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Nutrient balance in the soil
Background
Soil studies have confirmed that for healthy soils, plants and animals, the various
plant nutrients need to be present in balanced proportions. The amount of each
nutrient can be determined by soil tests, and it is important that subsoils are tested
as well as topsoils.
Plant nutrition
Plants need to be able to access all their food nutrients from the soil in sufficient
amounts to grow to their full potential. These nutrients must also be in the correct
balance (or ratios) for healthy plant growth. Incorrect balances will cause the
plants to become prone to disease or insect attack and they will not grow to full
potential.
Soil pH
Soil pH should ideally be around pH 6.5 (water) or pH 6.0 in calcium chloride for
best biological and chemical activity. At this pH, toxicities and deficiencies are
minimised due to chemical actions. The potential for the best macro- and
microbiological growth also occurs at this pH, but this is also dependent on
temperature and moisture. The soil chemical reactions that are dependent on acidity
still continue at this pH level, helping to make the nutrients available to plants.
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14 Soil Essentials
Animal nutrition
Animals grazing a diverse range of productive plants grown on healthy soils will
have a better chance to be healthy and grow to their genetic potential.
A healthy soil produces healthy plants that produce healthy animals.
Nutrient ratios
All plant nutrients are important for different reasons, some of which alter the
structure of the soil as well as providing nutrition for plants and soil microbiology.
The cations
This group of soil nutrients includes the elements:
Calcium (Ca)
Magnesium (Mg)
Potassium (K)
Sodium (Na)
Hydrogen (H)
Manganese (Mn)
Aluminium (Al). (Aluminium is not really a plant ‘nutrient’, but it is a cation.)
A soil analysis will usually show the ‘cation exchange capacity’ (CEC) of a soil,
that is, the amount of calcium, magnesium, potassium, sodium, hydrogen,
manganese and aluminium available. It has been found that the relative ratios of
soil cations influence how well plants grow in the soil.
Calcium (Ca)
Calcium (that is found in limestone) helps counteract soil acidity and makes the
soil friable. It is required in quite high amounts by some plants such as lucerne.
Calcium should be between 65% and 70% of the CEC.
Magnesium (Mg)
Magnesium is required in much lower quantities than calcium. In excess it causes
clay subsoils to become very dense and blocky, thus reducing root and water
movement downwards. Magnesium can also be used to raise pH levels in soil.
Magnesium should be between 15% and 20% of the CEC.
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Nutrient balance in the soil 15
Potassium (K)
Potassium is only required in relatively small amounts. Generally in soil it ‘cycles’
between plant available and plant unavailable forms as conditions within the soil
change. Deficiencies are very important in plant physiology and show up as slow
spindly growth or bright yellow leaves, lack of cold tolerance and ineffective use of
water. Potassium should be about 5% of the CEC.
Sodium (Na)
Sodium is required only in small amounts by plants, and an excess in the soil can
have severe effects on plants and the soil. An excess of sodium in the soil will cause
clays to have poor structure, becoming dispersive and settling into hard layers,
which resist root penetration and water movement. Soils with too much sodium
compared to calcium are said to be ‘sodic’ (this is different from saline).
Exchangeable sodium should be less than 5% of the sum of cations, that is,
calcium + potassium + magnesium + sodium + hydrogen added together (and
manganese and aluminium in acid soils).
Exchangeable Na 100
× = <5
Exchangeable Ca + K + Mg + Na + H 1
A guide to the level of soil sodicity is:
0–5 low sodicity
6–15 medium sodicity
>15 highly sodic
Carbon/nitrogen ratio
This is a characteristic of the organic part of the soil where most of the biological
activity occurs. It is a result of the formation of humus and organic matter, the
decomposition of dead plants and animals, and the amount of soil organisms
present, such as bacteria, fungi, earthworms, insects and other soil organisms. This
is all about ‘living soil’ and for many years it has been known that the amount of
production that can come from above the soil is directly dependent on what is
growing under the soil surface. A good ratio of organic carbon to soil nitrogen is
10:1–12:1.
Macro- and micronutrients
The macronutrients – nitrogen, phosphorus, potassium, calcium and magnesium –
are all essential for plant growth, and are required in larger amounts than other
plant nutrients. Needs vary between plant species and growing conditions, and
should be worked out on a crop by crop basis in conjunction with soil testing.
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