Phosphorus deficiency.Young plants, leaves and stems bluish green and develop strong purple tints; older leaves die off early.
Phosphorus
Phosphorus deficiency is one of the most widespread nutrient deficiencies.
Phosphorus forms part of the proteins in plant cells, so it is important in growing tissue where cells are actively dividing, (e.g. development of seedling roots, flowering and the formation of seed).
Phosphorus deficient plants are stunted, dark green plants with short, erect leaves and stout stems that often develop orange, red or purplish discolouration. Many soils in wheat growing areas will respond to the application of phosphate fertilisers.
Approximately 3.2 kg of phosphorus is removed in every tonne of wheat harvested. Phosphorus deficiency is more likely to occur after a long fallow due to low numbers of arbuscular mycorrhiza fungi (AMF, also known as VAM) in the soil. AMF are the beneficial soil fungi that help plant roots take up both phosphorus and zinc.
Crop demand for phosphorus can be considered in two distinct phases - during the early development (from emergence to the end of tillering, but before stem elongation), and then during the subsequent growth and grain-filling period. During early development, the requirement for P is small (perhaps 1 kg of P), but the root system is small and inefficient so the crop responds to a concentrated phosphorous source close to the seed and developing roots.
Ensuring these young plants have adequate phosphorous during this phase is essential to determine grain number (ie. yield potential) and ensure vigorous seedling development.
Subsequent P requirement is much larger, and largely mirrors the accumulation of crop biomass. As a general rule, crops require approximately 5 kg phosphorous to be accumulated by the plant to produce each 1 t of grain yield, so a 3 t crop will typically take up 15 kg/ha of phosphorus.
Only 1-2 kg will be taken up from the banded phosphorous fertiliser applied at planting (either in or below and beside the seeding row). The rest comes from the soil profile, with approximately half coming from the top 10-15 cm and the rest coming from the next 15-30 cm. These proportions will change somewhat with seasonal conditions, as root activity in surface layers will be minimal in dry periods. Having plant available phosphorous in the immediate subsoil (i.e. the 10-30 cm layer preferably) becomes a critical success factor for crop performance.
The need for phosphorus fertiliser can be determined through using soil tests (0-10 and 10-30 cm) and/or test strips of fertiliser. When interpreting Darling Downs soil tests, the best predictions of P response are obtained by using both the BSES and Bicarbonate (or Colwell) P test results as shown in the table below. These guidelines are currently based only on results from the 0-10 cm layer of the soil profile, but current research is developing guidelines to determine the soil phosphorus status in the 10-30 cm layer, and any resulting fertiliser requirement.
Table 1. Phosphorus requirements - Darling Downs soils
When fallow length exceeds 12 months, moderate to high rates of P fertiliser are often required to prevent long fallow disorder affecting the crop. A suggested rate is 10 kg P/ha.
