Nutrient balance in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 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.