Key Points

  • When evaluating a biological input for grain production, it may be useful to consider whether the input will alleviate yield constraints.

  • The major yield constraints in the northern grain-growing region are low nutrient use efficiency, low water movement through saturated soil and high soil density.

  • The biological inputs with the most potential to help alleviate these yield constraints are manure, compost, vermicompost, biochar and possibly some humic substances and seaweed extracts.


Figure 1: The northern grain-growing region includes Queensland and northern New South Wales. Image: GRDC.

Evaluating biological inputs

There are a wide range of biological inputs on the market including inocula, various biostimulants, composts, manures and biochar. It can be difficult to evaluate their potential effectiveness and return on investment for broadacre grain production for a number of reasons.

First, the properties of biological inputs are highly variable, even within a single type of input. This makes it difficult to evaluate whether one particular product is likely to be effective. Second, few previous scientific studies have been field experiments in broadacre grain cropping. Instead many were laboratory or glasshouse experiments with horticultural crops and farming systems. Finally, many of the mechanisms claimed by manufacturers have not been assessed by scientific investigations yet.

When considering using a biological input in broadacre grain production, it may be helpful to first consider what constrains yield on your property. A biological input is unlikely to improve crop growth or yield unless it alleviates a yield constraint. Previous scientific investigations can help determine which biological inputs have the most potential to alleviate particular yield constraints (table 1).


Table 1: The potential of different types of biological inputs to overcome particular yield constraints based on the findings of previous scientific investigations. (Black ticks indicate one or more studies observed the effect. Grey ticks indicate that the effect is widely claimed.)


Yield constraints in the northern grain-growing region

The northern grain-growing region has generally high soil fertility, although there is increasing evidence that this has been run down over time.

For example, vertisols, which occur throughout north-west NSW and southern Queensland, are generally well-structured and fertile and have pH close to neutral. On these soils, yield is not constrained by surface soil properties, but by subsoil constraints. These include low permeability, shallow rooting depths, sodicity, salinity and toxic concentrations of some elements.

In the northern region both summer and winter crops are important for profit. The region has relatively high seasonal rainfall and production variability compared with the western and southern regions. It experiences summer dominant rainfall and grain yield is significantly dependent on conserving soil water from summer rain.

In the northern grain-growing region, the most significant yield constraints are low nutrient use efficiency, low water movement through saturated soil (also known as low saturated hydraulic conductivity) and high soil density.


Alleviating yield constraints using biological inputs

When yield is limited by poor nutrient use efficiency, crop growth or yield may be improved by applying manure, compost, biochar, humic substances or seaweed extracts. All of these biological inputs have been associated with greater plant uptake of nutrients.

The mechanisms by which these biological inputs may increase nutrient uptake are not always clearly stated but there are several possibilities. Manures, composts and biochars may increase nutrient uptake if they contain significant amounts of nutrients themselves. They may also improve nutrient uptake by improving chemical and physical properties of soil that limit root growth. Some humic substances and seaweed extracts could increase nutrient uptake because they contain plant hormones and growth-promoting substances that can increase root growth. Many manufacturers of inocula claim that their products improve nutrient use efficiency without fully explaining the mechanisms involved.

When yield is constrained by low water movement through saturated soil, applying compost, manure or biochar may increase crop growth or yield. These biological inputs are largely composed of organic matter and they can increase soil aggregation and pore space in soil. These improvements in soil structure may increase water movement through soil when soil is saturated. Manufacturers of some biostimulants claim the products have the potential to improve water movement through soil, but the mechanisms remain unclear.

If low water movement through saturated soil is associated with waterlogging, applying humic substances may help alleviate this constraint. Humic substances have been shown to increase the tolerance of plants to excessive water in their rhizosphere. However, the mechanism for this effect remains unclear.

When the high density of a soil constrains yield, applying manure, compost, vermicompost or biochar may increase crop growth and yield. Because these biological inputs are largely composed of organic matter they can increase soil aggregation and pore space in soil, both of which can decrease soil compaction and the bulk density of soil. However, if yield is being constrained by subsoil compaction, manure, compost or biochar are unlikely to alleviate this constraint unless they are applied below the soil surface or incorporated into soil.


Author: Jennifer Carson (Ghost Media)

The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.

The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.

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