In this guide, we outline what types of soil might be the most fertile, and we also outline the various factors that might determine how fertile soil might be.
This guide compliments another guide we put together on where the most fertile soil is found worldwide.
Summary – What Is The Most Fertile Soil, & What Makes Soil Fertile?
What Is Soil Fertility?
Before reading about what the most fertile soils are, and the factors that determine soil fertility, you might like to read up on what soil fertility actually is first.
In that guide, we also compare and explain the difference between soil fertility, and also soil productivity, soil health, and soil quality.
What Is The Most Fertile Soil?
Additionally, there are a range of specific factors that play a part in soil fertility (such as the physical, chemical and biological characteristics and traits of the soil – we got through them below)
So, a description of ‘the most fertile soil’ might involve a detailed description of the different soil types, the factors that contribute to soil fertility.
A general way of describing what the most fertile soils according to their characteristics, might be soil that …
– Has a solid base of mineral and nutrients, has a good balance of different sized soil particles to retain and drain water, hold onto nutrients, help air flow through it, and also has good organic matter content (dark soil is sometimes known for it’s high organic content), along with beneficial microorganisms living in it.
When identifying specific soil types that are usually more fertile than others, those soil types might be:
– Soil Type By Traits & Particle Size
‘Loamy soil’ is often described as one of the most fertile soil types when describing soil by it’s traits or particle size, as it combines the best characteristics/properties of sand, silt and clay type soils.
It tends to have good water drainage, good moisture retention and allows good infiltration of air and water.
– Soil Type By Soil Order
In terms of soil orders, Mollisols and Andisols can be some of the most naturally fertile
Alfisols are also relatively fertile naturally, and are quite productive
Vertisols are naturally fertile, but their high clay content can create limitations in terms of how they might be used.
Although, there other ways other soil orders may be made more fertile for certain uses either by managing or adding to the soil in specific ways. For example, an Ultisol type soil might ‘generally [be] of low [natural] fertility although they can become productive with additions of fertilizer and lime’ (soilsmatter.wordpress.com)
– Soil Type By Geographic Location
Some soils are found in specific countries more than others, and some soils are either found predominantly in certain parts of a country, or even assigned to a specific State or region within a country as a ‘representative soil’.
We can’t go through the soils in every country, and soils within every region or State within a country, but we can give examples of what more fertile soils in two countries might be.
In the first example, in the United States, between the bbc.com, nrcs.usda.gov and soils4teachers.org resources, it is outlined that the State soils in Iowa, North Dakota and South Dakota, are all examples of soils with good to very high natural fertility.
In the second example, toppr.com outlines that Alluvial soils and Black soils may be some of the more naturally fertile soils in India.
Soil Fertility For Different Soil Uses
What is important to note is that soils can be fertile or productive for different uses.
For example, some soils may be fertile and productive for agriculture, whilst some may be more fertile and productive for forestry (such as hardwood forestry).
So, what the soil is being used for is an important consideration when linking it to soil fertility.
Natural Soil Fertility vs Modified Soil Fertility
Some soils are naturally more fertile, whilst other soils aren’t naturally the most fertile, but can become more fertile for specific uses (like agriculture) than they are naturally after they have been modified, or added to by humans.
Natural soil fertility is not controlled by humans.
Naturally fertile soils have a mix of natural soil traits and characteristics that make them more fertile and productive.
They inherit these traits and characteristics from natural factors responsible for soil development, which is mainly the parent rock material beneath them, and other factors like climate (that act against and help form the soil during the natural formation process).
Two examples of what might be naturally fertile soils are Mollisols and Andisols
Mollisols in particular have a rich based of minerals in their parent rock material (wikipedia.org mentions the parent rock material is usually limestone, loess, or wind-blown sand), but are also helped out by the organic matter that prairie plants can provide the soil.
Once soil is formed, other factors like the level of organic matter the soil is naturally exposed to in it’s environment can play a part too
This is the case with soil orders like Mollisols for example, that can naturally have prairie plants growing in them that provide organic matter to the soil.
On the other hand, some soils aren’t naturally the most fertile soils for certain uses (like agriculture), and need to be modified or added to, to increase their fertility.
We will use the same example from above to illustrate this – for example, an Ultisol type soil might ‘generally [be] of low [natural] fertility although they can become productive with additions of fertilizer and lime’ (soilsmatter.wordpress.com)
This brings us to to the next section …
Factors That Can Directly Determine Soil Fertility
Beyond the general indicators of what fertile soil might look like and be, there’s a list of different individual factors that can directly determine how fertile a soil is.
We identify and explain the full list of factors in the guide below.
What should be pointed out is that the factors aren’t always independent of each other – one factor can affect another, like for example how the availability of nutrients in the soil can be affected by the pH of the soil.
