Types of soils and their characteristics in agriculture

Responsible farming is all about growing and building healthy soils in addition to crops. Soil fertility and biodiversity are directly related to crop productivity and nutrition, and determine the long-term agricultural productivity of a piece of land. Farming communities around the world are experiencing the effects of poor soil management caused by industrial agriculture, where continuous soil degradation and erosion have created infertile dust bowls that are near impossible to cultivate. Understanding different soil types are key to their sustainable management, and this article will break down everything you need to know about soil types and stewardship so that you can make the most of your soil.

Types of soils and their characteristics with key features

Soils are typically grouped into six categories depending on their chemical composition, which determines how water and nutrients are retained and dictates which crops are most suitable for growing in them. Soil composition can be sand, clay, loam, chalk, peat, or silt-based, and many soil systems will have fluctuations throughout them with patches that have higher concentrations of one component than another. Let’s break each soil type down into its key features and characteristics, and how this translates in an agricultural context.

Sand

Sandy soils are, as you may have guessed, highly concentrated with sandy particles that create a very grainy but light growing medium. Characteristics include the quick drainage of water and other fluids, easy workability, and being soft and malleable for digging. Since water drains efficiently through these soils, they are often noted for their lower nutrient availability and tendency to heat up and dry out quickly. Sandy soils also often have a lower average pH, making the most appropriate for plants that appreciate slight acidity in their soil profile.

Crops that appreciate well-draining soil and hot, dry conditions thrive in sandy soils. This includes herbs native to Mediterranean regions like rosemary, thyme, and oregano, as well as several species of tree such as bay laurel, fig, and olive trees. The loose texture and lightness of the soil also make it easy for root vegetables to grow and expand without being impeded, so carrots, beetroot, parsnips, radish, and turnips are also compatible with this soil type.

Clay

Clay soils are just about the polar opposite of sandy soils, being very heavy with poor drainage capabilities. Since clay particles are so minuscule, the soil texture becomes much firmer and easily compacted, leaving few pathways for water to drain out. Although the poor drainage of clay soils often makes them undesirable for agricultural purposes, they do typically contain high levels of nutrients and minerals that can be beneficial for certain crops. Certain fruiting trees and vegetables in the Brassica family can tolerate clay soils but will grow best in a combination of clay/loam soil where they can uptake nutrients but also benefit from improved drainage.

Silt

Silty soils have a distinct silky and soft feeling, are typically quite fertile, and have the ideal balance of decent nutrient density without terrible drainage. Silt soils are usually easy to grow most crops in, although amendments for drainage may be needed for optimal crop performance. Silty soils don’t compact as easily as clay soils and they are softer and lighter, however, they do lack a robust structure in their soil profile that can be improved through the planting of perennial crops whose root presence holds them together. Perennial bushes and trees that enjoy moistly, fertile conditions are often the best option for silty soils.

Loam

Loamy soils are described as a balance between different combinations of the aforementioned soil types of sand, clay, and silt. This is one of the most desirable and fertile soil types due to its ‘best of both worlds’ characteristics which means it contains the benefits of all three soil types it is made up of. Loamy soils have good drainage, high nutrient availability, a well-structured profile, and are slow to heat up and cool creating a relatively temperature-stable environment for crops.

Most fruits and vegetables will grow very well in loamy soils, however since its composition is a somewhat delicate balance of three other soil types, it needs to be well maintained to prevent one component from taking over and tipping the scales. Crop rotation is one of the best things for this soil, as it prevents the repeated planting of one single heavy feeder from depleting the soil of all of its beneficial traits.

Chalk

Soils that are chalky or rich in lime are characterized by their alkaline pH, due to the high concentrations of calcium carbonate present. These types of soils and their characteristics usually originate from being on top of limestone or chalk bedrock and are often most arable when amended with organic matter and sulphuric fertilizers to improve nutrition and lower pH. Chalky soils do tend to have excellent drainage due to the presence of larger particles and rocks, but these can also impede the growth of certain root vegetables.

Peat

Peat soils present the opposite characteristics to chalk, as the presence of peat- which is decaying organic matter- creates acidic conditions that need to be alkalized for the successful growth of most crops. Peat soils are light and fluffy and have a springy texture that soaks up water like a sponge. Drainage is the main issue in peat soils, but they can be amended with lime or chalky soils (and vice versa) to balance out the acidity and improve drainage.

Determining Soil Types

Since the six soil categories are distinguished by particle size, testing where on the spectrum your soil lies is all about feeling the grittiness versus smoothness of your soil, how easily it falls apart or glues together, and leaving it in a medium of water to see how the particles settle. Soil test kits can provide detailed breakdowns of your soil profile, so for a conclusive diagnosis of the characteristics of your soil, you should purchase a professional testing kit.

