I've read a couple answers that have mentioned soil going sour. I've gathered that it's a bad thing, but what does it mean when soil goes sour? What causes it to sour?
Sour and sweet are different words for acid and alkaline, respectively. Going sour is usually a bad thing, but some plants (such as azaleas, blueberries, most conifers, etc) prefer sour soil.
Some people also refer to soil as sour when it lies unused and gets a putrid, or fishy smell, often caused by too much water and rotting organic matter, but can happen in simply unused soil that stays damp.
Over time, soil pH is naturally lowered for a variety of reasons. Rainfall naturally leaches Calcium out of the soil, lowering the pH, and increasing the soil acidity. This is especially true with acid rain, particularly in the Midwest and Northeastern United States, because the acid rain adds Hydrogen to the soil, lowering soil pH, and increasing soil acidity.
Soil acidity is also increased from fertilizer applications. Though it might sound strange, the main ingredients in fertilizer are acidic by nature, and can aid in lowering soil pH.
Even without rain or fertilizer applications, soil acidity naturally increases over time due to the decomposition of organic matter. Because soil is a living environment, it is constantly breaking down organic matter, naturally adding acid to the soil. So, whether you’ve got the most beautiful lawn on the block, or you’ve let your front yard become a field, unless you live in the desert, your soil is becoming more acidic all the time.
Btw, the acid from decomposing organic matter is humic acid, which causes a big drop in pH especially in potting soil, which is often mostly comprised of organic matter. So even if your garden is on a chalk bed, and isn't likely to go sour, the soil in your pots may still drop in pH.
People often talk about compost going sour when you have too much nitrogen and it starts smelling and rotting instead of composting because the bacteria release ammonia. Or, it could be caused by too much water and not enough oxygen so anaerobic bacteria start dominating the process.
The same phrase can apply to soil if it stays wet for a long period of time and the organic matter starts smelling.