We’ve already put together a guide outlining what the most fertile soil is, and what factors impact soil fertility.
In the guide below, we outline the different ways soil fertility might be able to be improved.
We’ve also expanded the guide to include how soil quality in general might be improved, and what some other helpful tips for working with soil might be.
(*NOTE – This is a general guide only. Ultimately, soil differs from on plot of land to the next, and will therefore require it’s own assessment of unique local factors, and it’s own soil management plan)
Summary – How To Improve Soil Fertility, & Overall Soil Quality
General Components Of Soil Fertility, & Factors Impacting It
Soil fertility has two direct components – natural soil fertility, and modified soil fertility.
Beyond those direct components, there are indirect factors that can impact soil fertility, as well as indirect factors that can impact soil production/productivity as a whole.
Read more about those components in this guide.
In the guide below, we outline potential options for addressing and managing each of these soil fertility and production components.
We also outline what the options might be if working with or trying to improve the existing soils’ fertility is not a good option.
What To Do About Natural Soil Fertility
The only realistic way of managing natural soil fertility is to pick the most naturally fertile soil available to use, prior to starting to use any area of soil.
We go into what that means specifically, and how to manage that, in the guide below.
Improving & Modifying Soil Fertility (Beyond The Natural Fertility Of Soil)
We go into each of the following soil fertility improvement areas in the guide below:
Nutrient Supply In The Soil
Water Supply In The Soil
Absence Of Toxic/Damaging Substances Or Conditions In The Soil
pH Of The Soil
Sufficient Organic Matter
Good Soil Structure & Traits
Biology Of The Soil, & Biological Activity In The Soil
Clay Content, & Cation Exchange Capacity Of The Soil
Bulk Density (Compaction Or Looseness Soil)
Saline Groundwater Sources Near
How To Manage Factors That Can Indirectly Impact Soil Fertility
The climate is one example of an indirect factor.
We go into options to address this factor, and others in the guide below.
How To Manage Factors That Impact Soil Production Other Than Soil Fertility
The type of plant or vegetation being grown is one example of a factor that impacts soil production outside of soil fertility.
We go into options to address this factor, and other factors, in the guide below.
Other Options If Existing Soil Cannot Be Improved, Or Is Not Suitable To Use
If the existing soil is not suitable to work with, or the fertility of the soil can’t be improved to the required level, we outline what the options might be to address that in the guide below.
*An Important Note
Obviously soil fertility is something that does not have a ‘one size fits all’ answer that applies to every situation.
The information in this guide is of a general nature only.
People obviously have to do their own research, and make their own decisions on what the specific (and best) solutions are for improving, or working with the soil they have.
What To Do About Natural Soil Fertility
Because natural soil fertility is pre-determined and can’t be changed, the best way to manage this component might be the pick the most naturally fertile soil available prior to starting to use the soil.
To do this, a person might:
– Pick the most naturally fertile square area of soil on their land to use
– Or, if purchasing a new property/plot of land, purchase in an area where there is more naturally fertile soil available compared to other areas
To manage both of these things, a person might:
– Test the soil on the land they own to find out where the most fertile soil is
– Get and understanding of the different types of soils in the world, and also get an understanding of what the most fertile types of soil are (and what factors make them more fertile)
– Get an understanding of where the most fertile soils are found worldwide, and in different countries
By doing these things, a person can understand what constitutes a fertile soil, where they might be found naturally, and how to test for and identify soils that are more fertile to use on a specific plot of land.
This information can be applied to land someone already owns, or to land a person might be planning on purchasing.
Ways To Improve Soil Fertility By Managing, Amending Or Modifying The Soil
Below are a list of potential options for improving soil fertility (beyond it’s natural fertility), across the range of factors that play a major role in determining soil fertility …
– Nutrient Supply In The Soil
Plant life needs nutrients as food.
The nutrient supply in the soil should not be too high or too low (so, adding too much fertilizer for example can sometimes be as much of a problem as not having enough nutrients).
Some options to address this might be:
1. Add fertilizer
Different synthetic fertilizers can add macronutrients, and also micronutrients and minerals to the soil
Some of the main types of synthetic fertilizers are nitrogen, phosphorus and potash fertilizers
Slow release fertilizers can be good.
