Drip-irrigation: watering the plant, not the soil

15/02/2016

François-Xavier Branthôme

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Systems designed to improve production results while saving water resources

Drip irrigation is a method of bringing water to the plants in limited and controlled quantities, either by depositing water on the surface of the soil near the plant, or directly in the vicinity of the developing roots thanks to underground irrigation devices. The water input uses a system of tubes or "drip-tape" that generally includes filters, valves, differently sized hoses and various kinds of drip dispensers. The goal is to increase production while reducing water usage to the strict minimum.

In a drip-irrigation system, the distribution of water on the surface of the soil, as close as possible to the plant, is achieved by using a tube into which drippers have been inserted. Drip-irrigation lines can be rigid, as in the case of installations that are designed to last for several years as with perennial crops like fruit trees, or flexible, when they are installed for annual crops. This second system is most frequently used by the tomato growing industry.

Although it is highly effective, underground drip-irrigation is still not very common, as it requires burying the irrigation tape so that the drippers are at a given depth in the soil, in direct contact with the plants' root system, but do not interfere with the cultivation processes that have to be carried out.
The concept of drip-irrigation is becoming more widespread both for herbaceous crops and for orchard crops, due to the interest in this technique in regions where water resources are limited, or where access to water (or its supply because of the thermal energy or electricity required to operate pumps) is technically difficult or costly, but also due to the improvements that this form of irrigation allows in terms of agricultural yields.

In addition to the essential primary source of water (spring, groundwater, rivers, etc.), drip-irrigation systems generally include a pump that takes up the water to feed into the system, a supply pipe to channel the water from the source to the crop, a regulating and filtering unit that can power up the installation or shut it down and prevent the occlusion of the drippers, a mainline pipe that runs along the ends of the rows of plants, and laterals or lines of tape into which the drippers are inserted. Drip-irrigation equipment can be operated manually or coupled to an automatic system that includes a regulating device with valves that are controlled electronically or hydraulically.

After going through a series of improvements that really only began in the sixties in Israel, the basic process consists in distributing water in the slowest and most precise way possible.
Pulse drip-irrigation (a method that uses the charge-discharge cycle of a pressurized reservoir) is sometimes used to further decrease the quantities of water provided to the plants at a predetermined moment, while reducing the negative effects of run-off and deep seepage. However, it is important to note that pulse irrigation systems are generally costly and require high maintenance. Drip-irrigation equipment manufacturers have focused their efforts on the development of new technologies that supply irrigation water at ultra-slow outlet flows, as little as 1 liter per hour.

Most drippers intended for agricultural use that are available commercially are pre-equipped with a system of micro-filtration and pressure regulation. However, it is often necessary to install an extra filtration device upstream, on the mainline, depending on the quantity and nature of the particles suspended in the irrigation water. Controlling this parameter implies that specific laboratory analyses need to be regularly carried out.

Many arguments in favor of drip-irrigation…

The basic conditions required to achieve satisfactory results with drip-irrigation, leading to notable improvements compared to other previously used irrigation methods, include the correct definition and sizing of the needs and reach of the project, careful installation and an intelligent agronomic approach to the irrigation of the crop.
It is a fact that some installations in the past were badly designed and operated, and therefore ended up costly and ineffective, but today's tomato growers (and other crop growers) can now reasonable count on maximizing their crop potential as long as the drip-irrigation system is correctly installed and used.
Among the advantages that are usually mentioned to justify the use of drip-irrigation systems, the following are the most frequent:

a greater uniformity of the water distribution and reduced losses;
better adaptability to automation;
easier to use;
big energy savings;
less compacting of the soil;
the possibility of fully using the distribution potential for fertigation through irrigation water; the risk of nutrient loss is limited thanks to the localized application and to a reduction of the run-off effect;
the possibility of applying certain specific kinds of chemical agricultural inputs to the crop by using the irrigation water as a carrier, to avoid them entering into direct contact with the plants or being dispersed in the air (chemigation);
the possibility of supplying very small quantities of water more often, in order to respond more precisely and efficiently to the needs of the crop;
an increase in production, both in terms of quantity and of quality.

If it is correctly designed, installed and controlled, drip-irrigation can help to achieve major savings of water by reducing evaporation. Water forms a humid "bulb" underground, while the surface of the soil mostly remains dry: this considerably slows evaporation and limits the growth of weeds. Furthermore, a drip-irrigation system noticeably reduces the appearance of a large number of plant diseases that are generally aggravated by the contact of water on the foliage.

In regions where the availability of water remains limited, using drip-irrigation is a particularly useful and desirable option in order to achieve maximum efficiency in the use of water resources and to optimize the volumes produced, while maintaining constant water consumption levels. Implementing drip-irrigation also leads to an increase in the efficiency ratio for water usage (crop volume produced /m³ of water consumed) and to performances that are better than those obtained with rainfall alone, and better even than in the case of furrow irrigation or sprinkler systems.

