|CERES No. 134 (FAO Ceres, 1992, 50 p.)|
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Conjuring crops from fire, mud and ash
by Paul Icamina
After the smoke, floods and volcanic violence of the erupting Mount Pinatubo finally subsided, all efforts in the Philippines focused on the task of rehabilitation - especially of the scorched and broken farmland surrounding the mountain, on which thousands of families had depended for sustenance (Ceres Nos. 131 and 133).
But when B.S. Vergara, a plant physiologist, and V. Coronel, assistant scientist, at the Philippines-based International Rice Research Institute (IRRI) searched their computer files for information on growing crops in farmlands covered with volcanic ash and lahar (volcanic debris and mud mixed with water), they found nothing. "Althrough the Philippines has several active volcanoes, we have little experiecnce on the controlled revegetation of volcano-affected areas", they admitted. So they, and other researchers to whom the country looked for guidance, turned to the earlier eruptions of Mount Saint Helens in the United States and Mount Galunggung in Indonesia, searching for answers.
From the Mount Saint Helens eruption, they learned that Central Luzon could expect floods because ash lacks continuous pores for capillary and saturated flows. Water tends to remain longer on the surfaces of the ash and lahar-covered soils, and since canals are clogged, lateral drainage is poor. Acting as a surface mulch, the ash layer also reflects solar radiation, and because of its white to pale gray color, it increases photosynthesis, but lowers soil temperature and water evaporation.
Studies near Mount Galunggung show that crop yields are still high in areas covered by up to 20 centimetres of volcanic material, but productivity declines in thicker deposits. Dr A.M. Fagi, a graduate of the University of the Philippines College of Agriculture who worked on crop establishment in the ash and lahar - covered areas around Mount Galunggung in 1982, found that yields of rice and other food crops were high when the proportion of volcanic materials to soil was up to 5:5 and 7:3. The addition of about 20 tons of manure or other organic material per hectare to the volcanic deposits improved the soil for crops. Dumping Manila's organic garbage has been suggested for the Mount Pinatubo area.
The Indonesian experience also indicated that, where the volcanic deposit is under 20 cm deep, the cropping pattern should be rice-rice-corn/soybean or rice-rice-leaf onion. For 20-30 cm deposits, volcanic material should be plowed dry and organic manure mixed during plowing. Food crops can be planted in the early rainy season but rice and corn are not recommended. If the volcanic deposits exceed 50 cm, pineapple would be suitable because it thrives well in sandy soil with a pH range of 4.5-7.5 and requires minimum care and inputs. Hybrid coconuts can also be planted. In Indonesia, fruits like guavas, jackfruit, papaya and banana grew even better than before Mount Galunggung erupted. The Philippine program Building on the knowledge gained from other countries' experiences, the Philippines' post-eruption Agricultural Rehabilitation Program - in effect, a kind of triage system - divided the Mount Pinatubo disaster area into four zones, according to the amount of sand cover and the severity of damage:
* Zone A - Land with more than 15 cm of ash fall and upland
areas deeply covered with sand, which is considered beyond recovery.
* Zone B - Land with 7.5 to 15 cm of ash fall and lowland towns currently inaccessible, which will need clearing for agriculture.
* Zone C - Land with less than 7.5 to 10 cm of ash fall and lowland towns slightly or moderately affected, which will need intensified assistance in order to produce enough food to meet the region's needs.
* Zone D - Land with less than 7.5 cm of ash fall, which is only slightly affected and can easily be rehabilitated.
To reclaim land for agriculture, the three basic considerations are availability of water, the depth of ash and sand deposits and the type of crop to be grown. Most of the areas to be reclaimed are Zone D lowlands, where ash and sand can be mixed into the soil and two crops of rice are possible because water is available to sustain growth over two seasons.
If the water supply during the dry season is insufficient, rice can be grown during the wet season and vegetables or corn during the dry season. Vegetables recommended for affected areas are string beans, mung beans, winged beans, peanuts, asparagus, cabbage, onions, tomatoes, eggplant, okra, cucumbers, watermelon, cantaloupe and garlic.
Corn and sugarcane, which have high tolerance to arid soil conditions, are highly recommended for planting in non-irrigated lowland areas.
In upland areas, mostly Zone C, and particularly where the volcanic deposits are greater than 20 cm, the recommended crops are fruit trees like jackfruit, guyabano, citrus, mango, cashew, guava, coconut and avocado, fruits like pineapple and root crops like cassava and sweet potatoes.
Fodder can be produced in rolling and sloping areas of both Zones C and D, where soil conditions are not suitable for crops. Under prevailing conditions of low soil fertility and poor water-holding capacity, leguminous fodder trees, leguminous crops and other pasture crops are highly recommended.
Growing rice in lahar
According to the Philippines Department of Agriculture, rice - the principal food in the area of Mount Pinatubo's eruption - could also grow well in lahar. In Zambales, Pampanga and Tarlac provinces, where rice is grown over more than half the cultivated areas, 18 per cent of the rice lands were hit by the eruption. About 20 000 ha of rice lands were heavily damaged by lahar and 10 000 ha by ash fall.
The department recommends four early-maturing rice varieties for planting on lowland or rain-fed farms: IR-72, IR-64, IR-66 and IR-61. "But fertilizers, particularly nitrogen fertilizers, may have to be applied in several doses", R.Y. Reyes, head of IRRI's Soil and Water Sciences Division, and H.U. Neue, plant physiologist at IRRI, said.
"Organic matter should be added to increase cation (positively charged ion) exchange capacity as well as water-holding capacity", Reyes said. "Farming practices that minimize water percolation should be studied. The practicality of compaction to increase soil density should be assessed. Compaction has been suggested as a way to get the benefits of puddling (a traditional method of tillage for lowland rice) without puddling".
