It’s only a slight overstatement to say that there are a million things to pay attention to when you’re growing vegetable, grasses, fruit, flowers, or whatever green growing thing that gets you excited enough to head outside and dig your hands into the soil when the weather is right. These variables include temperature, humidity, precipitation, bugs, pollinators, pests, wildlife, weeds, and on and on and on. And even if all that seems like a lot to keep track of, one thing to notice, is that all of these variables happen above ground out in the open where we can observe or experience them. But what about all the things happening underground with our plants? What about the roots and root nodules, and nitrogen fixing bacteria? It’s easy to forget about the importance of what’s going on with your plants underground because all the work (and reward) happens above it, but the elements in your soil, three in particular, have as much to do with the end product as any of those other variables.
The three elements that you need to be concerned about when growing anything are nitrogen (N), phosphorus (P), and potassium (K). Any sort of fertilizer bag in any gardening center will contain a series of three numbers (in a series that looks something like N - P - K) which represents the percent weight of nitrogen, phosphorus and potassium in the bag. These numbers alone aren’t particularly useful unless you know what each of these elements does. When nitrogen is transformed and then absorbed by a plant (via those previously mentioned nitrogen fixing bacteria and root nodules), one of its primary uses is as a special type of protein. In both human bodies and plants, proteins are important for making things bigger and stronger. In humans proteins are what gives each individual cell, and ultimately a person, their shape, and they are the primary component of the muscles that support us. In plants the role of proteins is similar, but one step removed. When absorbed by a plant, nitrogen is used to create enzymes which are proteins which control the functions an organism must perform in order to survive. So in plants, proteins (and by extension nitrogen) aren’t really part of the plant’s structural components, but control the process that creates the structural components. What this means in a practical and visual sense is that nitrogen is directly related to how big and leafy a plant gets. If you want those tomato plants or peas to get big and strong, make sure the soil they are planted in has enough nitrogen, but not too much.
Pictured: These tomato plants definitely have enough nitrogen, but do they have enough phosphorus?
Too much nitrogen, although it will produce big leafy plants, can hinder the flower and fruit producing process, which is where the importance of phosphorus comes in. While nitrogen will get the green parts of your plants going, phosphorus helps the plant produce flowers and/or fruit. Flowering and producing fruit requires a ton of energy, way more than producing green foliage, and plants store energy their energy in the form of ATP. ATP stands for adenosine triphosphate which is a long string of molecules that has three phosphorus atoms at one end of the string. When one part of a plant needs energy, the ATP molecule travels to that part of the plant and the three phosphorus break off from the rest of the string, and it is that breaking release of energy that the plant can use to accomplish a task. So if the soil doesn’t have enough phosphorus it might not be able to make enough ATP for the energy intensive process of producing fruit.
The final element that plants need is potassium, which doesn’t have as specific a role as either of the previous two, but more contributes to a plants general health. Potassium is said to increase the ‘vigor’ of plants, but it might be better thought of as a facilitator. Potassium is a key part of the molecules which control critical plant systems like moving water and ATP around, and making sure the plant has enough enzymes which help facilitate other systems. And in there you can see a sort of symmetry between these three molecules, that each requires the others in order to most effectively help the plant although they are present in different amounts. Generally the amount of each element that vegetables and fruits need follows the order of N - P - K from greatest to least (they need the most nitrogen and least potassium) although this varies depending on soil type or the time of season you choose to supplement the soil.
Pictured: Back to chemistry and biology class you go! The phosphorous molecules are in the "tail" of this diagram
More information, and answers to more specific questions about N - P - K ratios and how/when you may need to supplement your soil can be found at the following two links which were referenced in putting this piece together: