SOIL

For a garden soil to be well-structured, it should be composed of 45% mineral components such as gravel, sand, silt, and clay, 3-6% organic matter, and 50% unoccupied space, which should be evenly filled with 25% air and 25% water.

For effective gardening, it’s often suggested to aim for soil that has a 45% mineral content, broken down into 40% sand, 40% silt, and 20% clay for optimal water drainage, air circulation, and root growth. However, not all soils naturally meet this ideal.
What’s essential for gardeners to know is that soil fertility is not just about mineral ratios; the biological structure of the soil, shaped largely by organic matter and beneficial microorganisms, is even more critical for productivity. Even if your soil is too sandy or clay-heavy, it can still be made fertile through the right biological conditions. 

Minerals in the soil are broken down through physical, chemical, and biological processes. Physical forces include elements like wind and water, and chemical forces involve reactions such as oxidation. However, biological forces, largely driven by soil microorganisms, produce organic acids and enzymes that help dissolve rock material to release essential minerals. This biological activity significantly enhances soil fertility by making more nutrients available for plant growth.

The Earth’s crust has all the essential nutrients that plants need for growth, including both macro and micro nutrients. Experts in biological horticulture argue that most garden soils already contain these elements. What often lacks is an effective biological system in the soil to make these nutrients accessible to plants. For gardeners, the implication is clear: Enhancing the soil’s biological activity can unlock existing nutrients, reducing or eliminating the need for external fertilizers.

NUTRIENT AVAILABILITY WITHIN SOIL.

Nutrient mobility in soil varies due to several factors such as pH, moisture, and the presence of clay and organic matter. Highly soluble nutrients like nitrate-based nitrogen, sulfur, boron, molybdenum, and chlorine are prone to leaching, especially in wet conditions. Conversely, ammonium-based nitrogen, potassium, calcium, magnesium, copper, and nickel are less mobile as they bind with clay and organic matter. Phosphorous, iron, manganese, and zinc are the least mobile, as they readily form chemical bonds in the soil.

Managing soil pH is traditionally seen as crucial for making nutrients available to plants. However, it turns out that this is a simplified perspective. The chemical characteristics of soil can differ widely even within a single garden. Moreover, plants and soil microorganisms have the ability to adjust the pH in the root zone to facilitate their own nutrient uptake.

It’s also worth noting that the nutrient levels in the soil solution don’t necessarily correlate with what plants actually absorb. For instance, nutrients can be abundant in the soil but not efficiently taken up by plants, or conversely, low in concentration yet fully absorbed.

Furthermore, plants absorb nutrients most efficiently when they are chelated, or bound to organic molecules. Chelation is generally performed by biological processes in the soil. When nutrients are chelated, their solubility—and therefore their availability to plants—is less affected by soil pH.[]

Instead of constantly adjusting soil pH using chemicals like sulfur or calcium, a more effective and cost-efficient approach for gardeners is to nurture a balanced community of soil microorganisms. These microbes can naturally regulate and maintain the right pH levels for plant growth, making essential nutrients readily available to plants. 

SOIL ORGANIC MATTER  (SOM) .

Soil organic matter consists of varied carbon-based compounds that come from plant and organism biomass, as well as substances produced during microbial decomposition. Specific portions of this organic matter, often referred to as soluble or active carbon, serve as the primary fuel and nutrient source for soil microorganisms.

Organic substances get into the soil in two main ways:

(A) Through the breakdown of organic matter like leaves, dead roots, and manure. 
(B) Directly from plant roots, which inject sugars and other carbon-based molecules, in the form of exudates.

This organic content is crucial as it primarily supplies the soil with carbon and nitrogen. Most of the soil’s phosphorus and sulfur are also found in these organic materials.

Fostering healthy plant growth and encouraging organic matter decomposition leads to more fertile soil. This enhances microbial activity, which in turn aids in further decomposition and nutrient cycling.

Soil Organic Matter can be primarily categorized into three groups:

(1) Living organisms like bacteria, fungi, and earthworms, along with plant roots. These organisms are not an immediately source of carbon, but contribute to breaking down organic matter and producing certain soluble carbon compounds like microbial waste or root exudates.

(2) Dead organic matter, including decaying plants, droppings, and animal waste. This category contains both readily and less readily degradable organic substances like lignin and cellulose. It is a key source of soluble or active carbon.

(3) Humic substances, which are the end products of microbial decomposition. They don’t offer nutritional benefits but do significantly  contribute structurally and chemically to the soil.

For gardeners, the key takeaway is that focusing on enriching all three categories of SOM can contribute to a healthier soil ecosystem. Living organisms help in breaking down organic matter; dead organic matter provides active carbon and nutrients; and humic substances add to the soil’s structural integrity. By understanding these components, gardeners can better manage soil health to support robust plant growth.

HUMIC SUBSTANCES.

Humic substances are complex carbon-based molecules formed from the breakdown of organic matter by soil microbes. These substances turn into a stable, gel-like material called humus, which can persist in the soil for many decades. 

Humus is crucial for forming soil aggregates, the small clumps of soil that improve its structure and fertility. Humic substances combine closely with soil minerals to create complexes with clay and silt. These complexes are the building blocks of soil aggregates.

Humic substances act as a buffer for soil pH, neutralizing both acidic and alkaline soils. When the soil pH is balanced “naturally” through the presence of humic substances, a wider range of trace elements that were previously bound up due to extreme pH conditions become available for plant uptake.

Humic substances enhance the soil’s ability to hold water due to their large molecular structure and internal electrical charges. They function like sponges, retaining even more water than clay particles. For gardeners, this means that using compost or humate-based fertilizers can be especially beneficial. These practices help preserve humic substances in the soil, allowing for more resilient gardens during dry weather conditions.

Using either dry or liquid forms of humic substances, which are abundant in compost and compost extracts, significantly improves the effectiveness of fertilizers. As they prevent nitrogen and phosphorus from being lost through volatilization or immobilization in the soil.

The key to generating more humus in your soil is to increase the amount of organic matter and plant exudates (or photosynthates) that go into it.

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