, 32 16, 16 8, 8 4, 4 2, and < 2 mm in diameter. After sieving, the mass of soil aggregates obtained on each sieve was used to determine the aggregate size distribution. Image 9. Undisturbed large soil cores for soil drop test Image 10. Set up soil drop test Image 11. Result of soil drop test 3.7 Calculations The soils bulk density (ρ b, Mg m 3 ) was calculated from the dry mass of the soil divided by the total soil volume for the small soil cores sampled in each plot. Tensile strength (Y, kpa) of the aggregate size fractions was calculated for the natural airdried aggregates 16 8 mm and for the remoulded aggregates (two batches of 15 aggregates per size fraction) as suggested by Dexter and Kroesbergen (1985), assuming the natural aggregates are spherical: Y = 0.567 * F / d 2 Eq. 1 where 0.576 is a proportionality constant based on spherical form and perfect elastic behaviour material (Poison ratio = 0.5), F is the maximum force (N) at failure, and d is the effective diameter of the spherical aggregate (m). The diameter of aggregates was computed from method 4 of 16

Dexter & Kroesbergen (1985) in which d is adjusted according to the individual masses (g) of aggregates, assuming that all the aggregates have equal density: d = d 1 (m 0 / m 1 ) 1/3 Eq. 2 where d 1 is the mean diameter (mm) for the size fraction, m 0 is the mass (g) of the individual aggregate and m 1 is the mean mass of a batch of aggregates of the same size fraction. Specific rupture energy (E SP, J kg 1 ) of the aggregate size fractions was calculated for the natural airdried aggregates 16 8 mm and for the remoulded aggregates as elaborated by Perfect and Kay (1994): E SP = E ML / m a Eq. 3 where E ML (J) is the energy at the maximum load and m a (kg) the dry weight of the soil aggregates adjusted for water. Young s modulus (E) of the aggregate size fractions was calculated for the natural airdried aggregates 16 8 mm and for the remoulded aggregates was estimated from the gradient of the stress strain curve to the elastic limit, assuming a linearity up to that point: E = σ / ԑ Eq. 4 where σ is stress (Pa) and ԑ is strain. Friability (FI) was calculated of the aggregate size fractions was calculated for the natural airdried aggregates 16 8 mm and for the remoulded aggregates following the method of Utomo & Dexter (1981): FI = std(y) / μ(y) Eq. 5 where Y (kpa) is the tensile strength of a batch of aggregates of the same size fraction. Workability was calculated of the aggregate size fractions was calculated for the natural airdried aggregates 16 8 mm and for the remoulded aggregates following the method introduced by Arthur et al. (2014): W = FI * (1 / μ(lny)) Eq. 6 where FI the friability of a batch of aggregates of the same size fraction, and Y (kpa) the log transformed tensile strength of the same batch of soil aggregates. 17

The friability index (k) was estimated from the relationship between tensile strength (Y, kpa) and the average volume of the aggregates for each size fraction, based on the method proposed by Utomo & Dexter (1981): ln(y) = k ln(v) + A Eq. 7 where V (m 3 ) is the estimated average volume of the aggregates, and A (kpa) the predicted tensile strength of 1 m 3 of bulk soil (the intercept of the regression). The soil (SIn) was calculated as a method proposed by Watts & Dexter (1998): SIn = 1 (Yn / Yr) Eq. 8 where the strength of the natural soil aggregates (Yn, kpa) for each size fraction is normalised to the strength of the remoulded aggregates (Yr, kpa). The aggregate size distribution, expressed as the geometrical mean diameter (GMD, mm) and as the proportion of aggregates < 8 mm and > 32 mm, was calculated from the soil fragmentation from the soil drop test. 3.8 Evaluation of soil compaction risk The web based decision support tool Terranimo compaction during the experimental treatment in 2016. was used to evaluate the risk of topsoil 3.9 Statistical analysis Statistical tests were done using the R program. A generalized linear model (GLM) was used to fit the data for ρ b, Y, E SP, Young s modulus, friability, workability, SIn, GMD, aggregate proportions < 8 mm and > 32 mm, and yield. The Y and E SP were log transformed so it conformed to a normal distribution. Multiple comparison (Tukey s test) and t test were used to determine the differences in the means of treatments at the significance level P = 0.05. 18

