If you are looking for the most optimal results when planting tomato seedlings, you need to figure out how often to fertilize them. The best fertilizer for tomato plants is made up of macronutrients like nitrogen, phosphorous, and potassium, in addition to essential micronutrients like magnesium, calcium, boron, and zinc. This is the optimal combination for optimal plant growth. Tomatoes, on the other hand, have distinct dietary needs at every stage of their life cycle due to the fact that their development proceeds in stages. Tomato fertilization might seem to be a demanding and arduous procedure to someone who is new to gardening, especially if they have never done it before. Whether you are growing plants from seeds or seedlings, you do not want to be ignorant of the subject of feeding the soil since it is one that you do not want to be uninformed of. If you give your tomato plants some guidance, year after year they will provide you with a substantial harvest of tomatoes. But before we go any further, I want to make one thing very clear: There is no one tomato fertilizer that is excellent for all gardens throughout the year and at all times of the year. This is something that I want to make obvious before we go any further. If such a thing existed, gardening would not be nearly as challenging as it now is (and arguably also boring). Let's have a look at some information as well as a few different options for fertilizer so that you can choose the one that is going to be most beneficial to the stage of growth that the plant is now in. 
The quantity of fertilizer that tomato plants need changes as they go through their life cycle, which may be thought of as a spiral. It is imperative that all nutrients be present at all times; however, the ratios of nutrients that are recommended to be present change with each stage of development. It is necessary that all nutrients be present at all times. Because the roots of a tomato plant only grow to a depth of about 6-7 inches on average, it is imperative that you keep this information in mind as you are working fertilizer into the soil surrounding your plant. Regarding this topic, I've composed a comprehensive paper that I've titled "how deep do tomato roots grow." The number sequence that is used on commercial fertilizers, such as 10-8-10, is effectively an abbreviation for the components nitrogen, phosphorous, and potassium. The composition of the fertilizer is denoted by the number 10-8-10, which signifies that it is composed of 10% nitrogen, 8% phosphorous, and 10% potassium, with the remaining 2% being filler material. What specific kinds of fertilizer and nutrients does a tomato plant need in order to thrive? Phosphorous is essential for the development of roots, potassium is beneficial for blooming and overall growth, and nitrogen is beneficial for the development of leaves. Phosphorous is a vital nutrient at both the beginning and the end of a plant's life cycle since it is required for the growth and maturity of the plant's roots as well as its fruit. As a consequence of this need, phosphorous is required throughout the plant's entire life cycle. 
Nitrogen is important for the plant's leaves; nevertheless, an excessive amount of nitrogen may cause the plant to become bushy and yield little or no fruit at all. Nitrogen is responsible for the plant's growth. Potassium not only promotes rapid plant development and the production of flowers and fruit, but it is also an essential component in the process of photosynthesis and contributes to the plant's resistance to a variety of diseases. Potassium also helps the plant to maintain its overall health. When should I start planting the seeds for my tomato plants? Do I need to apply fertilizer? Tomato seeds do not require any additional nutrients to germinate because the seed already contains sufficient amounts to put up the first true leaves. Do the seedlings of my tomato plants need fertilizer when they first start growing? In response to your inquiry, the answer is "yes," they do. Once the tomato seedlings have germinated, they are going to develop extremely quickly, with an initial growth spike occurring just before to the flowering of the plants themselves. They typically start producing fruit around four months after being planted, which is the ordinary rule. If you want to make sure that your tomato plant is able to take in the nutrients that you have put to the soil, you are going to need to make sure that the way you water it encourages the plant to send out roots deeper into the ground. 
The plants are going to increase in height and girth, but they are also going to branch out, which will lead them to become top-heavy. You may wish to offer external support to avoid the plants from drooping and toppling over. The second piece of advice that I have for you is that you should either plant your tomato in the ground or in a container that is rather big. This is due to the fact that putting a plant in a container that is too tiny leads the roots to grow closer together. When a plant's roots are overloaded, it has a difficult time absorbing the nutrients that it needs. When transplanting tomato seedlings, there are two distinct ways that phosphorus may be added to the soil. It is recommended that bone meal and organic fertilizer spikes be used together in order to get the highest levels of phosphorous in the soil; however, it is quite reasonable if you do not like to use bone meal due to the fact that it is an animal product. If you do not like to use bone meal, you can get the same results by using organic fertilizer spikes instead (in which case, measure your soil and add a bit of traditional phosphorous fertilizer).
- Bone Meal
The use of bone meal as a fertilizer is a good strategy for making use of organic resources and, at the same time, ensuring that your tomato plants receive the phosphorus they need to build strong roots. This is because bone meal is an excellent source of phosphorus (which aid the production of fruit). 
The organic bone meal fertilizer made by Jobe's, which has a ratio of 2-14-0, is one of the products that I find very useful. This combination contains calcium, a vital vitamin, which helps prevent tomato blossom end rot, which is a frequent disease that may impact tomato plants. In addition to this, it helps in the strengthening of cell walls, which, in turn, assists in the overall structure of the plant. What exactly should I do with the bone meal that I purchased? It is advised that you check the pH level of the soil before adding bone meal, and if the level is greater than 7, you should focus on lowering it down to a level that is more manageable before adding the fertilizer. If the level is lower than 7, you may proceed with adding the bone meal. It might take up to four months for the slow-release fertilizer bone meal to completely break down in the soil after it has been added. Because of this, it is possible that you will only need one application of one pound of bone meal to ten square feet (one kilogram to two square meters) of soil. Take note that I used the phrase "may" in the previous sentence.
- Fertilizer Spikes
After you have transplanted your plants, you should immediately add spikes of organic fertilizer with the ratio of Jobe's 6-18-6 between three and six inches apart from the main stem of each plant. 
This step is particularly crucial if you are growing in a container since the nutrients in the soil are depleted more rapidly in plants that are grown in containers than in plants that are grown in the ground.