Indirect Factors That Can Impact Soil Fertility
Whilst some factors directly impact soil fertility, some factors can indirectly affect it.
Climate and weather are examples that we identify below.
Factors That Impact Soil Production Other Than Soil Fertility
Soil fertility is a key factor that can impact soil productivity, but other factors can impact production too.
Beyond soil fertility, things like climate and growing conditions, and what is grown in the soil, will also matter purely from a soil productivity perspective (along with many other factors)
We give examples of those factors below.
How To Improve Soil Fertility & Overal Soil Quality
What Is The Most Fertile Soil?
Soils that are dark and deep, with a good mineral and nutrient base, a high organic content, and a good mix of the right soil particles that allow for movement or storage of nutrients, water and air might be some of the most fertile soils.
Beneficial micro-organisms are also desirable.
These features help soil meet the physical, chemical and biological traits threshold for fertile soil.
By Soil Characteristics & Traits (Such As Particle Size)
Soil order is another way to classify soil types.
Mollisols and Andisols are both naturally highly fertile, and can also be some of the easiest soils to work with.
Mollisols in particular are said to be some of the most fertile, productive, and economically valuable soils in the world.
Alfisols also have high natural fertility, and can be productive for a range of uses.
Vertisols can be relatively fertile naturally, but because of their high clay content, can be more limited or more difficult to work with/produce with (because the clay expands, contracts, cracks, etc).
It’s worth mentioning that uidaho.edu mentions that ‘Inceptisols support approximately 20 percent of the world’s population, the largest percentage of any of the soil orders’, Alfisols ‘… support about 17 percent of the world’s population’, and Ultisols ‘… support 18 percent of the world’s population’
Mollisols are black, thick soils, with high organic content (that comes from organic matter from the roots of prairie plants).
They usually have a rich chemical base (from the parent rock material) of certain minerals.
They are some of the most fertile and important and productive agricultural soils in the world. Wikpedia.org mentions ‘As the world’s agriculturally most productive soil order, the Mollisols represent one of the most economically important soil orders’
Some are also supportive of, and found in hardwood forests.
Andisols are black soils formed from volcanic ash, are highly fertile and productive, and ideal for crops (can support intensive cropping).
In the Pacific Northwest USA, Andisols support very productive forests.
They have capacity to hold a lot of water and nutrients, but can fix phosphorus (and this can impact fertility).
More inherently fertile than Ultisols.
Relatively fertile, and quite productive for agriculture (as well as silviculture).
According to wikipedia.org ‘Because of their productivity and abundance, the Alfisols represent one of the more important soil orders for food and fiber production. They are widely used both in agriculture and forestry, and are generally easier to keep fertile than other humid-climate soils …’
They support about 17 percent of the world’s population
Highly fertile naturally, but has a high clay content that leads to shrinking and expansion (as water is added or leaves the soil), and can have water pool on top of it.
This soil can also develop deep cracks during dry periods.
Vertisols are generally used for grazing of cattle or sheep
When irrigation is available, Vertisols can be used for a range of crops.
May also have other limitations such as problems with rainfed farming.
In some places like Australia, Vertisols may be more appealing because they aren’t short on phosphorus.
– Other Soil Orders That Can Become More Fertile With Modification
It’s worth noting that other soil orders may not be the most naturally fertile compared to Mollisols and Andisols, but they can become more fertile with some type of modification.
These soils orders might include (paraphrased from soilsmatter.wordpress.com and uidaho.edu):
Usually a wetland type soil, but can become more productive as farmland when drained.
Although, draining them can come with problems.
Extremely low natural fertility because of how weathered they are, but can become more productive with the addition fertilizer and lime
Generally low natural fertility, but support forest growth, and can support agriculture and be productive with the addition of fertilizer and lime
Generally used for range, wildlife and recreation, and sometimes agriculture, but only if irrigation is available
Show little to no soil development in a lot of cases, and are usually made of made of weathered/eroded rock or sediment type soils, but some populations in some locations like river valleys and shore deposits can use for it for cropland and habitat in some instances
Naturally low fertility, but can support forests, and with the addition of lime, sometimes agriculture
– Soils With Low Fertility, Or No Agricultural Production Value
Paraphrased from soilsmatter.wordpress.com and uidaho.edu:
Tend to be more developed than Entisols, but have low to moderate soil development (and are usually made of degraded rock material).
They can be used for forestry, recreation and watershed in mountainous regions.
