Manure one time? Or minerals several times?

A question frequently asked by vegetable gardeners trying to
determine what is really best is “What’s wrong with just putting 3-
4″ of manure into the soil before planting, and letting that feed the
plants all year?” 

A three inch application of manure would weigh 200-300# per 18″ X
30′ bed.  This would contain between 2 and 6 pounds of actual
nitrogen and similar amounts of phosphate and potash.  This is ten
times the amount the plants need at that time, and there is risk of
burning your plants with excess salinity.  The nitrogen is volatile,
and most of it will return to the air before it can be used.  Also,
Phosphate becomes fixed – or unavailable – as does potash, and so
your garden – if it gets past the initial phase of too much – will
peter out in about July, because it then has too little available
nutrition.  And no-one knows how much of the other 10 nutrients the
manure has in it, so various deficiencies could develop at any time.