4. Results and discussion 4.1 Vertical stress in the soil profile The risk of soil compaction was estimated using the model Terranimo. The model did not allow for a front wheel load as low as used in the experiment, therefore was the risk of the front axle overestimated. The focus in the risk assessment was on the heaviest wheel, i.e. the rear axle, where, the risk of compaction was expected to be severe up to 40 cm depth and moderate at 80 cm depth at 30 hpa matric potential (Table 4.1). This is equivalent to the soil water conditions at early sowing. With more negative potential (i.e. drier soil), the risk of compaction decreased. For instance, at a 500 hpa matric potential the risk was only moderate up to 10 cm depth. This may imply that at the optimal and late sowing, which were conducted at a more negative matric potential than 500 hpa, the risk of compacted could have been expected to be very minimal. Table 4.1 The soil compaction index (SCI) for the experiment as modelled by Terranimo, at wet conditions (top, 30 hpa matric potential) and dry conditions (bottom, 500 hpa matric potential). NB. Front axle is overestimated due to restrictions by the model. SCI 0: no compaction risk; SCI 0 0.2: intermediate compaction risk; SCI > 0.2: high compaction risk. 4.2 Effect of compaction and sowing time on bulk density Although the risk of soil compaction was severe at early sowing, there was no significant effect of compaction, or of the interaction between sowing time and compaction, on the bulk density (ρ b, Mg m 3 ) at 1 5 cm depth (Table 4.2); the depth to which was harrowed. Sowing time alone, resulted in significantly lower ρ b at optimal sowing (P < 0.0001). The ρ b at early and late sowing was significantly similar, even though the soils were wet (matric potential 30 hpa) and dry (matric potential > 1,000 hpa) respectively. Smith et al. (1997) also found a high ρ b of dry loamy sandy soils, and attributed this to particle rearrangement after changing water contents. The effect of wetting and drying was studied by Utomo & Dexter (1982). The authors found a decrease of water stable aggregates with increasing occurrence (but an initial increase in tilled soil). When water stable aggregates decrease, aggregate instability must increase (defined as: the difference in mm between the average diameters of the aggregates before and after wet (sieving) (Utomo 19

and Dexter, 1982)). Peng et al. (2007) found severe modifications of soil structure after intense wetting drying cycles, and Salih & Maulood (1988) found wetting and drying cycles to cause soil shrinkage. Frequency of repeating cycles had less effect, as the changed soil structure was lasting. The experimental field in the current study may have been subjected to severe wetting followed by drying between April 24 (optimal) and May 9 (late), which may have increased ρ b for the latter. This thought was supported by the soil water contents at sowing, measured at 1 5 cm and 5 10 cm depth. As the soil water content increases with depth, it is at 10 15 cm depth again expected higher at late sowing compared to optimal sowing. However, the effects of wetting and drying could not be studied isolated as the treatments provided different conditions. At 5 15 cm depth ρ b was, as expected, significantly higher in the compacted compared to the control treatments (P = 0.013). Regarding sowing time, similar effects were observed as in the depth 1 5 cm. The interaction of sowing time and compaction was significantly different (P = 0.022), and highest for the compacted soil at early sowing, i.e. at the highest soil water content, which relates well to the compaction risk assessment. Within the interactions, the ρ b of the compacted treatment was only different from the control treatments at early sowing, i.e. at wet soil conditions. Table 4.2 Dry bulk density (ρ b ) for the different treated soils. Values with different letters in for compaction, timing, or interaction are significantly different (Tukey s test, P < 0.05). Depth (cm) Treatment ρ b (Mg m 3 ) 1 5 Compaction B1 1.10a B0 1.09a Timing 5 10 Compaction A1 1.10b A2 1.05a A3 1.14b B1 1.23b B0 1.19a Timing A1 1.24b A2 1.13a A3 1.24b Interaction A1:B1 1.28d A1:B0 1.20ac A2:B1 1.12a A2:B0 A3:B1 A3:B0 1.14ab 1.27cd 1.21bcd 20

