Maintaining soil organic matter to slow fertility decline and reliance on fertilisers

Abstract

Soil organic matter is critical for healthy soils and sustainable agricultural production. This is not ‘news’ to growers, agronomists, or indeed anyone with a vegetable garden or compost heap at home. We know that healthy soils with high organic matter levels grow better crops that are easier to manage. However, we also know that soil organic matter (SOM) and soil organic carbon (SOC) levels are declining, which means continued grain production and healthy crops are needing more fertiliser, especially for nitrogen (N).

To make sensible decisions, managers must understand how SOM and SOC work, why their levels are declining, the implications for enduring profitability, and what can realistically be done about it.

Data from 500 paired-site across grain regions of Queensland and northern NSW show the effects of landuse and farming practices on SOC levels across the northern grain region. Total Organic Carbon (TOC 0-10 cm) under remnant vegetation of 'agricultural soils' varied from 0.5 to 5.0% and typically declined with longterm cropping to 0.5 to 1.2%, representing up to $5000-8000/ha of lost natural nutrient capital. The good news is productive pastures with good nutrition were able to raise TOC levels by approximately 1 t/ha/yr compared to continued cropping, and so provide opportunity for long-term management of SOM and SOC in mixed farming systems