# Aggregate and Particle Size Distribution of the Soil Sediment
## 1 Introduction  
Soil erosion by water is commonly mentioned as a worldwide environmental problem. During a rainfall, the soil surface is disturbed by rain drops and surface runoff, the soil particles get mobilized and washed out of the surface [1]. The particle and aggregate size distribution of the eroded soil sediment could give us information about the erosion process and formation of the rills [2]. Topsoil is a non-renewable resource and a valuable asset of every country. Water erosion has very negative effect on the productivity of agricultural land and also on the service life of soil structure. It removes the most fertile topsoil, reduces the thickness of the soil profile and decreases the content of nutrients and soil organic matter. It causes significant degradation of agricultural soils and reduces the quality of surface water in which the eroded sediment is transported [3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Therefore, the soil must be protected as much as possible from degradation and the negative effects of water erosion must be reduced. The processes of water erosion must be thoroughly researched and understood in order to protect soil and reduce soil erosion effectively. The particle size distribution of the soil is given by the representation of individual mineral particles of different sizes. The soil is divided into fine soil and skeleton. Fine soil is formed by particles smaller than 2000 \(\upmu\)m and affects the basic soil properties [12]. There are many classification systems for classifying soils according to their particle size. In the Czech Republic, [PERSON]'s classification system is used which divides soils into seven soil categories, from sandy soils to the finest soils marked as clay, according to the representation of fine particles smaller than 10 \(\upmu\)m [13]. The classification system USDANRCS classifies soils using a triangular diagram for which the representation of the three basic fractions must be known: sand (50 \(\upmu\)m-2000 \(\upmu\)m), silt (2 \(\upmu\)m-50 \(\upmu\)m), and clay (smaller than 2 \(\upmu\)m) [14]. Furthermore, ASTM Unified Soil Classification System (USCS) can be applied. The combination of two letters to classify the soil is used, the first letter represents PSD (gravel, sand, silt, clay, organic) and the second letter represents the texture (poorly graded, well-graded, high plasticity, low plasticity) [15]. The soil aggregates are formed by cementing soil particles with organic matter and other cementing agents, thus increasing the stability of the soil structure, which makes the soil more resistant to the erosion. The aggregate's formation is influenced by many factors, like swelling pressure, capillary action, and biological and chemical cementing agents [16]. The organic matter increases the soil's ability to retain water [17]. Pores formed between the aggregates may contain air or may be filled with a loose material and allow water to infiltrate through the soil. The infiltration of water through the soil aggregates is influenced by their spatial arrangement. The destruction of soil aggregates by rainfall leads to soil degradation and increased soil erosion [18]. The aggregate size distribution can be determined by using a laser diffractometer [19; 20]. Recently, the popularity of laser diffraction for analyzing particle size distribution has increased. This method is faster than sedimentation methods, thus it is possible to analyze greater number of samples within the same time [21; 22]. In previous studies comparing particle size analysis methods based on different physical principles, it was found that the usability of the methods to obtain particle size distribution differ in the type of soil that is measured, mainly in the particle shape characteristics [23; 24; 25; 26; 27; 28].