Choosing and using irrigation
PUBLISHED: 08:11 28 March 2014 | UPDATED: 08:49 28 March 2014
Brian Callaghan suggests ways to keep up with your crops' watering needs
The efficient use of water is becoming a critical issue for everyone, but especially those involved in crop production.
Plant watering is often seen as a simple sort of task that can be entrusted to anyone, regardless of his or her knowledge or experience. While applying too little water, or applying it too infrequently, can result in plant death, more often, the result is much less dramatic: a reduction in overall growth, which is difficult to quantify. However, smaller or poorer quality plants mean a lower overall yield and a lower than possible return for all of the resources invested in producing the crop.
Applying too much water, on the other hand, is a waste of water, a waste of the time and energy needed to apply it, and washes expensive nutrients from the soil or compost, which might otherwise have fuelled growth. Persistent over-watering can also result in root death from oxygen starvation.
Far from being the least important job, efficient irrigation is the most critical, and begins with understanding the plant’s requirement for water and then supplying the right quantity frequently enough to maintain optimum growth.
Water movement in plants
Water will evaporate from any moist surface into the atmosphere as long as the air isn’t already saturated. So, unless the humidity is 100%, such as when it’s raining, plants will continually lose moisture when growing, and lose more moisture as the growth rate increases.
To maintain this flow, water enters the roots from the soil or compost, because there is a difference in pressure between the soil water and that water held in the roots. The water flows upwards via the plumbing system of the plant until it reaches the leaves where it finally evaporates into the atmosphere via the tiny pores, or stoma, which cover the leaf surface.
Water droplets like each other. Put two close together on a smooth surface and they will coalesce to form a larger droplet. As moisture is lost to the atmosphere, this same mechanism creates a pull sufficiently strong to draw water from below soil level up to the leaves at the top of tall trees.
Amazing as this process is, plants have very little control over water loss while they’re growing, hence the need for a constant supply at the roots. To give some idea of scale, a hectare of maize will transpire approximately 12,000 – 15,000 litres of water per day, while a mature broad-leaved tree like an oak will give off around 150,000 litres every year.
Of the total quantity of water taken up by a plant, less than 1% remains in the plant. Since around 60–90% of the total weight of many plants is made up by water, this gives some idea of how much flows through a plant over its lifetime.
Irrigation equipment can be divided into two basic groups according to the point of delivery. The first group throws the water into the air and lets it fall over the whole crop area, while the second group aims to route water only to where it’s needed – the plant’s roots.
This has been the standard means to irrigate plants in the UK and remains the most common method of application. Water under pressure is forced through a small orifice and the increase in pressure generates sufficient energy to hurl the water great distances. The equipment is readily available, simple to operate, and networks of pipes and irrigation heads can easily be moved into position with little disturbance to the crops. It’s a tried and tested solution and worked well while water was seen as a cheap and limitless resource.
As all water usage comes under greater scrutiny, the disadvantages of applying water ‘over the top’ are becoming more apparent. First, the spray pattern is easily disturbed by even light winds. Using large droplet irrigators can partly compensate for this but strong winds can render this form of irrigation virtually useless.
Second, it’s a wasteful system in that some water will inevitably fall on paths and non-cropped areas of soil which, of course, also encourages weed growth.
Third, large droplets constantly hammering the surface of poorly-structure soils can create a ‘cap’ or crust on the soil which germinating seedlings find difficult to penetrate.
Next, a great deal of water may need to be applied to penetrate the leaf canopy created by mature plants, and losses through evaporation from the leaf canopy can be considerable. The fifth main disadvantage is that the energy to throw water over the crop must come from somewhere. As the highest mains pressure will only manage to accommodate one or two sprinklers, more extensive systems necessarily require the introduction of a pump and reservoir, vastly increasing the complexity of the system.
Layflat tubing, leaky hose-type tubing, drip irrigation, and the many other variants all come into the second category: they carry water at low pressure and release it at soil level. Almost non-existent in the UK 20 years ago, these systems are rapidly gaining in popularity for applications as diverse as hanging basket and container irrigation through to conventional row crop watering. Most of these systems originated in areas such as the Middle East, where growers have had to contend with restricted and often expensive water supplies. This has focussed attention on developing more efficient means of irrigating crops, and their growing popularity in water-rich countries such as the UK reflects a similar growing concern about this valuable resource.
