Independent advice for soil phosphorus

Independent agronomist Graham Mussell speaking at the Soil Constraints event in Donnybrook

Livestock producers looking for independent advice on phosphorus application rates might want to look at a key document used by agronomists that’s available on-line.

Almost a decade ago, the Department of Primary Industry and Regional Development (DPIRD) published the Soil Test and Phosphorus Rate for High Rainfall Clover Pastures.

The document is one of several tools described by independent consultant Graham Mussell at a recent seminar in Donnybrook to help farmers identify their most limiting soil constraints.

Mr Mussell said that farmers just need to know their Colwell soil phosphorus level and phosphorus buffering index (PBI) to use the tables.

“The publication contains tables for different PBI values, so you find the table for your soils PBI and then find the column that best matches your Colwell phosphorus level from a recent soil test. That will tell you the yield you could get without applying any phosphorus, assuming there aren’t any other constraints, and the amount of phosphorus needed to reach your target yield.”

To use the tables, farmers need to select a yield target or production level, which is roughly related to profitability, stocking rate and grazing intensity. Higher stocking rates and grazing intensities utilise more pasture compared to more extensive operations so need to yield more pasture and can get a return on the cost of fertiliser. On the other hand, low stocking rates and/or set stocking typically utilises less pasture, so there is less benefit from high pasture yields and a big fertiliser bill.

“If I go to a set stock beef producer running 10-12 DSE (dry sheep equivalents) per hectare, I try to fertilise for 80% of maximum production because they won’t be utilising all the pasture grown. A dairy farm rotationally grazing at the three-leaf growth stage and applying nitrogen after each graze are typically targeting 95% of maximum production. Beyond 95% your investment in phosphorus is returning smaller and smaller increases in yield per dollar invested, while the risk of losing phosphorus off farm increases.”

A recent survey of 43 sites on 24 beef farms in the South West found that 40% of sites had phosphorus levels that would achieve greater than 95% production, assuming no other constraints existed.

“A lot of these farms are still applying phosphorus, but are we spending money wisely if we buy super phosphate when there’s enough in the soil and when there is most likely a more limiting constraint? Is potassium or pH something you should be looking at to use the phosphorus you have more efficiently?”

Super phosphate contains up to 10 percent sulphur, so if it isn’t applied, another source of sulphur such as gypsum, a nitrogen/sulphur product or hay fertiliser in spring may be required.

Critical to the correct calculation of phosphorus in the soil is best practice soil sampling. New soil testing guidelines released in 2019 are aiming to increase the effectiveness of soil testing by increasing the number or cores collected per sample.

“There’s a great deal of variation in paddocks, so you need to get 30 to 40 cores across a paddock or zone within a paddock to account for that variation.

“It’s also important not to scuff the surface before you take a core because there are nutrients in the top few millimetres that you won’t measure. Also, avoid unrepresentative areas of high nutrition like stock camps or feed-out areas.”

Soil testing in the same GPS locations over a number of years can start to build a powerful picture of how nutrients are trending in the paddock, which will help to fine-tune application rates and ensure any rundown in excess phosphorus doesn’t end in deficiency.

Mr Mussell asked farmer to consider their phosphorus removal rates particularly on grazing paddocks when soil tests show there is enough phosphorus.

“A 500 kg animal has about 4 kg of phosphorus in it. So think about how much phosphorus you are removing in the animals that you turn off. You’re doing pretty well if you can turn off 500kg of liveweight per hectare per year, so that’s 4 kg per hectare. If you’re adding 100 kg of single super, which is  9% phosphorus or 9 kg/ha,  you’re removing 4 kg/ha and adding 9 kg/ha, you’re putting on more than you need.”

By taking a reliable soil sample and understanding target yield, farmers can use the DPIRD document to take control of their phosphorus use and potentially free up resources to address other limitations to pasture growth.

This event was supported by South West Catchments Council and Western Beef Association Inc, with funding through the Australian Government’s National Landcare Program.

This figure from DPIRD’s “Soil Test and Phosphorus rate for High Rainfall Clover Pastures” shows the maximum production that may be achieved based on existing supplies of Colwell P in a low P fixing sandy soil (dashed curve: Phosphorus Buffering Index (PBI) below 5) and a high P fixing clay soil (solid curve: PBI = 280-840) where no other factors are limiting. Tables in the publication provide detail for other PBI ranges.

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