What is the antonym of "optimal" in the following context:
The utilization of organic fertilizer with "non-optimal" formula becomes a stimulant for farmers.
If you want the word to mean less than optimal, i.e. could still be good but isn't the best possible, you could say "suboptimal". If it is the worst possible, you could say "worst". I don't understand the meaning of the sentence well enough to tell which one you would want.
For example, say you're talking about rabbit diets, and there's a choice of hay, lawn clippings, and rocks. Hay is the optimal diet, lawn clippings and rocks are suboptimal diets, and rocks are the worst diet.