Geoscience Reference
In-Depth Information
Almost all farmed plants require oxygen in the root zone and oxygen
availability depends on the soil structure having sufficient pore spaces to allow air
to infuse into the soil profile. Often roots will only go as deep as the oxygen supply
and this not only depends on soil structure but can also be reduced by high water
tables or waterlogging.
Soil nutrient levels are critical to plant growth and a deficiency or excess of a
particular nutrient can drastically reduce growth rates or even kill plants. Nutrient
availability depends on the origin of the soil and the soil pH. For example, soils
derived from granite are naturally acidic, while soils derived from some basalts or
limestone are naturally alkaline. Farming practice may also change soil pH levels
over time due to nutrient export and fertiliser effects.
To summarise these comments, a fertile soil is one that provides the physical
conditions and all the chemical needs of plants and microf lora. Typically, it will be
a well-structured soil with a high proportion of air spaces between soil particles. It
will have sufficient levels of all the nutrients required to support plant and
microbiological growth, adequate water supplies with good infiltration rates and
good drainage.
It will be of medium texture, sufficient to hold nutrients without excessive
leaching, but loose enough to allow plants to extract the nutrients easily, that is, it
will have a high cation exchange capacity . The soil will also have structural
integrity. It will not easily be broken down by mechanical inf luences such as
pressure from livestock or reasonable levels of cultivation.
Obviously, different soils have degrees of soil fertility, ranging from highly
fertile to infertile. There are a number of ways of expressing soil fertility, one of
which is shown in Table 1.
Table 1
Soil fertility classes
Class 1
Excellent productivity for all agriculture and horticulture
Class 2
Very good for most agriculture and horticulture
Class 3
Good for a range of agriculture and some horticulture
Class 4
Fair for a limited range of agriculture
Class 5
Low yields, pastures and some crops only
Class 6
Unsuitable for general agriculture
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