Water Scarcity in South Africa
Water has always been a scarce commodity in many parts of South Africa, and with water restrictions becoming a reality in irrigated agriculture in some parts of the country we can safely assume that water availability for agricultural purposes may be significantly influenced in the future.
Advantages of Protected Crop Production Systems
The advantage that farmers with protected crop production systems have is that we can use water more efficiently if we manage it correctly. This is because crop water requirements are considerably less in greenhouses than in open fields when aiming for similar levels of production and is a consequence of the much lower evapotranspiration inside greenhouses because of less wind, reduced solar radiation and higher atmospheric humidity. Accurate management of the irrigation frequency and volume is very important to ensure that the crop uses the water as efficiently as possible.
Differentiating Crop Water Use Efficiency and Irrigation Efficiency
At this stage it is important to discriminate between crop water use efficiency and irrigation efficiency. Crop water use efficiency is defined by the total amount of marketable produce per volume of water lost through evapotranspiration. What this means is that the crop uses what it requires and will not necessarily use more water if it receives more. All the water that is not used by the crop is therefore wasted and this is where irrigation efficiency is differentiated.
Strategies to Reduce Unproductive Water Losses
The term irrigation efficiency therefore reflects on the marketable produce per volume water applied. This definition includes the total volume of water applied which includes productive (transpiration) as well as unproductive (evaporation, runoff and drainage) water losses. Unfortunately, drainage is something that we cannot go without when we fertigate on each irrigation event. So the question arises: how can we try to reduce these unproductive water losses?
Importance of Proper Irrigation Strategy
The suggested answer is, to develop and adopt an irrigation strategy of only supplying the plant with water when it requires it. It does make sense that if you irrigate less frequently, you will have less drainage and evaporation. This does not mean that we should only irrigate when we see the plant wilting, because wilting only sets in after the plant starts to experience moderate or elongated water stress.
Monitoring Soil Moisture for Efficient Water Use
The most direct way to monitor when the crop requires water, is to monitor the source from which water uptake takes place, i.e. the medium or soil. This may be done with the help of soil water sensors and the medium/soil may be irrigated as soon as the medium reaches a certain level of “dryness”. It is important to remember that we can only apply lower frequencies if we are working with a medium or soil with a good water holding capacity.
Practical Application and Results
This strategy of monitoring the medium/soil water content was tested in practice for tomatoes and cucumbers grown in coir. Depletion levels used comprised of a standard treatment (STD) and treatments where water was depleted to a level just before stress (BS) and at mild water stress.
For the STD treatment, the medium was held close to the drained upper limit by using frequent irrigation events (8 times per day). For the BS treatment, water was depleted to approximately 60% of field water capacity for tomatoes and 80% of field water capacity for cucumbers before irrigation was triggered. A drainage percentage was maintained over all irrigation treatments to prevent salt accumulation in the medium.
Various Methods for Irrigation Scheduling
There are a variety of methods used to schedule irrigation in greenhouses, of which irrigation based on time, irradiation sum and vapour pressure deficit is probably the most common strategies. The last two methods are used to predict a crop’s water requirement based on environmental conditions. In contrast, direct monitoring of the medium/soil’s water content provides realtime information of the crop’s water use and automatically adjusts frequency and volume according to crop requirement.
Application Beyond Soil: Soil Water Monitoring in Greenhouses
Soil water monitoring is an established practice in irrigated agriculture in the soil, but can be applied with just as much success in greenhouse mediums/soils if managed correctly. Given the water scarce times that we live in, it may be a good alternative strategy which can help the producer maintain yield while saving on fertilizer and water.
By: Dr. Rykie van der Westhuizen: Crop production specialist