They support approximately 20 percent of the world’s population, the largest percentage of any of the soil orders
Are usually found in cold climates, are permanently frozen, and usually have little agricultural value
Read more about the above soils orders in these resources:
The Twelve Soil Orders (uidaho.edu) (this resource also has graphics that show where the soil orders are predominantly found geographically, including worldwide and in the US, as well as photos of what each soil order looks in terms of their cross section, appearance and different layers)
Soil Orders Simplified (soilsmatter.wordpress.com)
USDA Soil Taxonomy, Soil Orders (wikipedia.org)
Naturally Fertile Soil vs Modified Soil Fertility
qld.gov.au indicates that the natural fertility of soil is determined largely by ‘… the parent materials from which the soil has developed and the original vegetation’
The parent material is the deposit that the soil developed from – soils.org has a good diagram that displays this
Two examples of soils that might have a collection of traits and characteristics that make more naturally fertile, are Mollisols and Andisols
Mollisols have a ‘parent material [that] is typically base-rich, calcareous and includes limestone, loess, or wind-blown sand. The main processes that lead to the formation of grassland Mollisols are melanisation, decomposition, humification and pedoturbation. (wikipedia.org)
In the case of soils with less than ideal natural fertility, these soils can have their natural soil fertility level improved with various soil management practices and additives.
Adding nutrients via fertilizer, adding water, adding another soil type to get the right mix of soil particles, adding organic matter, adding lime – are all ways of potentially doing this (and there are other ways too)
For example, an Ultisol type soil might ‘generally [be] of low fertility although they can become productive with additions of fertilizer and lime’ (soilsmatter.wordpress.com)
Soil fertility can also be maintained with soil conservation practices, or sustainable farming practices
What Plants & Crops Need To Grow
It’s important to note that the main things plants and crops need to grow are – water, nutrients (which is essentially their food), air, and light (sunlight).
The right temperature is also important for plant and crop growth, as well as physically having the space to grow in the soil.
Each species and variation of plant or crop may also have their own set of conditions and requirements that help them grow best (which is why different plants and crops grow better in different climates, soil types, with different requirements for watering and fertilizing, and so on).
What Makes Soil Fertile – Factors That Can Determine Soil Fertility
From the list of plant/crop growth requirements below, the most important soil fertility factors are probably:
Nutrient Supply In The Soil
Water Supply In The Soil
And (although we didn’t mention it above), Absence Of Toxic Substances Or Conditions In The Soil
Other important soil fertility factors other than the main ones above are:
pH Of The Soil
Sufficient Organic Matter
Good Soil Structure & Traits
Biology Of The Soil, & Biological Activity In The Soil
Parent Material Under The Soil
Clay Content, & Cation Exchange Capacity Of The Soil
Bulk Density (Compaction Or Looseness Soil)
Saline Groundwater Sources Near
Explaining each of the above factors in more detail …
– Nutrients Supply In The Soil
Nutrients need to be supplied to the plants or crops that are growing in the soil.
Nutrients refers to both the right nutrients, and the right amount of nutrients.
The right nutrients are both the macronutrients (such as nitrogen, phosphorus, potassium sulphur, calcium and magnesium), and also smaller quantities of micronutrients or trace elements (read more about these nutrients at qld.gov.au)
The right amount of nutrients is also key – there can’t be a shortage of nutrients, but too much nutrients can be a problem too.
Plant roots grow through the soil, and come into contact with nutrients that are stored as electrically charged ions (cations and anions)
The amount of nutrients in the soil is dependent on soil properties (like pH and texture), soil biology (such as organisms living in the soil that break organic matter down into nutrients for plants), soil organic matter (which can hold nutrients until they are needed by plants), soil water (which travels through the soil pores to deliver nutrients to plants), and fertilizers (which can deliver nutrients, but too much nutrients can also impede the uptake of others) (qld.gov.au)
Balancing nutrients in soil is important, as nutrients can be both added, but also lost over time.
Loss of soil nutrients can occur when crops are harvested, but also via soil erosion, runoff, leaching, burning of crop residues, and gaseous loss.
There’s a fine balance applying fertilizers to soil to increase nutrient inputs – enough has to be applied to maintain yields/soil productivity, but too much can affect other soil fertility factors, and lead to issues like environmental pollution issues from synthetic fertilizers.
Lastly, it’s also important the soil has good nutrient release capability to actually allow the plants and crops to absorb the nutrients.
– Water Supply In The Soil
Water needs to be supplied to the soil in adequate amounts.
Water enters the soil through the large pores, uptakes nutrients, and is stored in the small pores (the water that enters the small pores and takes up gases, organic matter, and minerals is called the soil solution), until plant roots need to absorb the water.
Some water that enters the soil drains out through the large pores.
This is also where the soil type can play a part, as good soils have a mix of large and small pores to keep the water and moisture content right.
How well water infiltrates soil, how well the soil retains a certain amount of water, and how well water can drain from the soil all matter as well.
– Absence Of Toxic Substances Or Conditions In The Soil
As important as it is that plants and crops have nutrients and water, it’s also important that there isn’t any toxicity in the soil that will kill the the plants/crops.
A few examples of soil contaminants in regards to plant growth might be heavy metals, and salts.
