If you want to buy, or use, any of the nasty chemicals that are needed to protect your crops, there is a new law which comes into force in November. You will need a permit called a cartão de aplicador
See details at http://snaa.dgav.pt/docs/FORMAÇAÕ.pdf
To obtain this permit you must attend a course (curso) of 36 hours, and will cost you about 70€.
If you are over 65, then a test of competency ( prova de conhecimentos) is needed instead. This will cost 12.50€.
To enrol in a class, or take the test, ask at your local supplier, or junta de freguesia.