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Is NPK Good For Tomatoes – Here’s The Facts!

When it comes to tomatoes, novice gardeners may find the task of learning best fertilization practices a bit overwhelming, but over time routines are developed, and providing plants with nutrients becomes a regular gardening task. NPK is a type of fertilizer that combines varying ratios of nitrogen (N), phosphorus (P), and potassium (K). While NPK is sometimes thought of as a chemical fertilizer, several organic options are available. NPK can be beneficial to many common garden plants when used appropriately. So what tomato enthusiasts want to know is if NPK is good for tomatoes?

Yes, organic NPK fertilizers are beneficial to the health, growth, and harvest of tomatoes. Positive NPK ratios vary from plant to plant, so if using NPK for tomato plants, gardeners must know and follow the best ratio specific for tomatoes regarding quantities and where they are at in the tomato’s life cycle.  

Tomatoes are known for being challenging to grow due to their high susceptibility to pests and diseases. Providing tomato plants with the appropriate types and amounts of nutrients is essential in keeping tomatoes healthy through a bountiful harvest. Read on to learn more about NPK and other nutrient sources for tomatoes.

Benefits of NPK For Tomatoes

Nitrogen, phosphorus, and potassium (NPK) are the primary macronutrients for plant development, including tomatoes. These nutrients are the most supportive to overall plant health as well as tomato color and quality at harvest. This combination of nutrients is used by growers everywhere on tomatoes and other garden plants.  

Nitrogen (N)Nitrogen specifically supports the process of photosynthesis and keeps the leaves and vines of the plant vibrant and healthy.  It helps the tomato plant to maintain strength throughout the growing process. 
Phosphorus (P)Phosphorus is supportive of the tomato plant’s root growth as well as flower and fruit production.  Phosphorus also helps plants to store energy that can be used to withstand extreme weather, disease, and pests.  

Potassium (K)Potassium promotes flowering in tomato plants and increases photosynthesis storage. This supports the plant in moving water and nutrients around more fluidly, helping it to access reserve supplies when exposed to less than ideal temperatures or weather conditions.  

Best Timing And Ratio Of NPK For Tomatoes

As mentioned earlier, it’s not enough to know what nutrients tomato plants need; gardeners must also follow each nutrient’s appropriate ratio, quantities, and timing. Tomatoes lacking any of these macronutrients may see slowed growth, browning, spotting or falling off leaves, and lack of flowering. All of these negative symptoms will also affect fruit production and the harvest’s success.  

Tomatoes do not require NPK or other fertilizers during germination, but as seedlings, some gardeners believe a reduced ratio of NPK can be beneficial. Ratio refers to the amount of each nutrient combined with a portion of water used to dilute the mixture. So a 5-5-5 NPK ratio is equal parts of each nutrient and 85% water. This is the NPK ratio that is ideal for tomato seedlings.  

As plants are maturing and pre-flowering, sticking with a low ratio, around 5-5-5, is still ideal so as not to overwhelm the plant with too many macronutrients in early growth. Overfertilizing can also cause negative effects on the plant, similar to nutrient deficiencies.

Once tomato plants begin to flower, a fertilizer with a ratio providing more phosphorus and potassium will support increased flowering and fruit production. One known successful ratio for flowering tomatoes is 10-10-10 with 70% water. However, there are some varying ratios that seasoned gardeners use and will argue work just as well at promoting continued plant health.  

Tomato plants producing fruit can continue with the same ratio as flowering to help extend a healthy harvest. Once the fruit begins to ripen, a reduced ratio of NPK is more appropriate again. In addition, calcium, a supportive micronutrient, is important during the flowering stage to keep the fruit strong and resist deficiencies like blossom end rot. Read more about calcium in the next section on micronutrients.  

Beneficial Micronutrients

While macronutrients are crucial to the health of tomato plants, micronutrients also play an essential role in the life of a tomato plant. These nutrients are called micro because plants consume quite a bit less of them than the macronutrients, NPK. 

Calcium Calcium supports the plant’s root and leaf growth as well as helping it to produce solid quality tomatoes.  Common household items such as egg shells and tums can work as calcium supplements to your soil. A calcium deficiency can severely affect a tomato harvest with blossom end rot.  
MagnesiumMagnesium supports the health of the tomato plant’s leaves and vines.  A plant with healthy levels of magnesium will be vibrant in color and produce high quality tomatoes.  
Boron & ZincBoron and Zinc support the tomato growth, flowering and fruit production.  In addition these micronutrients help tomato fruits to ripen evenly, resulting in a stronger, healthier harvest.  

Effect of Nutrients On Differing Tomato Types

There are thousands of tomato varieties and they all fall into one of two category types:  determinate and indeterminate.  Determinate tomato varieties are less viney and grow more like a tomato bush. They are typically not as tall as indeterminate tomatoes. Indeterminate tomato varieties tend to spread out more with long vines that grow horizontally and vertically, requiring more staking or trellis.  


Determinate tomato varieties have very clear growing and fruiting stages, making it easy to identify these tomato type’s phase of growth and provide it with appropriate nutrients based on this information. Many tomato fertilizers contain a healthy balance of macro and micronutrients and are specific to the growing or fruiting phases.  


Because of their vinier structure, indeterminate tomatoes’ growing and fruiting phases overlap as the plants become more mature, which means that a tomato fertilizer specific to a tomato’s phase of life may not always be appropriate with indeterminate varieties. For indeterminate varieties, choosing a tomato fertilizer that you can use consistently throughout the tomato’s adult life is ideal.  

Final Tips For Fertilizing Tomatoes

One of the most important and final tomato fertilization tips I have for you is to know what nutrients already exist in your soil before selecting a fertilizer. For example, suppose your soil has high levels of nitrogen. In that case, you may choose to adjust the ratio within an NPK mixture or purchase a fertilizer with lower nitrogen levels than what is recommended for your tomato’s growing phase.

Soil tests can be purchased at most garden centers and online and will provide the most precise picture of what macro and micronutrients exist in your soil. Using this information to guide fertilizing decisions for your tomato crop will result in healthier plants and a more abundant harvest.  

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