Dryland systems: environmental factors and cotton establishment

When you break it down, seedling establishment is dependent on two things:

• The placement of the seed into soil with adequate moisture (and temperature) so that enough surface contact occurs to take up water.
• The ability of the germinating seed to emerge, which depends on the root tip being able to penetrate the underlying soil to begin establishing a root system whilst the hypercotyl has to be able to elongate and push through the overlying soil enabling the cotyledons to emerge.

Sounds simple enough, but many of the establishment difficulties encountered under dryland conditions arise when suitable soil moisture (#1) is at depths greater than 5cm that then creates challenges for the emergence part (#2) of the establishment equation. It is often soil and system factors that limit part 2 of this equation - which is the focus of this article. 

Moisture-seeking cotton seed becomes more problematic in soil types that have shallow or poorly structured topsoil. A study by Hulme (2015) showed that moisture seeking seed in poorly structured topsoil is likely to lead to “Kinze” cracking whereby the soil shrinks away from the sown seed in the planting trench, resulting in a failure to take up moisture or establish.

Hulme suggested that the development of a Kinze crack following moisture seeking cotton at depths greater than 5cm was an inevitable outcome when cotton is sown into clayey soil types with peds* of 10 cm or more.

*A ped is the description of the natural, relatively permanent aggregates that occur in the soil which are separated from each other by small voids or natural surfaces of weakness. Peds persist through cycles of wetting and drying and the edges of peds are often where cracks begin to form.

The reason for this is that in poorly structured clay soils, the large peds act as one unit and the seed trench created by the planter then acts as the weakest part of the ped and becomes the location where a crack is most likely to develop as the soil dries. This soil behaviour will be very different to a seed trench made in well structured soil where the multiple small peds providing fracture points across the zone allowing multiple small cracks that prevents the seed from becoming stranded. Figure 1 is taken from the technical report prepared by Hulme (2015), and demonstrates this principal. 

Anything that can improve the soil structure by decreasing ped size in the top 15cm of soil to become more like Figure 2b, should greatly assist cotton emergence, particularly when moisture seeking at depth is required.

Strip tillage

Tillage can create ideal seedbed conditions for plant emergence, development, and unimpeded root growth. In a dryland system strip tillage offers the potential to provide the benefits of conventional tillage in the plant row whilst leaving the inter-row undisturbed with intact residue cover for soil and moisture conservation purposes. Hulme (2015) suggested that strategic strip tillage conducted straight after the harvest of the last winter cereal crop while the soil is dry, could effectively shatter poorly structured or compacted soil into smaller peds. The following 12 month fallow lead in period prior to cotton sowing would allow the disturbed soil to settle forming improved tilth with subsequent wetting and drying from rainfall. A potential risk that needs to be considered with strip tillage is that the resulting seed bed is dependent on timing and intensity of rainfall between when tillage is enacted and later planting. Factors such as soil type, likely rainfall patterns and field slope should be considered in terms of identifying the timing for strip tillage and the likelihood of potential soil structure benefits. As a result this tactic is likely to be better suited to some soil types and systems than others. 

Soil additives for overcoming structure related emergence issues

Whilst strip tilling can offer short term relief for blocky hard setting soils, tactics that increase organic matter or utilise chemical amelioration of structure through the addition of gypsum may also be beneficial.

Benefits from adding gypsum to soil depends on a number of factors, including the gypsum source (purity), particle size, following rainfall duration and infiltration, depth of incorporation, and the inherent characteristics of the soil and the extent to which structural issues are linked to soil sodicity (McLean-Bennett 2010).

The application of gypsum to the plant line during strip tillage would allow gypsum rapid soil entry to provide chemical amelioration. The cost:benefit ratio for gypsum usage will depend on a range of factors including – quantity of gypsum required, soil characteristics, expected length of affect, and the frequency of cotton (as the main crop beneficiary) in the crop sequence and associated productivity gain.

Farming systems that encourage stubble accumulation through the purposeful planting of high residue crops such as winter cereals are also likely to assist in improving soil surface structure. For soils with inherent topsoil structural issues, the frequent production of low stubble legumes is likely to exacerbate emergence difficulties. Similarly cotton itself leaves minimal surface stubble and therefore on difficult soil types where cotton features as a pillar crop rotation it is essential to ensure subsequent rotation crops provide high levels of stubble and organic matter return.    

Water injection

Placement of water into the seeding trench (water injection) during the cotton planting operation has been considered by many growers and occasionally put into practice over the last 30 years. The premise is to inject water into the seed trench to accelerate the imbibing process and limit drying back of the seed trench post-planting.

Marshall (1998) reported the practice of water injection by growers with rates varying between 500-2000 L/ha. In most instances plant vigour was notably improved with water injection but establishment percentages were rarely different. A drawback of water injection was reduced planting operation efficiency with a 20-40% reduction in the amount of hectares sown in a day.  Recent studies of water injection at a number of sites during 2016 and 2017 gave similar results, with an improvement in establishment only being achieved on one out of three occasions. Critically, water injection did not provide a substitute for insufficient planting moisture in the soil.

The options for overcoming soil related constraints affecting cotton establishment particularly under marginal moisture conditions are limited and indirect. In a strategic sense the number one priority should be the use of crop sequences and practices that increase organic matter and soil cover and to minimise factors that contribute to surface compaction where ever possible. The regular growing of low stubble crop rotations such as legumes or canola in dryland systems where cotton is a pillar crop is only likely to further exacerbate emergence problems for poorly structured soil types over the medium to long term.