Water and Crop Production
Crop production, especially under rainfed farming has declined in tropical areas because of frequent drought occurrences. Annual rainfall totals have been on the decrease in the last ten years. Even in areas where total amounts may be adequate, the distribution has been so poor such that crop failure has always been the result thereby causing food insecurity. With reduced rainfall over the years, total available water from the rivers and underground reservoirs have also been reduced tremendously. This water is essential for use in crop production under irrigation during the dry season where high value crops like vegetables can be grown to fill the hunger gap created by poor rains. With scare available water resources, the use of ceramic pitchers (clay pots) less water is needed to produce the same or higher yields than when conventional methods are used. The system maximizes yields per unit of available water.
The Ceramic Pitchers
Ceramic pitchers (clay pots) are made from sand mixture mostly by rural women. The sand and clay mixture is normally mixed in the ratio of 20% sand 80 % clay. The pitchers are usually made uniform in shape and can hold up to least 5 litres of water. The pots are not glazed after they have been mounded so that natural pore space of clay and sand are kept intact. In order to suppress the shrinking and expansion properties of clay, the pitchers are tempered with fire until they are red hot and then allowed to gradually cool down before they are ready for use. Suitable covers (lids) are also made with the same sand and clay mixture and treated in the same way.
Using the Pitchers
Seed beds with a size of 4 m by 1 m were prepared and clay pots were placed at the centre of each bed and at distance of 50 cm between them. The mouth openings of the pots were left above ground. The pots were filled with water and covered with clay lids in order to avoid evaporation. It is normally desirable that soil be brought to field capacity by flooding it with water and allowing it to drain to until it reaches field capacity before the pitchers are installed. Pitchers which have not been glazed, have pore spaces which cannot be seen by a naked eye. When buried in the soil, filled with water and covered by a lid (wooden or clay), the water is under sub-atmospheric pressure. Water will ooze out of the clay pot depending on the soil conditions around the pot until it is in equilibrium with the surrounding area. The movement of water is as a result of the uptake by the crops and it continues as long as the plants take it up and some of it evaporates. Should the surrounding area become saturated with water and the pot emptied, water will tend to move back to fill up the pot. The system is therefore self-regulating. The surrounding soil is almost always at field capacity as long as the pot is not allowed to dry up completely due to evapotranspiration. From the authors findings, an irrigation cycle of 10-15 days appeared to be the optimum for this system. Such a cycle would assist in saving water as most backyard gardens are in practice irrigated every day with light irrigations.
Cabbage, rape, cauliflower, beans and maize were then planted in the seed beds at the recommended seed rates. Each crop treatment was replicated three times in a completely randomized block design. The same treatments were repeated in bare beds without clay pots and these were irrigated by buckets. The latter treatment acted as a control to check this new innovation.
In addition, fertilizer was applied directly into the clay pots at recommended rates (VRT, 1992) and broadcast on the soil surface and incorporated into the soil in the beds that were used as a control.
Water requirements were determined by quantifying the amounts of water applied at each irrigation cycle. A measuring cylinder was used to measure the quantity of water required to replenish the existing level for pitchers at each irrigation cycle whereas in the bare conventional beds a 10 litre watering can was used. For the bare plots, water was added to the soil to bring it to field capacity at each irrigation cycle . In the case of the pitchers, the depleted amount water was added to fill up the pots.
Yield response to water was recorded and relative comparisons were made in terms of the effectiveness of the systems and sustainability as regards soil management and use of scarce water resources in crop production.
Results and discussion
Yield responses of rape, cauliflower, beans, cabbage (Riana F1), maize (Pool 16) and tomato (Money Maker) to water are indicated in Tables 1 and 6 respectively for the two comparative irrigation systems. Figure 1 shows clearly graphical representation of these results. The results showed water use efficiencies of 12.1, 21.82, 96, 7.26, 38.60 and 13.44 m3/ton/ha for rape, cauliflower, beans, cabbage, maize and tomato respectively under the pitchers as compared to water use efficiencies of 48.25,79.5, 297, 23, 126.49 and 44.1 m3/ton/ha for the same crops under the conventional bucket irrigation system. In addition, higher yields were obtained under pitcher irrigation than the conventional bucket irrigation as shown in figure 1. Cabbage, though more efficient in water utilisation under pitchers than bucket irrigation, did not show corresponding higher yield perhaps due to its shallow and non-fibrous root system. The results in tables 1 and 6 clearly show that total seasonal crop-water requirements differ significantly when the two systems are compared. Up to 70% of the water was saved when ceramic pitchers are used to irrigate the crops under compared to the bucket irrigation system.
Before applying fertilizer to the soil it was first mixed with water in the clay pots instead of broadcasting and incorporating it into the soil as was the case for the crops treatments under the bucket irrigation system. Crops in clay pitchers responded quickly as early as three days from the time of application to fertilizer application compared to one week in the bare plots. It was evident from crops in the pitchers that there was no need to adhere to the recommended application rates as it would be an over application. Some plants could easily get destroyed particularly that all the fertilizer is dissolved and taken by plants almost immediately. It is therefore necessary, under the pitcher irrigation system, to introduce new fertilizer application rates which would reduce the amount of fertilizer to be applied.
Implications of soil texture
This system is like drip irrigation applied on the subsurface. It is highly suitable to medium textured soils. In heavy soils like clays, the wet perimeter may be small and water application to the plant may be inadequate. Low infiltration rates are not be recommended as this affects water movement laterally and vertically.
On a moderate enterprise, e.g. 1 ha, it is possible to grow high value vegetable crops like tomatoes in combination with maize and rape split on a cropping pattern of 0.25 ha, 0.5 ha and 0.25 ha respectively. This combination will require 140m3, 260m3 and 100m3 of water respectively under ceramic pitchers to produce the above crops to marketable level. This combination gives a total of 500m3 of water as compared to a total of 1350m3 of water for the same cropping pattern under the conventional system. This gives water savings of 63% under ceramic pitchers. The fact that with limited available water resources, you can maximize water utility makes this technology a sustainable one since the raw materials for making pitchers are easily available and the fact that they are made locally, creates employment in rural areas especially for women.
Conclusions and Recommendations
The indigenous ceramic pitcher sub-irrigation is no doubt a water saving system. It is an efficient innovation suitable for places with water shortages. It saves up to 70% of the water when compared to conventional systems.
Fertilizer uptake is made more efficient than in conventional systems since it is directly applied in the pitchers and allowed to dissolve for later uptake by the roots.
Unlike in the conventional system, the surrounding soil around the pitchers always remains at field capacity since water is replaced instantaneously during uptake and loss via evapotranspiration. It is therefore recommended to replenish water in the pots before they are completely empty.
The soil surface from pitcher plots remains dry but wet in the subsurface. It reduces the gross evaporation and weed infestation. The soil also remains loose enough to allow for good aeration unlike in the overhead bucket irrigation where the drop impact of applied water compacts and seals the soil surface.
It is possible to use domestic effluent under pitchers and this ensures maximum utilization of water resources. Crop security against stress is ensured as the plant takes up water as per its requirements.
V.R.T (1992) Vegetable Research Team Annual Coordination Report. NIRS, Mazabuka Daka A.E. (1991) Conservative Irrigation using ceramic pitchers as ancillary media for Water conservation. Greece Belkema Press pp5.
Copyright Chris Lungu, SACCAR, Private Bag 00108, Gaborone, Botswana email address: firstname.lastname@example.org