agronom.me
Menu

Agrochemistry

The role and significance of silicon fertilizers for crop productivity

For students

5 min read

AGROCHEMISTRY A

Why silicon is critical for the crop and soil

Silicon is the second most abundant element in the Earth's crust after oxygen, accounting for 29.5% of its mass. In the soil, this element is primarily found in the form of quartz, as well as primary silicates and aluminosilicates. Together, they form from 65 to 95% of the entire inorganic part of soils. However, intensive agricultural production rapidly disrupts this balance, annually removing silicon with the harvest in enormous volumes.

The need for the element depends directly on the biology of the cultivated crop. All plants are divided into two groups based on their ability to accumulate silicon:

  • Silicon-phobic (mainly dicots): contain a minimum of the element — from 0.01 to 0.1% of dry mass.
  • Silicon-philic (the majority of monocots): actively accumulate silicon in the range of 1–10%, and in some cases, up to 20% of dry mass.

The global annual removal of silicon from fields reaches 210–225 million tons. Because of this, the concentration of available monosilicic acid in the arable layer is falling rapidly. The growing deficit destroys the organo-mineral complex of the soil, accelerates the degradation of organic matter, and worsens its mineralogical composition.

Crop Silicon removal with harvest, kg/ha
Potato 50–70
Cereal crops 100–300
Sugar cane up to 700

If the silicon content in rice straw falls below 11%, this indicates silicon deficiency. Under such conditions, the crop sharply reduces productivity and resistance to adverse factors.

How silicon fertilizers work in the field

Plants assimilate silicon in the form of true solutions of monomeric orthosilicic acid or silicate ions. Absorption occurs both through passive diffusion and moisture transport, and via active transport against the electrochemical potential. Inside cells, special enzymes — silicases — are responsible for silicon assimilation, while in rice, cytochrome oxidase participates in this process. The availability of the element in the soil is monitored by the amount of SiO2 extracted by water or an acetate buffer solution of citric acid.

The application of silicate lime fertilizers enriches the soil with colloidal silicic acid. This reduces the activity of sesquioxides harmful to plants and prevents the binding of phosphorus. As a result, the mobility of soil phosphates increases, and the applied phosphorus fertilizers transition into a form easily accessible to plants.

  • Share of silicon in the Earth's crust — 29.5%
  • Share of silica in the inorganic part of soils — 65–95%
  • Global annual silicon removal with harvest — 210–225 million tons
  • Deficit threshold of silicon in rice straw — 11%

The use of silicon fertilizers significantly improves the physical properties of the soil. The resulting polysilicic acids bind soil particles with peculiar "bridges," increasing the water-holding capacity, exchange capacity, and buffering of light soils. Thanks to the coagulation of colloids, water infiltration is improved, which reduces the risks of water erosion. Furthermore, the high adsorption capacity of silicon allows for the retention of other mineral fertilizers in the root zone, preventing their leaching.

The idea of using silicon has a long history. The first field experiment with the annual application of silicon fertilizer was established as early as 1856 in England and continues to this day, confirming a stable yield increase. In Russia, the use of amorphous silicon dioxide was proposed in 1870, and a commercial patent for silicon fertilizer was registered in the USA in 1881.

 Low <105 <11 High Medium 105–130 11–13 Medium High >130 >13 Not effective

The following compounds can be used as silicon fertilizers:

  • sodium metasilicate Na2SiO3 (Na2SiO3∙9H2O, ionohydrate);
  • potassium metasilicate K2SiO3;
  • sodium orthosilicate Na2SiO4;
  • sodium silicofluoride Na2SiF6.

The listed fertilizers provide the greatest effect during pre-sowing seed treatment and foliar top dressing of growing plants.

Method of treatment Concentration of aqueous solution
Pre-sowing treatment of cereal seeds 0.5–1.0%
Foliar top dressing of plants 0.10–0.50%

Silico-calcium compounds are suitable for application to the soil. This method is most effective on soils depleted of mobile silicic acid, which include soils with high acidity.

Despite its low solubility, calcium silicate is capable of exchange reactions in acidic soils. As a result, calcium enters an adsorbed state, causing a decrease in soil acidity, while silicic acid becomes freely active. The positive effect of silicic acid on acidic soils is noticeably manifested only when soil acidity is simultaneously eliminated or reduced. Consequently, the ability of calcium silicate to neutralize soil acidity is a prerequisite for the positive effect of the silicic acid itself.

Silicic acid has a protective effect against the harmful influence on plants of aluminum hydroxide, which forms in the soil as a result of the decay of complex silicates. Its neutralization occurs as a result of the formation of ferric-aluminosilicates, which quickly precipitate due to their negligible solubility.

Read next