Good phonics teaching in the early years should lead to good spelling but I'm afraid this just doesn't happen in some schools where phonics is used to teach reading but spelling is somehow treated as a completely different from reading. In reality spelling and reading are two sides of the same coin in that words are constructed by spelling each of the sounds in them.
Having said that, spelling is more difficult than reading because of the fact that some sounds can be spelled a number of different ways and one spelling may spell more than one sound; in reading, the sound spellings can be seen which obviously makes it easier to try the different sounds associated with that spelling and so arrive at the correct pronunciation of the word. In spelling, the child has to remember just how the sounds are spelled in that particular word. If the school has not integrated the spelling and reading instruction so that children gradually acquire a 'bank' of words spelled with the lettter/sound correspondences they are learning to 'read' then spelling will be more difficult for them. However, what they should will have learned is an awareness of the individual sounds in words, and at least one way of spelling them, so that they are likely to spell words 'phonetically' rather than correctly.
You don't say 'how' your child spells (i.e phonetically? or just wild guesses at letter strings?) so it is not easy to offer any advice until we know.
The only alternative to using phonics for spelling is to learn each and every word as a series of 'letter strings'. This is actually impossible for all but a very few exceptional children. (imagine trying to learn about 30,000 telephone numbers)
Reading helps children to become familiar with the 'look' of discrete words but the idea that extensive reading will enable anyone to be able to mentally visualise every single word they can read when they are required to spell it is very flawed. In most cases, all it will do is enable one to see that a word 'looks' wrong when one has written it down. And of course, if one has never seen the word written down one is completely flummoxed..
Please tell us a little more about the spelling problem 
N.B The idea that 'overreliance on phonics' leads to poor spelling is a myth..most adults actually do use phonics when faced with spelling an unfamiliar word.