# Aggregate and Particle Size Distribution of the Soil Sediment
## 4 Discussion
### Comparison of Three Tested Methods  
From the mutual comparison of the three methods tested, it is obvious that the results from the hydrometer are overestimated compared to the other two methods and suggest that the number of particles finer than 1 um is almost 40% in all tested soil samples, which corresponds to the PSD analysis of the original soil. The shape of the particle size distribution curves obtained by the PARIO is similar to those from laser diffractometer, while the curves with a linear trend obtained from the hydrometer differ; see Figure 3.  
The representation of particles smaller than 2, 10, and 50 um were compared in Table 1. It was found that the fraction smaller than 2 um obtained by the LD was lower compared to the sedimentation method, and the fraction smaller than 50 um had an opposite result. This confirms the conclusions from the previous studies [21; 23; 27; 28; 29; 30; 31] which state that the laser diffraction underestimates the clay fraction content and overestimates the silt fraction content. Furthermore, it was found that LD and hydrometer gave very similar results for particle size <10 um. Note that this value is a boundary used in the [PERSON]'s classification system [14].  
The time required for the methods tested varies greatly. To determine the PSD using both the hydrometer and the PARIO, the standard sample preparation is required before the measurement to destroy soil aggregates and separate the fine fraction from the sand fraction. This preparation takes about 25 min per sample. In addition, a sieve analysis required for the sand fraction of each sample takes another 10 min. The hydrometer measurement takes 48 h and the PARIO measurement takes 8 h for one sample. However, in 2021 the METER Group developed the innovative device PARIO Plus which can reduce the analysis time to 2.5 h [46]. The PSD analysis by the LD requires no prior preparation of the soil sample except for thorough mixing. The particle size distribution is analyzed for 15 min per sample. Evaluation of the PSD of all 156 samples by LD took 39 h. If the PARIO was used for the particle size analysis of all samples, the total measurement time would be 340 h and the hydrometer evaluation would take 1590 h, i.e., more than 66 days (if five PARIO devices and five hydrometers were used).  
The main advantage of using of the laser diffractometer is that the soil aggregates are destroyed during the measurement by ultrasound which is sufficient and a very time and user-friendly method [20]. Also, the particle size can be analyzed before the aggregates are destroyed. Such results are satisfying thanks to the principles of laser diffraction. On the other hand, the sample preparation for the hydrometer and PARIO measurements is demanding and analysis of the soil samples with aggregates using sedimentations principles (Stokes' law) has physical imperfections.  
A significant advantage of the laser diffractometer's particle size analysis is the amount of sample that is required for evaluation--while the weight of the soil sample for both sedimentation methods must be at least 50 g, for the laser diffractometer, about 1 g of material is sufficient. This advantage is beneficial, especially when the amount of eroded sediment is small and, in some cases, it could be even less than 50 g. On the other hand, the small amount of the material can decrease representativeness of the whole sample during the measurement.