One of the most common questions we as horticulturists today are faced with is; what is good irrigation practice and how much water should I give my garden! Before we can get into the detail we first need to understand why we irrigate and what the factors are to consider when irrigating our gardens.
Let’s look at the basic three soil types and what their characteristics are. In our next article we will return to the soil types to further calibrate our irrigation systems, for now let’s look into the basics.
The three basic soil types are:
Sandy
– High water intake
– Low water retention
Loam
– Medium water intake
– Moderate water retention
Clay
– Very slow water intake
– High water retention
Secondly we need to look at how water is drawn into the soil and the water content in soils. Now we need to understand the two different methods of water distribution in soils namely the infiltration rate and capillary action. The infiltration rate is a measure of the soils ability to allow for vertical drainage of the water applied to the surface. This is made possible by gravitational pull. The phenomenon of capillary action is by which water is transferred from pore to pore in soils, this is basically the movement of water in a lateral movement in soils.
Now that we have a basic understanding of how water acts within the soil we can look at how we lose water and available moisture. Available moisture is the usable volume of water that remains in the soil following the extraction of a portion of its former water content.
How do we lose water?
Evaporation is one of the most common causes of water loss, factors influencing evaporation are; the sun, wind, temperature and humidity. Another factor of water loss to consider is Transpiration, This is the expiration of evaporation of water from a plants leaves and stems to the atmosphere. The combination of the loss of water by evaporation and the plants transpiration is called evapotranspiration (ET). Irrigation must be scheduled to replenish the water that is lost by evapotranspiration.
Good irrigation practices
The ultimate function of an irrigation system is to replenish the water in the soil that is lost by evapotranspiration. Irrigation mimics rain “Natural replenishment”. A common mistake is to schedule irrigation frequency to water turf and the landscape every day.
This is bad for the following reasons:
– The presence of air and water in the soil is essential for the uptake of water and nutrients from the soil. Irrigating every day will reduce the ratio of air and water in the upper root zone of plants.
– When plants receive water every day, a superficial root system will develop. This occurs because the roots have no need to venture deeper in the soil to seek out water. A negative influenced by this is that a plant will only extract nutrients from the upper reaches of the soil.
– When a superficial root system develops it will result in a reduction of the plants anchoring ability and may lead to the plants unable to withstand strong winds, also when a plant develops superficial root systems they are extremely susceptible to droughts and or the reduction of the irrigation schedule they have become accustomed to.
– This will also lead to increased susceptibility to fungal infestations and occurrences. Fungi require moisture for survival and reproduction. Watering daily will increase the risk of a fungal attack.
– The increased competition of weeds will have a detrimental effect on your garden plants.
This brings me to the second part of your question, how much water and when do I apply it. Firstly lets again look at some important information. We are going to use the information gathered above and by using the rate of precipitation of different nozzle types calibrate our system accordingly. The rate of precipitation is the rate at which water is applied to the irrigated area per unit time, usually expressed in mm per hour of operation. Before we apply this we need to understand that that the water application norms used in Gauteng by Horticulturists in the last 30 years are:
– Summer 25mm per week
– Spring 20mm per week
– Autumn 15mm per week
– Winter 8mm per week; NB only where necessary – certain plants undergo a seasonal dormancy in which little sap flows and transpiration occurs.
It is important to note the above figures are the average “no rain” requirements for irrigation. The scheduled applications must be reduced by the amount of rainfall.