Sandy soils usually give this kind of problem. Sandy soils are also easily impoverished, because of their free draining nature the nutrients just leach out with the water.
In hot weather the first layer of soil will always be dry - water evaporates whatever the soil type, adding manure does not prevent this - even manure will dry out completely in the sun.
Mulching is the only way to prevent evaporationn from the soil surface. The best mulch I have found over the years is grass clippings. If the grass is applied as a thick mulch, it's surface will dry but the soil underneath will remain damp. Because of the texture of grass clippings water does not transfer easily from the soil surface to the mulch.
If you have sandy soil adding generous amounts of humus when tilling will improve the situation.
The proportions of sand, silt and clay have a major influence on soil structure. In sandy soils the particles do not bond together strongly and therefore have little structure, but are free draining. As the clay content of the soil increases from loam to clay loams to clays, the particles become held together more strongly and generally structural strength increases. This bonding is due to the high level of chemical activity exhibited by clay particles.
For production, good soil structure relies on the formation of small soil aggregates which do not break down when wetted, with good pore spaces between the aggregates. In the right conditions, soil particles will cluster together and become stabilized by organic matter, fungal hyphea and polysaccharides produced from root, fungi and bacterium exudants and organo-metallic complexes. In the wrong conditions, aggregates will not form, or will break apart when wetted, reforming into large, dense and impermeable clods with few pore spaces.
Therefore, soils with poor structure will probably be low in clay or organic matter and high in minerals, such as sodium, that reduce the ability of clay particles to bind together. Management practices that break down organic matter cause soil structural degradation. This is indicated by hard setting soil surfaces and crusting in tilled soil.
It should be remembered that organic matter not only helps to hold particles together, but it also helps hold them apart, creating pore spaces.
I hope that helps. There's lots more information on my web-site at http://aberaeronallotments.org