Proper phototransistor for detection of small object
given only 1/3 of the light will be blocked, you need a linear light detector.

Notice "linear".

You'll need to operate the photodetector into moderate-size resistor, or into a TIA.

The light intensity should be adjustable, so you can learn about what provides "linear" behavior.

Borrow a scope, and watch the voltage across the resistor as you drop the seeds. If you don't see about 30% drop in amplitude, you are overloading the detector with photons.