Components of soil fertility - Soil Essentials: Managing Your Farm's Primary Asset - page 86

Table 19 The main essential plant nutrients

Macronutrients Chemical

symbol

Micronutrients Chemical

symbol

Nitrogen N Iron Fe

Phosphorus P Manganese Mn

Potassium K Copper Cu

Sulphur S Zinc Zn

Magnesium Mg Boron B

Calcium Ca Molybdenum Mo

Nutrients in the soil may be taken up directly by plants, or nutrients may be
accessed by soil organisms and thus made available to plants indirectly. Where
there are insufficient nutrients available to meet the needs of the plant, nutrient
deficiency symptoms can develop. This may occur when nutrients are leached out
of the topsoil down beyond the root zone of the plants.
However, plants growing in soil with adequate essential nutrients present may
still suffer from a deficiency of one or more elements. This is because some of the
nutrients present may be in forms that are unavailable to the plants, as chemical
reactions in the soil have changed them into insoluble compounds. Nutrients may
also be tied up within soil particles, and are physically unavailable to plants.

Leaching
A soil may start off with an adequate supply of nutrients, but they may be leached
down the soil profile and become inaccessible to plant roots. This is especially true
for sandy soils in high rainfall areas. In comparison, heavier soils do not leach as
readily, and in areas of relatively low rainfall there may be little, if any, leaching,
especially when good plant cover is present.
Heavy rainfall events during normally dry times of the year can lead to
nutrient losses by runoff as well as leaching.

Soil structure
Soil structure is related to the sand, silt, clay and organic matter content of the soil
and its ability to form aggregates. A soil with good structure will have soil particles
formed together to form stable aggregates. These aggregates can be quite stable over
time, especially in a soil with a high organic matter content, although some
aggregates can be fairly fragile and easily broken down by machinery or stock
pressure, especially when wet. Stable aggregates provide the framework for soil voids
that allow easy entry of air, water and plant roots. They also provide areas in which
the soil's macro- and micro-organisms can thrive. Poor structure is inhospitable for
plant growth and leads to waterlogging and plant diseases (Figure 24).