Effective rooting depth and water use of lawn grasses, shrubs and trees.

Introduction:

New roots and root tips are the primary organs for absorbing water and nutrients from the soil. Older roots become suberized and are not active in water/nutrient uptake. These older roots are mainly for transport and support. This study investigated the rooting depth of nine grass species/cultivars with an irrigation cycle of 30 days.

Methods:

The nine grass species /cultivars selected are commonly planted grasses in Israel as they are heat tolerant (Table 1) and suited to local conditions. 30 m² of each grass was planted in duplicate, in an unshaded area away from neighbouring trees and their root systems.

Table 1.

Gravimetric water content of the soil* at different depths was used to determine active uptake by root systems.

Three boreholes 1.5 meters deep were dug at each replicate site to assess watering depth. Samples were taken three days after watering, 14 d and 28 d i.e. at the end of the irrigation cycle. Water consumption was compared to that from a Class A evaporation pan installed adjacent to the experimental plot.

Results:

Rooting depth: most species/cvs were able to use between 9% and 12% of the soil water at the maximum sampling depth of 1.5 m. S. secundatum cvs' root systems were found to take up water only from the top 1.2 m.

The potential evaporation coefficient (plant water consumption (as measured by gravimetric water content) / potential evaporation (Class A pan)) by for the first two weeks of the irrigation cycle ranged from 0.89 to 1.32, depending on species /cv. This fell to a range of 0.36 to 0.76 for the second two weeks. The average total consumption of water per irrigation cycle ranged from 0.74 to 1.01. This equates to a water consumption (species/cv. dependent) ranging from 4.8 to 7.0 mm per day.

The first signs of water stress appeared 28-32 days after irrigation regardless of the amount of water consumed by species/cv.

Introduction:

In the experimental plant acclimation sites we have many shrubs and trees growing without additional irrigation. The soil type is sandy loess soil with approximately 60% sand to a depth of tens of meters (>25m).

In 1998, 251 mm of rain fell (the multi -year average), but because of the precipitation distribution pattern the late winter wetting depth was only 70 cm. The following year, 1999, was considerably drier with only 65 mm total precipitation.

In autumn 1999 all plants flowered as though they were in a well watered ornamental garden. We took advantage of this situation to understand from where in the soil profile plants take up water.

Methods:

We tested rooting depth using a construction drill to remove soil samples at 60 cm intervals for 18 m final depth. Soil samples were tested for water content. The bushes and trees were six years old at the time of the experiment.

A year later in 2000, we had another drought year (177 mm). In late summer we repeated the drill test near two species: Eucalyptus populnea and Eucalyptus torelliana. Samples were checked for presence/absence of roots. Both species had seven years growth.

Results:

In the 1999 experiment the soil was completely dry to a depth of 7 m, and only from >7 m did we find wet soil. This indicates that the active root system for these all the surviving species is > 7 m deep.

In the 2000 experiment we found roots of E. populnea at a maximum depth of 17 m, E. torelliana reached a maximum depth of 15 m.

General conclusions:

Given the active rooting depth of the tested plants it is worthwhile to prepare the garden soil for maximum water harvesting. This can be achieved by deep cultivation and a20 cm upper layer of pure sand. This should be done for the entire garden area. This method has the advantage of helping to store water from wet years for plant use in drier years.

We recommend 'training' grasses to grow deeper by lengthening the irrigation cycle (without unduly stressing the grass). Using this method for over 30 years we have achieved water savings of 15% or more after a good winter's rain.

*Gravimetric water content of soil can be calculated in the following way: ((wet mass of soil – oven dry mass of soil)/oven dry mass of soil) and is often multiplied by 100 to express it as a percentage.