Low flow irrigation and a booming citrus industry

Efficient water management

In crop production, the main goal of water management must be to minimise non-beneficial water loss. As we know, water loss can occur through evaporation, deep percolation, subsurface outflow, run-off, and transpiration. The main concern is with evaporation and deep percolation. If these two culprits can be limited, we can ensure highly efficient water use. Remember, efficient water use is about preventing negative water loss on the one hand and ensuring optimal water intake by the roots on the other hand. In an agricultural extension brief in the April/May 2021 edition of the SA Fruit Journal, Dr Pieter Raath of the University of Stellenbosch explains that the irrigation water application rate must never be higher than the soil’s infiltration rate.

This is to prevent deep percolation, run-off, and other negative water losses. “The irrigation duration must be sufficient to wet the root zone to field capacity without saturating the root zone and causing excess draining or runoff. The difference between field capacity and the actual soil water content is what we refer to as soil water depletion. Irrigation timing and the amount of water to be applied are determined by monitoring or estimating soil water depletion and applying water when the depletion reaches a pre-selected level, called the management allowable depletion.” This explanation points to the value of low flow drip irrigation and the associated scheduling theories.

With Centralised Low Flow Drip Fertigation we can better control the depth of wetting and reduce under and over irrigation- therefore improving water-use efficiency and soil aeration. Low flow drippers are used to irrigate at a very low level over a longer time. The application rate of the irrigation system is balanced with the maximum daily water use over the total consumptive period of the crop by decreasing the application rate of the irrigation system. The biggest change is moving from several irrigation shifts to one irrigation shift. Chris explains that the soil water movement dynamic completely changes when making this switch as the dominant activity changes from extraction to infiltration.

Johan du Preez of Bavaria Fruit Estate near Hoedspruit reports that they have seen amazing results with low flow drip irrigation in their soft and hard citrus orchards. “It also earns great results in our Mango and Avocado orchards. My advice to farmers would be to switch as soon as possible.”

According to Johan, low flow drip irrigation allows them to meet the water needs of the trees more accurately during the different phenological stages which leads to better and more consistent yields. “I can report more efficient uptake of water, which leads to reduces fertiliser use. Furthermore, we have seen better oxygen levels in the root zone that leads to healthier roots.”

Click here to read more about continuous irrigation.

Paradigm shift

It is important that we change our way of thinking about irrigation design to adapt to the possibilities set out by low flow technology and concepts such as continuous irrigation. Remember, the total success of new technologies depends on the correct application of these technologies.

Danie Malan of Tierhok Citrus near Patensie in the Eastern Cape says that the greatest benefit of low flow drip irrigation and the associated precision irrigation methods is that you can plan ahead. “This does not mean that we do not measure, but we now plan, apply and then measure to determine whether we are on par with the water requirements in the orchard.” Danie explains that he irrigates according to a set annual plan. “To prevent over or under irrigation, we apply 80% of the planned irrigation water. We then measure to determine the status of the root and supplement or adjust the 80% irrigation accordingly.” This is one of the important paradigm shifts to make, to use data to plan ahead and measure for exact accuracy, rather than measuring and making day to day decisions according to these measurements. “Since we have made the switch to low flow drip irrigation, we never have to guess anymore, we work with precision and we can irrigate and feed the trees in line with the yield tonnage we are aiming for,” says Danie. Danie says that switching to low flow drip irrigation requires you to change your way of thinking completely. “But as soon as you put your head in the low flow gear, it is the most logical method and makes management much easier.”

According to Danie, success will depend on many factors. The first is that you have to carefully choose the experts you will have by your side during the journey. “Be sure that they have enough experience and are properly trained in the areas they must advise you on. Secondly, he says, you cannot switch to drip irrigation without also switching to fertigation. “You have to go all the way, to ensure optimal benefit.

Netafim’s Chris Malan supports this. “In order to succeed with drip irrigation, it is further essential to fertigate through the system; think in terms of concentrations rather than amounts of fertilizer application.” There are a few prerequisites for successful drip irrigation, the most important of which is a well-established deep root system. Chris Malan further recommends that it is crucial to implement the correct type of emitters and spacing for the soil type and topography and control the irrigation depth. “Thorough planning and preparation are required for successful drip irrigation. Two of the important parts of this are soil preparation and scheduling calculations.” Experience with drip irrigation on citrus has shown that the principles traditionally applied for conventional irrigation do not necessarily apply for drip irrigation, or other localised irrigation methods, such as micro-sprinkler irrigation. “We have to change our approach to successfully implement drip irrigation in citrus and other fruit tree orchards,” says Malan.

Malan shares principles* for preparation and management, to help achieve very efficient use of water, accurate irrigation volumes and finally optimal tree performance.

The four most important activities:

• - Establish the soil’s water holding capacity and the optimal percentage extraction of the readily plant available water for each phenological stage of the tree. This is to establish the amount of water that needs to be applied per irrigation cycle. Read more here.
• - Calculate the length of the irrigation cycle by using the calculated delivery rate in mm/hour.
• - Calculate how long the period between irrigation cycles must be by using the ETo, Kc and amount of water needed per cycle.
• - Check the soil water content regularly and adjust if necessary, to avoid gradual trends of under- or over-irrigation. *This is a very basic rendition of principles discussed by Pieter Raath, Chris Malan and Teunis Vahrmeijer in the article, Irrigation of citrus trees: A practical approach, first published in the South African Fruit Journal.

Finding our way

As both our understanding of the water requirements of plants and our ability to measure aspects such as soil and plant moisture have improved, our understanding of irrigation scheduling has evolved. The concepts associated with low flow drip irrigation technology have already had major impacts on the citrus and other tree crop industries. Co-operation between producers, Citrus Research International, Universities of Stellenbosch and Pretoria and Netafim has led to research projects currently in progress to investigate various soil physical, soil chemical and plant response aspects related to Continuous Low Flow Drip Fertigation. As increased research is being done and our understanding of water movement and distribution under continuous drip irrigation, knowledge of irrigation depth, understanding of the impact of tree stress and more expands, the impact of improved irrigation technology further expands. This will allow the irrigation industry to continue to play a major role in the growth of the citrus industry and many other agricultural industries.