4.3 The effects of compaction and sowing time on aggregate strength 4.3.1 Airdried natural aggregates 16 8 mm For the natural aggregates at 1 5 cm depth (Table 4.3), the tensile strength (Y, kpa) was highest at early sowing, and decreased for optimal and late sowing (P < 0.0001). Previously research, for example by Causarano (1993) and Munkholm and Kay (2002), showed increasing soil Y with decreasing soil water content, which implies that a soil is weaker when wet: tillage performed on wetter soil destroys the existing soil structure, which in turn increases Y. A reduced Y can also be caused by for example wetting and drying cycles (Salih and Maulood, 1988; Ma et al., 2015), yet Peng et al. (2007) found the effects of such cycles on shrinkage and swelling of a soil also related to its pore structure. In the current study, the decrease of Y regarding sowing time is unlike related to such cycles, as there was only a short period of time between the treatments and soil sampling. Moreover, wetting and drying could not be studied isolated from the treatments. Regarding the interaction of compaction and sowing time (P = 0.014), a comparable trend was found for the compaction and timing treatments individually. Similar to Y, there was no significant difference for the specific rupture energy (E SP, J kg 1 ) of the compacted and control treatments. E SP was higher for the aggregates at early sowing compared to optimal and late sowing (P < 0.0001). Smaller difference of E SP compared to Y were also found by Munkholm & Kay (2002), in a topsoil compaction experiment on a sandy loam. The Young s modulus (MPa), friability, and workability were not significantly different within the compaction and timing treatments, even while an increase Y generally results in reduced workability (Arthur et al., 2014). As to the depth of 5 15 cm (Table 4.4), no significant effect of the interaction was found on Y, but there was significant effect of both compaction (P = 0.031) and sowing time (P < 0.0001) separately. The Y was highest in the compacted treatments, and in the treatments conducted at early sowing (where the risk of soil compaction was severe). Increased values of Y on compacted treatments were also found by (Munkholm, Schjønning and Kay, 2002), and was by the authors significantly negatively related to macroporosity, which decreased by compaction. The E SP was not significantly different within compaction treatments, nor within the timing treatments. The Young s modulus was not significantly affected by compaction, but tended to be higher in the compacted treatments (P = 0.060). Munkholm & Kay (2002) did find a higher Young s modulus in compacted treatments, and suggested that this could result from the reduced pore structures of compacted soils. In a wellstructured soil, the crack propagation is expected to be slowed down by complex pore structures, which causes a less efficient use of applied stress to fracture aggregates. The Young s modulus at early sowing was significantly higher than at optimal sowing, which can be interpreted as the 21

aggregates at wet sowing being stiffer in comparison with the aggregates of the soil sowed in drier conditions. Both friability and workability of the aggregates were again not significantly different within the compaction treatments, indicating that the plots of the treatments would be tilled with the similar ease. Regarding sowing time, a trend was observed in both friability (P = 0.201) and workability (P = 0.185), with the highest values at optimal sowing. Table 4.3 Geometric means of tensile strength (Y), specific rupture energy (E sp ), Young s modulus, and arithmetic means of friability and workability of the air dried natural aggregates, 16 8 mm, 1 5 cm depth. Values in the same column for compaction, timing, or interaction with similar letters are not significant different (Tukey s test, P < 0.05). Treatment Y (kpa) E SP (J kg 1 ) Young s modulus (MPa) Friability Workability Compaction B1 84a 2.9a 12a 0.48a 0.11a B0 83a 3.0a 15a 0.46a 0.10a Timing A1 114c 4.3b 14a 0.46a 0.10a A2 84b 3.3b 13a 0.51a 0.11a A3 62a 1.9a 14a 0.44a 0.11a Interaction A1:B1 A1:B0 A2:B1 A2:B0 A3:B1 A3:B0 135d 96cd 76abc 93bd 58a 65ab Table 4.4 Geometric means of tensile strength (Y), specific rupture energy (E sp ), Young s modulus, and arithmetic means of friability and workability of the air dried natural aggregates, 16 8 mm, 5 15 cm depth. Values in the same column for compaction or timing with similar letters are not significant different (Tukey s test, P < 0.05). Treatment Y (kpa) E SP (J kg 1 ) Young s modulus (MPa) Friability Workability Compaction B1 106b 3.5a 19a 0.47a 0.10a B0 90a 2.9a 16a 0.43a 0.10a Timing A1 123b 3.7a 21b 0.39a 0.08a A2 84a 2.8a 15a 0.57a 0.13a A3 91a 3.0a 16ab 0.38a 0.08a 4.3.2 Remoulded aggregates The overall Y of the remoulded aggregates was higher than the Y of the natural aggregates (p < 0.0001 for both 1 5 cm and 5 15 cm depth). Watts & Dexter (1998) suggested that the lower Y of natural aggregates results from the internal structure of aggregates, which is destroyed in the remoulded aggregates. Surprisingly, the Y of the remoulded aggregates at 1 5 cm depth (Table 4.5) was higher in the control treatments in comparison with the compacted treatments (P = 22

