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Why is Soil Black: Humus, Nutrients, Minerals & Moisture

The black color attributed to soil is due to factors including nutrients, minerals, and moisture content.

Black soil is an indicative sign that the soil has been enriched with nutrients caused by the breakdown of organic matter, done by microorganisms. The presence of carbon-rich compounds and minerals indicates the soil is highly fertile. Increasing the moisture content also darkens its color.

In this article, we explain what causes soil to be black and how it affects the growth of both potted and garden plants.

What Gives Soil A Black Color?

Soil Minerals and nutrients

There can be various reasons behind this. Here are some of them:

  • The decomposed organic matter, known as Humus, has different compounds, namely Humic Acids and Humins. A high amount of carbon causes the black hue of these compounds. As a result, your potting soil also has a black color. 
  • The mineral content in some potting soil makes it black.
  • Manganese oxide, iron sulfide, and iron pyrite also cause a black hue.
  • The Micro-fauna population in the soil is one of the reasons.
  • If the moisture level is higher, the growth of organic matter gets augmented. It, too, causes the black color of the soil.

Again, the high water content in soil may make the earth seem black by hindering the oxidation rate. The charcoal content in the soil may also give it a black tint.

See our detailed article – Why is topsoil dark in color

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Is Black Soil Good For Potted Plants?

To answer this question, I have to see if black soil has the qualities that a good potting soil or a soil good for potted plants should have.

The following qualities make up good potting soil:

  1. Potting soil first soaks up the water and then discharges to remain evenly hydrated and not soggy. 
  2. It is well-draining as well as moisture-retaining.
  3. Potting soil has slow-release, water-soluble organic ingredients that break down over time to provide the plants with nutrients throughout the season.
  4. Good potting soil has a uniform texture. It contains a sufficient amount of clay particles and sand or gravel to help in good drainage of excess water and root growth.
  5. It cares to provide ample air to promote the growth of roots.

Now, let’s see how many of these criteria black soil meets.

  1. Black soil can provide good hydration to the earth. It is moisture-retaining as well.
  2. It is well-drained too.
  3. The humus in black soil works as its slow-release fertilizer by being degraded by anaerobic organisms.
  4. It contains a higher proportion of magnesium, calcium carbonate, and potash. So, it is highly fertile
  5. It even provides good aeration to the roots.

In view of the above, I can say that black soil can serve well as potting soil. But not for all plants. Why?

  1. This soil is clayey and not fluffy like potting soil.
  2. It lacks significant elements, namely nitrogen, and phosphorus.
  3. It gets too compacted and sticky when watered.

Therefore, Black soil, also known as Regur, performs well if used for growing crops such as cotton, wheat, tobacco, millets, etc.

Are All Planting Soil Black?

wet potting soil

Not really. 

Potting soil turns black when it contains ample portions of humus. Potting soil may have different other hues too.

  • Red-colored soil has a considerable percentage of iron oxide and aluminum oxide, which helps in excellent drainage.
  • The soil having the yellow color contains oxidized ferric iron oxides. This soil can retain moisture.
  • Orange soil indicates the presence of excess iron in the water.
  • Sulfur causes the gray color of the soil.
  • The soil turns white when too much salt and silicates are present in it. 

Hence, it is pretty clear that the hue of potting soil depends solely on the elements present in the ground.

What Is Black Earth Soil?

Black Earth Soil is also known as Chernozem. It can be seen in the low-lying, marshy zones. This type of soil is rich in humus which consists of high amounts of NPK, humus, phosphoric acid, ammonia, and phosphorus. 

Abundant nutrients in the Black earth soil help improve poor and sandy soil’s water and nutrients retaining capabilities. This soil is too compelling for flowering and vegetable plants that are highly nutrient-dependent.

This soil is suitable for planting annuals, perennials, etc. as well. It is generally mixed up with compost and then used at the bottom of the pot.

Topsoil Vs. Potting Soil

Let’s see the differences between topsoil and potting soil at a glance.