Some people also choose to use organic/natural fertilizers like animal manure
There’s also specialized soil nutrient mixes designed to be added to soil to increase specific nutrients or minerals
A gardening professional might be able to help identify what type of fertilizer to apply, and how often to apply it (and in what quantities).
The fertilizer packaging should also have instructions on what the fertilizer is for and application rates.
2. Add organic matter
When organic matter breaks down, one benefit is that is adds nutrients to the soil
Mulch, compost and manure are examples of organic matter
3. Other ways to regulate nutrients in the soil
Consider planting deep root plants like stinging nettles and comfrey that can ‘mine’ minerals from deep in the ground (read more at motherearthnews.com)
Consider applying a rock powder when first preparing soil to correct mineral deficiencies in the soil (read more at motherearthnews.com). Soils can benefit from minerals like iron, manganese, sulfur and calcium
Consider livestock integration such as allowing chickens to forage in the soil – they scratch/break up organic matter, eat insects and bugs, and their droppings provide nutrients
– Water Supply In The Soil
Plant life also needs water to grow.
The water content in the soil should not be too high or too low.
gardeners.com mentions that a good/healthy soil is about 25 air, and contains about 25% water.
Some options to address this might be:
1. Make sure the soil is getting supplied enough water
Rainfall will provide a certain level of water to the soil in some areas
Additional water can be added to the soil via manual watering, or automated watering with the use of irrigation/sprinkler systems for example
2. Make sure the soil retains/holds enough water
How well soil is able to hold water depends largely on the type of soil particles it has – very sandy soil for example is loose and might not hold water well, a very clay rich soil might hold too much water, and a well balanced loam soil might hold just the right amount of water.
Beyond the soil particles, the addition to the soil of materials like mulch can stop the top of the soil drying out, keeps the soil underneath moist, and can reduce watering in some instances.
A layer of mulch (roughly 75mm depth) on top of soil can help prevent water evaporation in some instances, but, over time will break down and can help with soil structure too.
Straw, dried grass clippings and deciduous leaves can make a good mulch, but, buying a commercial mulch is also an option.
3. Make sure the soil is able to drain adequately
Again, this largely relates to the type of soil, and the same soil traits described above apply here.
Good soil can hold enough water, whilst also allowing the draining of water so it doesn’t get too waterlogged or sticky.
The pores in the soil play a role for soil retention and draining.
Other factors like making sure the soil is located somewhere where water can run off and not pool in the soil can also play a part (e.g. soil in between a valley might get flooded or waterlogged during periods of heavier rainfall)
– Absence Of Toxic Substances Or Damaging Conditions In The Soil
The soil should be free of contaminants, but also free of unfavorable substances, chemicals and conditions that negatively impact the soil and the plant life growing in it.
Examples can include pollutants getting into the soil, waste getting into the soil, or the soil becoming very saline.
Pests and pest control may also factor in here, but we discuss this further down in this guide.
Some options to address this might be:
1. Make sure the soil isn’t contaminated, or doesn’t get contaminated
Before starting to use the soil, do a soil test to make sure it isn’t contaminated.
Additionally, whilst using the soil, make sure contaminants and chemicals (that could change the soil conditions) don’t leach into the soil from nearby sources.
– Ensure The Topsoil Depth Is Adequate
The topsoil is usually where most of the nutrients is, and is where plants establish their roots.
Different sources detail what a good topsoil depth is – with some saying that the first few feet matter.
gardeners.com as another example indicates ‘… most soil life and plant roots are located in the top 6 inches of soil, concentrate on this upper layer [with the organic matter]’
It might be different for the purposes of an amateur garden though vs topsoil depth for industrial level agricultural production.
Topsoil depth requirements may also differ for different plant life whose roots have different ways of establishing themselves.
So, determine what depth of topsoil is required for the type of soil production being undertaken.
Some options to address this might be:
1. Grow in naturally deep topsoil
This is obviously the best option i.e. find naturally deep and fertile topsoil that is already established, and grow in it.
2. Or, add commercial topsoil to the existing soil
Sometimes the topsoil depth across a particular square area of land is not adequate.
In this instance, using commercial topsoil as a top up for the topsoil depth may be required.
– pH Of The Soil
Soil can be acidic (pH 6.5 and below), alkaline (pH 7.5 and above), or neutral (pH 7).
