16 Soil Essentials
The micro, or trace, nutrients, are all required in very small amounts, but are
essential for healthy plant growth. The availability of these nutrients is often
dependent on soil pH, and in strongly acid or alkaline conditions they may be
either unavailable to plants or available at toxic levels. (Refer to Chapter 9.)
Keeping soil at near neutral or slightly acid pH overcomes many problems with
trace elements, but if deficiency signs occur in crops, or if a known deficiency is
present, the required trace element should be added.
Antagonism between nutrients
When talking of nutrient deficiencies or excesses we are talking about fine tuning
the balance of each plant food in the soil. Too much of a nutrient may be even
worse than too little for the reason that it may be hard to remove excesses (e.g.
sodium or manganese). Some of the plant nutrients in excess can cause other
deficiencies in plants by interfering with the uptake of a nutrient that would
normally be in adequate supply. Overdosing with one plant nutrient may change
the availability of others. Table 2 shows some examples.
Just as it is possible to have too little of a nutrient in the soil, it is also
possible to have too much.
Table 2 Some soil nutrient interrelationships in plants
Nutrient in excess Induced deficiency
Nitrogen Potassium
Magnesium Potassium, nitrogen, phosphorus. Calcium shows as
magnesium deficiency
Potassium Magnesium, sodium, calcium
Sodium Calcium, potassium
Calcium Phosphorus, magnesium, trace elements
Boron Potassium, magnesium
Chlorine Potassium
As well as soil nutrients affecting plant health, nutrients supplied to animals
from plants grown on the soil also affect animal health, as shown in Figures 7 and
8. If an arrow points from one nutrient to another, it means a deficiency in the
nutrient the arrow is pointing to may be caused by excess of the first nutrient. A
nutrient in excess can affect more than one other nutrient.
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Nutrient balance in the soil 17
Calcium
Iron Copper
Sodium
Manganese
Zinc
Magnesium
Cobalt
Cadmium
Potassium
Phosphorus
Figure 7 Mineral interrelationships in animals. (Soil Sense C-01)
Manganese
(mg)
Potash Calcium
(K) (Ca)
Copper
Iron (Cu)
(Fe)
Phosphate Magnesium
(P) (Mg)
Molybdenum
(Mo)
Boron
(B) Zinc
Nitrogen (Zn)
(N)
Figure 8 Mineral interrelationships in plants.
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18 Soil Essentials
Determining the nutrient balance of the soil
The amount of nutrients present in the soil is best determined by taking soil
samples and having them tested in a laboratory. Soil testing is a very useful tool to
help understand what the current nutrient status of the soil is, and to decide on a
fertiliser program. The cost of soil testing is nearly always returned many times
over by better crops or pastures. An alternative to soil testing is plant tissue
analysis, which will indicate nutrient availability to the plant at the time the
sample was taken, but will not indicate total nutrient levels in the soil or balances
between nutrients.
Reasons for soil testing
Advantages of soil testing are:
• to improve the balance of exchangeable cations for optimum soil structure and
friability
• results show the relative levels of nutrients in the soil
• results show pH, which can influence nutrient availability
• enables calculations of nutrient ratios
• enables fertiliser programs to be targeted to soil and crop needs
• saves money on applying fertilisers – only necessary ones need to be applied
• normally means better crop responses from applied fertilisers.
Alternatives to soil testing
Plants growing in nutritionally unbalanced soils will show it in their growth
habits, for example, leaves may be yellow or have dry edges, roots may be stunted
or fruit may be deformed. Knowing what these signs mean can give an indication
of what nutrient is missing or is in excess, but a great deal of experience is needed
to use this method successfully, as it is difficult to decide between nutrient
deficiencies or toxicities, or whether the symptom is caused by antagonism
between nutrients.
Action following soil testing
To fully benefit from soil testing, the following steps should be taken:
(1) Determine pH
Ideally the pH should be 6.5 in water or 6.0 in calcium chloride. This allows soil
acid chemical reactions to occur, but will not cause toxicity problems. A pH as low
as 5.0 in calcium chloride may be acceptable if well managed and manganese is not
in toxic amounts. Aim to have a slightly acidic pH so that soil conditions are ideal
for soil micro- and macro-organisms, which in turn improve plant growth by
supplying nutrients from the decomposition of organic matter.
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