[Max Lobovsky]

I'm not sure how an automatic fertilizer mixer works, but a venturi pump is pretty simple. In its most basic form, have a T-junction of pipes, blow air down the not-dead-end pipe of the T and you have negative pressure on the dead-end pipe.

Because of Bernoulli's principle (faster fluids have lower pressure than slower ones), The speed of the driving air cause's its pressure to drop into negative range. The T junction lets you access this negative pressure.