The big pluses with these systems are that they apply water slowly and directly to the soil. In the vast majority of cases, mains pressure is adequate, so there’s no need for pumps, and the water is applied only where it’s needed – at the plant roots. Applying the water slowly also allows sufficient time to infiltrate the soil, so waste is minimised.
While soil-level delivery systems have many advantages, they can be an absolute pain to install among established crops, as each plant requires a length of hose or dripper placed close to its roots. They’re just as time-consuming to gather up and store at the end of the season.
Because the orifices through which the water flows are very small, it’s vital to install adequate filtration in the system, as blockages can, and will, occur otherwise. If you’re thinking of installing such a system, don’t be tempted to do without the filters.
The final major problem is that they’re easily damaged and often difficult to repair, especially later on in the season when the tubing is partly covered by soil and you get a bit carried away with the hoe.
Throwing money at problems rarely provides an effective solution, so take a close look at what, and how, you’re growing before deciding that an irrigation system is for you. Even if installing a system is unavoidable, it still makes sense to minimise its use.
Know your plants' water needs
On the whole, plants that grow quickly tend to be water guzzlers, while a slow grower’s water needs tend to be more frugal. Read up on the origin of plants: for example, many from areas such as the Mediterranean tend to be drought tolerant. These are typified by small and/or tough leaves, deep roots and a medium rate of growth. Somehow, they can cope with most of our winters and still flourish during summer conditions without supplementary irrigation.
Next, maximise the effects of irrigation by watering according to individual crop requirements. For example, once established, most legumes can manage without supplementary watering until the flowering stage when regular irrigation can greatly increase yield.
Other crops, especially leafy ones like brassicas, benefit from watering during the whole of their growing lives. Unless it’s very dry, hold back on watering potatoes until the tubers reach the size of a large marble and then irrigate to increase tuber size. Once established, both onions and carrots should only require supplementary watering during very dry periods up until harvest.
Water thoroughly when required.
Small doses of water applied frequently encourage the development of roots close to the surface. These roots in the top few centimetres of the soil will quickly absorb small doses of water applied, leaving little to infiltrate to the lower reaches. As a result, a large proportion of the plant roots are near the surface which reduces the ability to survive periods without irrigation, so water well and water thoroughly, but not past the point of run-off.
All soils have an infiltration rate – they can absorb a set amount of water at a time. Applying more water than the particular soil can absorb merely result in the excess being lost as ‘run-off’. The best way to avoid this is to part water each area in turn until they’re all saturated to a sufficient depth. If you can afford an automatic system covering a number of crops, set it up to apply, say, 25% of the total at each watering until the whole amount has been applied.
Timing the irrigation
The rate at which water is lost from a surface is governed by four factors:
- air temperature
- solar energy
- wind speed
During the very early hours of the morning, air temperature and sunshine levels are lower than during the day, and humidity is generally higher. In total, this means that the bulk of water applied at this time will find its way down to the plant roots and not be lost from the soil via evaporation. Irrigate during the day if it’s unavoidable but less of what you apply will actually reach its target.
It’s easy to get lost in the technicalities of irrigating crops and forget about the watering can. The real reason that water companies impose hosepipe bans is because they know that once the pipe is unravelled the energy required to apply water is minimal. The watering can, on the other hand, can impose a self-rationing system of the first order. Each litre of water weighs 1kg (try finding such a simple relationship in imperial) and as I have two arms, I tend to fill and carry two 10 litre cans each trip, which means 20kg total weight. There’s a limited amount of time that you will want to carry that sort of weight back and forth so the mind quickly becomes focussed on using that water very efficiently. And no pumps to fail, filters to block or irrigators to fall over. Keep it simple.
Information on water conservation, use and reuse:
CAT, Machynlleth, Powys, SY20 9AZ
Tel: 01654 702400
This article is from the August 2006 issue of Country Smallholding magazine.
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