Using Compost on Coonawarra High Carbon Soil

Trial Overview

This trial studies the plant response of beans grown in black loamy soil in the Coonawarra when different rates of compost are applied prior to sowing. This paddock has rocks present and was worked with a reef crusher 2 years prior to compost application. A reef crusher is a specialised machine that is used to break down and prepare rocky terrain for agricultural use. The compost applied to the paddock may enable a faster healing process and return the paddock to productivity more quickly.

Figure 1 (above): Image of trial site, 15km North of Penola, South Australia

Purpose of Trial

The purpose of this trial is to evaluate different application rates of compost on a bean crop, compared to control (no compost) in a paddock that was previously amended with a reef crusher. This trial aims to assess this by looking at plant biomass, estimated yield and grain weight, to determine if compost can improve productivity under these conditions.

Methodology

Trial Objectives

• Assess the impact compost has on the growth and productivity of beans. 
  

• Assess the ameliorating properties of different rates of compost in a high-carbon Coonawarra soil
  

Location

15km North of Penola, South Australia (Figure 1)
Crop Type: Beans
Soil Type: Black Clay Loam
Start date: 20/05/2025
End date: 31/12/2025

Treatments

• Control – No compost applied

• AgriGro – Spread at 5t/ha

• AgriGro – Spread at 10t/ha

• AgriGro – Spread at 15t/ha

AgriGro is a high-quality base compost designed for broad agricultural use.

Measurements

• Biomass and phenology – Monitored 2-3 times during the growing season
  
  
• Grain yield – Measured at harvest

Figure 2: Shoot biomass collected 11/8/2025 from different treatment samples of beans

Figure 3: Shoot biomass collected 23/9/2025 from different treatment samples of beans

The results below refer to Figures 2 and 3.

• Six biomass samples were collected from each plot on 11 August, then three on the 23 September

• Compost treatments have greater shoot biomass when compared to the control

• The treatments demonstrate similar trends over the two sample dates

• Unexpectedly, the 10t/ha treatment has greater biomass compared to the 15t/ha plot. This may be due to unknown factors influencing trial results, for example, variation in soil or pressure from disease.

Figure 4: Estimated yield samples collected 9/12/2025 from each treatment

Figure 5: Grain weight collected 9/12/2025 from each treatment

The results below refer to Figures 4 and 5

• Yield estimates were collected on the 9th of December. Four samples were collected from each plot. 

   

• Plants and beans were mostly green, although the hilum had blackened, indicating beans had reached their maximum size. 

   

• Patches of lodging were evident across all plots, with a visibly greater number of dried beans in the 10 t/ha treatment compared to the other treatments.

   

• The control performed unexpectedly well compared to the compost treatments in terms of estimated yield, despite a smaller bean size.

   

• Grain weight is positively correlated with the rate of compost applied. 
  
  

Key Findings

• The 10t/ha treatment had the greatest biomass in comparison to the other treatments

• Grain weight is positively correlated with rate of compost applied

• The trends in biomass do not directly correlate with estimated grain yield

• Rocks in the soil may have impacted trial results.

Summary 

Plant responses varied across the plots, and some of the observed differences could not be explained solely by the rate of compost application. Biomass samples collected in August and September displayed similar trends across treatments. However, these patterns did not directly correspond with estimated grain yield. In contrast, grain weight showed a positive relationship with compost application rate, indicating that yield response was more closely associated with compost input than early biomass alone. Trial results may have been influenced by soil variability, particularly the presence of rocks, with the 5 t/ha and 15 t/ha plots appearing to contain more rock than the 0 t/ha and 10 t/ha plots. Additional factors, including pest pressure and disease incidence, may also have contributed to treatment variability. To obtain more conclusive results, this trial should be repeated in the following season, and consideration should be given to conducting future trials at an alternative site to reduce the influence of rocky soil conditions.

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