All types of soil and how to recognise them

Soil supports plants by providing anchorage. At the same time, it functions as a reservoir for water and nutrients, which plants absorb through their roots. Viewed from close up, soil is a system made up of:

• A solid mineral part with elements of various nature and size

• A non-mineral solid part (organic matter)

• Multiple living organisms (bacteria, fungi, insects, earthworms and the like)

• Water and air inside the pores, that is, the spaces between the solid components

The fertility of a soil depends on the entirety of its physical, chemical and biological properties including texture, porosity, structural stability, nutrient availability, pH and the microorganisms that inhabit it. Today we’ll take a look at some of the key properties – texture, structure, pH – and see how to classify soil types based on them. Better understanding the soil of your garden or vegetable patch helps you to understand its "pros" and "cons", and if necessary intervene in the most appropriate way to improve it.

Sandy, silty and clay soils: texture

The solid mineral part of soil is composed of a ‘skeleton’ (or matrix) and fine soil. The skeleton includes the coarsest elements (i.e. those larger than 2 mm in diameter) from gravel to pebbles. Fine soil includes particles with a diameter from 2 mm to a few thousandths of a millimetre, so in decreasing order by grain size: sand, silt, clay.

The texture or particle size of a soil indicates the composition of the fine soil, that is to say, its content of sand, silt and clay. So, depending on its predominant elementary particles, a soil is defined as sandy, silty or clayey. Within these primary types, the many possible particle size combinations give rise to “intermediate” soil types; for example, soil can be sandy-clayey or sandy-silty. A soil with a balanced mixture of sand, silt and clay is called loam or medium-textured soil. The relative amounts of these components in soil significantly impact its chemical, physical, and biological properties, ultimately affecting the productivity of your vegetable patch and the growth of the plants in your garden.

Texture impacts on your gardening work by determining how easily water and air can flow through the soil (permeability), its capacity to hold water, the availability of nutrients, and how workable the soil is for cultivation. Sandy soil is very porous, so it allows for good aeration and drainage, but it has low water and nutrient holding capacity, making it a poor substrate for plant growth. However, it is good for growing vegetables: it requires more frequent but less abundant irrigation, and the same principle applies to fertilisation; it is also easy to cultivate both dry and wet.

On the contrary, clay soil has a very fine porosity, which makes it less aerated and permeable, as well as more fertile because it retains nutrients better. It also retains water, potentially causing stagnation and making it difficult to work with (muddy when wet, hard when dry). Silty soils have intermediate characteristics between sandy and clayey soils. Soils are categorised as light or heavy based on how easy they are to cultivate, with light soils being mainly sandy and heavy soils being mainly clayey.

How do you determine if the soil in your garden is sandy or clayey? You can take a sample to the laboratory for analysis or you can get a general idea of the texture with a simple hand-feel test. All you need to do is wet a small amount of soil, knead it with your hands into a ball, then rub it between your palms and feel it with your fingers: sandy soil feels gritty and crumbles, whereas clayey soil is sticky and plastic – that is to say, it is mouldable, but remains compact. Silty soil is also plastic but slippery, not sticky.

Texture is an intrinsic property of soil, in other words, it is a relatively stable or intrinsic property. In a small plot like a vegetable patch, however, you can lighten heavy (clayey) soil by adding and mixing in sand.

Soil types and structure

Unlike texture, the structure of soil is a dynamic characteristic that changes over time. The structure of a soil defines the way in which the elementary particles – sand, silt and clay – are arranged in space or clump together. The structure determines the porosity of the soil and, in part, its fertility. The growth potential of plants specifically depends on adequate water and air circulation in the soil, suitable moisture and temperature, sufficient rooting space and the ability of living organisms to thrive. This occurs thanks to the pores, or gaps, between soil particles, the size of which is related to the size of the particles themselves, i.e. the soil’s texture.

The ratio between micropores and micropores in the soil should be balanced: macropores contain air and facilitate the drainage of excess water, while micropores retain it. When macroporosity is excessive – as occurs in sandy soils – the soil is unable to store water. This is why, as we said, if you have sandy soil you need to water it less but more often. Conversely, when microporosity is excessive – as in clay soils – water stagnates and a lack of air is created, thus the soil becomes inhospitable for roots and living organisms.

We mentioned above that soil structure is dynamic, i.e. not stable. It is affected by various factors, such as heavy rain, trampling, the availability of certain chemical elements and the presence of organic matter. All of these things contribute to weakening soil stability over time, especially in heavy soils: for plants, the ideal structure of fine-textured soils like silt or clay is granular or lumpy, with the right proportion between solid aggregates and pores.

What can you do to restore good soil structure so your crops can thrive? Cultivating soil with a spade, rotary tiller or two-wheeled tractor can improve the degree of aggregation of a compacted substrate. Be careful though, because if you cultivate soil badly you could damage its structure: this can happen if the cultivation is done too frequently or aggressively, or on very wet/dry soil.

On the other hand, to improve the structural stability of any type of soil, you can add organic matter and incorporate it during cultivation, for example in the form of compost, manure or pelleted manure. As we said before, another soil improver to improve the structure of a very clayey soil is sand.

Acidic and alkaline (or basic) soils: pH

The texture and structure of a soil must be evaluated also considering its pH or reaction, that is, the measure of its acidity or its alkalinity/basicity. A soil can be categorised based on its pH, with those having a pH below 5.5 considered highly acidic, those with a pH above 8.5 considered highly alkaline or basic, and those with values around 6.6–7.3 considered neutral. Why is knowing the pH of a soil important? Because this property affects various aspects, in particular the availability of nutrients for plants and therefore the fertility of the substrate.

How do you measure soil pH? Just use some litmus paper or a pH meter. Litmus papers are inexpensive and easy to use: take a soil sample and mix it in a container with an equal amount of distilled water. Dip a litmus paper in it and compare its colour with the colour scale on the litmus paper packaging.

Individual plant species have specific pH preferences, although most plants can tolerate a pH range between 5.5 and 8.2. However, to modify or correct the pH of an acidic soil you can use substances containing calcium – such as hydrated lime or wood ash – or water the soil with calcareous water. Conversely in the case of alkaline soil, you can add peat or acidic compost, or make an acidic mulch with natural material (for example, sawdust and conifer needles). Here you can learn more about how to acidify soil and how to make your own compost.

In addition to climate and the use of fertilisers, pH depends on the inherent nature of the soil itself, including its buffering capacity. This means that over time, soils tend to return to their original pH value, so pH measurement and correction should be repeated periodically.

A soil that is no longer fertile is defined as depleted: to round off, we recommend this article on how to prevent soil fatigue and regenerate exhausted soil.