Take home messages

• Dry sowing offers a viable option for Mid North growers but requires careful management and risk assessment to maximise its benefits.
• Anecdotally, there was often reduced and patchy germination of seed and emergence in paddocks that were dry sown when the first effective rainfall was more than 3-4 weeks later.
• Ensuring good seed to soil contact with sowing operations is vital, especially when sowing into dry soil.

Background

The 2024 season certainly tested and challenged growers in many ways, largely as a result of limited rainfall and the subsequent flow-on effects following low rainfall cropping seasons. Soil management and overcoming constraints to root growth is a large part of understanding how to maximise water holding capacity of the soil and crop performance in these dry years. It is through an understanding of the soil and crop interactions that an understanding of dry sowing techniques (risks, reversals and rewards) has evolved.

Dry sowing, or sowing winter crops into dry topsoils in the absence of recent prior rainfall, is increasingly being practiced in the Mid North of South Australia. Water scarcity and unpredictable rainfall patterns present additional challenges for growing crops, especially for traditional tillage systems where evaporation was an issue. Dry sowing involves planting crops before substantial rainfall (no choice in 2024), sometimes relying on moisture stored deeper in the soil profile (critical) and recent follow up rains (even more critical) to support good crop establishment.

Every grower approaches dry sowing in their own way, and knowledge in this space has built rapidly in a collaborative, transparent way, largely from grower experience. In an ideal world, crops are sown into soil with adequate moisture, but that will never be the case in every season, especially for cropping programs that are extensive. To move forward and take learnings from this year, it is important to reflect on the successes and unintended consequences of dry sowing.

Benefits of dry sowing

Logistics - Dry sowing allows growers to sow crops early in the season and at scale, with crops germinating and establishing rapidly over a large area when the rains arrive, compared to delayed sowing into soil moisture. These improvements in overall farm management and efficiency are critical for large sowing programs where machinery and labour are limited, and sowing may take 4-6 weeks if everything goes well. By sowing into dry soil, growers don’t have to wait for rain and can get more of their crop in the ground ready for germination earlier. Also, this potentially avoids crop temperature stress (both hot and cold), drought or other abiotic (and potentially biotic) stressors once the crop is established.

Fracture patterns - On heavier soils with relatively high clay percentage in the Mid North, dry sowing can result in a tillage fracturing effect, opening up cracks to depth, especially with knife points. This is much more pronounced in dry soils compared to sowing into moisture, and can allow better water infiltration to depth in the furrow, effectively dismantling dispersive clay in the topsoil and redistributing them to depth. In wide furrows, there is potential for water harvesting and preferential flow patterns are usually observed. Depending on soil type, a disc can also create soil fracture effects, but the benefit of discs is mainly due to more precise seed placement and germination.

Issues with dry sowing

Rainfall uncertainty - The main risk of dry sowing is the reliance on uncertain rainfall. If rains are delayed excessively, crops may emerge in late June or July and may flower outside their optimum window. By contrast, if growers wait for rain the break is very late, they have the option of changing variety, crop type, or rotation e.g. fallow or pasture. Follow-up rains that are crucial to crop establishment may be patchy and light, and as a result, issues such as fertiliser toxicity, enzyme depletion and malting of seed can lead to reduced crop vigour and yield potential.

Soil erosion - Dry sowing can leave the soil exposed to wind erosion, particularly in areas with light soils or little ground cover, as there is less material to protect the soil from autumn winds and potential heat stress on germinating crops. Obviously, the longer the period between dry sowing and the first rains, the greater the erosion risk, and if crop emergence is patchy, the risk can persist into winter.

Weed pressure - Early sowing may give some weed species a head start, as they can also germinate with the first rain, competing with crops for nutrients and moisture. While there can be herbicide rotations and post-emergent options to suit dry sowing situations, by far the best option is to avoid dry sow paddocks with large weed burdens, if possible.

Lessons from 2024

Overall, dry sowing offers a viable option for Mid North growers providing significant logistical advantages for large croppers, but it requires careful management and risk assessment to maximise the benefits. So, from a soil and crop perspective, what did we learn in 2024?

