The Mystery of Soil: What is 'Aggregate Structure'!

One of the elements always mentioned for good soil is 'aggregate structure'!
The benefits of soil with an aggregate structure include:'good drainage,' 'good water retention,' and 'fertilizer works effectively.'It seems strange when you think about it—how can it have 'good drainage' while also having 'good water retention'? Let's take a look, as it makes sense once you properly understand the structure of aggregate structure.

・Good drainage
　There are large gaps between large aggregates (macro-aggregates), so even when heavy rain
　falls, water flows out quickly from there. After the water flows out, air enters,
　ensuring the aeration necessary for root respiration.

・Good water retention
　Moisture is held in the gaps inside small aggregates (micro-aggregates). This water
　does not drain away immediately even when dry, so it acts as countless water storage tanks.
　Plants can extend their capillary roots to absorb water from these storage tanks.

・Fertilizer works effectively
　Fields with a developed aggregate structure have many microorganisms. Organic matter is
　decomposed one after another and used as fertilizer for crops.

～How is aggregate structure created?～

・Micro-aggregates are made by bacteria!
　Micro-aggregates are formed by the binding of soil particles such as clay and silt,
　which are the substances that make up the soil, as well as humus and plant debris.
　Various substances stick together due to the sticky substances (polysaccharides, proteins, peptides) secreted by bacteria.
　These aggregates are relatively strong and do not collapse from just tilling or rainfall.

・Macro-aggregates are made by fungi!
　Macro-aggregates are formed by the gathering of micro-aggregates. Large ones
　are several mm (0.25 mm or more) in size. They are created by the hyphae of fungi and mycorrhizal fungi
　and the sticky substances secreted by plant roots. They are easier to break than micro-aggregates,
　but they form immediately when organic matter is added.

～Adding organic matter that is difficult to decompose makes macro-aggregates last longer.～

・Without fungi, macro-aggregates collapse
You can tell by observing soil where fungi have developed that fungi extend their hyphae to stick various substances together, creating relatively large clumps in the soil. (Macro-aggregates)
　Macro-aggregates are mainly formed by fungi, but when the food runs out and the
　fungi die, the sticky substances disappear and they collapse. If the fungi live long,
　the macro-aggregates also last long.

When rapeseed oil cake or clover leaves with a low C/N ratio are added to the soil, microorganisms multiply rapidly, but they also decrease quickly afterward. On the other hand, with rice straw, which has a high C/N ratio, the microorganisms that have multiplied do not decrease much. In other words, adding organic matter with a high C/N ratio allows microorganisms to live longer, and macro-aggregates to last longer.

～Types of organic matter and how macro-aggregates are formed～
〇 Organic matter that is difficult to decompose (C/N ratio of 50 or more)
　・Rice straw
　・Rice husks
　・Pruned branches
Because they decompose slowly, the multiplied microorganisms do not decrease much, and macro-aggregates last a long time. However, rice husks and pruned branches, which contain a lot of hard-to-decompose lignin and cellulose, make it difficult for microorganisms to multiply on their own. Since aggregates cannot be formed unless microorganisms multiply to some extent, it is better to add rice bran or the like at the beginning.
　* Cellulose (chemical formula (C6H10O5)n)
　　Cellulose is a carbohydrate represented by (C6H10O5)n. It is the main component of plant cell walls and plant fibers, accounts for 1/3 of natural plant matter, and is the most abundant carbohydrate on Earth. It is also called fiber.
　* Lignin
　　Lignin is a high-molecular phenolic compound involved in the lignification of higher plants, and is also called wood substance. It plays a role like an adhesive that firmly binds the fibers of wood consisting of cellulose and hemicellulose, and also fixes cell walls together.

〇 Organic matter of medium hardness (C/N ratio 20-30)
　・Rice bran
　・Coffee grounds
Since microorganisms multiply all at once, many macro-aggregates are also formed. However, compared to hard organic matter, decomposition is faster and the decrease in microorganisms is also faster, so the aggregates do not last as long.

〇 Soft organic matter (C/N ratio 10 or less)
　・Fish meal
　・Oil cake
Microorganisms multiply temporarily, but are quickly decomposed completely. Because carbon is insufficient, microorganisms decrease, and nitrogen becomes inorganic. With this alone, macro-aggregates are not formed much.

～Thanks to aggregates, diverse fungi can coexist～
・The habitat of bacteria is inside micro-aggregates
In terms of total weight of microorganisms in the soil, we fungi are more abundant, but in terms of numbers, bacteria are far more numerous, and about 90% of them live inside micro-aggregates. One micro-aggregate is like a single room, where the same type of bacteria live. Because there are countless micro-aggregates, diverse bacteria (6.4 million types in 10g) can coexist in the same soil.

・The habitat of fungi is the gaps between macro-aggregates
Fungi gather multiple micro-aggregates to create macro-aggregates, and live between the macro-aggregates.

・Plant roots multiply microorganisms
Plants send half of their photosynthetic products to the underground parts, and half of that is released from the roots into the soil, becoming food for fungi, bacteria, and mycorrhizal fungi. As a result, there are many microorganisms around the roots, and aggregates are easily formed.

~~~~~~~~~~~~ Main Fungi at Work ~~~~~~~~~~~~~
○ Filamentous Fungi (Molds)
Filamentous fungi with the strongest decomposition power. Filamentous fungi is a general term for what we call molds. While some molds cause diseases in plants, filamentous fungi are useful for soil building, such as by decomposing organic matter. Filamentous fungi have functions such as:
・Converting starch ((C6H10O5)n) into sugar (C6H12O6)
・Secreting various enzymes to decompose organic matter
The sugar produced by the action of filamentous fungi becomes food for other microorganisms.
※Organic acids are a general term for organic compounds that exhibit acidity. Most organic acids are carboxylic acids and have a carboxyl group (-COOH).

○ Bacillus subtilis (Natto bacteria)
As the name suggests, these are aerobic bacteria that work when making natto. They are often found in rice paddies. Even with hard organic matter like cellulose ((C6H10O5)n), they secrete alkaline enzymes to decompose it and produce glucose (sugar C6H12O6).

○ Lactic acid bacteria
They are not very good at decomposing organic matter, but they decompose sugar (C6H12O6) to produce organic acids such as lactic acid (C3H6O3). They are facultative anaerobes and can live without oxygen. By lowering the pH with lactic acid, they can also be expected to have the effect of suppressing pathogenic bacteria.

○ Yeast
Using sugars and amino acids decomposed by koji mold, Bacillus subtilis, etc., as food, they synthesize various amino acids, hormones, and vitamins within their bodies. When oxygen is present, they continue to synthesize while proliferating rapidly, but when oxygen is scarce, they cannot proliferate and instead produce alcohol (ethanol: C2H5OH) and carbon dioxide (CO2). Foods in which yeast has proliferated are flavorful and healthy, and as fertilizer, they activate crop growth. They also serve as food for other bacteria.

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