Growing Food: A Guide to Food Production

Soil Types
Sand, silt and clay, as well as variable amounts of organic matter, are mixed together in
soils in many combinations, giving soils their characteristic texture:

• Sandy soils

weathering, or worthless quartz, or a mixture of the two. Very sandy soils are

generally poor soils, often low in nutrients and moisture.

• Light soils


• 
  

aerated and drained, and often produce plants with retarded/stunted root systems.

Soils may become acidified over time as a result of a number of factors:
the bedrock, loss of calcium (lime) due to uptake by plants, the addition of nitrogen to
the soil by either fertiliser or legumes (where N is converted to nitrates) and/or leaching.
An indication of the acidity or alkalinity of soil and other substances is given by its
pH value, the measure of the concentration of hydrogen cations (H+) and hydroxyl
anions (OH).
Neutral substances have a more or less equal proportion of hydrogen ions (positive
charge) and hydroxyl ions (negative charge), alkaline substances have a higher
proportion of hydroxyl ions and acid substances have a higher proportion of hydrogen
ions than hydroxyl ions.
pH is expressed on a scale of 0 to 14; pH7 is neutral, less than 7 is acidic and more
than 7 is alkaline. In practice, soils with a pH of between about 6.6 and 7.3 are often
described as being “neutral”.
pH values are reciprocal logarithmic expressions to the power of 10, so that a soil
with a pH of 5 is ten times as acid as a soil with a pH of 6, and so on.
Although all soils are inherently either acidic, alkaline or neutral, their pH can be
altered a little. If the soil is too acid lime can be added; if it is too alkaline, organic
fertilisers, sulphate of ammonia or leguminous green manure crops can be added.
Alkaline soils contain bases such as lime, soda or potash in large amounts.

Signs of Soil Acidity
Some indications can be observed by eye if a field, or parts of a field, are too acidic:

1. Bare patches, or poor growth, of acid sensitive crops such as barley, wheat and
   sugar-beet (see list of crops below).


2. Poor growth of clovers, and an accumulation of a mat of undecomposed organic
   matter in grassland.


3. A lack of earthworms.

Heavy soils—

that they are difficult to cultivate, and not their weight. Very heavy soils are poorly

these have a high proportion of clay and silt. The name indicates

—

with which they can be cultivated, not their weight.

these have a high proportion of sands. The name indicates the ease

—

ticles may be either mineral fragments that slowly become available to plants by

to plant roots), but have low water holding capacity (retention). The sand par-

these are well drained and well aerated (oxygen is readily available

20 TONY WINCH

1Cc. Acid Soils / pH Value