Why Do We Need Fertilizers?

By 2050, there will be more than 9.7 billion people on the planet [1]. To feed all these people, farmers will need to produce more food than they do today. To ensure that no one goes hungry, we need to come up with farming techniques that both produce lots of food and are safe to the environment. Farmers generally use fertilizers to help their crops grow. Fertilizers provide important nutrients that plants need to thrive. The problem is that the common chemical fertilizers that farmers use consist of large particles, which generally do not enter plants very easily because of their size. As a result, during heavy rains and floods, these fertilizers are washed out of fields and into waterways, like rivers and groundwater. This can be bad for the environment and unhealthy for humans [2]. Good farming methods require fertilizers that are fully used by the plant, with less washout during rains.

Nanoparticles are very tiny particles that are measured in nanometres. A million nanometres is equal to a single millimeter (mm), so nanoparticles are much smaller than the smallest known ant, which is about 1 mm long (Figure 1). Nanoparticles have been used in several kinds of industries [3]. For example, nanoparticles can be found in medical test equipment and cleaning solutions like detergents, because they can fight germs. Nanoparticles are also used in agriculture as nanopesticides to protect crops and as nanofertilizers to help them grow. Because of their tiny size, nanoparticles can enter crop plants more easily than traditional fertilizers can, so they can be used to efficiently deliver nutrients that boost plant growth [3]. When used in this way, they are called nanofertilizers. If they are taken up by plants more easily, nanofertilizers might have less washout, which means these fertilizers could be less harmful for the environment.

However, not all nanoparticles can act as fertilizers as some nanoparticles can be too dangerous for the plant. In most cases, the nanoparticles that are mostly helpful in plants are those that are known nutrients such as zinc, iron and others. Some nanoparticles such as silver and titanium are used to clean contaminated water and can also be incorporated in membranes or filters to allow the water to pass through for purification.

The use of nanoparticles as nanofertilizers is very helpful however, there is another factor farmers must consider in their choice of fertilizers—microorganisms that live in the soil, such as bacteria. Soil contains bacteria that are both good and bad for crops. Bad bacteria can delay or stop plant growth, while good bacteria can help plants grow by making nutrients available to them. When nanofertilizers are used, they might affect the growth of bacteria in the soil [4]. We did not know whether nanoparticles would inhibit or improve the growth of soil bacteria, so we decided to do an experiment to test this.

How Do Nanoparticles Affect Soil Bacteria?

First, we studied the effect of nanoparticles made of a substance called iron oxide on a type of bacteria called Bacillus subtilis. Bacillus subtilis can be grown in the laboratory, so it is easy to do experiments on, but this type of bacteria was also chosen because it is found in soil, where it is good for plant growth. The experiment was done by growing Bacillus subtilis on a culture plate containing nanoparticles along with the nutrients that the bacteria need to grow.

To clearly show whether nanoparticles negatively affect bacteria, we needed to include a control in our experiment—something that was known to have an effect on bacteria, so that we would have something to compare the effect of nanoparticles to. Antibiotics are medicines doctors use to kill bacteria, so we placed small discs of paper soaked in antibiotics on our control culture plate. In Figure 2A, you can see the clear zones around the antibiotic discs where the bacteria cannot grow. No clear zones (cloudy surface) around the discs mean bacteria can still grow, and this is what we saw in the presence of our iron oxide nanoparticles (Figure 2B), meaning that these nanoparticles did not hinder the growth of Bacillus subtilis.

How Do Nanoparticles Affect Seeds?

We also wanted to test whether seeds could still germinate in the presence of the iron oxide nanoparticles. This was done to prove whether the nanoparticles can also act as a fertilizer. To test this, we placed carrot seeds on damp filter paper in culture dishes, either with or without nanoparticles. As our control, we did the experiment with water alone and with no addition of the nanoparticles, as shown in Figure 3A. We found that the nanoparticles in Figure 3B were able to improve germination of carrot seeds compared to the control test without nanoparticles in Figure 3A. The length of the overall plant when nanoparticles were used increased to 82 mm, while the length of the plant when water with no nanoparticles was used increased to 27 mm. Both seeds in Figures 3A, B were germinated for 12 days. This means that even though plants can still grow using water alone for germination, adding nanoparticles for seed germination of carrot can increase the growth of the plant 3 times in comparison to using water alone.

What Can We Learn From These Results?

To feed the increasing number of people in the world, it is important to test new ways of promoting crop growth that are safe for the environment. We tested nanoparticles to understand whether they can help carrot seeds germinate and grow faster. We also looked at the effect of nanoparticles on a helpful type of soil bacteria, Bacillus subtilis. This was done because we need to know if nanoparticles are used as fertilizers, will they affect the growth of helpful microorganisms in soil. We discovered that small amounts of nanoparticles can help carrot seeds to germinate faster, and that the nanoparticles did not harm the bacteria. These results show that it might be safe to use low amounts of nanoparticles as nanofertilizers to promote plant growth.

Glossary

Fertilizer: ↑ A natural or factory-made product containing chemical elements such as nitrogen, phosphorus and potassium, which act as food for plants and help them grow.

Nanoparticles: ↑ Tiny structures that have a size range between 1 and 100 nanometres. One nanometre is 1/1,000,000 of a millimeter.

Nanofertilizer: ↑ A fertilizer formulated with nanoparticles to help the plant grow

Bacteria: ↑ Living organisms that are found everywhere on the planet but cannot be seen with the naked eye, only through a microscope.

Control: ↑ Part of an experiment that shows the expected result, proving the experiment setup works. It helps scientists compare and confirm their results.

Germination: ↑ The development of a seed to form a plant, also called sprouting.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We acknowledge L’Oréal-UNESCO FWIS Programme and National Research Foundation (NRF); Grant numbers (121924 and 129651). The Agricultural Research Council (ARC) is also acknowledged for the support provided to undertake this research. Opinions expressed and conclusions reached are those of the authors and not necessarily endorsed by L’Oréal-UNESCO, NRF and ARC.