Effect of Added Nutrients on Photosynthesis

Eutrophication – When there is an increase in the rate of supply of organic matter in an ecosystem usually caused by runoff of nutrients from land and food industries. It causes a dense growth of plant and algae life, decreasing the oxygen supply leading to death of animals; the phenomenon being called Eutrophication.

The two main nutrients which it’s caused by are nitrogen and phosphates.

Role of nitrogen -Nitrates are a source of inorganic nitrogen, the most essential element and nutrient for strong plant growth. In the form of nitrates, nitrogen is quickly released from a fertilizer and taken up by plants in a form they can directly use.

Nitrogen is responsible for strong vegetative growth of stems, leaves and shoots. As the main constituent of chlorophyll, which is directly responsible for photosynthesis, nitrogen promotes the production of food used by the plant.

Role of phosphates – Phosphorus is a root activator and is quiet essential for plant growth. Though its excess presence will not do the plant any good. In plants, phosphate stimulates root growth and helps prevent disease.

By the above we know that if these two nutrients were added to any plant or algae they would should significant growth within a given period of time.


To investigate the effect of added nutrients (nitrates and phosphates) of different concentrations on the growth of photosynthesizing duckweed by counting the increase in the number of leaves over the period of 15 days.

Variables and methods to control variables:

Independent variable – Nitrates and phosphates concentration

N1 (1g of nitrate)

N2 (5g of nitrate)

P1 (1g of phosphate)

P2 (5g of phosphate)

N1P1 (1g of nitrate and 1g of phosphate)

N2P1 (5g of nitrate and 1g of phosphate)

N1P2 (1g of nitrate and 5g of phosphate)

N2P2 (5g of nitrate and 5g of phosphate)

Control (no nutrients added)

I used an electrical balance to measure the mass of the nutrients added.

Dependant variable –

The number of leaves (the growth in number of leaves after the nutrients are added from the initial number of leaves)

Control variables –

Sample size/5 trials – all concentrations need to be repeated the same amount of times (5) in order to get an analytical data.

The glass beakers used (500ml) – the area of light passing through each beaker should be the same so that the rate of growth in number of leaves is uniform. So I used the similar 500 ml glass beakers.

Volume of water taken in each beaker (250ml) – so that the growth in all leaves is not affected by the change in volume of water.

Type of water used in all beakers (distilled water) – I preferred to use distilled water as it doesn’t contain any previously acquired nutrients which might affect the result. I used distilled water from the same can so that not even minute differences would be present in the nature of water.

The number of duckweed leaves added initially (20) – as the concentration of nutrients increases the number of leaves will obviously increase but to see the difference one needs to keep in mind the number of leaves added initially, so kept same in all beakers.

The amount of sunlight received – all placed in the same line where they receive equal amount of light so that the rate of photosynthesis is maintained in all the beakers and it will not affect the result.

Time for the leaves to grow – for the leaves to show significant difference in numbers it has to be left as it is for a while in all the beakers as the highest concentrated beaker might take 1 week to show a particular number of leaves whereas the lowest concentrated beaker may take 1 month to show the same growth. Hence for data to be valid and reliable all the beakers should be left as it is for the same time (15 days).

Selection of plants – the plant which is selected for the experiment will have the same size of the root and this will not affect the absorption of nutrients and food production of the leaves.

Hypothesis – I expect the beakers which have more nutrients to show more growth of leaves than the beakers which have only one or lowest amount of nutrients. But the beaker which has the maximum amount of both nutrients will show the most growth of leaves in number. I predict

The control variable (which has not nutrients) will show no growth and the plant will die.

P1 and P2 to have the least number of leaves to increase (as it only stimulates root growth not leaves);

N1 and N2 to have slightly more number of leaves;

N1P1 and N1P2 to again have slightly more number of leaves than N2 and P2; (N1P2 is also included here as the amount of nitrate – which helps in food production – is same whereas phosphate only stimulates the roots growth and not leaves).

N2P1 will show more number of leaves than others mentioned above;

Lastly N2P2 will show the same amount of growth as N2P1 (nitrate amounts being same in both) or if not slightly more but I don’t think it will show a vast difference in the number of leaves as the phosphates presence doesn’t make a lot of difference.

Materials used:

45 glass beakers – 500 ml

Distilled water (250ml in each beaker)

Nitrates (18g for each trial)

Phosphates (18g for each trial)

Duckweed plants (Lemma) (20 leaves in each beaker)

Electrical balance


I took 9 beakers and fill all with 250ml of distilled water.

I then took 20 duckweed leaves and immersed it in each.

I then measured both the nutrients (on the electric balance) and put all of them in the respective concentrations (as per indicated in the independent variables) – control, P1, P2, N1P1, N2P1, N1P2, and N2P2.

After this is done I arranged them all in such a way that they all receive the same amount of sunlight.hat increased over the time.

Then I leave them as it is for 15 days and observe the number of leaves

This whole experiment is done 4 more times to get a better average.

I will process the raw data by means of finding out the average and standard deviation.

Also I will draw a graph using Excel sheet. The concentration of the nutrients is plotted on the x – axis and average number of leaves counted is plotted on the y – axis. I will plot it in a line graph as the concentration of the nutrients increases and combines making it more nourishing enabling the plant to grow more. Hence the gradual increase of growth in the number of leaves can be shown through this graph.