Better germination of seed and subsequent crop emergence in paddocks was noticed when they were dry sown close to the first effective rainfall. There is a degrading effect from seed being in dry soil for an extended period. This is relatively slow if the soil is completely dry, but while a soil may seem dry, there is often small amounts of moisture present in the pores. Partial moisture can result in the instigation of germination and enzyme depletion which reduces vigour. Consider the same falling numbers deliberations used for rain-affected grain, and only use good quality seed for dry sowing. Partial soil moisture can also exacerbate fertiliser toxicity effects. This can be mitigated by separating seed and fertiliser by at least a couple of centimetres, especially if banding urea.

Summer rainfall provides soil moisture when temperatures are warm, which boosts N mineralisation and the release of N from organic matter. Small amounts of phosphorus (P), sulphur and other nutrients can also be mineralised. During the very dry summer and autumn in 2024, there was a lack of mineralisation due to inadequate moisture. Without sufficient soil moisture prior to planting, crops start with little available soil N. A better understanding of mineralisation in soils is needed to plan more effective dry sowing programs, but as it turned out in 2024, winter rainfall was poor and yield potentials were low, so crops were not N limited. Soils with water repellent surface layers do not wet up effectively with small rainfall events. This not only results in patchy crop establishment, but mineralisation is reduced. These soils are also prone to erosion. Hence, growers should be cautious when dry sowing water repellent soils.

The 2024 season provided challenges with the set up of seeder bars. What was perfect for 2023 was not ideal for 2024 in many instances. This created interest in refining the physical structure of ground-engaging sowing tools to better manage cloddiness, seed depth, seed to soil contact, and speed of operation while dry sowing. Shallow seeding generally performed better in 2024, given the small winter rainfall events. On some soil types, better crop establishment was achieved by slowing down which improved soil throw, the depth of soil fracture, and seed bed preparation.

Applications of lime and gypsum in the summer or autumn of 2024 as a soil amelioration strategy did not benefit following crops. This is simply due the fact that there was not enough rainfall to solubilise the ameliorants in the soil and crop yield potentials were very low. Gypsum is more soluble than lime and usually benefits crops in the year it is applied.  Depending upon the lime quality, it can take 2-3 years to dissolve significant and increase soil pH. Soil acidity and dispersion (causing poor structure) are present in many Mid North soil types, so lime and gypsum are needed depending on the main constraints. Money invested in these ameliorants in 2024 was not wasted, as the benefits will occur with rainfall in 2025 and beyond.

Nutrition

Calcareous and alkaline grey soils in the Mid North have been of much concern, with growers noting poor crop growth early in the season. This is largely related to poor P availability particularly in dry soil conditions which limit the dissolution of granular fertilisers and root growth. Granular fertilisers in dry soils can absorb small amounts of moisture over time, however, the P can be locked up in the granule and may fail to diffuse into the soil in forms available to plants. This can be overcome to some degree by adopting liquid P, which is more available on these soils than granular forms. Additional costs of liquid P is an issue, and some growers have had benefits from a hybrid approach applying liquid P alongside granular to reduce the cost.

Alkaline grey soils have been responsive to iron applications in 2024 in the field. There is research actively occurring in this space led by SARDI (Wilhelm and Pearce 2024) and there’s a need to further understand the use of iron in dry seasons.

Antioxidants and biostimulants

Use of seed priming compounds, foliar antioxidants and biostimulants in dryland cropping systems is being investigated by Flinders University, including laboratory and greenhouse trials in 2024 involving Upper North Farming Systems and Field Systems. The most interesting developments are a wheat seed treatment with potential to provide abiotic stress tolerance (e.g. heat, chilling, drought, Ramesh and Booth 2024), and a leaf-applied antioxidant compound with similar effects (Anderson et al. 2024). A 2025 field assessment will further determine the effectiveness of several seed treatments and foliar-applied options, especially in relation to dry sowing.

References

Anderson P, Sweetman C, Rainbird B, Soole K, Lowe J (2024) Bioactive antioxidative compounds project. UNFS Grower Expo Booleroo, Flinders University.

Ramesh S, Booth N (2024) MicroBiome (FAME) research workshop. UNFS Grower Expo Booleroo, Flinders University.

Wilhelm N, Pearce A (2024) Correcting iron deficiency in broad beans growing on highly calcareous clay loams. Proceedings Agronomy Australia Conference, Albany, 21–24 October 2024 (https://www.agronomyaustraliaproceedings.org/images/sampledata/2024/ASAwilhelm_n_213-213-1042-Wilhelm-Nigel.pdf)

Contact details

Michael Eyres
Field Systems
michael@fieldsystems.com.au