Image segmentation for image processing

I have a soil image to do analysis. I need to extract the size(diameter) of each soil particles from the image to determine the type of soil. I have done the segmentation using image filters but it seems like not working. It can't extract minute particles from the image. Please help! Thank you! Below is the sample image.

The below is the code I have used :-

rgbImage = imread('S01-15.tiff');
grayImage = rgb2gray(rgbImage);
se = strel('disk', 1,8);
filteredImage = imbothat(grayImage, se);
binaryImage = filteredImage > 1;
cc = bwconncomp(binaryImage,8);
n= cc.NumObjects;
Area = zeros(n,1);
Diameter = zeros(n,1);
MajorAxis = zeros(n,1);
Sand = zeros(n,1);
Silt = zeros(n,1);
Clay = zeros(n,1);

k = regionprops(cc,'Area','EquivDiameter','MajorAxisLength');
numClayParticles = 0;
numSiltParticles = 0;
numSandParticles = 0;
for m=1:n

      Area(m)      = k(m).Area;
      Diameter(m)  = k(m).EquivDiameter;
      MajorAxis(m) = k(m).MajorAxisLength;

       if      Diameter(m) < 0.0236
        Clay(m)=Diameter(m);
        numClayParticles = numClayParticles + 1;

       elseif  Diameter(m) >=0.0236 && Diameter(m) < 0.5906
        Silt(m)= Diameter(m);
        numSiltParticles = numSiltParticles + 1;

       elseif  Diameter(m) >= 0.5906  
        Sand(m) = Diameter(m);   
        numSandParticles = numSandParticles + 1;

       end
  end