To begin with, we can quickly go through the three letters that make up NPK .
N is for nitrogen. Nitrogen is an element with chemical symbol N and atomic number 7. Nitrogen means from the Latin and Greek words to form! Nitrogen is your building blocks for your plants. The grass family is extremely fond of nitrogen as nitrogen gives vigorous growth to green plants, so it is not without reason that you use heavy nitrogen fertilizer for lawns and bushes. But when it comes to tomatoes, large green plants are not as convenient. If you use too much nitrogen on a tomato, flowering and fruiting will be affected, and the number of tomatoes you get in one season will be greatly reduced.
P is for phosphorus. Phosphorus is also an element with chemical symbol P and atomic number 15 and comes in many forms and varieties. Phosphorus is a key ingredient for the establishment of roots and promotes flowering in your tomatoes. If your phosphorus source is natural, for example from guano, it will also promote in your soil.
K is for potassium and is also an element with the letter K and has atomic number 19. The word potassium comes from Arabic al-qali which means from ash. Potassium is needed by your tomatoes when they have flowered and set tomatoes and helps to ripen the tomatoes and make fruits, berries and vegetables sweeter. Excessive use of the same with nitrogen can cause severe leaf blight.
It is not certain that you became smarter yet, but this is quite simple. If you buy potting soil in bags, it has been added a form of fertiliser, chemical or organic. Based on the average of what they contain, we can say that the nutrition you add to such soil should be in the 5-10-5 range. It is also important to consider that if the soil is organic and contains organic nutrients, this will release nutrients more slowly and over a longer period of time as the decomposition begins. If, for example, you had clean soil with no nutrients added, the npk would look different and something like 10-15-10.
So when you are now standing there mixing the soil and putting your tomatoes where they will be for the season, it is important to think about certain things. Hold back on nitrogen, you don't want lush, big tomato plants with very few tomatoes and think back to previous seasons. Did I get enough tomatoes? How close were the tomato clusters? If you yourself think that you did not get enough or that there were many branches between each tomato cluster, you must consider adding more phosphorus. The second, and perhaps most important for everyone with a busy everyday life, is how to do this easily and most efficiently.
Method 1: You do as instructed above and you plant the tomatoes in a 5-10L pot. When the tomatoes start to bloom, you need to start adding more nutrients. This means either watering in liquid nutrients, or diluting solid nutrients in water and watering with this. This is a process that you need to do every two to every week, depending on how many tomatoes you have and how big your plants are. This is a process you do throughout the season until the season ends and the plants have to be cut down or the frost takes them. Another thing is that tomatoes drink an incredible amount of water. With pots this size, prepare to water every day or even twice a day in the hot summer. Possibly install a drip water system.
Method 2: You mix (See retraining-part-3-tomato-chili) the soil as directed in this post and pot your tomatoes in larger pots. 15-30L. When you have mixed the soil as you found in the link, you do not need to do much more than watering. But now you have one large volume, the soil will not dry out between waterings and you do not need to water as hard or as often. Two advantages to it. One is that you don't flush the soil, the other is that you can spend more time tending to the plants and observing without it going beyond your time that you have set aside for other things. Many people have probably experienced that especially bagged soil with a lot of peat can be difficult to get moist again if it has dried up properly. When you water, the water flows straight through. The other advantage is that you don't need to add more fertiliser! You have long-acting fertilizer in the soil, micro-life in the soil, organic material that breaks down and becomes nutrition, you have symbiosis between the plant's root structure and the soil around it. But it is wise to add supplements. For example, 1 tablespoon of epsom salt every week that you put on top of the soil and water in, and once in May you run a round of compost tea.