There are a large range of plants that will grow well in soils with neutral pH levels (5.5 to 7)
So, it makes sense to find out the soil pH required for the specific plant life being grown, and also test the soil pH to find out what level it is currently at (and see if it needs to be changed).
Commercially available pH test kits are usually more of a guide, whereas laboratory testing can be much more accurate.
Some options to address this might be:
1. Match the plant life to the soil’s pH
A very easy approach is to match the plant to the soil pH (pick a plant type that grows well in the soil’s natural ph)
Even with more acidic or alkali soils, it may be easy to access a type of plant that can grow in that soil.
2. Amend the soil pH to meet the pH requirements of what is being grown
There are instances where the soil pH will have to be changed to meet the pH requirements of the plant life being grown.
To make soil more acidic …
Fertilisers such as crushed sulfur and some ammonium-based nitrogen fertilisers lower pH and make soil more acid (qld.gov.au)
Other sources say adding iron or aluminum sulfate or sulfur to soil can lower its pH (and make it more acidic).
To make soil more alkaline …
When soils are too acidic for a particular crop, lime or dolomite can be used to increase the pH to the desired level [and the amount varies depending on the soil type]
Soils with high organic matter and clay content will be more resistant to changes in pH and will require larger application rates. Therefore soil pH, while indicating the need for lime, is not a reliable guide as to how much lime is required.
To bring soil to a more neutral level …
Use either one of the above options to increase or lower the soil pH back to neutral, depending on the current pH of the soil.
There’s other amendments that can be added to change soil pH too.
– Sufficient Organic Matter
Organic matter has several important benefits.
These benefits include but aren’t limited to – decomposing and providing nutrients to the soil, giving better structure to the soil and the particles to allow better movement of water and air through the soil, retaining moisture, and acting as a form of food for beneficial microorganisms.
Options to address organic matter in soil might include:
1. Growing in a soil that has a natural form of organic matter available
bbc.com also mentions how ‘prairie grass growth, death and decomposition’ over many years has left the soil thick and fertile in Iowa, United States
2. Adding organic matter to the soil
Some sources indicate that a diverse array of organic matter is best, whilst others say one can be ok.
There a range of organic matter than can be used, such as manure, organic fertilizer, compost and organic scraps and leftovers, peat moss, cover crops, and more.
In terms of manure, well rotted livestock manure can work well – but allow a few months between application and harvesting edible plant food. Chicken droppings can also be very good if chickens are foraging and scratching around garden patches.
In terms of compost, greens and grass clippings can work well – apply around one-quarter inch per season.
Find a good explanation on composting and compost application at motherearthnews.com
In terms of mulch, straw, dried grass clippings and deciduous leaves can work well.
Other plant and animal organic matter can also be suitable.
Organic matter can take years, and even up to a decade or two, to start having a significant impact on soil’s fertility, health and quality – so be patient, and add it consistently over time. It’s not just a quick fix.
Some sources say to add it at least twice a year in a thin to moderate layer, and to make sure to dig it in, spread it, and break it up.
Read more about adding organic and inorganic materials and fertilizers at provident-living-today.com
gardeners.com indicates ‘… most soil life and plant roots are located in the top 6 inches of soil, concentrate on this upper layer [with organic matter]’
– Good Soil Structure & Traits
Soil structure, and traits and characteristics, impact how well the soils retains and drains water, aerates the soil, and does other important things like hold onto nutrients.
It is mainly determined by the type of soil, and the soils’ particles.
Clay, sandy, silt and loamy soils for examples all have different traits and characteristics, as well as structure.
Mixed soils can have unique traits and structures too.
An option to address this is:
1. Assess what type of soil the existing soil is (and the traits and characteristics it has), and decide to work with it, add to it, or not work with it
So, it may have favorable traits and characteristics (and structure), and can be used as is.
It may need a better balance of soil traits, characteristics, and structure, and therefor may need to be mixed with another soil, or other soils
Or, the soil may not be suitable for use, and new soil may need to be imported (a soil with more favorable traits, characteristics and structure).
– Biology Of The Soil, & Biological Activity In The Soil (Soil Life)
This is the level of living organisms and beneficial microorganisms in the soil.
They contribute to things such as breaking down organic matter in the soil, along with other important soil functions.
