Generally, gardeners aim to have 45% mineral, 5% organic matter and 50% pore space in their soil.
The composition of Ultisol in North Carolina, for reference, is approximately 16% pore space, 2% organic matter and 82% mineral.
The use of mulch is widespread in the Piedmont region of the United States as a solution to the high temperatures and saturation of the soil.
The addition of mulch helps to make the soil more porous.
 
Adding manure and/or compost can help to boost the amount of organic material present in the soil which in turn helps to add essential nutrients.
Specifically, the addition of a 2- to 3-inch layer of compost and/or manure should be mixed into the soil to approximately match the depth of a shovel.
 The addition of organic material also helps to improve the drainage of the soil and also helps to decrease the overall weight of the soil.
 
However, microorganisms in the soil consume the same nutrients that plants use to grow so certain nutrients will remain unavailable to plants until the microorganisms completely break down the organic material and release nutrients.
Living organisms within the soil use, and subsequently convert, organic material into usable humus.
To avoid the delay presented by this process, adding manure in the fall is advisable.

Some gardeners who live in areas with large amounts of red clay soil use raised beds to avoid having to amend the soil.
By using raised beds, gardeners avoid having to deal with Ultisol and can effectively make their own soil composition from scratch.
This allows a large degree of freedom in choosing what to plant in a garden.
Generally, gardeners aim to have 45% mineral, 5% organic matter and 50% pore space in their soil. The composition of Ultisol in North Carolina, for reference, is approximately 16% pore space, 2% organic matter and 82% mineral. The use of mulch is widespread in the Piedmont region of the United States as a solution to the high temperatures and saturation of the soil. The addition of mulch helps to make the soil more porous.  
Adding manure and/or compost can help to boost the amount of organic material present in the soil which in turn helps to add essential nutrients. Specifically, the addition of a 2- to 3-inch layer of compost and/or manure should be mixed into the soil to approximately match the depth of a shovel.  The addition of organic material also helps to improve the drainage of the soil and also helps to decrease the overall weight of the soil.  
However, microorganisms in the soil consume the same nutrients that plants use to grow so certain nutrients will remain unavailable to plants until the microorganisms completely break down the organic material and release nutrients. Living organisms within the soil use, and subsequently convert, organic material into usable humus. To avoid the delay presented by this process, adding manure in the fall is advisable. 
Some gardeners who live in areas with large amounts of red clay soil use raised beds to avoid having to amend the soil. By using raised beds, gardeners avoid having to deal with Ultisol and can effectively make their own soil composition from scratch. This allows a large degree of freedom in choosing what to plant in a garden.
Ultisol