what is NPK
Tomato fertilizer isn’t just any old thing you see on the shelves of your local garden center. What’s right for other veggies and plants might not be what you want for growing tomatoes. So here’s a quick discussion on the components of fertilizer (what those numbers mean) and how they relate to tomatoes.
Your average plant fertilizer is primarily made up of three components: nitrogen, phosphorus and potassium (N-P-K). While there can be various micronutrients present, the numbers (like 8-3-2) you see on the fertilizer refer to the relative amounts of nitrogen, phosphorus and potassium (in that order). And that’s what this post will discuss.
Nitrogen is the first number indicating the fertilizer strength, and nitrogen is primarily responsible for foliage — making it strong and healthy. And while the tomato is first growing, that’s exactly what we want — a good strong plant. When just setting out plants into the garden, something along the lines of 10-5-5 is nice — you want the foliage to be plentiful and strong.
Phosphorus is of interest to tomato growers because it’s a necessary nutrient for plenty of blossoms. Without blossoms, you won’t get tomatoes! After the tomato plant has adjusted to being out in the garden and has plenty of new growth, it’s time to increase the ratio of phosphorus. Now you want more phosphorus than nitrogen (example, 5-10-10).
Potassium is used for regulating the water content of a plant; its transpiration. If there isn’t enough potassium available, the plant is more vulnerable to droughts, be they temporary (forgetting to water one day) or longer-lasting. Once the plants are established, something along the lines of 5-10-10 are good.
The Strength of the Numbers
Don’t go thinking the higher the numbers the better — if 5-10-10 is good, then 10-20-20 is great. Not so, as fertilizer that is too strong can burn the roots as they try to develop. I prefer to use a weaker fertilizer with the same relative percentages, and even then, I don’t always use it at full strength. Instead, I prefer to fertilize a little more often; this seems to give me a good balance between feedings and how much the plant can use at a time.
As you’ve probably gathered in my other posts, I prefer to use organic fertilizer wherever possible. In additional to Terracycle and sea kelp, one fertilizer I like to add in is a fish emulsion. Since I can’t seem to find it locally, I get it online and I find that my plants really like it. It’s concentrated, so just a little bit mixed in with some water goes a long way.