5.4 Key Nutrients

Soil analyses for phosphorus, potassium, surface pH and organic carbon are best done on a bulked sample from approximately 30 sub-samples from the 0-10cm surface layers, collected from the same soil type. With increasing interest in sub-soil fertility, judicious sampling of deeper layers is recommended, especially for sulphur and nitrate.

Nitrogen

Nitrogen deficiency is probably the most widespread nutrition problem in South Australian agriculture. Soil nitrogen (N) can take two basic forms in soils: organic and inorganic. Organic forms include that associated with soil organic matter, plant residues, soil organisms and animal waste; at least 90% of soil nitrogen originates from organic sources. Inorganic N can take the form of ammonium, nitrite and nitrate, but is typically dominated by nitrate in agricultural soils. The conversion of organic to inorganic N is called mineralisation and is driven by micro-organisms.

The micro-organisms use the organic N as a nutrient source. Any organic N in excess of their maintenance or growth requirements is released to the soil as ammonium. This ammonium is converted by specific bacteria to nitrate that can then be used by plants. The conversion of ammonium to nitrate N is called nitrification.

The nitrogen cycle illustrated in Figure 27 demonstrates the flow of plant residue N, through soil micro-organisms and soil organic matter and into the inorganic fraction as ammonium.

Organic carbon percentage (OC%) gives an indication of likely plant available N. The higher the OC% the more N available, but seasonal conditions will control the mineralisation of this N.

Nitrate levels vary considerably and should be measured in the whole root zone and deep nitrate testing at 60cm is becoming more widely used. SAP nitrate testing and near infra-red reflectance (NIR) are used to assess the nitrogen status of growing crops.

Rhizobia, nitrogen-fixing bacteria, form a symbolic relationship with leguminous plants, eg clover, lupins. The Rhizobia provide N to the plant and in return the plant supplies sugars to the bacteria. This is an important source of crop N.

When assessing a crop N requirement, the following factors should be considered:

Paddock history

• previous crops

• previous fertiliser inputs

• residue management

Crop targets

• yield

• quality/protein

Phosphorus

Phosphorus is almost universally deficient in Australian soils in their natural state. Added phosphorus becomes attached to clay particles, organic matter and compounds of calcium, iron and aluminium. Only part of the total content is available to plants. The source of phosphorus will determine how much is readily available to plants at a given point in time. This is illustrated in Figure 3.

Soils with a high pH and those with high levels of iron, calcium and aluminium have a reduced capacity to supply phosphorus, because in these situations, phosphorus is less soluble. These soils, together with sandy soils in high rainfall areas, which are prone to leaching, are the most likely to be phosphorus deficient.

Potassium

Except on very sandy soils in the higher rainfall districts, deficiency of potassium in South Australia’s agricultural soils is rare.

This is because the rocks and sediments from which most soils are formed contain clay minerals naturally high in potassium. Deficiencies are most likely where large amounts of potassium have been removed in farm products, particularly hay.

Trace Elements

Trace elements are required by plants and are often only present in small quantities. Deficiency symptoms may only occur sporadically. These may be due to seasonal conditions, reduced availability caused by addition of other fertilisers, or a whole lot of other factors

The situations where the main trace element deficiencies are most likely to occur are:

Copper: Acid and calcareous sands

Manganese: Highly calcareous soils

Zinc: All soils, especially calcareous and sandy types

Molybdenum: Sandy, acid soils in high rainfall areas

Iron: Calcareous soils

Boron: Leached sandy soils 

Note that boron can also be toxic; if test results are more than 15ppm

Soil tests for manganese and iron currently have little interpretative value, but values of less than 0.2ppm for copper and 0.5ppm for zinc indicate likely deficiencies.

Plant tissue testing is necessary to establish if plants are obtaining adequate trace elements from the soil.

Sulphur

Sulphur occurs in both organic matter and as sulphate compounds in the soil. Sulphur deficiencies are not widespread and are most likely in sandy soils in high rainfall areas and in soils low in organic matter. Removal of sulphur in produce reduces soil sulphur levels. Additions are now less, as many fertilisers are now low in sulphur.

Calcium

Calcium occurs in limestone, calcrete and soft ‘lime,’ all forms of calcium carbonate and a common constituent of South Australian soils. It also occurs in gypsum (calcium sulphate) and attached to clay minerals. Calcareous soils and soils with high clay contents, are unlikely to suffer calcium deficiencies.

Magnesium

Like potassium and calcium, magnesium is present in large amounts attached to the clay minerals of most soils. Leached sandy soils are the most prone to deficiencies.

5.4 Key Nutrients

[ Back ] [ Next ]