# Aggregate and Particle Size Distribution of the Soil Sediment
## 4 Discussion
### Influence of External Factors to the Particle Size Distribution  
Graphical representation of statistical analysis of PSD of all 156 collected samples is shown in Appendix A in Figure 11.  
Table 2 shows comparison of samples with and without aggregates. The volume of particles larger than 50 \(\upmu\)m was almost 50% in soil with aggregates, while it was 10% in the soil without aggregates. The number of particles smaller than 2 \(\upmu\)m was less than 5% in the sediment with aggregates and 18% in the soil without aggregates. This means that the particles before using ultrasound were coarser than those analyzed after its use. This proves that the eroded soil is clustered into the soil aggregates [18]. As the table presents all samples which were collected under different conditions (slope, initial condition), the particle size distribution curves of the samples without aggregates had less SD and were more consist than those with aggregates. If the results with aggregates are divided into each slope separately (Tables 3-5), the SD is much smaller compared to all samples together (for samples without aggregates the difference in SD is not so significant). This is because the PSD with aggregates depends more on the external conditions (slope) than the size of the soil particles that are forming the aggregates. This fact is the motivation for our further research. Influence of initial condition and slopes of the experimental plots was analyzed. [PERSON] et al. [40] found that the sediment washed out during rill erosion is formed by coarser particles compared to the sediment eroded at the beginning of the rain during sheet flow. In our research, we found (Tables 3-5) that samples eroded as a dry variant (D1 and D2) contained more particles in fraction <2 and <10 \(\upmu\)m compared to those eroded as a wet variant (W1 and W2). This could mean that at the beginning of the experiment the sheet flow is present, then the runoff was stabilized and the rill erosion occurred. After ten days without rain, at the beginning of the second part of the experiment (D2), the eroded samples contained more particles in fraction <2 and <10 \(\upmu\)m than the W1 sample, even though the erosion rills were already made. The number of particles smaller than 50 \(\upmu\)m increased over the sampling time. This means that in general, the eroded particles became finer throughout the experiments. Tables 12-13 show the percentage representation of the aggregates calculated as the difference between the median values. The smaller the difference, the less aggregate representation percentage the sample contains. The soil sediment eroded at the beginning of the rain from the steep slopes contained more aggregates compared to the sediment eroded later. However, in the plot with the slope 22\({}^{\circ}\) this trend was the opposite. If the aggregate representation percentage in fraction smaller than 2 \(\upmu\)m is subtracted from the fraction smaller than 10 \(\upmu\)m, the fraction content 2-10 \(\upmu\)m is obtained. Such values had almost the same percentage representation of particles for all three slopes (13.6% to 15.9%), thus it is apparent that the main difference was generated by particles smaller than 2 \(\upmu\)m.  
During the first part of the experiment (when the erosion rills were forming--D1 and W1), the largest percentage representation of aggregates in a sample was eroded from the 30\({}^{\circ}\) slope, while during the second part of the experiment (when the erosion rills were already formed--D2 and W2), the largest percentage representation of aggregates was eroded from the 22\({}^{\circ}\) slope. The least percentage representation of aggregates was eroded from the 34\({}^{\circ}\) slope throughout the experiment. The percentage representation of aggregates increased for the 22\({}^{\circ}\) slope and decreased for the 30\({}^{\circ}\) and 34\({}^{\circ}\) slopes throughout the experiment. This means that for steep slopes, the fill erosion was present and only smaller percentage representation of aggregates was eroded.