An option to address this is:
1. Make sure the soil has adequate organic matter, aerated soil, and soil with good water content
These are usually good conditions for beneficial organisms like bacteria and fungi, protozoa and nematodes, and earthworms to live in the soil (per gardeners.com)
bbc.com also mentions how ‘photosynthesis from cover crops and intercropping helps to feed the soil system and keep the biology there active’
Additionally, synthetic pesticides and inorganic fertilizers can lead to living organisms in soil suffering
bbc.com mentions that some companies even offer microbe ‘cocktails’ that help revitalize degraded soils
Fungal spores and beneficial fungi like Mycorrhizal fungi can be added to the soil, but, keeping the soil healthy will normally create a natural environment for micro-organisms like fungi, bacteria, worms etc. to thrive in the soil and break down organic matter for nutrients in the soil.
– Clay Content, & Cation Exchange Capacity Of The Soil
The CEC impacts the soil’s ability to hold onto nutrients as well as other factors.
The CEC varies according the clay %, the type of clay, soil pH and amount of organic matter
An option to address this is:
1. Make sure there is the right balance of these factors that impact CEC
Both when the soil is being chosen and prepared, and during the time when the soil is being used
If we look at loam type soil for example, which is seen as a fertile soil for growing many types of vegetables and plants, a good loam composition might consist of 40 percent sand, 40 percent silt, and 20 percent clay.
Ask a soil expert if unsure about this.
– Bulk Density (Compaction Or Looseness Soil)
How compact or loosely the soil is packed impacts how well the roots can penetrate the soil and establish themselves (and this consequently impacts whether the roots can access the nutrients, water and other things in the soil that they need)
Soil should be packed in a way allows this establishment.
Soil that is too compact can also have poor water drainage and aeration.
Options to address this might be:
1. Don’t compress/compact the soil too much at any time
Both when first preparing the soil, or when managing it throughout its use
2. Have the right type of soil particles
Extreme clay soils and heavy soils tend to have a high level of bulk density, so, it’s important to have a soil that strikes that balance between having decent soil structure that clumps together well enough, but is also loose enough to allow air and water to pass through it.
– Saline Water Sources Near Soil
Saline water sources like groundwater aquifers for example can leach into the soil , and cause soil salinity issues.
An option to address this might be:
1. Don’t grow near water sources that are highly saline
This is a pretty simple one – don’t use soil that is likely to be contaminated with nearby saline/salty water.
How To Manage Factors That Can Impact Soil Fertility Indirectly
Climate, weather and natural events can indirectly impact soil fertility
As can factors like topography, and where the soil in located within a local region.
These are just a few example, but there can be more.
Options to address these factors might be:
1. Make sure the climate isn’t too dry or too wet for the soil
Places with heavy consistent rainfall can be very wet, whilst hot places without much rainfall can be dry (such as some deserts).
For example, heat makes clay soils dry up and go solid or crack, and wet conditions can lead to wet clay soil that goes sticky and too compact.
So, choose a suitable climate for the soil
2. Make sure certain types of soil degradation won’t cause topsoil loss, or damage the soil
Prevention and management of this things can lead to preservation of topsoil as opposed to topsoil loss, and also preserve soil fertility and health
Intensive agricultural practices (such as intensive tilling, and overgrazing by livestock) as one example not only break up the soil’s structure and make it easier to wash away, but it can also impact the soil’s ability to hold water and support healthy microorganisms
bbc.com mentions one example of a potential solution in some areas might be to grow perennials (as opposed to an annual crop), as ‘[a perennial] wheatgrass will regrow without having to be re-sown [and therefore there is less need to till and break up the soil]’. And added benefit of this approach is that there are less opportunities for weeds to grow in a field [because the soil doesn’t loosen up as much to allow weeds to establish themselves]
One limitation of this approach though might be that yields start declining after three or four years
bbc.com also mentions how some places with biosphere environments are currently researching how degraded soils might be ‘eco-engineered’ to be revitalized, or what conditions lead to the creation of new soil
3. Consider the impact of the weather on the soil
Wind can cause topsoil loss via wind erosion
Heavy rain can also sometimes contribute to water erosion by washing topsoil away, but regular rainfall can too in places where water flow and drainage are poor
Ground cover/cover crops can be a solution in some instances to manage wind erosion and water erosion. Cover crops can include legumes, grasses and clovers (read more at motherearthnews.com)
bbc.com mentions how [prevention of heavy tilling and disturbance from farm vehicles might reduce the ability of wind and water to erode the soil]
4. Make sure natural events won’t damage the soil in a growing area
Consider whether areas prone to natural events like droughts are suitable to grow in – droughts may leave the soil dry
Floods can also be an issue as they can carry topsoil away and in some cases contaminate soil
5. Consider the impact of growing on certain topography
Such as sloping land, or land in a valley … as opposed to flat land
There can be issues with both, so try to grow on flat land if there will be potential issues on sloping land or in valleys
How To Manage 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 (and there’s no one size fits all answer for soil productivity)
Just a few are climate and weather, soil health and soil quality, the type of plant being grown (and where and how it’s grown), and pests.
