Learn Russian: Если почва очень сухая, то овощи плохо растут. - If the soil is very dry, then the vegetables grow poorly.

то acts like 'then' in English conditional sentences. It makes the 'if... then...' connection feel more complete, but it is actually optional. You could perfectly well drop it and say Если почва очень сухая, овощи плохо растут.

Where is the verb 'is' in the first half of the sentence?

In Russian, the present tense of the verb 'to be' (am, is, are) is almost always left out. So почва очень сухая literally translates to 'soil very dry', but it is understood as 'the soil is very dry'.

Why does the verb растут look so different from its dictionary form?

The infinitive is расти (to grow). It has an irregular stem in the present tense: you replace the -сти with -ст- and add stressed endings. For 'they grow' (the vegetables), the ending is -ут, making it растут.

Why does сухая end in -ая while плохо ends in -о?

сухая (dry) is an adjective describing the feminine noun почва (soil), so it takes the feminine nominative ending -ая. плохо (poorly) describes the action растут (grow), so it is an adverb. Adverbs do not have genders or cases, and they typically end in -о.