Agriculture is a trillion-dollar industry and employs about a billion people.
To produce enough food for a growing population, farmers and innovators found new ways to increase yields.
The most well-known innovation is fertilizer, and it has been around for centuries.
However, historically fertilizer has come from compost, manure, or other byproducts.
Today we have isolated the three most important macronutrients Nitrogen, Phosphorus, and Potassium, which together is called NPK fertilizer.


In this article, I will show you how Nitrogen, Phosphorus, and Potassium affect plant growth and their protein contents, by using different concentrations of fertilizer on radish plants.
Seeing the effects on plant growth, color, and general visual differences is easy with the naked eye.
However, there is no easy way to see protein contents, so I will perform a Biuret Test using copper sulfate and sodium hydroxide.

Background Info (Theory):

Nitrogen, Phosphorus, and Potassium are all very important for a plant, but they all contribute to the plant in different ways.
Nitrogen is responsible for generating green pigments, the growth of leaves and the stem, and protein/enzyme production.
Phosphorus is responsible for encouraging water intake, and therefore also nutrient intake, and the fruits and flowers of a plant.
Potassium is responsible for root growth, strength, and spread/reach and it is a critical component of photosynthesis.
In a Biuret test, Copper sulfate forms a complex with two peptides with four Nitrogen lone pairs.
The Biuret reagent solution contains that copper sulfate, but also sodium hydroxide, which stabilizes the copper ions in the solution.
Copper sulfate is naturally blue, but the copper-peptide complex produces a deep purple color, which can be used to detect protein contents, and the darker the purple, the more protein is present, and many tables of approximate protein content values can be found online.

The Procedure:

Step 1 (Let’s Plant):

The first thing to do is to plant the different radishes in separate pots.
I had 4 different concentrations:

  • Concentration: 0, as control/organic
  • Concentration: 1:500, as the too little compared to the recommended amount
  • Concentration: 1:200, as the recommended amount
  • Concentration: 1:50, as the too much compared to the recommended amount

I let these plants stay in my backyard for 1.5 months, where I would add the specific water-to-fertilizer ratio to each pot and I would add 50mL.

Step 2 (Visual Differences):

After 1.5 months, I took pictures of the 4 pots and compared the heights, leaf sizes, and color.

As you can see the more fertilizer you give to the plants the more they grow.
In all of the pots with added fertilizer, you can see a darker green than in the control.
All of these results are expected, however for the “1:500 too much” plant, the leaves grew more than the stem could support and the leaves aren’t as strong as the recommended concentration plant.
This makes sense, as this shows why there is a recommended amount/limit and why farmers don’t over-fertilize their crops.

Step 3 (Protein Gains):

Aside from the visual differences, the composition is also important.
We can expect that the fiber contents and glucose amounts are larger in the plants with fertilizer, because of the effects of NPK fertilizer.
Something that isn’t as easy to see is the protein contents.
To find it you take the radish up from the ground and cut it open.
Then you add a couple of drops of 0.4M of CuSO4 and NaOH to the radish halves and wait to see the color change.
For radishes, the protein contents aren’t high enough to see a difference with this, but this method can easily be applied to other experiments.

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