The color of your soil can also be indicative of certain soil types, for example, peat soils are dark in color and can be almost black depending on the percentage of peat content. Chalk soils, their opposite, will often have a white layer of dust or obvious chalk particles in the soil that make it instantly recognizable. Aside from this, there are two main tests you can do yourself to figure out what kind of soil you have:

See how it holds its shape

Grab a small handful of soil and squeeze it tightly with your hand for a few seconds before release. Observe the soil to see how it maintains or does not maintain its shape after squeezing. Clay soils will be very mouldable and will keep the shape they have been squeezed into for a long time after release. Sandy soils will typically fall apart upon squeezing or become very crumbly. Peat soil may release moisture upon squeezing, and bounce back a little upon release, like a sponge. Loam and silt soils will feel similar, very smooth and silky, and will keep their shape for a short period after release until they fall apart.

Observe how the particles settle in water

Place a good scoop of your soil in a large container of water, stir it, and then let it sit for around 10-12 hours. Afterward, observe how the particles have settled or dissolved in the water, as this indicates particle density and can be used to assess soil type.

Sandy soils have heavy particles that will settle at the bottom of your container in a thick layer and leave the water almost completely clear. Both clay and silt soils have the opposite effect, leaving cloudy water with just a thin layer of residue at the bottom of the container. Loamy soils will also leave a thin layer of particles at the bottom of the container, in addition to a layer of very light particles at the surface, and the water will be mostly clear but just a little cloudy. Peat soils look similar to loamy soils except they will have more of the lightweight particles floating on the surface of the liquid and only a very fine layer of heavy particles at the bottom. Chalk soils will leave the water tinged with grey, and the particles settled at the bottom of the container will likely be white or grey in appearance.

How to Make the Most of Your Soil

Utilizing your specific soil type to your advantage is dependent on what crops you are trying to grow and their preferred conditions, but no matter the type of soil you have you should make sure to steward it by employing good and healthy soil management techniques.

Usually, soils that have an even balance of good drainage, nutrient availability, and robust structure are ideal for crops, like loam or silt-based soils, and if you are planning to grow a variety of frequently rotated crops then a fairly neutral pH is best. If you have clay-heavy or super sandy soil, you can add amendments to balance out their undesirable characteristics or grow crops that are well-suited to your soil type. You are never 100% limited by the type of soil you have, but amendments will need to be consistently added in order to maintain a type that is opposite to yours – like clay to sand. Soil is also not monogamous over large areas, so try to grow well-adapted crops wherever they are expected to thrive the most to save yourself time and money trying to permanently change conditions.

However, for cases when you do need to amend your soil to better suit your needs: lime can be added to very acidic soils, like peat-based ones, to raise the pH and make them more alkaline. Conversely, aluminum sulfate will lower your soil pH and create more acidic conditions in very alkaline or chalky soils. Organic matter can be consistently added to chalky soils over time to create a build-up of nutrients and minerals that will gradually make them more hospitable to more crops. In fact, organic matter like compost or well-rotted manure is a very productive addition to most soil types to improve their structure and balance them out. Clay soils that suffer from poor drainage will become more aerated and loose with the addition of organic matter, and sandy soils will benefit from the nutrient addition and moisture retention they offer.

Aside from the addition of organic matter, key techniques for maintaining great soil health include mulching around crops and covering cropping beds in the winter. Mulching with straw, wood chips, dead leaves, or using a living mulch-like clover benefits your soil by cooling the soil surface, retaining moisture, and suppressing weed growth. Exposed soils are more likely to be eroded by wind and rain, or to become baked and deserted by the sun, neutralizing beneficial microorganisms and reducing their overall fertility. For this same reason planting a cover crop, like clover, alfalfa, or legumes, in exposed soil beds after you have harvested your crops at the end of the season ensures that the soil is protected during the winter. Cover crops offer the same benefits as mulches, but also support good soil structure and drainage with their root systems and can be harvested in the spring for use as green manure. Another key component of healthy soils is their richness in organisms and life-like mycorrhizal fungal networks, beneficial bacteria, and a diversity of insect species. These are often at high risk of being destroyed through excessive applications of synthetic chemical fertilizers and pesticides, or through the consistent disturbance of soil through practices like rototilling.

Employ responsible practices and use sustainable inputs that will encourage biodiversity in your soil ecosystem, so that your land can be farmed for many years to come and that the crops grown in it will be highly nutritious. Making the best use of your specific soil type is all about maintaining good soil health and growing region-appropriate crops, whilst adding amendments when needed. Consider what crops are native to your area and how they might be well suited to both your soil and climate and conduct soil tests to better understand the specifics of your soil chemistry. Whatever your soil may be, build on it constantly by mulching, adding organic matter, and sowing cover crops and you will see the benefits it offers your crop quality and yields over time.