Review: The effect of pot volume on the performance of pot irrigation system (Al-Mohammed, 2012)

Table of Contents

Objective of the study
Description of the experiments


Title: The effect of pot volume on the performance of pot irrigation system
Authors: Al-Mohammed, Fadhil Mohammed
Type of Study: Experimental
Journal: Al-Taqani, Volume 25, Issue 3, 2012, E88-E98.
Open Access: Yes


Objective of the study

The objective of this study is “to investigate the effect of pot volume on water use efficiency and surface wetting edge by comparing the performance of large pots to that of smaller ones by using three different crops, namely, tomato, beans, and cucumber.” [E88]



Type of experiment: Greenhouse experiment
Country: Iraq
Place: Technical Institute of Karbala (TIK)
Timespan: 12.01.2010-30.06.2010
Crops: Tomato, bean, cucumber
Soil Type: Clay loam (40.3% Sand, 27% Silt, 32.7% Clay)
Number of pitchers: 36
Parameters analysed: Yield, seasonal crop water requirements (CWR => note: this term is misleading, the author actually refers to the amount of water used for irrigation and not to the water actually consumed by the crops; see comment section below), water saving (WS), water use efficiency (WUE), surface wetting edge (SWE)



The paper is structured in four sections plus references.


Materials and methods
Clay Pots
Pot Irrigation System

Results and Discussion
Yield responses
Crop water requirements
Water use efficiency
Surface Wetting Edge




Description of the experiments

Pitcher characteristics
The unglazed clay pots had a conical shape and were produced locally. The x-ray analysis of the clay provided the following results:

Clay components
SiO2 Al2O3 Fe2O3  CaO SO3 MgO Na2O L.O.I Total
42.92 8.23 6.94 18.78 0.31 5.63 0.36 16.5 99.67


Overview of the characteristics of the clay pots

Clay pot class
Volume (ml) Average wall thickness (mm)
Height (cm) Lower end diameter (cm) Upper end diameter (cm) Surface area (cm2)
L(arge) (PSI1) 7.841 8.6 29.7 14 26 1.894
S(mall) (PSI2) 2.023 8.6 17.2 11.1 17.2 779


Soil characteristics
The soil texture was clay loam (40.3% sand, 27% silt, and 32.7% clay). The following soil properties were measured in the lab. pH: 7.8 at 30°C; extract electrical conductivity (EC): 2.4 dS/cm; soil bulk density (ρ): 1.34 gm/cm3.


Experimental Set-up
“Two water tanks were installed to supply water to the clay pots that were used in the experiments, the first is to supply water and provided with a plastic tube at its side to measure the vertical distance of water fall down. The second tank was installed to maintain the water in clay pots at a constant level.” [E90]

36 clay pots were used in the field study. They were buried up to their necks with the rim about 2 cm above the soil.

It remains uncertain what exactly the author means by writing: “For each experiment, the two pots were connecting by using plastic pipe 1 cm in diameter.” [E90] . Since there is one experiment with – as it seems – six set-ups and each set-up consisting of six pots and three different crops, this may be related to two pots around which the same sort of crop was planted.

The buried pots were closed with galvanized lids and connected to the “water supply tank and constant level tank by using a plastic tube of 1 cm diameter.” [E90]

Three crops were planted: tomato, bean and cucumber. “Carrying out seeding in the wetted area (in four situations) surrounding the pots, three days after filling up the clay pots by operating PIS.” [E90] This probably means that four seedlings were planted around each pot after a three day period in which the pots were filled with water.

The pots in this experiment were continuously supplied with water.

The soil type was clay loam (40.3% Sand, 27% Silt, 32.7% Clay), soil properties were: pH=7.8 at 30°C, electrical conductivity (EC) = 2.4 ds/cm, bulk density = 1.34 gm/cm3.

The exact type of crop is not mentioned but photographs show that, for the tomatos, it was a ground covering type.



“PIS1 gives greater yield than that with PIS2 for all crops used in the experiments. Tomato, beans, and cucumber yield under PIS2 is 46.35%, 37.75%, and 56.54% lower than that under the PIS1, respectively.” [E92]

“(…) it is observed that water savings between 22.3% and 41.3% are achievable with PIS2 as compared with PSI1 where the pots used in PIS2 are small, that minimizing of seepage to soil.” [E94] See comment below for this misleading description.

