There are many ingredients that must be present in living cells. Some of these are: C - carbon, S -sulfur, N - nitrogen, etc. It is more convenient to deal with compounds that supply these elements. Commonly, we refer to the carbohydrate component such as glucose as the growth limitation for carbon although some carbon may be derived from a compound that contains carbon in addition to another element such as nitrogen. With nutrients that are defined, it is fairly easily to deal with one in low proportion to the others as the growth-limiting nutrient. With complex nutrients such as soybean meal or distillers solubles, it is not straightforward to decide where the carbon and nitrogen are coming from and to state the growth-limiting nutrient.
One way to identify what is growth limiting is to reduce concentrations of the various nutrients systematically one at a time. When this produces little or no effect, the nutrient that is being tested is probably in excess. If there is a definite effect, the nutrient is very likely the "growth-limiting nutrient".