Economics of high phosphorous rates on pulses

Jan 6, 2021


Field trials funded by SAGIT (AS118) conducted by Agronomy Solutions in 2018 at Brinkworth in the Mid North and Urania on the Yorke Peninsula indicate pulses require higher phosphorous rates than wheat for biomass production.

The common fertiliser input practice for growing pulses on soils types prone to phosphorus deficiency is to reduce phosphorus inputs to avoid increasing soil nitrogen levels which potentially affect the nitrogen fixation process. In most cases the grower might be unaware that selected parts of their paddocks are running at P deficiency. However, there is the potential that this practice results in lower nitrogen fixation and lower yields for current and future crops.

By conducting this research, Agronomy Solutions is working towards creating an economic analysis of the benefits of increasing phosphorus rates when growing selected pulses. This work involves growing lentils, chickpeas and wheat and applying five rates of phosphorus at sowing to find the optimal input amount for crop biomass, nodulation, yield and associated nitrogen fixation rates.

Project Details

One year field trial

Project participants 

Brinkworth and Urania, South Australia

Future work 
An economic analysis is being conducted to measure the benefits of increasing phosphorus rates when growing selected pulses for the following crop in rotation.

Project Objectives

This project aimed to determine the optimal phosphorous rates for two pulse crops compared to wheat. We also worked to assess the flow-on effects of extra pulse biomass in regard to nitrogen fixation.


In-season biomass assessments showed higher phosphorus requirements for both pulse crops when compared to wheat. This was particularly the case for lentils where the optimal phosphorus rate was approximately 50kg/ha, compared to 26-47kg/ha for wheat.

Benefits from optimising pulse biomass growth occurred at phosphorus rates higher than what is considered district practice for these crops. It is important to consider that these low phosphorus sites were targeted within the paddock and associated with soil types that expressed higher phosphorous fixation capabilities. The extent of phosphorous deficiency across these paddock landscapes is currently being explored. Meanwhile, nitrogen fixation measured by natural abundance just prior to harvest was also related to measured nodule numbers, particularly for lentils.

Grain phosphorus uptake results supported early vegetative responses and confirmed the higher phosphorus requirement of both pulse crops to maximise grain uptake. Soil mineral N nitrogen assessments taken soon after harvest revealed an increasing trend of soil nitrogen with increasing rates of phosphorus applied, particularly for lentils.


Increasing phosphorous rates corresponded with an increase in pulse crop biomass, nodulation numbers and nodule weight per gram of root.

The form of phosphorus applied could be key in obtaining the right balance of higher phosphorus inputs while keeping soil nitrogen levels low to ensure peak pulse performance in terms of both yield and nitrogen fixation.