A typical good soil is made up of about 45% mineral, 5% organic matter, 20-30% water, and 20-30% air, though the percentages vary depending on the climate, soil characteristics, and farming practices. Water and air are familiar to us, but mineral and organic matter might not be. Let’s take a closer look at them.
Mineral: This is the major component of soil. It refers to rocks that have been crushed into small particles. These rock particles have different sizes; the largest is sand, and the smallest is clay.
Comparing the sizes (diameters) of different soil particles. Image from (Ref.1).
Organic matter: This comes from plants and animals. It includes plant residue and dead animal bodies in various stages of decomposition and living microorganisms that decompose these materials. The final product of decomposition is humus, which makes up about 70% of the soil organic matter and gives soil its dark color (Ref. 2).
High water-holding capacity: Like human beings, plants and organisms in soil need water to live. Water also moves nutrients in soil and keeps the soil temperature steady. If all of the water passes through the soil, plant roots cannot access water. Good soil should hold water and provide enough moisture to plants.
Sufficient drainage: Although water is important, if too much water stays in the soil, the roots will not be able to breathe. Excess water should be able to pass through the soil. A balance is important here.
High air-holding capacity: The plant roots and many soil organisms need to breathe oxygen. Soil organisms also need some empty space to move around. Soil should have some pore space to hold air.
High nutrient-holding capacity: Good soil can hold nutrients so that they are not easily washed away (See note 1 below).
Appropriate pH: The soil’s pH tells us how acidic or alkaline (basic) the soil is, measured on a scale of 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline. If the soil is too acidic or alkaline, not only will it harm certain plants and soil organisms, but also some of the essential nutrients in the soil will become unavailable to plants, so it is important to keep the soil near neutral (pH 5.5 - 7.5).
The effects of soil pH on the availability of various nutrients for plants. The thicker the white bar is, the more available the nutrient is. We can see that as soil gets acidic (red region in the image), many nutrients including nitrogen and phosphorus become less available. As the soil gets alkaline (blue region in the image), nutrients such as nitrogen and iron become less available. We can maximize the overall availability of nutrients by keeping the soil neutral (yellow green region in the image). Image from (Ref. 3).
Contains a lot of organisms: Earthworms, moles, and microorganisms such as bacteria and fungi play many important roles in soil. They are the ones that break down organic matter and provide nutrients for plants. They can also help improve the physical and chemical qualities of soil, as we will see later. Good soil provides a nice living environment for organisms.
We now know what good soil is. Before moving on to discuss how to obtain good soil though, we must recognize that to some degree, soil quality is affected by factors we cannot control. One such factor is the mineral composition of soil. The percentages of sand and clay in soil affect its ability to hold water, air, and nutrients (See note 2 below). While it is difficult to change the soil’s mineral composition, there are other ways to improve soil quality, which we will focus on here.
One important principle that we follow at ARI is to learn from nature. We can apply this principle to our discussion on obtaining good soil because the best soil can be found in natural forests. Nobody applies artificial fertilizers in natural forests, but plants grow well there. Soil in natural forests has high physical, chemical, and biological qualities. How is this possible?
What stands out about soil in natural forests is that it is full of life and decaying organic matter. In forests, there is plenty of organic matter such as fallen leaves, dead animals, and poop piling up on the floor. Soil organisms feast on this abundant organic matter and create a thick layer of humus. This is actually the key to obtaining good soil: maintaining a high biological activity in soil and creating humus. Let's see why.
1. Soil organisms provide nutrients to plants
As mentioned above, under "the biological quality of soil," we depend on soil organisms to eat organic matter, break it down, and release nutrients for plants.
We first need to understand what soil structure is. Even if the mineral compositions of two soils are the same, they can have different structures. Low-quality soil tends to have a single-grain structure, while good soil has a crumb structure (or an aggregated-grain structure). With the single-grain structure, individual grains (soil particles) exist separately.
The characteristics of soil with single-grain structure depend on whether the soil has more clay or sand. For example, clay soil (soil with a high percentage of clay) is better at holding water because of the smaller grain size and pore space, whereas sandy soil, with bigger grain size and pore space, holds air and drains excess water well, but is not good at holding water. Neither of them has well-balanced physical characteristics.
