Soil organic matter what is it worth to grain production and what practices encourage it

David Lawrence, Suzette Argent & Rod O’Connor, DAFF Queensland
Graeme Schwenke, Sally Muir & Malem McLeod, New South Wales DPI

Introduction

Soil organic matter is critical for healthy soils and sustainable agricultural production. This is not ‘news’ to grain growers, agronomists, or indeed anyone who has a vegetable garden or a compost heap at home. We all know that soil organic matter is good for the soil and that crops growing in healthy soils with high soil organic matter levels are easier to manage and perform better.

However, the recent  dialogue about ‘climate change’, Australia’s emerging ‘carbon economy’ and opportunities to trade sequestered soil carbon have renewed interest in better understanding soil organic matter (SOM) and soil organic carbon (SOC) levels, and how best to manage them on commercial grain farms. Of course, this requires a clear understanding of how soil organic matter improves the soil, the subsequent benefits to the crops and pastures grown and their contribution to profitability of grain farms.  Our new project is helping growers to answer these questions as part of the GRDC national RDE program on soil organic matter that began in June 2012. Here we present findings from on-farm sampling done throughout the northern grains region over the past 3 years.

Soil organic matter and carbon decline under cropping

Our on-farm measures from over 500 sites in Qld and northern NSW confirm that soil organic matter, measured as soil organic carbon, declines dramatically when land is cleared and continuously cropped. This decline affects all soils and land types but is most dramatic for the brigalow/belah soils because their starting organic carbon levels are so high (Figure 1). 

Our results show that 30+ years of cropping of brigalow/belah soils will reduces soil organic carbon levels in the 0-10 cm layer alone by about 2%, or 20-25 t/ha of soil carbon!

These are big changes in the soil; what is going on? Everyone needs to understand that soil organic carbon levels are simply a snapshot of the current balance between inputs (e.g. plant residues and other organic inputs) and losses (e.g. erosion, decomposition) constantly happening in each soil and farming system. The decline over time is overwhelmingly driven by the extent of fallowing in our farming systems. Most fallow rain in the northern region (as much as 75-80% in a summer fallow) is lost as runoff or evaporation. This wasted rain does not grow dry matter to replenish the organic matter reserves in the soil. However, increasing moisture in the fallowed soil continues to support microbial decomposition that accumulates available nitrogen for the next crop, but reduces soil organic carbon. The soil organic matter and carbon levels will continue to decline until they reach a new lower level that the dry matter produced by the new farming system can sustain. Put simply,

‘Crops may make more money than trees and pastures, but do not return as much dry matter to the soil’

Figure 1. The decline of soil organic carbon with long-term cropping systems

The benefits of soil organic matter

A paddock’s “carrying capacity” for organic matter varies with its long-term average rainfall and soil type. Critical levels are not defined for each soil because the many and varied functions of soil organic matter are difficult to match with crop productivity. Basically, more organic matter is better.

Soil organic matter is perhaps the best single indicator of soil health, long-term crop production and the need for continual inputs to maintain this productivity. While soil organic matter contains about 60% organic carbon, it is decomposition of the organic matter that drives many physical, chemical and biological soil processes in the soil, and supplies a range of nutrients needed by both plants and soil biota.  Organic matter can help the major soil functions:

However, the impact and value of these different functions varies with different soils and the types, or fractions, of soil organic carbon that we use to measure soil organic matter. For example, Figure 2 shows that the contribution of organic matter to cation exchange capacity (CEC) is large on sandy soils but small on heavy clays, and that the bulk of nutrients come from the humus fraction, while the particulate fraction provides much of the energy for microbial activity.

What is soil organic matter really worth to grain producers?

The prospect of being paid $23 /t CO_2-equivalent (which equates to ~$85 /t soil carbon) has attracted growers’ attention, but prices of $5/t CO_2-equivalent provide little incentive to trade carbon. Organic matter and carbon are currently more valuable for their contributions to production.

