01-21 soybeans

[Image above] Researchers found that fertilizers based on iron oxide nanomaterials may promote better soybean growth than some traditional fertilizers. Credit: United Soybean Board, Flickr (CC BY 2.0)

Just like humans, plants require a well-rounded diet to grow big and strong. And because plants get their nutrients from the soil in which they are grown, farmers are constantly researching ways to ensure the fertility of the soil, for example, by harnessing established techniques such as crop rotation and the ever-evolving science of fertilizers.

Fertilizers are chemical or natural substances that are added to soil to increase its fertility. While most fertilizers contain the three basic plant nutrients—nitrogen, phosphorus, and potassium—some fertilizers also contain certain “micronutrients,” such as zinc and other metals, that are necessary for plant growth.

Iron fertilizers are one type of micronutrient fertilizer that requires more research. To date, iron chelates are commonly used as iron fertilizers. “Chelate” refers to the pincer-like way in which an iron ion is encircled by a larger organic molecule, usually a ligand, which prevents the iron from oxidizing and thus improves its uptake by the plant.

While iron chelates are significantly more effective and efficient than nonchelated iron fertilizer sources, they can have negative effects on the environment, either directly or indirectly. For example, some studies have found that certain iron chelates increase soil pH and can cause phytotoxicity in sensitive species and can cause stress to soil microorganisms.

Developing alternative iron chelates is one way to improve iron fertilization. However, fertilizers based on nanomaterials is another approach that is gaining traction.

The use of nanotechnology in fertilizer development remains relatively low compared with nanomaterials in pharmaceutical research. The research that does exist, however, is encouraging.

Of the studies that have explored iron-based nanomaterials, some published reports found that foliar application of iron-based nanomaterials—which involves applying liquid fertilizer directly to the leaves as opposed to in the soil—can lead to greater plant growth than conventional iron chelates.

“Given these promising findings and the known sensitivity of soybean to [iron] deficiency, future intensive investigation into the mechanisms of action and optimization of efficacy is warranted,” researchers write in a recent paper.

The researchers come from Jiangnan University in China, along with colleagues from the Chinese Research Academy of Environmental Sciences, the Connecticut Agricultural Experiment Station, and the University of Massachusetts Amherst. In their new study, they investigate the effects of variously sized iron oxide nanomaterials (γ-Fe2O3) on the development, gain, and plant nutritional value of soybeans.

The nanomaterials in this study were sorted into three categories: small (4−15 nm), medium (8−30), and large (40−215 nm). Examination of the foliarly treated soybeans revealed that those treated with the small iron oxide nanomaterials exhibited the greatest enhancement on growth, yield, and nutritional quality. In addition, these soybeans outperformed ones treated with a conventional iron fertilizer of an equivalent iron dose.

The researchers attribute the good performance of the small iron oxide-based fertilizer to a modulation of the nitrogen fixation process, or the mechanism by which atmospheric nitrogen is assimilated into organic compounds. They identify three mechanisms in particular that are modulated:

  1. The small iron oxide nanomaterials enhance carbon assimilation in nodules, thus supplying more energy for nitrogen fixation.
  2. The small iron oxide nanomaterials activate nodule antioxidative systems, with subsequent elimination of excess reactive oxygen species.
  3. The small iron oxide nanomaterials upregulate synthesis of cytokinin and downregulate synthesis of ethylene and jasmonic acid in the nodule, which promotes nodule development and delays nodule deterioration.

“These findings offer significant insight for understanding the potential of γ-Fe2O3 NMs [nanomaterials] as a high efficiency and sustainable agricultural amendment,” the researchers write.

They conclude by saying future studies should focus on optimizing material chemistry properties and application regimes to maximize sustainable benefit, “particularly under scenarios related to a changing climate (e.g., increased temperature, drought, salinity, light intensity) that will maximize abiotic oxidative stress.”

The paper, published in ACS Nano, is “Foliar application with iron oxide nanomaterials stimulate nitrogen fixation, yield, and nutritional quality of soybean” (DOI: 10.1021/acsnano.1c08977).