2.8 How Nutrients Work

Manipulating nutrient availability and correcting deficiencies

There are three key sources of nutrients 

1. from the soil pools - organic matter and clay complexes. 

2. biological fixation of some nutrients e.g. nitrogen, 

3. organic and inorganic fertiliser inputs. 

Each source can be manipulated to increase the supply of plant available nutrients. 

Following are the nutrient sources and the soil factors that affect plant use of added nutrients

NITROGEN (N)

Soil Pools

• 95% of nitrogen in the soil is held in organic matter. Approximately 10 - 40 kg N/ha will be released (mineralised) from this source each year.

• Higher levels of mineralisation occur in soils with higher organic carbon percentages, warm moist conditions, biologically active soils, where tillage occurs.

Biological fixation

• Carried out by Rhizobium in association with legumes can result in returns of 22-25 kg N per tonne of above ground legume dry matter.

• Legumes: generally have high nitrogen demands. If nodulation is not effective they require additional nitrogen fertiliser.

Gaseous fertilisers 

• Anhydrous Ammonia - 82% N (commonly used in Canada)

Liquid fertilisers

• Aqua Ammonia - 20.5% N

Solid fertilisers

• Urea - 46% N - in expensive but susceptible to losses by volatilisation

• Ammonium Nitrate - 34% N - less susceptible to volatilisation as half N is already as nitrate.

• Sulphate of Ammonia - 21% N - together with MAP and DAP these products can be more acidifying than urea.

• MAP - 12% N

• DAP - 19% N

• Potassium Nitrate - 13% N

• Sodium Nitrate - 16 % N

• Calcium Ammonium Nitrate - 20.5 - 23% N

The percentage represents the kilograms of nitrogen in 100 kg of product. The remaining percent may be made up from other nutrients or carrier.

PHOSPHORUS (P)

Soil Pools

• About 50% of soil P is held in organic matter.

• Mineralisation an lock-up can occur in the same way as for N but less organic-P is made available - approx 10 - 15kg P/ha each year.

• Soil organic carbon percentage, moisture, temperature, pH and biological activity all influence mineralisation and lock-up rates.

• Phosphorus is released when the pasture is cultivated.

Biological fixation

• Increased availability due to VAM is not generally accounted for in nutrient plans. 

• In Queensland trials VAM accounted for 400 mg P/kg soil.

Fertilisers:

Generally applied as solid fertilisers

• Superphosphate 8% available P

• Double Superphosphate - 16.8% available P

• Triple Superphosphate - 20.2% available P

• MAP -22% P

• DAP -20% P

• Rock Phosphate - P in an unavailable form except in high rainfall (over 600mm) situations.

Liquid fertilisers:

• These are most likely to be of some real benefit in the grey highly calcareous soils of Upper Eyre Peninsula where the conventional form of solid phosphorus fertiliser are quickly fixed in the soil and made unavailable to the plant.

• In 1998 at he Miltaburra site an 11% (significant) yield increase was recorded for cereals by using Technical grade MAP + Ammonium nitrate in solution compared with conventional MAP + solid ammonium nitrate.

SULPHUR (S)

Soil Pools

• 95% of sulphur in the soil is held in organic matter. 10 - 15 kg S/ha are mineralised each year.

• Mineralisation and lock-up are driven by the same mechanisms as for N and P.

Fertilisers

• Elemental sulphur contains the highest sulphur content of all the potential fertilisers but it must be finer than 0.5 mm diameter to be of agronomic value. 

Fine sulphur can be difficult to apply and a fire hazard. Sources include

• Double Superphosphate - 4.5% S

• Single Superphosphate - 11.0% S

• Sulfate of Ammonia - 24% S

• Sulfate of Potash - 16% S

• Gypsum - 12-18% S

At current prices single superphosphate is an economical way of applying reasonable quantities of S. Gypsum applied at 500 kg/ha will supply 70 to 80 kg/ha S, which should be sufficient for 15 to 20 years of intensive cropping.

ZINC (Zn)

Soil Pool:

• Held in organic matter and in the clay complexes these nutrients are released by cation exchange  or are released by soil organism activity on organic matter (mineralisation).

Biological fixation:

• No mechanism for biological fixation of these nutrients are currently known

Fertilisers:

• Pre-sowing applications of zinc enriched fertilisers which must be worked in provide long-term correction to zinc deficiency.

• Only homogenous Zn-enriched fertilisers are effective in correcting Zn deficiency n the year of application. (The fertiliser is manufactured so all granules contain some zinc).

• A rate of 2kg Zn/ha should persist between 3 - 10 years, depending on soil type. Repeat applications of 1kg Zn/ha should be sufficient to prevent deficiency re-occurring.

• Regular repeat applications are required on heavy and calcareous soils in high rainfall areas. 7 - 10 year repeat applications acceptable in low rainfall areas.

• Foliar sprays provide in season correction (250 - 350 g Zn/ha)

• Zn seed coats, or seed with high internal Zn will help young crops but will not overcome severe deficiencies or increase soil Zn reserves.

MANGANESE (Mn)

Soil Pool:

• held in organic matter and in the clay complexes these nutrients are released by cation exchange  or are released by soil organism activity on organic matter (mineralisation).

Biological fixation:

• No mechanism for biological fixation of these nutrients are currently known

Fertilisers:

Generally plentiful supplies in soil pools except in high pH soils.

• On highly calcareous soils correction can require the use of Mn- enriched fertiliser banded with the seed (3 - 5 kg Mn/ha) plus 1 -2 follow up foliar sprays (1.1 kg Mn/ha). This is due to Mn lock-up in high pH soils.

• Mn seed dressings are cheap and probably the most effective approach in conjunction with foliar sprays or enriched fertilisers.

• The use of acidifying fertilisers e.g. N in the ammonium form, may partially correct Mn deficiency in alkaline, high pH, soils.

• No treatment has residual benefit and must be reapplied every year.

• Lupins: If soil is Mn deficient treat with foliar spray at mid flowering on primary laterals.

COPPER (Cu)

Soil Pool:

• held in organic matter and in the clay complexes these nutrients are released by cation exchange (see latter and Better soils book page 29) or are released by soil organism activity on organic matter (mineralisation).

Biological fixation:

• No mechanism for biological fixation of these nutrients are currently known

Fertilisers:

• Traditionally corrected by pre-sowing applications of Cu-enriched fertilisers but these must be incorporated by tillage.

• Most soils require applications of 2kg Cu/ha to fully correct a deficiency.

• Excellent residual benefits of soil applied copper - 10 - 30 years.

• Foliar sprays offer in season cures with no residual benefits. A foliar spray of Cu (75 - 100g Cu/ha) on deficiency diagnosis is very cheap. A second spray immediately prior to pollen formation may be necessary in severe situations.

Note on foliar fertilisers:

The nutrient status of the plant can be altered directly by the addition of foliar fertilisers. These compliment rather than substitute for solid fertilisers. They are used strategically to ensure an adequate supply of nutrients throughout the growing season. As these are applied to the plant foliar fertilisers have no residual impact on soil nutrient levels.

Figure 1: 
Nutrient Content of Common Animal Manures

2.8 How Nutrients Work

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