It is hard to quantify the direct economic value of many of the soil functions that organic matter supports. However, they are clearly higher than the value of soil carbon alone. Indeed, 1 tonne of soil carbon is typically associated with 100 kg organic nitrogen, so when the soil organic carbon levels of a brigalow/belah soil decline by 2% (20-25 t/ha), it means up to $3000 /ha of nitrogen has been released for crops to grow on. For all nutrients, including phosphorus and sulphur, this figure totals over $4000/ha.

These nutrients may not have been wasted. They were from decomposing organic matter reserves and enabled cropping for 30+ years with little of no fertiliser application. Fertiliser use is now increasing as the supply of nitrogen and other nutrients declines with soil organic matter levels.     

Figure 2. The relative contribution of soil organic matter (carbon) fractions to key functions of soils with different clay contents (source: Jeff Baldock).

How do practices effect soil organic matter and soil carbon levels

Soil tests on paired-paddock of the same soils with different histories have been used to compare the effect of other key practices of total soil organic carbon levels (Figures 3-6).

Figure 3. The impact of grain versus forage cropping systems on total soil organic carbon (0-10 cm)

Figure 4. The impact of tillage and fertiliser practices on total soil organic carbon (0-10 cm)

Figure 5. The impact of pastures on total soil organic carbon (0-10 cm) in crop land (Darling Downs)

Figure 6. The impact of pastures on total soil organic carbon (0-10 cm) in crop land (western Downs)

These data show clearly that changes in land-use have much bigger effects on soil organic matter and carbon levels than agronomic practices that ‘fine tune’ the system for profitability:

-        Grain and forage systems had similar soil organic carbon levels (Figure3), which underlines the overwhelming impact of fallowing. While lower levels of removal from forage crops may be expected to maintain higher soil organic carbon levels, management of forage crop is often not as good as for grain crops, and stock may deposit dry matter in manure away from the cultivation in shade lines and around watering points. Regardless, these results

-        Best management practices with minimal tillage and nutrient replacement for good crop production (Figure 4) are having minor impacts. However, sites with very high nutrient addition (e.g. SOC18) may be providing long-term benefits. In the northern NSW SCARP project, differences in soil organic matter between conventionally cultivated and minimum tillage soils were confined to the top 0-10 cm only. Questionnaires at the time of soil sampling revealed that 20% of surveyed paddocks under conventional tillage cropping had had no nitrogen fertiliser applied in the previous 5 years.

-        Pasture phases (Figures 5&6) are the practice with the greatest ability to rebuild soil organic matter and carbon levels in cropping land. However, these pastures must be well grown and productive with high dry matter production to maximise their contributions to soil organic matter reserves. This means that pasture phases must have good nutrition to grow well. If they do, they will build soil organic matter (that is soil organic carbon and the associated nutrients) for future decomposition and use by subsequent crops.

Our work to date confirms that declines in soil organic matter and carbon are inevitable when the land use changes from native vegetation to cropping. The data really shows ‘historical effects’, which included many practices that are now considered ‘poor’ by modern standards. For example, few people now use long bare fallows with burning and excessive cultivation. The data also shows that well-grown pastures can make major improvements in old croplands with soil carbon levels under pastures up to 1 t/ha/yr higher than with continued cropping. This is an area for further research and better pasture agronomy to ensure swift transitions into pastures. There is also scope to research the use modern farming practices to slow down the decline of soil organic matter reserves when new country is developed, or indeed when land comes out of restorative pasture phases.

Contact details

Dr David Lawrence
Department of Agriculture, Fisheries and Forestry, PO Box 102 Toowoomba 4350
Ph: 07 4688 1617
Email: david.lawrence@daff.qld.gov.au 

Dr Graeme Schwenke
Department of Primary Industries, 4 Marsden Park Rd Calala NSW 2340
Ph: 02 6763 1137
Email: graeme.schwenke@dpi.nsw.gov.au