Eight principles to better manage soil water availability

By Dr Andrew Hirons
With climate projections indicating that the growing season in most parts of Europe will become warmer and drier, it is essential that we think holistically about how to provide water security to our trees. Here are eight principles that will help provide our urban trees with the rooting environment that they need to thrive.
I. Maximise the volume of soil that roots have access to
Assuming soil texture and structure are similar, larger soil volumes will hold more water, dry more slowly and allow the fine roots of trees to proliferate more extensively, including within soil that is less prone to surface drying (Figures 1, 2 and 3). It is critical to remember that potential rooting volume only includes soil that is of low bulk density (typically <1.4 g cm-3) as compaction is highly detrimental to healthy root development. Therefore, ensure that soil is not compacted (Figure 4). In urban areas, it may be possible to increase soil (rooting) volumes by intentionally designing good soil volumes and designing root paths to ‘breakout zones’ that have additional uncompacted soil for roots to grow within. Providing trees with good soil volumes will always enhance tree health and resilience to water deficits.
Figure 1: Oak (Quercus robur) trees growing on the slope of this escarpment with shallow soils of limited volume are suffering from severe drought symptoms, whilst those trees with access to larger volumes of soil still appear healthy.
Figure 2: Small soil volumes, combined with insufficient irrigation can cause serious decline in young trees trying to establish. These pin oaks (Quercus palustris) are really suffering in a period of dry weather.
Figure 3: Isolated tree pits with small volumes of soil lead to trees that perform very badly in this paved environment. The sparse crowns on these cherries (Prunus maackii) indicate chronic water deficits.
Figure 4: Even when trees appear to have large volumes of soil, high levels of compaction can seriously inhibit the ability of roots to proliferate within the soil thereby reducing the effecting rooting volume available to the tree.
II. Improve the soil water holding capacity where possible
In very sandy soils it is worth considering ameliorating the soil with a combination of silt, clay, organic matter and biochar to increase water retention so that the soil volume can support – and hold onto – more water. However, it is always worth checking that any ameliorants will increase water availability to the tree and not lock up soil water, making matters worse for the tree.
III. Understand the water release characteristics of the soil
It is vital to remember that it is soil water potential – not soil water content – that the tree experiences. A small investment in laboratory analysis in understanding the relationship between the soil water content and the soil water potential of your soil can substantially increase the confidence that you have in subsequent irrigation decisions (Figure 5).
Figure 5: Soil water release curves for a sand and a loam soil, showing the typical relationship between the soil water content and the soil water potential (a measure of the ease in which a plant can extract water from the soil. The permanent wilting (turgor loss) point – below which the plant cannot extract water from the soil – is generally considered to be -1.5 MPa for agricultural crops, but it ranges between -2 to -4 MPa in temperate trees. Redrawn from Hirons and Thomas (2018).
IV. Reduce soil evaporation and competition from other vegetation
Ensure that water applied to the tree is taken up by the tree and not lost via evaporation. Consider the time-of-day water is applied and, wherever possible, avoid watering during the middle of the day when evaporative demand tends to be greatest. The application of organic mulch around the base of the tree will reduce evaporation and the competition for water from other vegetation (Figures 6, 7 and 8). The application of organic mulch will also provide food for soil life and thus increase infiltration of water into the soil.
Figure 6: Young lime (Tilia cordata) trees have to compete with grass for water and nutrients. These trees would be much happier if they were well mulched and irrigated. Soil sensors can provide valuable feedback to tree managers and can help ensure trees such as these do not go into decline as a result of periods of dry weather.
Figure 7: A young Wollemi pine (Wollemia nobilis) that is well mulched and cared for should make an excellent contribution to this treescape.
Figure 8: Even larger, mature plantings benefit greatly from mulching and reduced competition for water and nutrient resources.
V. Ensure water that is being applied gets to the roots
When relying on pre-installed irrigation infrastructure (such as watering pipes), there is a danger that much of the water being applied is hydrating soil beyond the reach of the existing fine root network. Especially on recently planted trees, make sure that water is being applied to the root ball and not just preinstalled pipes that may not deliver water to where it is needed most. Applying water gradually will improve infiltration: delivering water in high-volume pulses tends to lead to significant run-off and should be avoided.
VI. Decide on the timing of irrigation based on the tree’s water demand
If the timing of irrigation events is not based on tree physiology or soil water status then it is likely that water resources will be used inefficiently, and tree development will be constrained (Figure 9).
Routine (e.g., once a week during the growing season) watering events will never be as effective a watering protocol that is responding to actual need. Soilmania’s sensors are designed to aid irrigation decisions through their highly accessible dashboards that deliver real-time information on the soil water status to those responsible for watering trees (Figure 10). This allows water resources, and contractor time, to be allocated in the most efficient way.
Figure 9: By the time that you see wilting leaves and chlorosis, water deficits are well advanced and it is likely to be too late to prevent serious hydraulic damage. Getting real-time feedback from soil sensors allows tree managers to intervene below stress gets this serious.
Figure 10: Soilmania sensors provide feedback to a cloud-based dashboard that can be accessed from anywhere. This allows interventions to be made in a timely manner and helps to prevent serious water deficits developing in trees.
VII. Avoid over application of water
Saturated soils are very low in oxygen and these conditions are very damaging to most trees. Fine roots rely on soil oxygen for their function and survival. Hypoxia will lead to rapid mortality of fine roots and diminish the tree’s ability to take up water and nutrients. High levels of root mortality also leave the tree more vulnerable to subsequent stress, such as drought. Therefore, it is critical not to apply too much water, or to irrigate when the soil is already sufficiently hydrated. Soilmania’s sensors can help managers decide when irrigation events should be postponed to avoid this problem. Take care to place sensors carefully so that you get feedback from the best region of soil to inform tree management.
VIII. Ensure that the osmotic potential of the soil solution does not hinder root absorption
Excessive salt or fertiliser can reduce the osmotic potential of the soil to such an extent that roots find it very difficult to take up water. Where the osmotic potential is being affected by high levels of electrical conductivity (a measure of the salts in the soil solution), flushing soils to leach the salts through the soil profile is the most effective way to ameliorate this. Soilmania’s sensors can provide feedback to managers to help them decide when this might be necessary.

If these eight key principles are followed, then trees are much more likely to establish effectively and thrive for generations to come. One thing is for sure, if we fail to provide trees with high quality rooting environments and fail to nurture them when they need it most, then our arboreal assets will be severely compromised.
Want to learn more on how tree biology can help inform the management of trees, then please check out my book Applied Tree Biology (Hirons and Thomas, 2018).