Stanghellini, C.

English

Wageningen Yield 

Water management
IN THE GREENHOUSE 18  Fruit & Veg Tech 9.2  2009
Limited leaching saves money
Nowadays there is much interest in ‘closed 
systems’. However, it is often forgotten that 
there is a ‘leak’ of some 70% of water supply: 
that is the water taken up by the crop. A small 
fraction of that is accumulated into fresh 
weight; the rest disappears altogether in tran-
spiration. All minerals that the plants do not 
use will stay behind. When the water used to 
re-fill the system contains salts, these will 
accumulate in the ‘closed’ loop. That shows 
up as ever increasing EC, and that costs pro-
duction.
A high EC means less production, no doubt. With some crops more than with others, but at some point it will be necessary to leach the sys-
tem to lower the salt concentration. 
In this way, the concentration of the salt in the closed 
loop will increase, until the leaching ‘ceiling’ is reached 
and the system is drained. After closing the loop, the 
concentration will rise again and so on. Figure 1 shows a 
real-life example, of time trends of EC in rock wool 
slabs with a tomato crop grown in closed-cycle, the re-
fill water containing 12 mmol/l of Na, which is rather 
extreme. The very variable trends in Figure 1 show that it 
is difficult to predict how fast the EC will reach the 
leaching ‘ceiling’. This means that, even if about the 
same amount of water is leached each time, it is diffi-
cult to say how often leaching is necessary and how 
much leaching will take place in total.
Amount of leaching
The problem with Figure 1 is that although salinity will 
increase with time, it is not time that determine what 
happens. The following example makes it clear. Let’s 
assume a grower has irrigation water with a Na concen-
tration of 2 mmol/l. That will be, obviously, the concen-
tration in the slabs at the start of the crop. Each litre 
refill brings in another 2 mmol Na. When the crop has 
taken up the equivalent of the whole volume of water in 
the system (and all Na has been left behind), each litre 
has been replaced, and the concentration has doubled 
to 4 mmol/l, and so on. 
So, what determines the rate of increase of concentra-
tion is the ratio between cumulated water uptake (U) 
and the water volume (V) of the closed system, and the 
resulting concentration is expressed as a multiple of the 
concentration of the irrigation water, Cirr. Then it can be 
calculated that the real water requirement (W, including 
leaching) is a multiple of crop water uptake, given by:
W = CMAX / (CMAX – Cirr)
*U
in which CMAX is the maximum allowed concentration in 
the solution.
The leaching (L) then can be easily calculated as:
L = W – U = CMAX /(CMAX – Cirr)
*U – U = U*Cirr/(CMAX – Cirr)
For instance, transpiration of a rose crop in the 
Netherlands is about 1,000 litres per square metre per 
year. With irrigation water containing 1 mmol/l Na, and 
By Cecilia Stanghellini, Wageningen UR 
Greenhouse Horticulture, the 
Netherlands
Water management
IN THE GREENHOUSE 19  Fruit & Veg Tech 9.2  2009
Limited leaching saves money
allowing for a CMAX of 4 mmol/l, the water requirement 
would be 1333 l/m2, of which only 1,000 used by the 
crop and the rest leached. Tomato is more tolerant for 
salinity, so a CMAX of 8 mmol/l is prescribed by Dutch 
environmental regulations. As water uptake of tomatoes 
(in Dutch greenhouses) is 750 l/m2 per year, leaching 
would be in this case ‘only’ 107 l/m2 per year. This is not 
only water, but it contains a lot of fertilisers (160 kg/ha 
nitrogen, for instance, in this example) and, sometimes, 
plant protection chemicals. It is therefore no wonder 
that authorities all over Europe try to limit leaching. 
In the Netherlands – where closed irrigation systems are 
in principle mandatory – there are rules indicating for 
each crop the sodium concentration at which a grower 
is allowed to leach. Sodium chloride is the most com-
mon ‘problem’ salt in the whole world, by no means the 
only one. 
Costs and benefits 
Ideally, the choice of the leaching ‘ceiling’ (the maxi-
mum concentration allowed, CMAX) should be based on a 
cost and benefit calculation, that is: the loss of income 
(by postponing leaching) due to yield decrease with 
salinity, against the cost of the water and the fertilisers 
that would be leached – possibly including a cost of the 
impact on the environment. 
The European project EUPHOROS (see Box) is aimed at 
decreasing the resource use of European greenhouse 
production and thus limiting the leaching. Therefore, 
we have calculated the ‘optimal’ watering strategy for a 
number of different scenarios (crops, climates, value of 
produce). Except for very salt-tolerant crops (such a 
cherry tomato), the result is invariably that there is no 
advantage in allowing a higher salinity than the irriga-
tion water has. In other words, most closed systems fed 
with poor quality irrigation water are better not closed 
at all. In the Netherlands, the environmental regula-
tions therefore compel growers to have good water. 
Usually this translates into minimal dimensions of a 
compulsory rain collection basin.
Value of water
Rain water is cheap, obviously. However, rain water in a 
basin that occupies land that is otherwise potentially 
productive, may not be cheap at all. The Dutch 
Agriculture Economics Research Institute has calculated 
that the real cost may well exceed €1/m3, which makes 
other water sources competitive, such as desalinization, 
or private providers of cleansed surface water. As a 
cubic metre of water produces anything between for 
instance 40 kg tomatoes (in Spain) and 80 kg tomatoes 
(in the Netherlands), even at €1 /m3 water is several 
times cheaper than what it may produce. Our calcula-
tions about the optimal management have shown that – 
except for very low value crops – it is always worthwhile 
for a grower to have good water, even if it is more 
expensive. 
Good for the environment
Leaching has a serious environmental impact. In the 
example of tomatoes leaching only 100 l/m2 per year, 
that means a leaching of 270 kg/ha of potassium and 60 
of phosphorus, besides 160 kg/ha of nitrogen. In view 
of this, irrigation or local authorities willing to reduce 
agricultural pollution should consider providing incen-
tives for growers to switch either to less salt-sensitive or 
to more valuable combinations of crops. Then there will 
be a need for good irrigation water, but also the revenue 
to pay for it. n
EuropEan co-opEration for a sustainablE horticulturE
EUPHOROS is a four-year project aiming to develop a sustainable greenhouse system that does not need any fossil energy 
and which has a minimum carbon footprint. There must be no waste of water nor emission of fertilisers. Substrates have 
to be fully recycled and the need of plant protective chemicals must be minimal. Yet, this greenhouse system must realise 
high productivity and use resource efficiently.
The project is a European co-operation between research institutes in Spain, Italy, the United Kingdom and the 
Netherlands. Also five commercial partners from Latvia, Switzerland, the Netherlands, Hungary and Italy and a Hungarian 
growers’ organisation are participating. To respond to the diversity of climatic, economic and environmental constraints 
across Europe, techniques will be tested locally in relevant combinations of crops (tomato and/or rose). 
The focus is on three main items: 
1. Systems to reduce energy, water, fertilisers, pesticide consumption, and waste.
2. Optimising the growing environment.
3. The balance between environment and economy.
Figure 1. Trend oF eC in The roCkwool slabs, oF a TomaTo Crop in a Closed sysTem 
reFilled wiTh waTer ConTaining 12 mmol/l oF sodium. whenever The eC reaChed 9 ds/m The 
sysTem was leaChed as Far as possible.


Limited leaching saves money

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