– Topsoil Depth
Topsoil depth is important because the topsoil is usually where the plant or crop gets it’s nutrients from, and where it establishes it’s roots.
– pH Of The Soil
The pH of the soil is a measure of how acidic, or how alkaline the soil is.
Soil can also be neutral in pH.
Soil pH affects the amount of nutrients and chemicals that are soluble in soil water, and therefore the amount of nutrients available to plants.
Some nutrients are more available under acid conditions while others are more available under alkaline conditions.
However, most mineral nutrients are readily available to plants when soil pH is near neutral, and there a wide range of plants that might grow well in more neutral 5.5 to 7 pH soil.
A soil might be naturally more acidic or alkaline, but soil pH can also be changed with soil amendments.
Different plants and crops are also going to grow better in different pH soils.
Soil tests can be done to determine the pH of the soil.
– Sufficient Organic Matter
Organic matter is usually found on top of the topsoil, either in a humus layer, or in the form of things like plant and animal matter (manure, mulch, leaves, and so on)
Organic matter is important because not only is it broken down or decomposed into nutrients that can be stored in the topsoil for plants to absorb, but it can also help with healthy soil structure and soil moisture retention
– Good Soil Structure & Traits
The structure and traits in the soil are important for two key things (amongst other things) – 1. Allowing drainage of the soil so it doesn’t get waterlogged, and 2. Allow aeration of the soil
As we mentioned in the water supply section, soil pores contribute to drainage and aeration.
– Biology Of The Soil, & Biological Activity In The Soil
Fertile soil usually has beneficial organisms living in the soil that can break down organic matter (such as animal and plant matter) into nutrients
Some examples of beneficial organisms can be small organisms like bacteria and microbes, all the way through to bigger ones like earthworms
– Parent Material
The parent material is the underlying layer of material (usually bedrock) from which soil forms.
Soils gets a large amount of their nutrients, structure and other characteristics from parent material, and therefore parent material can have a significant impact on the natural fertility of the soil.
This material can be different in different locations, which is why you can get different types of soils in different locations.
Wikipedia.org has two good descriptions of the parent rock material that Mollisols and Vertisols come from if you follow the respective page links in the Soil Order Taxonomy resource
– Clay Content, & Cation Exchange Capacity
The CEC influences the soil’s ability to hold onto essential nutrients and provides a buffer against soil acidification.
As plants and crops grow, they come into contact with cations and nutrient supply occurs.
Low CEC indicates the possibility of easily losing nutrients by leaching – so, soils with a higher CEC tend to have more nutrients present.
The CEC varies according the clay %, the type of clay, soil pH and amount of organic matter.
Read more about CEC at soilquality.org.au
– Bulk Density
Compacted soil is not desirable.
Looser soil is preferred as it allows plant and crop roots to penetrate the soil and reach nutrients in the soil.
– Soil Conservation, & The Prevention Of Soil Degradation & Erosion
Soil and land degradation, as well as topsoil loss via wind and water erosion, intensive farming practices, and other causes, all impact various factors of soil fertility.
As just one example, topsoil erosion leads to loss of topsoil and shallower topsoil.
The conservation of soil is therefore important to soil fertility.
– Surrounding Saline Water Sources
If for example the soil has saline groundwater near it or underneath it, this can create problems for soil fertility.
Other Descriptions Of What Makes Soil Fertile
bbc.com describes what makes soil fertile: ‘The evolution of a fertile soil, either from parent rock material or from a degraded system, is mediated by a complex set of tightly interwoven hydrological, geochemical, geomorphic and biological processes’
Factors That Can Indirectly Impact Soil Fertility
One example of a factor that can impact soil fertility is the climate and the weather, along with natural events.
As soils.org outlines, dry climates and tropical climates can impact soil locking and soil leaching within the minerals in the soil.
Additionally, windy weather and droughts can both rearrange soil, and dry it out of it’s water.
Each of these effects ties back to a factor of soil fertility.
In some cases, topography like sloping land may also impact different aspect of soil fertility.
Factors That Can Impact Soil Production Other Than Soil Fertility
Ultimately, there’s a whole range of factors that can impact soil productivity other than soil fertility.
Some of those might include:
Climate and weather
The type of plant or crop being grown, and how and where it’s grown. Just as one example, the soil conditions required for growing a Gao Tree in Niger will be different to soil conditions required for growing wheat in Australia, and will be different again for growing cotton in Alabama.
Whether additives like fertilizers, pesticides, organic matter, etc. are used
The presence of weeds and other pests
Whether or not genetically modified plants and crops are used
… Ultimately, there’s no one size fits all answer for soil productivity.
How To Improve Soil
Where Is The Most Fertile Soil In The World Found?
Different Types Of Soils Found WorldWide
We’ve put together this guide which outlines the different soil types found worldwide.