Over time, if not given the correct amount of nutrients, many plants will start to turn light-green, then eventually start to yellow if they are malnourished, stressed, or in need of fertilizer.
An easy way to prevent undernourishment is to find or add soil to your garden that is rich in minerals and/or organic material for you plants to grow in such as Sweet Peat, which can be purchased here! Another option is feeding plants an All-Purpose Fertilizer, in which you mix 1 tablespoon into a gallon of water and pour it on the base of the plant down to the roots. Try to feed when it's sunny so the plant can use the feed efficiently.
To keep your plants healthy you should feed them at least once a week, and if you really want bragging rights - feed them a few times in a row (maximum of 3) then, when ready, fresh water thoroughly to wash the 'salts' out of the soil, and repeat! For more on watering techniques click here!
If anyone was wondering - The featured image is a close-up picture of a barrel of fertilizer mixing up :)