Silt is a soil particle size in between the size of clay and sand, probably the best size of particle for growing plants , because as you mention sand drains the soil very fast and is hard to hold nutrients in , clay the opposite wont drain and has too high a cation capacity so locks up too much of the nutrients.
For our blue berries (350 plants) before planting we added gypsum to the rows where the strips where the Blue berries were to be planted , we then skimmed the top soil (which contains organic matter reason it is black) from between the 1.6 meter wide rows making a hollow and put it on the 1 meter wide strips where the blue berries are planted making a hump , effectively increasing the organic matter there by 500% and also creating a raised bed and also made an effective hump and hollow form of drainage with the blue berries on the hump.
8 years on and we are still getting around 500Kg of blue berries produced per year , as the "hump" the berries are on means it is free draining when it rains but also holds the moisture because it is mainly top soil .
If you are having issues with adding organic matter ( ie straw and grass clippings ) and it not retaining moisture , I would give the straw longer time to break down and make an environment for worms to help break the straw down faster , the more the straw breaks down from its hexicellulose structure , the larger the surface area of the organic matter it has is and the higher the cation exchange capacity is to retain moisture and nutrients once introduced back into the garden