Options to address these factors might be:
1. Climate and weather
We already addressed this above, but, make sure the climate is suitable, and the weather is manageable.
Soil health and soil quality consider some factors that soil fertility does not.
View the linked guide to see what some of these factors might be
3. The type of plant or crop being grown, and how and where it’s grown
4. Controlling pests
Make sure to control pests like weeds, insects, vermin/rodents, and other pests that can damage or destroy plants
Synthetic pesticides can help, and so can biological pest control methods
5. What growing and farming practices are used
Some practices like sustainable farming can have long term benefits for soil health and soil enrichment
bbc.com outlines on example of this by growing cover crops (native crops to an ecosystem like prairie strips in Iowa), along with buffer strips and no till farming
6. Minimize soil disturbance, tilling and foot traffic
Tilling, walking on and disturbing the soil are all ways to break up the soil structure – which is not ideal for soil fertility and health long term.
Creating permanent gardening beds and growing patches, and having dedicated footpaths and walkways, are both ways to prevent this.
Differences Between Soil Fertility, Soil Productivity, Soil Health & Soil Quality
Read more in this guide – Differences Between Soil Fertility, Soil Productivity, Soil Health & Soil Quality
What About Soil Drainage & Soil Structure?
These two are interrelated.
Soil drainage and structure is really a part of soil fertility (it’s one of the physical characteristics of soil to consider, along with the chemical and biological characteristics)
Soil structure is how the soil particles arrange or clump together to form pores, spaces and gaps in the soil as a whole.
Soils like clay tend to clump together very tightly because of the small soil particle size, and soils like sand tend to be far more loose and have less structure due to the large particle size.
Good soils needs space for both water and air to move between the soil particles, but should also be able to retain some water and nutrients for the plant roots to absorb, and not allow them to completely wash away.
Soils with good drainage can do this.
How To Improve Soil Productivity
Soil productivity is simply the yield or production capacity of the soil.
Improving soil fertility is one way to do this, but, in reality, there’s a whole list of ways to improve soil productivity, and it can be dependent on what is being grown, as well as local conditions.
But, nrs.fs.fed.us outlines some ways to improve soil productivity specifically in rainforests might be with fertilization, genetic improvement with selective breeding, prevention of flooding, shortening rotation, controlling competing plants and vegetation, and matching compatible plant species with the soil and geographic location they are being planted in.
How To Improve Soil Health & Quality
Soil health and quality goes beyond soil fertility.
It might be improved by improving soil structure, biological activity in the soil, and preventing soil health issues like soil erosion, acidification, salinity, and so on.
Soil erosion can be minimised by protecting the soil from water and wind damage (cover crops, using flat soil, good soil drainage can all help with this).
Soil acidification can be minimised by controlling the amount of nutrients added to the soil – mainly nitrogen based fertilizers. Acidification happens when there is an excess of hydrogen cations added to the soil, which lowers the pH.
Soil salinity mainly occurs due to saline groundwater reaching a high level just below the soil level.
How To Improve Soil Drainage
Good soil structure contributes heavily to good soil drainage.
Heavy soils and clay rich soils can have poor soil drainage due to their structure.
Some people have had success adding sand (and sometimes silty soil) to clay soil to loosen it up.
But, for others like those at provident-living-today.com, they’ve found adding sandy soil to clay can in fact harden it up.
They found that adding organic matter consistently over an extended period of time worked better.
Some soils may be unworkable, or may only be workable for experienced gardeners and growers.
In this case, raised garden beds with imported soil, or growing plants suited for the existing soil may be the best options rather than trying to amend the soil too much (and risk wasting time and money).
