Organic matter is widely recognized for its ability to enhance soil fertility, which is why gardeners often add it in substantial quantities to their soil. However, The benefits of organic matter in garden soil may not be seen right away. A key factor influencing soil fertility is the dynamic relationship between decomposers, such as fungi and bacteria, and the nitrogen available to plants. The carbon-to-nitrogen (C:N) ratio refers to the amount of carbon in relation to nitrogen in soil organic matter.
In fertile soils, this ratio tends to remain relatively stable, with carbon levels typically being about 10 to 12 times greater than nitrogen levels.
Soil microorganisms require both carbon and nitrogen for growth, but when carbon is in short supply, microbial activity often slows down.
But what happens when carbon rich organic materials like leaf litter, bark, or wood chips are added to the soil? Or when plant residues from previous crops are left to decompose?
The decomposers in the soil are presented with an abundant food source that is rich in carbon but low in nitrogen. As the decomposer populations grow and their activity intensifies, they consume both carbon and nitrogen, depleting the soil's nitrogen reserves. This results in nitrogen becoming a limiting factor for plant growth. This process results in microbes directly competing with plants for inorganic nitrogen.
The duration of nitrogen limitation depends on the quantity and type of organic matter added to the soil. As humus formation progresses and decomposers begin to die off, their activity diminishes. Other soil bacteria then break down the dead decomposers, converting the organic nitrogen in their bodies into nitrate nitrogen, which can be used by plants. Over time, soil fertility improves as nitrogen availability increases, and soil structure may also benefit from the creation of humus.
When organic matter with a high C:N ratio is used as mulch (applied to the soil surface without being mixed in), nitrogen deficiency is typically not a concern because mulch decomposes at a much slower rate. Additionally, organic mulches help suppress weeds, retain soil moisture, and promote healthier root systems, resulting in more vibrant trees and plants.
So, keep applying mulch to the surface and enjoy the long-term benefits of having fertile, well-structured soil that supports healthy plant growth.
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