“For the three crops, WUE of the PIS1 is higher than that of the PIS2. Tomato, beans, and cucumber WUE under PIS2 is 7.4%, 15.1%, and 42.7% lower than that under the PIS1, respectively. This means that these crops irrigated by the PIS1 require less water to produce a higher yield per unit applied water.” [E95]

“Observation shown that for both pots class L and S, the SWE still continue and reach maximum values after approximately 7.5 days.” [E96]



The author summarizes the results in the conclusions section, no additional thoughts are provided.



The description of the set-up lacks some details.

Considering that the author measured the “surface wetting edge”, the more dense, unprepared soil in which the holes for the clay pots were dug has a strong chance to influence the size of the area around the pots that gets irrigated but this potential influence is not mentioned in the paper.

The author calculates the decrease in yield irrigated with small pots based on the yield of crops grown with large pots for irrigation. It is even more significant to take a look at the increase of yield gained by irrigating with large pots compared to small pots. Compared to the yield and water use of the smaller pots’ set-up PSI2 the increase in yield and water use in set-up PSI1 looks as follows, based on the data presented in tables 3 and 4:

Crops Large pots

Average yield (kg)

Small pots

Average yield (kg)

Large pots

Average water use (l)

Small pots

Average water use (l)

Increase in yield large pots compared to small pots Increase in water use large pots compared to small pots
Tomato: 11.657 6.254 1.370,9 804,8 +86,4% +70,3%
Beans: 2.188 1.362 1.108,3 806,2 +60,6% +37,5%
Cucumber: 3.173 1.379 1.132,8 879,9 +130% +28,7%


Since the title of the paper reads “The effect of pot volume on the performance of pot irrigation system” the author should have outlined and discussed these effects to a greater extent, especially the effect and degree of increase in yield from large pots compared to small pots. This increase means for farmers that investing in larger pots could lead to a higher income in the long term.

Of importance for farmers is also the result that cucumbers grown with large clay pots for irrigation produced 130% more yield with less than 30% increase in water use compared to cucumbers grown with small pots for irrigation.

The authors makes two fundamental mistakes in his “conclusions”:
“The difference in crop water requirement under PIS1 and PIS2 is significant, higher crop water requirement is obtained under PIS1.” [E96]
“Crop water requirement” is a misleading term since what is measured here is not the water explicitely used by the crop but the amount of water used for irrigation. It is not the case that 100% of the irrigation water is consumed by the plant. Parts of the water, e.g., wet the soil, seep away or evaporate.

It goes without any further argument that a clay pot with a higher volume and a larger surface area provides more water to the soil and thus creates a higher wetting radius (as the author also shows in fig. 6) than a smaller pot of the same material and firing temperature. It is not necessarily the crop that requires more water, it is the pot that provides more water based on physical laws. In addition the pots in this experiment were continuously supplied with water which also increases the amount of water seeping into the soil since the pots were always filled.

As a consequence “crop water requirement” should be replaced with “water use”.


“PIS2 is a conservation irrigation system, which saves between 22% and 41% water when compared to PIS1.” [E96]
This claim is misleading, too. The experimental set-up labeled “PIS2” used smaller pots compared to the set-up labeled “PSI1”. In both systems the pots were continuously filled with water up to their necks. Of course the seepage rate of a smaller pot with a smaller surface area is lower than the seepage rate of a pot almost fourth the size when both pots were made of the same material and fired by the same temperature. So yes, less water was provided to the crops by the smaller pots, but this must not be confused with “saving” water, because the lower water use resulted in less yield – totally as well as relatively. The PIS2 set-up with the smaller pots did not “save” any water, as the author writes, the pots deliverd less water to the plants and could not provide enough water to fulfil the crop’s water need to produce a higher yield.In other words: If both set-ups were irrigated with the same amount of water the larger pots would have provided a higher yield. Table 3 clearly shows that the water use efficiency of the large pots is much better for all three crops than that of the small pots. Knowing this, the author should have recognized that there is no water “saving” by using the small pots. He actually points this out on page E95: “This means that these crops irrigated by the PIS1 require less water to produce a higher yield per unit applied water.” but still draws this erraneous conclusion.

The author does not differentiate between results and conclusions. He basically summarizes the results in his conclusions section.

Within the replications of PIS1 as well as of PIS2 some results concering yield and water use differ strongly but the author does not address these differences. Since the experiment was conducted in a greenhouse with identical soil, water quality and weather conditions for all the pots and crops, these differences are unexpected and would have needed an analysis and discussion of the potential underlying reasons.

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