Science Buddies: "Ask an Expert"

How much potassium, phosphorus, or nitrogen should I add to a plant? Im doing a science project where I will grow four plants. One with added nitrogen, one with added phosphorus, one with added phosphate, and one will all three. I was thinking I could mix it into the water but idk how much of each element to add or which plant to use. So my question is how much of each should i mix into much much water? Also what plant should I use?

Hi,

The kind of plant you use depends on where you are able to grow them. If you live in a cold climate then I assume you will be growing the plants indoors under lights in a reasonably warm (55-65 degrees F) room. You could sow bean seeds or radishes or turnips or sunflowers--anything that will grow well under your conditions of light and temperature.

When you use a plant fertilizer that has all three nutrients in it, nitrogen (N), phosphorus (P) and potassium (K), the percentages of the N, P and K are given on the bag (e.g., 10-10-10) and it tells you how much fertilizer to use for a certain size of pot. Some fertilizers are water soluble and they will tell you how much to dissolve in the water that you water the plants with.

You are fertilizing with each nutrient individually, however, so how much you use will depend on how much of each nutrient is in the mixture. This should be expressed as a percentage and you will have to calculate how much to use by comparing this to the percentages on a bag of fertilizer that contains all three. A common fertilizer would be 10-10-10, which means that it contains 10% of each of the three nutrients, N, P and K. For example, if your N fertilizer is 30% nitrogen then you would use only one third the amount per pot than what is recommended on the bag for the 10% fertilizer. If your P fertilizer is 50% phosphorus, then you would use only one fifth as much.

If you are sowing the seeds in a potting mix, read the bag carefully to make sure it does not already contain fertilizer. If you want to have only the one nutrient present then you may have to plant the seeds in vermiculite or sand that does not contain nutrients. Remember to do one planting (at least three pots per fertilizer amount) with all three nutrients present.

If you need more help, please post again.

Sybee