Determine Site Conditions

Sunlight and soil moisture are the two most important factors to consider when selecting plants for a specific location. If you can manage to match plants with the conditions that they are best suited for, you can drastically reduce your maintenance needs in the long run. This approach is also much more practical and sustainable than trying to amend your soil. 

Sunlight

This might be the single most important factor for successfully growing plants in a landscape. Soil can be watered or amended to accommodate certain plant needs, but none of that will matter if there is too much or not enough sunlight. Light conditions are influenced by the time of day, the season, and proximity to larger objects such as buildings or trees. The page linked here can help you determine your light conditions.

Soil Moisture

Soil moisture will be affected by a variety of factors such as elevation, drainage pathways and soil type. Soil tends to dry out more quickly toward the top of a hill or on a slope while bottomlands tend to retain more moisture. Soil type is also a major factor. Most landscapes in the St. Louis area have heavy clay soil which drains very slowly and stay wet longer whereas the sandier soils west of the city tend to drain quickly and will therefore tend to be drier overall.  Heat is also a factor; urban heat islands can be up to 20 degrees hotter in summer, causing the soil to dry out very quickly, especially next to hot surfaces like unshaded streets and driveways. 

The simplest approach to determining soil moisture is to use your senses. Touch the soil. Stick a finger in if you can at various locations and times. Pay attention to how quickly the soil dries out after a rain in summer compared with how long it stays moist in spring. Another clue is the color of the soil; drier soil is often (but now always) lighter in color while moist will often be darker. 

For more precision, it may be worth investing in a soil moisture probe or soil moisture meter. These are commonly available at hardware stores or garden centers.