0.006). These results are contrasting with research by Watts & Dexter (1998), who found no significantly different Y of remoulded aggregates of a compacted wheel track and of an arable control soil (heavy swelling clay). The Y was again highest at early sowing (P < 0.0001). E SP was, as Y, higher in the control treatments compared to the compacted treatments (P = 0.021). Moreover, E SP was significant different for each of the sowing times (P < 0.0001). The Young s modulus showed similar trend to Y with regards to the sowing times (P < 0.0001), but was also affected by the interaction of sowing time and compaction (P = 0.033). These results can be interpreted as the aggregates of the soils at early sowing being stiffer compared to the aggregates of the soils at optimal and late sowing. Both friability and workability of the aggregates were not significantly different between the compacted and control soil, nor between early, optimal, and late sowing. At the depth of 5 15 cm (Table 4.6), the Y was significantly higher in the compacted treatments compared to the control treatments (P < 0.0001), and higher at early sowing compared to optimal and late sowing (P < 0.0001). E SP was also significant higher in the compacted treatments (P = 0.001). Regarding sowing time E SP was significant different for all of them (P < 0.0001): highest at early sowing and lowest at late sowing. Moreover, there was an effect of the interaction of sowing time and compaction on E SP (P = 0.007). This implied that the compaction treatment at optimal sowing more energy required to break the aggregates compared to the control treatment at corresponding sowing time. The Young s modulus had the same trend as the Y, however with lower significant differences (P = 0.002 for compaction, and P = 0.006 for sowing time). Again, there was no significant difference in friability and workability. Table 4.5 Geometric means of tensile strength (Y), specific rupture energy (E sp ), Young s modulus, and arithmetic means of friability and workability of the remoulded aggregates, 1 5 cm depth. Value in the same column for compaction, timing, or interaction with similar letters are not significant different (Tukey s test, P < 0.05). Treatment Y (kpa) E SP (J kg 1 ) Young s modulus (MPa) Friability Workability Compaction B1 289a 4.8a 77a 0.10a 0.02a B0 306b 5.2b 84a 0.09a 0.02a Timing A1 341b 6.0c 94b 0.09a 0.01a A2 286a 4.9b 77a 0.10a 0.02a A3 270a 4.3a 72a 0.10a 0.02a Interaction A1:B1 A1:B0 A2:B1 A2:B0 A3:B1 A3:B0 87bc 100c 80ab 74ab 66a 79ab 23

Table 4.6 Geometric means of tensile strength (Y), specific rupture energy (E sp ), Young s modulus, and arithmetic means of friability and workability of the remoulded aggregates, 5 15 cm depth. Value in the same column for compaction, timing, or interaction with similar letters are not significant different (Tukey s test, P < 0.05). Treatment Y (kpa) E SP (J kg 1 ) Young s modulus (MPa) Friability Workability Compaction B1 341b 5.5b 101b 0.09a 0.02a B0 307a 5.0a 91a 0.09a 0.02a Timing A1 342b 5.7b 97ab 0.08a 0.01a A2 319a 5.6b 89a 0.09a 0.02a A3 310a 4.5a 101b 0.10a 0.02a Interaction A1:B1 A1:B0 6.0cd 5.3bc A2:B1 6.1d A2:B0 5.2ab A3:B1 4.4a A3:B0 4.6a 4.4 Friability index and strength index The friability index was similar within both the compaction and timing treatments in both the depths of 1 5 cm and 5 15 cm, and between similar treatments in both depths (Figure 4.1). According to the classification as proposed by Utomo & Dexter (1981), the soil was classified as friable (friability index 0.10 0.25) or very friable (friability index 0.25 0.40). No significant difference of the friability index in a compaction experiment was also found by Munkholm & Kay (2002). At 5 15 cm depth, the friability index was however nearly Log (Tensile strength, kpa) Log (Tensile strength, kpa) 7 6 5 4 3 2 7 6 5 4 3 A1 A2 A3 Compacted Control 2-28 -27-26 -25-24 -23-22 -21-20 Log (Aggregate volume, m 3 ) -28-27 -26-25 -24-23 -22-21 -2 Log (Aggregate volume, m 3 ) Figure 4.1 Friability index for the sowing time and compaction treatments at 1 5 cm depth (a and b) and 5 15 cm depth (c and d). A1: early; A2: optimal; A3: late, and B1: compaction, and B0: control. significantly higher at optimal sowing in comparison with early sowing (P = 0.058). This indicated that at optimal sowing large aggregates tended to fragment easier in comparison with small aggregates, whereas at early sowing the strength of the different size fractions differed less. (c) (a) Friability index A1= 0.23 A2=0.30 A3=0.27 Friability index A1= 0.21 A2= 0.29 A3=0.26 (b) (d) Friability index Compacted=0.27 Control=0.29 Friability index Compacted=0.26 Control=0.25 24