… and some potential inconveniences

Drip-irrigation nonetheless requires good implementation and control. As for any technology or expertise, the use of such equipment can be difficult and exposes a grower to a number of errors or inconveniences:

Installation requires an investment that is usually higher than conventional irrigation systems.
Sunlight can lead to a deterioration of the tubes and pipes, and reduce their life-span.
Even when the water is correctly filtered and the installation is adequately maintained, problems of occlusion can occur over time.
In the case of a buried drip-irrigation system, the grower has no visual means of checking on the quantities of water released, which can lead to irrigation being either excessive or insufficient, particularly when the grower does not have enough experience (or the right monitoring equipment) to correctly manage the water input.
Drip-irrigation may at times be inadvisable in the case of certain surface herbicides and fertilizers that require sprinkler irrigation in order to be activated. Uninstalling the dripper tape after the harvest can lead to extra costs. Users need to plan and budget for removing the system after use, and disposing of it, either by recycling or by re-use.

The use of this type of equipment is not neutral: it requires an in-depth study of relevant factors, like field topography, soil type and content, quality of the water, type of crop and agro-climatic conditions in order to determine the relevance of using drip-irrigation as well as the nature and quality of the components to be installed.
In lighter soils, a drip-irrigation system can turn out to be unable to humidify the surface of the ground enough to cause germination to happen: a careful evaluation is required of the depth at which the tape is buried.
The main objective of drip-irrigation is to decrease water consumption by reducing the leaching effect. However, when the available water presents high levels of salinity or alkalinity, the soil can progressively become unsuitable for agriculture, due to the heightened salinity or to harmful infiltrations into the soil. In some cases, drip-irrigation can convert agricultural land into fallow land, in the semi-arid or arid regions where natural leaching by rainwater is insufficient.

An ideal support for fertigation

Crops need complementary nutritional inputs, which growers supply in various applications in the form of different kinds of fertilizer throughout the crop cycle. Sometimes, the fertilizer input can turn out to be insufficient, as when the application does not coincide with the phonological phase when the plant most needs it: the distribution of fertilizer, when it is suitably dissolved in the irrigation water thanks to drip irrigation systems, can be an efficient and reliable way of breaking free from this kind of difficulty.

This type of application allows a larger number of nutritional inputs to be administered throughout the life-cycle of the plant: the process, called fertigation, is based on the use of a system that injects the soluble fertilizer solution (in water) into the irrigation flow, which is then distributed to the roots of the plants through suitable injection nozzles.

According to the type of application, users generally differentiate between two types of fertigation.

Fertigation by discontinuous application is when the input is administered along with the irrigation water, but not every time that the crop is watered; the nominal doses distributed are lower, but the total number of applications is higher than with conventional methods of soil improvement (solid fertilizer applied on the surface of the ground).
Fertigation by continuous and proportional applications is when the fertilizer is added to the irrigation system each time the crop is watered, but in markedly lower doses; the distribution is carried out during most of the irrigation period. This last method is more complex and more difficult to operate than the previous one, but it allows growers to achieve very big savings in terms of quantities of fertilizer. It causes less stress to the plants and results in an increase in final productivity.

Latest research results

Among many different studies, special mention should be made of the results presented by A. Battilani and T. Letterio ("Irrigation technologies and strategies impact on quality characteristics of processing tomato"), during the 13^th ISHS Symposium at Sirmione, which was held in parallel with the World Processing Tomato Congress in June 2014.

"The survey was conducted on a collection of data from different projects at European and national level, carried out over a period of more than 20 years (1991-2013). Results show that total yield has greatest influence on tomato fruit quality: yield higher than 80 mT/ha results in about 0.5-0.7 °Brix losses. Reversely, as farm yield increases, harvested soluble solids and crop profitability are augmenting. Irrigation methods do not influence °Brix, but drip irrigated tomatoes harvested more °Brix per ha with better color and gross margin. Irrigation strategies affected °Brix: severe water stresses increase °Brix but depress both yield and gross margin, while light regulated deficit does not affect quality or farmer’s income. A cultivar’s earliness (genotype) greatly influences fruit quality. Fertigation affects neither fruit soluble solids content nor Bostwick, but improves fruit color and harvested °Brix.
The analysis shows that it is possible to wisely reduce water usage without negative impact on crop profitability, and even increase farmers' income through a better quality product. Fruit quality forecast semi-empirical algorithms with sufficient robustness and reliability to carry out a risk analysis can be implemented in irrigation management models.”

Other research results and technical presentation documents are appended to this article on our site www.tomatonews.com.

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Drip-irrigation: watering the plant, not the soil

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