MOBILITY AND AVAILABILITY IN SOILS AND THE VINE
Phosphorus (P) is available via the slow breakdown of organic materials in the soil, or as an applied fertiliser. Most soils used for viticulture in Australia have low native concentrations of phosphorus and therefore phosphorus fertiliser inputs are normally required.
Phosphorus is taken up by vines from the soil water in its soluble phosphate form.
•Phosphorus is not readily leached from the root zone in most soils, but availability is reduced when it is ‘fixed’in a soil by reactions with high levels of calcium, iron and aluminium. It can also be bound to organic matter and soil particles, making it slowly available to vines.
•Where phosphorus has been applied, soil reserves may be sufficient for long periods as this mineral is relatively immobile in most soils.
Phosphorus is mobile within the grapevine, and can be moved from mature organs to areas of new growth. This gives rise to deficiency symptoms first appearing on older leaves, which may fall prematurely.
SOURCES AND LOSSES
Once applied, phosphorus can remain in the soil profile for a considerable time, however it can be leached from sandy, acid soils and lost when surface soil is eroded. It has been estimated that around 0.6kg of phosphorus is removed at harvest per tonne of grapes.
NUTRIENT MANAGEMENT WITH PHOSPHORUS FERTILISERS
Phosphorus fertilisers are usually available in three forms:
•Water soluble
•Citrate soluble
•Citrate insoluble
The water soluble and, to a lesser extent, citrate soluble forms are available to plants relatively quickly, whereas the citrate insoluble form, e.g. rock phosphates, act as a slow release source of phosphorus and can take years to become available to vines depending on the pH of the soil. Phosphorus fertilisers containing the highest percentages of water soluble phosphorus have the most P readily available for use by vines (see Table 1 for P fertiliser characteristics).
APPLYING FERTILISERS
Consultation with your winery is always advisable when considering phosphorus applications.
Traditionally, superphosphate has been applied to soils by deep ripping prior to planting vines. However, a common method now used to apply phosphorus fertiliser prior to planting involves applying it with any applications of lime, organic matter
and gypsum in a band on the surface along the proposed vine row and mixing it into the soil.
The rate of superphosphate to apply should be determined with pre-planting soil tests. Although most Australian soils are naturally low in phosphorus, previous land use may have seen substantial amounts of phosphorus fertiliser applied to the soil.
When applying superphosphate to soils with low background levels of phosphorus (less than 25–30mg per kg Colwell P), additional P can be applied to the inter row for the cover crop or sward if required. (Seek advice on rates from a cropping or pasture agronomist as these will differ from the rates required by vines.)
Band application of phosphorus fertiliser is generally more effective than broadcast application. Banded fertilisers are applied in a narrow band along the under vine bank after weeds have been cleared, where the majority of vine feeder roots can be found. In low P soils the equivalent of 0.5–1.0 t/ha of single super phosphate is typically banded along the vine row.
Phosphorus fertiliser is best applied in autumn or early spring to take advantage of any rain to move it into the root zone. Some forms of phosphorus can also be delivered to vines by fertigation.
As phosphorus only moves away from the point of application very slowly, except on very sandy acid soils, the application strategy must be thought out carefully to avoid fixation of the phosphorus in forms that are unavailable to plants. In some highly fixing soils, repeat applications of phosphorus fertiliser may be needed to saturate the fixation sites on the soil particles so that applied phosphorus fertiliser remains available to the vines.
TIMING APPLICATION FOR EFFICIENT UPTAKE
Pre-planting
Applications of phosphorus fertiliser prior to planting should be made on the basis of soil tests. If indicated, phosphorus fertiliser should be applied as a surface band along the proposed vine row and cultivated in. This increases the chance of young vines accessing the phosphorus.
Established vineyards
In established vineyards, periodic testing of plant tissue and/or soil for phosphorus is advisable to determine the current reserves of phosphorus in the vines and soil. Phosphorus fertiliser only needs to be applied when testing indicates it is in short supply.
It is important to ensure a supply of phosphorus is available for use by vines before the two major uptake periods, which occur:
•3–4 weeks after budburst, and
•for approximately four weeks after harvest up to leaf fall.
