Early to mid-tillering (about six weeks after emergence) is a good time to check crop root growth.
The aim is to see whether subsoil constraints are impeding root growth and are likely to limit crop yield - with a view to ameliorating any problems for next season.
Checking roots earlier in the season helps show:
- what challenges the crop is up against at a critical time, when much of final yield is being determined; and
- how quickly roots are growing to access moisture and nutrients.
Crops with poor root development, especially poor downwards growth, are more dependent on ever-variable in-season rainfall.
They become more water-stressed during dry periods and hay-off more rapidly at the end of the season.
Breaking up compacted soils and/or fixing acidity increases root growth, allowing crops to access water and nutrients deeper in the soil.
Later in the season, the roots may have found a way down. Checking roots then can mask how the crop was coping earlier in the season.
How to check the roots
The best way to check roots is to dig a hole to see how deep the roots have grown and assess why they have not grown deeper.
Pulling up plants is not the best method as it tends to break the roots, especially the finer roots, making it harder to gauge whether there is an issue. Pulling up plants also makes it harder to pinpoint where the soil issues start.
Compare root growth in good and poor areas within your paddock and your property.
Comparing ameliorated and non-ameliorated areas, such as ripped versus unripped, is the best way to show how the amelioration is affecting (ideally improving) root and plant growth.
Productive areas without any known soil constraints are another 'good area' option.
Small patches of better growth can also show up in previous tree sites or burn piles. These areas showcase how better root exploration, whether from higher pH (where trees have been burnt) or less compaction (crop roots can follow old tree root channels), lead to better crop growth.
What to look for in roots
By early to mid-tillering, a cereal crop should have both seminal (primary, vertical) roots and nodal (secondary, more horizontal) roots. Wheat roots can be 300 millimetres deep at the start of tillering.
Clues that a subsoil constraint is limiting root growth include:
- roots are shallower compared to elsewhere across the paddock/property - provided the comparison is fair (i.e. plants are from the same crop variety and similar emergence date);
- roots are growing mostly horizontally, rather than vertically and are concentrated near the surface (i.e. upside-down umbrella-shaped, rather than cone-shaped root system). This is a clear indication there is a physical and/or chemical barrier to deeper root growth;
- no roots are in the upper part of the soil profile or between furrows. This usually indicates the soil is too dry (probably non-wetting) for roots to grow into the nutrient-rich topsoil;
- there are thicker/swollen root tips - suggests roots are struggling to push through the soil;
- there are twisted/distorted roots that are struggling to find an easy route to grow;
- there are few fine roots and branches, with only some larger, thicker roots present;
- roots are only following old root channels (commonly put down by the native vegetation) in the soil, which indicates hard/dense soil; and
- there are brown roots - healthy roots have white growing tips.
Sometimes, in an attempt to deal with a subsurface constraint, the plant will have increased branching above the issue - for example, more branches above an acid or saline layer. Where there is a compacted layer, you might see some roots trying to grow into the compacted layer, but most roots growing and branching in the softer soil above.
What to look for in the soil
If the problem is deeper in the soil, look where the root growth peters out. Acidity and compaction have similar-looking effects on roots and the two can go hand-in-hand, so check for both.
In compacted subsoil there will not be many visible soil pores or cracks. In deep sands, press your finger down the pit face, feeling how tough the soil is. If the soil is softer where roots are happy, and hard where root growth dwindles, you have probably got compaction. Using a penetrometer near the pit will confirm how hard the soil is getting at depth. Root growth restriction starts at 1.6 megapascals and severe restriction at 2.5 MPa or more in soils at field capacity. In heavier soils it is harder to judge compaction by feel alone; use a penetrometer.
If roots reach a dense clay or cemented gravel, you have a dense subsoil. Most broadacre crops and pastures will struggle to get into this layer, as will a penetrometer if it hits gravel.
Because sodic soil is often hard, crop root symptoms tend to look similar to compaction. A dispersion test will show whether compaction is caused by too much sodium. Crops with strong root systems, such as lucerne, are better at growing into sodic subsoil.
If there is no obvious compaction but there is moisture below the roots, there may be a chemical choke. Take a soil sample from the soil layer where the roots are growing well and another where they are unhappy. Have the samples tested for pH, salt (E.C.), and boron and chlorine if they are issues in your area.
NOTE: This article was produced as part of the GRDC 'Maintain the longevity of soils constraints investments and increase grower adoption through extension - western region' investment (PLT1909-001SAX).