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Soil and Nutrition

United States Department of Agriculture


Soils consist of layers which can vary in thickness enormously from one habitat to another.


  • O-Surface litter– covers the soil, typically only a few centimeters thick and consisting of fallen leaves.
  • A-Topsoil – just beneath the surface litter, the uppermost layer of soil extending usually 10-30cm deep. Typically not acidic and contains 10-15% organic matter (gives dark colour).
  • B-Subsoil – layer 30-60cm below the soil surface consisting of larger soil particles. Contains relatively little organic matter (lighter in colour). In many regions, this layer contains large amounts of minerals washed through from upper layers. Mature roots commonly extend into this layer.
  • C-Weathering bedrock – consists primarily of rock fragments, and usually lack organic matter.


Weathering breaks rocks into progressively smaller pieces, the smallest of which are called soil particles. These particles consist of minerals, which are naturally occurring inorganic compounds that are used by plants as building blocks. All soils contain three kinds of soil particles: sand, silt, and clay. Sand particles are the largest (0.02-2mm), clay particles are the final product of weathering and the smallest of these particles (<0.002mm), and silt particles are between sand and clay particles in size. Different kinds of soils contain different proportions of sand, silt,and clay. Soils having a mixture of sand, silt, and clay-the soils in which most plants grow best- are called loams. Sandy soils are usually nutrient deficient and poorly suited for growing crops.

Humus is the decomposing organic matter in soil (usually 1-30%). The lightweight and spongy texture of humus increases the water retention capacity of the soil. Humus swells and shrinks as it wets and dries aerating the soil. Acts as a reservoir of slow release nutrients, and a habitat for many organisms that mix and concentrate nutrients.

Soil acidity, measured as pH, directly affects the availability of nutrients in the soil.  In soils with low organic materials, pH has a greater influence on nutrient availability.  pH of about 6.5 is best

The most important things to know about the soil in your allotment or garden is the pH - whether it is acid or alkaline, the amount of organic matter present, the soil type and nutrient levels. Once you have determined some basic characteristics, you can manage your soil in such a way that it will reach its peak condition."

Taking Matters Into Your Own Hands

Knowing what your soil consists of will help with managing it. Pick up a handful and roll it into a ball - if it feels gritty and falls apart, it is a sandy soil. If it contains clay, it will roll into a ball and stick together. Peaty soil is almost black and spongy to touch, while chalky soil is light and contains chunks of white flint or chalk. Loamy soil is brown and crumbly, and silty soil feels silky and won’t form a ball.

Get Digging

Soil can vary within your allotment, regardless of how small an area it is, so find out if there are any significant variations in soil depth. Dig down a couple of feet to find out what lies underneath the soil. For example, new houses often have turf lain on top of building rubble, meaning that the soil needs a lot of work to improve its condition.

Do You Want Lime With That?

pH is an important soil property to consider for maximum productivity. In Scotland the soil tends to be acidic, but it’s important to check the pH to establish this. An application of lime is a cheap and easy treatment to increase soil pH and lower its acidity, thus increasing the soil’s fertility.

Finding The Right Match

Different plants thrive in certain soil types, so it’s worth considering the soil you have and the plants that grow best in that environment. For example, tubers, such as potatoes and turnips, like a lot of potassium in the soil, while hydrangea macrophylla flowers change colour depending on soil pH.


To request a soil testing kit from the Macaulay Land Use Research Institute’s soils service, contact 01224 395115 or visit 'macaulaysoils', for more details.

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