How To Improve Soil Structure
As discussed above, soil structure can be improved by firstly identifying the type and texture of soil.
Then, work on adding other types of soil to the mix, and adding and mixing in organic matter.
Planting cover crops, minimising tillage and disturbance to the soil, and adding a thin mulch layer to the soil can also improve soil structure to differing extents.
A layer of mulch (roughly 75mm depth) on top of soil can help with water retention (it can help prevent water evaporation), but, over time will break down and can help with soil structure too.
Straw, dried grass clippings and deciduous leaves can make a good mulch, but so can a commercial mulch.
Consolidated Checklist Of Tips & Steps To Working With Soil
Below is a summarized checklist of tips for managing, amending and improving soil, whether it’s existing natural soil, mixed soil, or imported new soil:
– Test the soil (to find out what it is, and the different properties and traits it might have)
– Get a second opinion from knowledgeable or expert sources about the soil (gardening store, online groups of people growing successfully in the area, professional soil testing and horticultural companies)
– Decide whether to work with/improve the existing soil, or whether to import new soil
– Consider whether to mix in other soil types with the existing soil
– Consider adjusting the soil pH
– Choose a type of plant that grows well in the local soil, and climate, weather and region
– Consider organic matter, mulch, fertilizer, pest control, and other soil fertility, soil productivity, soil health and soil quality practices
– Consider sustainable soil practices during the growing stage
Improve The Existing Soil, Or Import New Soil?
It makes sense to work with the existing soil if it is either naturally fertile, or the time and resources required to improve it will be worth the increase in fertility or production
But, there may be instances where it’s too costly or time intensive to improve the existing soil, and it’s better to import and work with new soil (for example – it may be too time intensive to do all of – adding organic matter (compost, manure etc.) to improve nutrient supply, adding fertilizer to improve supply of nitrogen and other nutrients, adding other soil types to the existing soil to improve soil texture, structure and drainage, adding mulch surface layers to improve water retention and structure, and adding various materials and soil amendments to increase or decrease the soil pH)
‘Problem soils’ like extreme clay soils that don’t significantly improve, even when mixed with other soil types, or when they receive soil additives, may be an example of a soil that is better to move on from.
Some people have found that adding sand to clay has hardened the clay up – just as one example.
And, when adding organic matter for better soil structure – this still takes time to see results.
Read more about two different people’s experiences growing in extreme clay soils in the provident-living-today.com, and the empressofdirt.net resources
Other soils that simply don’t grow anything in them, or aren’t productive, may be best to move on from too.
A very stoney/rocky soil might be one example of this.
Additionally, if it’s preferable to start growing straight away as opposed to waiting to have to improve the existing soil, it may be best to move on from the existing soil.
Mixing New Soils With The Existing Soil
There are some instances where the existing soil might be able to be significantly improved with the addition of one or two other types of soil to get a better balance of different sized soil particles.
We’ve put together guides on the different soil types, how to work with them and amend them, and what might grow well in each of them:
If we look at loam type soil for example, a good loam composition might consist of 40 percent sand, 40 percent silt, and 20 percent clay.
It should be noted that adding soil types together isn’t always a quick fix. ProvidentLivingToday.com for example found that adding sand to existing clay soil only hardened it up. Read more in the provident-living-today.com resource
Importing New Commercial Topsoil
Another option to to buy commercial topsoil.
This topsoil can usually either be spread on top of the existing soil base, or, be used completely independently by itself.
It’s also very feasible to start growing in above ground planter beds filled with commercial topsoil first (in a smaller area), and then move across to the natural soil once plants are ready.
Using Only Newly Imported Soil (& Not The Existing Soil)
In some instances, it’s better to neglect the existing soil, and import and use new soil only.
A few ways to do this might be by using:
– Raised garden beds with imported soil
– Grow in a greenhouse or polytunnel (the benefit of a greenhouse or enclosed space like a polytunnel is that factors like temperature and exposure to the elements and environment can better be controlled)
Why Some People Can’t Grow Anything In Their Soil
It usually has to do with either:
– the soil
– the climate (for example, in can be hard to grow a plant in a colder climate if it needs warm temperatures and lots of sunlight)
– the plant being grown,
– the way the plant is being grown,
– or some other local environmental or conditions based factor