Replacement phosphorus after two high rainfall seasons

Key messages

• The case study highlights the value of integrating multiple data sources, including historical yield data, biomass imagery, and routine soil test results to provide clarity on soil constraints and develop targeted fertiliser strategies.
• Simple soil type zones were developed to tailor a phosphorous replacement strategy and ensure high performing areas are receiving adequate amounts to meet expected yield potentials.

Background

• Soil type: Grey clay, deep sand, duplex sandy gravel
• Average annual rainfall: 250 mm

The 249-hectare paddock, located in Gairdner, contains varying soil types including , deep sand, and duplex sandy gravel. Previous soil testing showed that Colwell-P ranged from 13 to 66 milligrams per kilogram across the paddock, with the eastern end of the paddock containing lower levels of phosphorus. Phosphorus buffering index (PBI) measurements were low, allowing the phosphorus to be taken up readily from the soil. The soil pH_Ca ranged from 4.9 to 7.5, with some of the lower-pH sites also containing lower phosphorus levels. Potassium levels ranged from 40 to >300 milligrams per kilogram. The soil chemical properties show that there is a greater requirement for nutrition in some areas of the paddock, but also adequate levels in other parts.

Data layers

The grower has been collecting and storing yield data since 2018. By reviewing historical yield data as well as biomass imagery they were able to see how much the yield varied across the paddock. This combined with routine soil test results was starting to show how product removal also varied across the paddock.

The average annual rainfall for Gairdner is approximately 450 mm. In 2021 and 2022, annual rainfall was above average, exceeding 600 mm and the paddock produced above average barley yields.

Normalised difference vegetation index (NDVI) from satellite imagery showed the grey clay and deep sand had the lowest levels of biomass in both the 2021 and 2022 barley crops.

Strategy

By averaging the 2021 and 2022 barley yield maps, using Ag Leader® SMS ™, a multi-year barley yield map was produced showing lower and higher-yielding areas of the paddock. The average yield of the barley multi-year yield map was 3.5 tonnes per hectare, with parts of the paddock averaging greater than 4.5 tonnes per hectare while others averaged less than 2.5 tonnes per hectare.

The grower had historically applied 80 kilograms per hectare of monoammonium phosphate as a blanket rate across the paddock. As a starting point for applying variable rate fertiliser, the grower wanted to use a very broad replacement strategy to move product from the lower yielding areas to higher yielding zones.
The multi-year yield map was simplified to four yield ranges (Figure 10) that were as follows:

• Low zone = <2.5 t/ha
• Low-medium zone = 2.5–3.2 t/ha
• Medium-high zone = 3.2–4.2 t/ha
• High zone = 4.2–7.6 t/ha

Working on 4 kilograms per hectare of phosphorus removed for every 1 tonne per hectare of barley grain produced a replacement strategy for the four yield zones of 60, 70, 80 and 100 kilograms per hectare of monoammonium phosphate fertiliser was decided on.

This resulted in the same average rate of 80 kilograms per hectare of monoammonium phosphate fertiliser being applied to the paddock. The final step was simplifying the prescription map to be applied by a 24-metre-wide seeding system.