We all know fertiliser, and phosphorus in particular, are at historically high prices. Combined with low cash flows from previous seasons, it’s no wonder many landholders are considering reducing their application of Phosphorous.

While it is true that there will potentially be some additional available P because of failed crops, a soil P (with a phosphorous buffer index) test is essential for understanding the actual status of the soil.
 
However, what may be overlooked in your considerations is the timing of seeding. The earlier a crop is sown, the greater the surface area of soil the seedlings are able to explore (intercept), and therefore have a greater potential to capture the residual P.

This is important because P does not move in the soil to any significant degree, particularly in Calcareous soils. Effectively, the plant roots then have to either grow near to the P in the soil to be able to take it up
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This is in contrast to a plant nutrient such as Nitrogen which is highly movable in the soil, and moves with the water in the soil.

The later a wheat crop is established, the more it will be dependant on finding ‘easy’ P such as from a fertiliser granule.

Narrow tine spacings create greater distances between each granule of P, while wider spacings in effect concentrate P. This is also one of the benefits of liquid P systems because they are able to create near continuous bands of P around the seed rather than just high concentrations at only certain points.

The implications then should be to NOT start with high rates of P at seeding, if seeding commences when you consider it still early, but rather perhaps start at the low end, and tend towards increasing the rate of P as the seeding program progresses. Therefore, if the break in the season is late, sowing crops with little or no P is more likely to cause problems as crop growth slows over the June-July period, and quite possibly contribute to a greater risk of erosion during those high risk wind events in late winter.

• Soil test for P including a PBI for an estimate the availability of P
• If planting with low P, the risks increase the later the sowing
• Consider starting lower and finish higher- rather than starting high and finishing low, particularly when $’s or availability become a limiting factor.

AUTHOR: Josh Telfer, Soil and Land Consultant, Rural Solutions SA

CONTACT: Josh Telfer, Pt Lincoln Rural Solutions SA office, Telephone: 8688 3400 Email: telfer.joshua@saugov.sa.gov.au