TopsoilPotting Soil
Topsoil is a mixture of sand and clay used in the garden.The mixture of peat moss and other organic matter is called potting soil. It is for use in pots or containers. 
It has a heavy texture for holding a lot of water.It has a fluffy texture as it dries out rapidly.
It does not cost much.Potting soil is expensive.
Topsoil may contain clay and composted manure. So, different soil bacteria and fungi get accommodation in it.Potting soil is formulated stringently. The soft materials in it give the roots ample space to grow further. Moreover, there cannot remain any kind of diseases.
It holds moisture for a more extended period.It lets the excess water run off the pores at the bottom of the pot.
Topsoil requires to be mixed with compost or vermiculite to get a better result.Potting soil is already too fertile, refined, and ready to use.
It is generally used to fill in low areas and sidewalks.It is made especially for planters, window boxes, hanging baskets, and container gardening.

Albeit topsoil and potting soil seemingly look the same, there are specific differences between the two.

Why is My Soil Turning Black?

 Soil will turn black due to an increase in organic matter and minerals, or when watered. The change in soil color affects both potting and garden soil. 

Here are the explanations for each case that cause soil to turn black – 

When Watered –

When Watered the soil is moistened with water which enhances the dark color of the soil itself. Unwatered or dry soil will tend to have a much lighter color which may sometimes even seem closer to a grey hue.

But as soon as water hits the surface the color changes and deepens to the black color soil is known for.

Note: A dark color soil when watered doesn’t mean that the soil is fertile as it should be.

Organic Compounds –

Soil with high levels of organic matter will be darker. This is due to the fact that all living organisms are made of almost entirely of carbon-based compounds.

Carbon is dark in nature which is a characteristic of decaying organic matter because the compounds are being broken down into their basic elements. 

These elements eventually break down further in the soil by microorganisms that release nutrients for plants to use.

Organic matter such as leaves, grass clippings, and manure is good for the soil. They provide nutrients and help keep the soil loose and friable. These materials also add organic matter to the soil, making it darker.  

Minerals –

Black soil is also caused by iron oxide (rust) or manganese oxides. It occurs when there are high levels of iron or manganese in the soil.

Iron is an essential element for plant growth. Iron deficiency leads to stunted growth.

Manganese is also needed for optimum plant health. Manganese deficiency results in leaf chlorosis and reduced fruit yield.

I use this cost-effective soil amendment which enhances the soil and gives my plants the best growing conditions.

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Signs of Infertile Soil

Infertile soil lacks the essential nutrients required for plant growth.

Macronutrients: Nitrogen, Phosphorus, Potassium (N,P,K)
Micronutrients: Iron, Boron, Calcium, Magnesium, Sulphur, Zinc etc.

One of the first signs of infertile soil is when plants do not grow well. If you notice that your plants are dying or look sickly, then you should check the soil quality.

Little or no microbiological activity is also a sign that the soil may not be fertile 

Infertile soil may be caused by :

Overfarming – 

Overfarming is when farmers grow the same crop over and over in the same plot of land, which causes the nutrient availability in the soil to significantly decrease, which increases the risk of crop failure.

This problem is especially common where farmers do not know how to properly use fertilizer and pesticides. The solution to this problem is to teach farmers about proper farming techniques and help them implement these methods into their crops.

Erosion –

Erosion is when water flows away from a landmass, leaving behind a depression. This is caused by either gravity or wind. Erosion occurs when the soil gets exposed due to natural causes such as drought, excessive rain, or human activities like construction.

Soils lose their ability to hold water because they become compacted. Plants need water to survive. When soils become compacted, they become hard and dry. As a result, they no longer retain water.

Compaction happens when the soil becomes so dense that it can’t absorb water anymore.

Soil compaction leads to erosion.

Leaching –

Soil leaching is when rainwater seeps into the ground through cracks in the soil and then flows out of the ground. This water may be polluted and can cause serious damage to plants and animals. The best way to prevent soil leaching is to keep your lawn watered properly. If you live in a dry area, you should install a drip irrigation system.

To fix these issues, you need to dig up the affected area and remove any debris from the soil. Then, add organic matter such as composted manure or peat moss to increase the fertility of the soil.

The Takeaway:

Several reasons may cause the soil to be black. But, black soil is well-aerated and nutrient-rich. So, it is excellent for growing specific plants.  

Black soil does much good to the plant. It is replete with carbon-rich humus, which gives the earth a black hue. Black soil alias Regur is also called Cotton Soil. Deccan Trap Region is the hotspot of this type of soil. 

Again, the black color may point to anaerobic bacteria. Then the earth might be stinky with a malformed appearance.

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