By Paulette Baumgartl, Applied Horticultural Research

The Mulgowie Farming Company, a family-owned producer, grows conventional and organic sweet corn and green beans across Australia.

Known for its strong focus on innovation and sustainable farming practices, Mulgowie teamed up with the Soil Wealth ICP project to examine the potential of legumes to effectively supply nitrogen to a sweet corn crop in a Queensland trial in 2023.

Cover crop treatments

The trial area of 1.1ha included three different cover crop treatments plus one control, as follows:

GREEN: Faba bean crop terminated green at flowering by incorporation (using a rotary hoe). AUG 2023

SPRAY: Faba bean crop terminated at flowering by spraying with herbicide (using glyphosate and carfentrazoneethyl) and rolling, leaving the residue brown on the soil surface. AUG 2023

FALLOW: with no faba bean crop (control). JULY 2023.

HARVEST: Faba bean crop grown to maturity and harvested for grain with stubble left on the soil surface. OCT 2023.

Fertiliser treatment

At sowing of the sweet corn crop via direct drilling in December 2023, the trial area was split in half:

• Half of the treatment area received base fertiliser (300kg/ha containing 10.5 percent nitrogen).
• Half of the treatment area received no base fertiliser.

Leaving half of the trial area without base fertiliser facilitated an investigation into whether the cover crop alone could supply sufficient nitrogen to the corn crop.

Soil & climate data

Soil was regularly sampled across treatment areas to monitor the effects of termination methods on plant-available nitrogen and other soil properties.

This including monthly soil tests for available nitrogen, organic matter, C:N ratio, bulk density, and particle size.

Using probes and data loggers, soil moisture and temperature data was collected to assess how treatments influenced these parameters and their potential impact on nutrient cycling.

Rainfall data was accessed via an on-site rain gauge, supplemented by Bureau of Meteorology data from nearby weather stations.

Plant tissue & biomass analysis

Plant tissue and biomass data were collected to understand nitrogen cycling and to create a nitrogen budget.

Faba bean was tissue tested at flowering and harvest for carbon percentage and nitrogen percentage, and measurements were taken of above-ground biomass (t/ha), with grain yield assessed separately at harvest.

A full nutrient status of the sweet corn at the V4 stage as measured via tissue testing of the fifth leaf from the tip.

At one week before harvest, the following was measured:

• Corn ear leaf testing for full nutrient analysis.
• Above-ground biomass testing (excluding cobs) for nitrogen (percent) and biomass (t/ha).
• Corn cob analysis for nitrogen (percent) and pre-harvest quality and yield assessment.
• Above-ground biomass of Johnson grass was measured (t/ha) to track its growth alongside corn.

The results & what they mean

Faba beans contributed significantly to nitrogen management in the trial by absorbing excess nitrogen and storing it in their biomass. In the fallow treatment (control), soil nitrogen levels remained at 146kg N/ha due to the absence of heavy rainfall that could cause nitrogen loss (Figure 1).

FIGURE 1. Soil available nitrogen (kg/ha) under different treatments

In the faba bean plots, nitrogen levels dropped to 47kg N/ha by July as the plants absorbed nitrogen. By flowering, faba beans produced 5t/ha of biomass containing 242kg N/ha. At harvest, they produced 5.3t/ha of biomass and 3.5t/ha of grain, with 228kg N/ha stored in the plant.

Harvesting the faba beans for grain removed 152kg N/ha, redistributing nitrogen from the shoots to the grain. This shift increased the carbon-to-nitrogen ratio (C:N) from 11.4 at flowering to 37 at harvest, affecting nitrogen cycling by promoting nitrogen immobilisation (Figure 2).

FIGURE 2. Nitrogen (kg/ha) in different plant parts at flowering and at grain maturity

Termination method also had an influence on nutrient cycling. Incorporating green residues into the soil led to a rapid nitrogen release from September to November, while spray termination resulted in slower nutrient release. Fine-cut residues decomposed faster, making nitrogen more available. Timing nitrogen release is crucial to match sweet corn’s needs, with the crop’s nitrogen demand peaking from the V6 to R1 growth stages.

Corn crop – biomass, yield & nitrogen trends

In late September and October, there was a significant flush of Johnson grass weeds, with a noticeable difference in foliage colour between treatments. The green treatment had darker foliage compared to the spray treatment. The Johnson grass was sprayed with herbicide in November before planting corn, and the corn crop received a light scuffling in the interrow to manage the weeds. Despite these efforts, Johnson grass pressure remained a problem throughout the crop’s growth.

At the tasselling stage of sweet corn, the green treatment showed darker and more uniform foliage compared to the spray and harvest treatments. Due to substantial Johnson grass pressure, data on above-ground biomass of sweet corn was combined with Johnson grass and interpreted as a combined unit. Johnson grass pressure was highest in the spray treatment, followed by the harvest treatment, and lowest in the green treatment, with no pressure in the fallow area. There was a direct correlation between weed pressure and packable yield: increased weed pressure led to a reduction in corn crop size and yield.

Unexpectedly, the total above-ground biomass in areas without base fertiliser was consistently higher than in those with base fertiliser across all termination treatments. The reason for this is unclear.

In the no base fertiliser areas, the fallow, green, and harvest treatments returned similar biomass levels, while the spray treatment had significantly more biomass. Late November soil tests showed that the fallow and green areas had significantly more readily available nitrogen (119kg N/ha) just before planting, compared to the spray treatment area (57kg N/ha), potentially giving the crop a growth advantage over the weeds (Figure 3).

FIGURE 3. Above ground biomass dry matter of sweet corn and Johnson grass (t/ha, left axis), and packable yield (t/ha, right axis) by treatment, at end of corn crop. FEB 2024

It is unclear why the harvest from the no base fertiliser area returned similar biomass levels to the green and fallow treatments. However, the harvest no base fertiliser area did have lower nitrogen in the above-ground biomass of corn and Johnson grass at the end of the season than the other treatments, likely due to the large amount of nitrogen
removed during the faba bean grain harvest.

What can we conclude

Did nitrogen availability from legumes match crop demand? While it is difficult to draw definitive conclusions due to the skewed results from weed pressure, it appeared that nitrogen was more readily available in the green incorporated treatment area. The area where faba beans were terminated by spraying contributed the same amount of nitrogen at termination, yet it became available much more slowly than when it was incorporated green. While the fallow area also showed good results and simplified management, there was a real risk of rapid nitrogen loss with heavy rainfall. Fortunately, the conditions during the trial allowed for nitrogen retention.

The Soil Wealth ICP team would like to thank Andrew Johanson, Markus Sinnott, and the broader Mulgowie Farming Company team for their time and contributions to the demonstration site.

FIND OUT MORE
Mulgowie demonstration site and the full case study download.