Humus

Humus is dark soil.

It can be black or brown. It is made from old plants and animals. Tiny living things help break them down. Earthworms also help make it.

This dark stuff helps the ground hold water. It also has food for plants to eat. This makes the soil very healthy. It is a special part of the earth.

Humus is a dark part of the soil. It is often black or dark brown. It comes from dead plants and animals. Tiny living things like fungi and bacteria help break them down. Earthworms also help make it. This way of making humus is called humification.

Humus is full of nutrients. These are like food for plants. It has a lot of nitrogen and phosphorus. It also has sulfur. Humus helps the soil hold water. This is very helpful in sandy soil. It also helps the soil stay healthy and strong.

There are different kinds of humus. In some places, it is called Mull humus. This happens when earthworms mix it with the soil below. In other places, it is called Mor humus. This happens when the soil is very acidic. In those places, the humus stays in a thick layer on top. It does not mix with the soil underneath. This can happen under certain types of woods.

Scientists use letters to study soil layers. The organic layer at the top is the O horizon. The topsoil is the A horizon. Humus makes these layers look very dark.

Humus is a dark and important part of our world. It is the dark brown or black material found in soil. This material comes from organic carbon. Organic carbon is the stuff that makes up living things. Humus is found in the organic horizon, which is the O layer at the very top of the soil. It sits just below the leaf litter. This layer is very important for keeping the Earth healthy.

How does humus form? This way of working is called humification. It starts when plants and animals die. Tiny living things like bacteria and fungi begin to break them down. Earthworms and other small creatures also help a lot. They eat the dead matter and move it through the soil. Some parts of the dead matter break down very fast. Other parts, like wood or bark, take much longer. Eventually, these pieces turn into a dark, spongy substance.

Scientists have studied this for a long time. A scientist named Selman Waksman shared ideas about it in textbooks. He thought humus was different from regular decomposing matter. He described humus as having a jelly-like look. It does not have the same shapes as the plants it came from. Today, researchers use special tools to look at it. They use light microscopes to see tiny remains. They also use radiocarbon dating to see how old it is.

There are many facts about what is inside humus. It is full of nutrients like nitrogen and phosphorus. It also has smaller amounts of sulfur. The ratio of carbon to nitrogen is usually between 8:1 and 15:1. The middle number for this ratio is about 12:1. Humus also helps the soil hold onto water. This is very helpful for sandy soil that usually dries out fast. It can even help make the soil better for farming.

Different places have different kinds of humus. In forests with acidic soil, you might find Mor humus. In this type, the humus stays in a thick layer on top. It does not mix with the soil underneath. In places with less acid, you find Mull humus. Earthworms mix this humus into the soil below. This makes the soil very rich. Some special soils, like the Amazonian Dark Earths, were made by this process long ago.

Humus is a complex, dark substance found in the earth. It is formed by the accumulation of organic carbon. This material is essential for healthy soil and agriculture. Humus typically appears black or dark brown in color. It is located in the organic horizon, also called the O horizon. This layer sits on the very surface of the soil. It is found just below the layer of leaf litter.

The creation of humus is a process called humification. It begins when plant and animal substances die. This dead material is known as detritus. Small organisms like bacteria and fungi start to break it down. Earthworms and other invertebrates also play a massive role. They eat the organic matter and move it through the soil. As they work, they help transform the material. This process turns rough organic remains into a dark, spongy substance. Humus is amorphous, meaning it lacks a clear cellular structure.

Humification follows a specific sequence of chemical changes. First, simple compounds like sugars and starches decompose very quickly. Next, more complex materials like cellulose and lignin break down slowly. Lignin comes from parts like wood and bark. White-rot fungi are especially good at transforming this lignin. During decomposition, polyphenols are chemically modified. These molecules join together to form very large, complex structures. Proteins, amino acids, and amino sugars then attach to these bases. This chemical process gives humus its unique properties.

There are three distinct types of humus based on soil conditions. Mor humus occurs when decomposition is very slow. It is usually found in acidic soils under conifer woodlands. In this type, a thick layer of litter sits above the humus. There is little mixing with the mineral soil below. Mull humus is the opposite type. It occurs in low-acidity soils like deciduous woodlands. Earthworms mix the humus into the underlying mineral soil. This creates a very rich, well-mixed environment. Moder humus is a transitional form between these two types.

Scientists have worked for decades to understand this substance. In the 1940s and 1960s, researchers tried to analyze it chemically. However, they found it very difficult to study. The chemicals they used often reacted with the organic matter itself. This created many errors in their results. Some scientists, like Selman Waksman, described humus as a jelly-like substance. They believed it was completely different from regular decomposing matter. Today, many researchers use the Soil Continuum Model to explain it. This model suggests a gradual transition from detritus to humus.

Humus provides many vital nutrients for plant growth. It contains high amounts of nitrogen and phosphorus. It also contains smaller amounts of sulfur. The carbon-to-nitrogen ratio, or C:N ratio, is very important. This ratio typically ranges from 8:1 to 15:1. The median value for this ratio is about 12:1. Humus also helps the soil's physical structure. It decreases the bulk density of the soil. This makes the soil less packed and easier for roots to grow. It also helps sandy soils retain much-needed moisture.

Some forms of humus are incredibly stable. Much of the humus in our soil has lasted over 100 years. It stays stable because it is hidden inside small soil particles. This is called occlusion. It can also be tightly bound to clay minerals. This stability helps with carbon sequestration. A famous example is the Amazonian Dark Earths. These fertile soils may have formed from the slow oxidation of soil carbon. This process was likely aided by the mixing of charcoal and earthworms.