With crumb structure, the grains stick together and form aggregates, creating a fluffy soil. This happens when there is a high biological activity in the soil. Bacteria and plant roots produce sticky substances that glue soil particles together. Fungi and root hair also wrap soil particles into balls, creating aggregates.
The image above shows crumb structure in clay soil. We can see a wider range of pore space sizes; small ones within aggregates and big ones between aggregates. This allows a certain amount of water to stay in the small pores while the excess water passes through the big pores, leading to high water-holding capacity and good drainage. Air can also stay in the big pore spaces. Thus, having a crumb structure can help improve the physical quality of clay soil. Note that it is uncommon to see very sandy soil with crumb structure because sand particles do not stick together well. However, most soil has a mix of clay and sand, which allows aggregates to form.
3. Soil organisms add humus to the soil
When soil organisms break down organic matter, humus remains as the end-product. Humus is very effective for improving soil quality in the following ways:
Holds air and water and drains excess water, improving the physical quality of soil
Stabilizes soil aggregates, which strengthens the crumb structure
Has a very high nutrient-holding capacity
Helps soil resist changes in pH by acting as a buffer (Ref. 4). Note that humus itself is neutral.
What farming practices promote high biological activity in the soil?
We have seen that high biological activity in soil is essential for soil improvement. What this means is that soil must be a good host to soil organisms by providing enough food (organic matter) and a comfortable living space, which involves good aeration and moisture. Below are some examples of helpful farming practices:
Add organic matter to soil: Adding crop residues (straws, stalks, leaves, roots), compost, green manure, animal manure, or mulch to soil provides a varied food supply for soil organisms (See note 3 below).
Keep the soil surface covered: Mulching beds prevents moisture loss and high temperature in addition to providing food for soil organisms. Living mulch (cover crop) also releases special substances that attract soil organisms.
Minimize physical disturbance: Too much tilling can kill soil organisms and destroy their habitats. It also destroys soil structure, which lowers the physical quality of soil.
Avoid chemical pesticides: They can kill soil organisms.
Maintain appropriate pH: Most soil organisms cannot tolerate highly acidic or alkaline environments, so we should keep the soil pH neutral for them, as well as for our plants.
There are many ways to improve soil. Here are some materials that we use to improve soil at ARI.
Compost is one of the best materials to improve soil. It contains a lot of organic matter and organisms.
Charcoal improves the soil’s physical quality, increases its nutrient-holding capacity, and provides shelter for microorganisms in the soil. It can also neutralize acidic soil because it is alkaline.
Indigenous microorganisms (IMOs) are soil organisms that are helpful for soil improvement. They are mixed with other materials and added to soil to promote a healthy biological activity.
Bokashi is more often used as a fertilizer, but is also effective for soil improvement as it contains IMOs, organic matter, and charcoal.
Tsuchi-koji and sumi-koji are particularly helpful for restoring a healthy population of microorganisms in the soil. They contain IMOs, clay soil, and carbohyrates (food for IMOs). Sumi-koji also contains charcoal.
1. The ability to hold nutrients is measured by the cation exchange capacity (CEC). Cations are positively charged ions. Most essential nutrients in soil exist as cations, such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+). Humus and clay, both of which have high CEC, tend to be negatively charged, so they can attract and hold onto these cations.
2. Soil’s minerals consist of rock particles of varying sizes: sand, silt, and clay. The percentages of sand, silt, and clay (soil texture) affect many aspects of soil quality. For example, clay soil (soil with a high percentage of clay) is better at holding onto nutrients than sandy soil because clay has a relatively high CEC.
3. There are a few things to be careful of when adding organic matter to soil. One is that incorporating raw (not decomposed) organic matter into soil can be risky. In the initial stage of decomposition, a lot of oxygen will be consumed, leading to oxygen deficiency in the soil. Harmful gas, such as methane and ammonia will also be produced, which can hinder plant growth. To avoid this problem, we can put raw organic matter on top of soil as mulch or incorporate only decomposed organic material such as compost into soil. If we do incorporate raw organic material such as green manure into soil, we should wait until the decomposition has completed (2-3 weeks). Another thing to consider is the carbon to nitrogen (C/N) ratio of the organic material added to soil. If too much carbon-rich material is added to soil with little nitrogen, microorganisms feeding on the carbon will also consume nitrogen in the soil, making it unavailable to plants. This essentially creates a nitrogen deficiency in the soil. For more information about maintaining an appropriate C/N ratio, read this page about composting.