There are several devices available that allow you to feed your plants at the same time you are watering your plants. These "injectors" either siphon dilute liquid fertilzer from a pail into the water hoses or slowly dissolve pelleted fertilizer as the water passes over the pellet on its way to the garden. Either approach provides a very, very dilute level of fertilizer to your plants every time you water. Research indicates that plants will respond better to a steady source of very dilute fertilizer than they do to large inundations of feeding normally occuring in the home landscape.
Various degrees of automatic watering can be achieved with simple timers to complex multi-station solid-state controllers (which may need an electrian to install).
Timers - There a many mechanical timers on the market that work just the same way the cooking timer for the stove works. You turn it on to a preset number of minutes and the watering system runs for that time and turns itself off automatically. They are inexpensive and easy to use. You can turn on your watering system and go shopping knowing that the system will turn itself off. Beware, however, because some of the mechanical timers depend on a certain minimum water flow and will not perform properly with drip irrigation systems -- be sure before you buy.
Computers - With the cost of computer chips coming down, there are now a number of relatively intexpensive computer devices that allow you to program your watering activities so that the system goes on and off on preset days of the week. It will water your landscape all summer long without you ever having to touch it. The devices in the $50 to $80 range control one watering section but allow up to six different watering programs each week. The more expensive devices ($150 +) allow you to have varying programs on up to six different sections of your property. You can have a different watering program for your lawn than you do for your vegetable garden.
There are now sensors on the market that will monitor the moisture level in the soil, at various depths, so that the watering system only is turned on when the actual moisture level in the soil demands it. These devices can accompany a computerized controller that does the turning on and off of the watering system. Such a device allows you to have a watering system that works only when it doesn't rain for a few days.
There are also devices available that stand alone and are useful in measuring the moisture condition of the soil. These devices tend to be expensive and are used primarily by research scientists.