A small effect on the strength index (SIn) was found within the different treatments (Table 4.7), indicating that the different treatments had nearly similar effect on the structural conditions for a suitable seedbed. This relates well to the aggregates friability, which represented the ease of a soil to crumble into the desired aggregate size distribution. However, at 1 5 cm depth, the SIn was significant higher at late sowing (P < 0.0001). This indicated the greatest difference in Y between natural 16 8 mm and remoulded aggregates, i.e. the soil structure was most developed at late sowing. The SIn was not significantly different in the compaction treatments, but it was for the interaction between compaction and sowing time (p = 0.002). The compacted soil at early sowing had the lowest SIn, implying the poorest structural conditions and seedbed quality (Watts and Dexter, 1998). Yet, this SIn was not significantly different from the control soil at optimal sowing. As to the depth of 5 15 cm, the only significant difference was a lower SIn at early sowing (P = 0.009). Table 4.7 Soil strength index (SIn) at 1 5 and 5 15 cm depth. Values with different letters for compaction, timing, or interaction are significantly different (Tukey s test, P < 0.05). Depth (cm) Treatment SIn 1 5 Compaction B1 0.66a B0 0.69a Timing A1 0.61a A2 0.66a A3 0.75b Interaction A1:B1 0.53a A1:B0 0.69b A2:B1 0.69b A2:B0 0.63ab A3:B1 0.75b A3:B0 0.74b 5 15 Compaction B1 0.65a B0 0.67a Timing A1 0.61a A2 0.69b A3 0.68b 25

4.4 Soil fragmentation There was no effect of the interaction of the sowing time and compaction treatments on the results of the soil drop test of the subsurface layer (5 15 cm depth), whether drained to 100 hpa or 300 hpa (Table 4.8). At matric potential 100 hpa, the geometric mean diameter (GMD, mm) was higher in the compacted treatments (P = 0.004), and at early sowing (P = 0.007). The proportions of aggregates < 8 mm and > 32 were not affected by compaction. Regarding sowing time, early sowing had a significantly smaller proportion of aggregates < 8 mm (P = 0.018), and larger proportion aggregates > 32 mm (P = 0.012) than optimal and late sowing. As for matric potential 300 hpa, the compacted soils had significant higher GMD (P = 0.023), smaller proportion of aggregates < 8 mm (P = 0.047) and larger proportion of aggregates > 32 mm (P = 0.017) than the control soils. More larger aggregates, accompanied by reduced pore volume, after compaction on a wet soil (typical Haplaquept, Ås) were also found by Bakken et al. (1987). In dry conditions, they found no such effect. Regarding sowing time, the GMD was again highest at early sowing (0.015). The aggregate proportioning was significantly different between early and late sowing (0.023 and 0.047 for < 8 mm and > 32 mm respectively). Small aggregates form a finer seedbed and improve crop production. Research by Håkansson et al. (2002) showed an increase of 5% of the number of plants and yield on a fine seedbed (dominated by fragments smaller than 5 mm) when compared to a coarse seedbed (silty soil, Sweden). In the current study, the seedbed quality is thus favoured at the control treatments, and at optimal or late sowing. The subsequent poorest seedbed quality was at early sowing which is in line with the results on SIn. Table 4.8: Aggregate size distribution at 5 15 cm depth of the soil at 100 and 300 hpa matric potentials. GMD: geometric mean diameter; proportion in g/g. Values in the same column for compaction or timing with the same letter are not significant different (Tukey s test, P < 0.05). Matric potential Treatment GMD (mm) 100 hpa Compaction Proportion of aggregates < 8mm B1 25.7b 0.17a 0.55a B0 19.2a 0.22a 0.42a Timing 300 hpa Compaction A1 29.7b 0.11a 0.66b A2 19.8a 0.22ab 0.44ab A3 17.9a 0.25b 0.36a B1 30.8b 0.21b 0.70a B0 21.5a 0.12a 0.46b Timing A1 34.8b 0.08a 0.77b A2 21.0a 0.20b 0.47a A3 22.7a 0.21b 0.49a Proportion of aggregates > 32 mm 26