Sandy, rocky soil of the Hyderabad Collective, displaying yellow to brown colour ranges

Like the soul is the core of a being, for us at Beforest, soil is our soul. It is where invisible life resides and where our food begins. On our journey to growing food forests full of diverse and colourful fruits and vegetables, our major efforts lie in building soil health. While there are many scientific methods to determine soil health, is it possible to know if the soil is healthy or fit for any type of cultivation just from its colour? Let’s find out.

What is Soil Made of?

Soil is a complex composition of minerals, organic matter, water, air and living organisms, and it varies in composition depending on factors like location, climate, and geological history. A combination of these essential soil elements is what makes or breaks its soil health. 

Healthy soil is the foundation of a robust plant system. It is characterised by balanced nutrients, good structure for aeration and root growth, sufficient organic matter, neutral pH, proper drainage, and a diverse ecosystem of microorganisms and macroorganisms. The lack of these can make soil unhealthy and hinder plant growth and soil vitality, posing a challenge for agriculture. Hence, proper soil management is key to improving soil health.

To learn more about what makes soil healthy, how to check soil health and what is needed to bring soil to health, read our blog on Components of Healthy Soil for a Thriving Food Forest.

It is important to note that the overall soil composition is what determines its suitability for a particular land use. While soil colour indicates health, crop suitability, and environmental conditions, soil composition reveals nutrient content, texture, and pH levels, helping tailor fertilisation, irrigation, and pH adjustments. Combining both aspects informs crop selection, nutrient management, and irrigation practices, optimising productivity and sustainability in agriculture.

Soil can be found in a range of colours. You must have learnt about the common red and black soils in school, upon which specific types of crops are cultivated. But is there more to soil colour and what it speaks of? And as a farmer or gardener what should you know about soil colours? Let’s explore below.

What Gives Soil its Colour?

The colour of soil is influenced by several factors, including the mineral composition, organic matter content, and environmental conditions. Different soils can range from red and brown to grey, black, yellow, and even white. These colours of soil are chiefly determined by its mineral composition, especially iron oxides. Red and yellow soils have high iron oxide content, making them reddish or yellowish. Dark, black soils contain abundant organic matter, such as humus, giving them a dark, fertile appearance. Grey or pale soils result from poor drainage and reduced oxygen levels. Sandy soils appear light due to their larger, less moisture-retentive particles.

While organic matter darkens soil, environmental conditions, like drainage and aeration, impact soil colour vibrancy. Weathering over time can alter the colours as iron oxides form in response to oxygen and moisture exposure. Soil scientists use standardised colour charts such as the Munsell system for accurate colour description and classification.

Now, let;s dive into what are some of the common soil types and the types of growth they are ideal for.

Common Soil Colours and What They Indicate

Red Soil

Red soil derives its colour from iron oxides, particularly hematite. This soil type is often found in well-drained, aerated areas with a history of weathering of iron-rich rocks, releasing iron into the soil. Naturally it is common in regions with high temperatures and rainfall, particularly in tropical & subtropical regions, which encourages soil development. 

Red soils are typically fertile with good drainage, making them suitable for a range of agricultural production. They are suitable for a variety of staple as well as cash crops, including cotton, soybeans, groundnuts, millets, pulses, oilseeds, and certain fruits like pomegranates and grapes. They are also stable and less susceptible to erosion due to their particle aggregation, and can add to the aesthetic beauty of some landscapes.

Black Soil

Black soil owes its colour to a high content of organic matter, particularly humus. This type of soil is often very fertile, as organic matter enhances nutrient content and moisture retention. Hence, these soils are ideal for agriculture and are well-suited for a variety of high-yielding crops due to their fertility and moisture-holding capacity. They are often referred to as ‘regur’ or ‘black cotton soil’. They are ideal for growing cotton, oilseeds like soybeans and groundnuts, cereals such as sorghum and pearl millet, pulses like chickpeas and pigeon peas, and various vegetables. 

They are generally resilient and have good structure, making them less susceptible to erosion. The dark colour can also provide a striking contrast in landscapes and can be aesthetically pleasing and for all these reasons they are highly valued in the Indian subcontinent.

White/Pale/bleached Soil 

White/pale/bleached soil often has its colour due to a lack of organic matter and low mineral content, leading to a pale appearance.  These soils often lack the nutrients and moisture-retaining capacity necessary for robust plant growth. Pale soils may also indicate poor drainage or water logging issues, which can affect soil quality and make it more susceptible to erosion. Some pale soils may be acidic and may require significant amendments and management practices to make them suitable for agriculture or other uses.

Yellow-brown Soil

Yellow soil obtains its colour from the presence of iron oxides, particularly goethite, which imparts a yellow hue to the soil. They often form in areas with good drainage and aeration, promoting healthy soil conditions. These soils may contain essential nutrients, indicating moderate fertility and making them suitable for agricultural activities with proper management. Yellow soils are also often close to neutral pH, making them suitable for a wide range of crops.

Brown Soil

Brown soil typically derives its colour from the presence of iron oxides and organic matter (lesser than black soils). Like yellow soils, they are moderately fertile, making them suitable for agriculture with proper management. They contain a range of essential nutrients for plant growth and require fewer amendments than paler soils to maintain fertility and productivity.

Grey Soil

Grey soil gets its colour from poor drainage and reduced oxygen levels, often leading to a pale, greyish appearance. Prolonged waterlogging in these soils can deplete nutrients and organic matter, reducing soil fertility. The lack of aeration can result in acidic conditions in some cases 

Is the Colour of Soil the Same at the Top & in its Depths?

Soil colour can differ between the surface and deeper layers due to several factors. Topsoil, the surface layer, often contains more organic matter, like leaves and humus (formed from the decay of plant and animal organic matter), making it darker in colour. In contrast, deeper layers may have less organic matter, appearing lighter. The mineral composition varies with depth; specific minerals like iron oxides can be concentrated at certain depths, affecting colour. Environmental factors like moisture and aeration impact the surface soil more, leading to colour changes. Soil compaction from human activities and natural disturbances can alter surface soil colour as well. Weathering processes also vary with depth, as exposure to oxygen and water can change mineral composition and colour.

Whether it is a dry landscape like that of the Hyderabad Collective, or a tropical rainforest ecosystem like our 3 collectives in Coorg (Poomaale 1.0, Poomaale 2.0 and Co-Forest), understanding soil colours is crucial for farming and landscape development. This knowledge helps us make informed decisions, implement effective soil management practices, and optimise resource allocation, ultimately leading to more productive and sustainable agricultural and landscaping projects.