If you don't know how to determine the best fertilizer for healthy tomatoes, start with figuring out which nutrients they need the most. Phosphorus is also directly related to a tomato plant's early cell division, so it's a key nutrient for young plants. While you should not overuse phosphorus, there can be consequences to neglecting it in your tomato fertilizer. The best tomato fertilizer — especially for transplants — typically includes higher levels of phosphorus than other nutrients. One of the best tomato growing tips every beginner needs for a successful first harvest is knowing when to use certain fertilizers.