This project will examine how soil type affects crop emergence under different levels of available soil moisture and the impact of this variation on yield and profit. The work will also explore how remote sensing can be used to examine variation in soil moisture content as an aid to decision making. The project was developed in consultation with the four grower groups involved in the project– Hart, Northern Sustainable Soils (NSS), Upper North Farming Systems (UNFS) and Mid North High Rainfall Zone (MNHRZ). In this proposal, dry sowing encompasses sowing into completely dry soil as well as sowing into soil with marginal soil moisture. 

This project aims to improve the effectiveness of dry sowing, especially at early sowing times. It will:

• Describe the effects of soil moisture content on crop establishment in different soil types
• Describe how sowing rate and sowing depth can be used to optimise crop establishment under marginal soil moisture
• Examine the value of remote sensing technologies to map variation in seedbed moisture
• Improve the guidelines for dry sowing to allow more informed decisions and improved risk assessment.

Project description: 

There has been a shift to earlier sowing in SA as average farm sizes have increased and cropping programs have become larger. Apart from the logistical advantages of earlier sowing, there are also benefits from higher yields when varieties with an appropriate maturity type are used. However, over much of SE Australia, the reliability of autumn rainfall has declined and the autumn break has become later.  In much of SA, the median time of the opening rains has been delayed by up to 2 weeks, with up to a 4–week delay in some parts of the State.  The concurrence of these trends means dry sowing or sowing under marginal soil moisture is now common, with the attendant risks of low and uneven crop establishment and poor weed control.  Most growers now sow at least part of their farm dry.  

Despite the widespread uptake of dry sowing, it is not without risk. There are still a number of issues with dry sowing that limit yields, many of which relate to crop establishment.

It is unlikely the trend to dry sowing will diminish; indeed, it is likely to increase as cropping programs continue to get larger.  The pressure to sow crops on time and the momentum to continue sowing once it has started can result in seed being sown under less than optimal conditions, resulting in poor or staggered emergence.  In severe cases, some of the crop may need to be resown.  There is a need to optimise the management of dry-sown crops to reduce the risks of yield loss and to provide better guidelines to help with decision making and risk assessment. This will be achieved by a better understanding of the interactions between soil type, rainfall and soil moisture and of how changes in sowing practices can affect crop establishment and yield under different levels of soil moisture availability. 

Fundamentally, germination and seedling emergence are driven by the availability and uptake of moisture by the seed and this becomes the overriding influence on establishment when crops are sown into dry soil.  Soil characteristics such as texture and structure, and their interactions with the amount and frequency of rainfall, will then determine emergence rates.  Variation in establishment across a field will reflect the variation in soil type.  Improving our understanding of the interactions between soil type and rainfall on establishment and yield and how these can be influenced by sowing practices will improve the reliability of dry sowing and/or allow a more objective assessment of risks of dry sowing, leading to development of recommendations related to soil-specific or variable rate practices such as variable sowing rates or sowing depths.  This will be the focus of the proposed work. 

Key achievements and results: 

The main result from 2023 was the completion of five field experiments at three sites.  These examined the effects different management practices – time of sowing, sowing rate and sowing depth – on crop establishment, growth and yield in crops sown. 

Following good rainfall in early-mid April, there was a long period of dry weather until the end of May.  Therefore, the early two sowing times at Hart emerged and established in declining soil moisture and establishment was lower and patchier than at the later sowing times, when crops were sown into good moisture.

Field trials. 

Six experiments were conducted at three locations in the Mid North of SA (Giles Corner, Hart and Bute) to examine the effects of time of sowing (between late April and June), sowing depth (shallow, standard and deep sowing) and sowing rate (standard and high) on crop establishment, growth and yield.  The experiments included canola (2 sites), wheat (3 sites) and lentil (1 site).  All experiments were completed successfully.   The main results from 2023 were: 

• Low soil moisture in late April-early May reduced establishment in crops sown at this time 

• Sowing deeper than the standard sowing depth reduced establishment while higher sowing rates increased plants/m2 

• Despite lower establishment, the highest grain yields of canola were achieved with the early sowing times in late April and early May at both Hart and Giles Corner 

• Wheat yields at Hart were highest with the earlier sowing data, despite low crop establishment. 

• Responses to time of sowing at Giles Corner depended on depth of sowing with the greatest reduction from April to early May sowing, when the seedbed moisture was low, occurring with deep sown crops. 

• Deep sowing reduced grain yields in wheat at all sites, but canola yields were not significantly affected by depth of sowing. 

• Yield was most affected by deep sowing in the heavier vertosol soil at Giles Corner when the crop was sown into a dry profile with little follow up rain.  When soil moisture was high there was better establishment and little effect of sowing depth on yield. 

• Increasing sowing rates above the standard rates did not improve yields when sowing into soil with low moisture except in wheat at Giles Corner except when establishment rates were below 50%.  The additional cost of a higher sowing rate may be considered as insurance mitigating the effects of low establishment but without providing any benefit at higher establishment rates in 2023. 

• Sowing deeper than the standard sowing depth into dry soil provided no benefit to crop establishment and was detrimental to yield in many cases. The effect of deep sowing on yield was influenced by its effect on establishment. 

Laboratory and glasshouse work 

Surface soil was collected from the trial sites and will be used for the studies on emergence in the future.  The generation of the moisture release curves has been slower than anticipated; curves have been produced for two of the four soils and the remaining curves will be generated in early 2024.  Completing the KPI for the moisture release curves will be delayed by about a month. 

Proximal sensing 

The purpose of this work is to explore the feasibility of remote sensing technology to map spatial variation in seedbed properties to assist with decision making.  Preliminary aerial measurements of soil temperature were made at Giles Corner and Hart in May with the main aspect of the work planned for 2024 and 2025.  Very high winds at the time of measurement limited the flight times and precluded measurements at Bute and limited the quality of the data. The initial plan to use microwave sensors to monitor soil moisture was first delayed and then found to be not possible because the commercial arrangements to use the technology with an overseas partner were not able to be finalised.  A meeting was held with Airborne Logic in early 2024 and a modified approach was mapped out which will deliver similar outcomes to what was originally proposed. 

• The yield penalty from delaying sowing may not be overcome by changes to sowing rate or sowing depth 

• Crops have a high capacity to adjust to variation in plant density and there may be little variation in ground cover later in the year or in yield from poor establishment.  

• Variety maturity type needs to be matched to sowing time to take full advantage of early sowing.