To ascertain whether rice could be grown on volcanic material, a preliminary study was made in the IRRI greenhouse using four kilograms of lahar set in five-litre pots. Pre-germinated seeds of IR-64 were dibbled into each pot.
Nitrogen fertilizer treatments were applied either as a single basal dose or in three equal doses (basal, 3 WAS, 6 WAS) at the total rate of 50 ppm. To some pots, phosphorus and potassium were added at the rate of 25 ppm as a single basal dose or in three equal doses as in the nitrogen fertilization. An additional treatment involving 10 equal doses of nitrogen applied weekly at the total rate of 50 ppm was included to examine the effect of sustained nitrogen fertilization.
Results: where water is not limited and fertilizer, particularly nitrogen, is added, rice could grow fairly well. It appeared that nitrogen alone boosts rice growth. However, a high rate of nitrogen fertilization is required. Rice plants showed nitrogen deficiency symptoms seven weeks after sowing. Plants fertilized with a single basal dose of nitrogen showed initial better growth. At six weeks after sowing, no apparent difference in growth was observed between the basal nitrogen application and the split applications. There was no further improvement in growth with the addition of phosphorus and potassium.
A field experiment
Percolation of irrigation water and leaching losses of soluble nutrients could not have occurred in the container pots, but field tests on a rice field covered by more than one metre of lahar in Bacolor, Pampanga Province, showed results comparable with the greenhouse experiment. With proper fertilizer management, growth of rice plants is normal both in the greenhouse and in the field. Corn plants planted on lahar in Bacolor exhibited early leaf yellowing, but this was expected because corn normally needs a high nitrogen level.
By and large, if grown to rice, the texture and structure of lahar implies high percolation rates, poor water economy and high soluble nutrient losses. The single-grain loose structure of the volcanic material makes it highly permeable to percolating water. Water would be difficult to impound. Bunds are difficult to stabilize as the material easily slakes in water. High amounts of sodium in the ash favor slaking. When flooded, leaching of applied fertilizers would be high.
IRRI studies show that some rice varieties are better established on the lahar than others. Of the 40 varieties/lines tested, there were seven outstanding entries. The three top entries were those grown in the acidic areas of Indonesia.
One immediate problem is irrigation where there is no water. The Bureau of Soils and Water Management's Soilsearch Centre, which carries out research on soil management, is promoting drip agriculture using two-gallon (7.6-litre) plastic containers.
"Applying lime and organic fertilizer will solve the acidity", Dr Rogelio N. Concepcion, Soilsearch project manager, said. "As soil material, lahar is better than volcanic ash as it is composed of clay soil and volcanic ash".
Areas covered in 5-10 cm of volcanic ash can recover in three weeks, he added. In areas with highly sulphuric soil of more than 300 ppm, the easy solution is to apply "plenty of organic material", about 500 kg per ha, to nurture micro-organisms that remove the sulphur. "In just five days, instead of three weeks, the soil's sulphur content is back to normal", Concepcion said. Trials showed that when fresh ash is incorporated with the original soils the initial increase in the available sulphur content levels off after about 30 days. In good drainage, it may diminish even faster.
Sugarcane, banana and other deep-rooted perennial crops may be grown immediately in areas covered by more than 10 cm of volcanic deposits. With aging and weathering, the plowed ash and sand deposits decompose, producing a very fertile soil for rice.
Ash deposits of up to 15 cm are manageable because plows can reach that depth. "Places covered in one metre or more of lahar should not be treated as a problem but as a resource base", Concepcion said. "For example, use these areas as an anchor for plants, not for tilling". The Department of Agriculture's instructions for do-it-yourself rehabilitation include:
For coconut trees, cut and burn broken or dead leaves. Then apply I kg of table salt per tree for fast recovery. Follow this with 500 grams of urea or four kg of organic fertilizer per tree, applied one metre away from the tree base. Mix fertilizers thoroughly with topsoil.
For mango trees, prune broken or dead branches. Paint the cutoff portion with coal tar.
Replace dead trees, getting planting materials from the Department of Agriculture.
For fish ponds, first collect pond soil and water samples and send them for analysis to the Bureau of Soils and Water Management Laboratory. Tests will show if ash and sand deposits are dangerous to fish stocks. Experts will then recommend the kind and amount of fertilizer needed to enhance the growth of natural fish food like planktons.
The Soilsearch Centre recommends "basket farming" in which plants or trees are planted in biodegradable bamboo baskets filled with healthy soil and transplanted, basket and all, to lahar-covered areas. As the plant matures, it outgrows its basket and takes root in healthy soil beneath the lahar.
Basket farming was developed from planting techniques of local farmers. Some farmers in Cavite Province plant their crops by digging holes several feet deep through adobe layers. Root crop farmers in Bohol Province do the same in farms covered with rocks where plowing is impossible.
Soil manure "basket farming" is recommended in areas where the
scraping of ash is almost impractical because ash-and-lahar deposits are more
than 15 cm deep. It is also a good alternative in areas
where farmers have no access to tractors and in poorly drained upland areas where soil acidity has already set in.
The seedlings are planted in baskets containing 70 per cent soil, 20 per cent organic manure or fertilizer and 10 per cent urea fertilizer. If the crops are vegetables and other annuals, an ordinary native basket about 30 cm high will do. If tree crops are planted, then the container must be at least 45 cm high, the equivalent of a medium-sized native basket or a rice sack cut in half. A fruit tree can be planted in the centre of the basket and one or two legume vegetables added.
A hole large enough to accommodate the soil mixture is dug in the lahar and ash-covered soil, making sure to remove the thin, impermeable layer of fine ash that impedes infiltration of rainwater into the subsoil and to expose the surface of the original soil. The basket's circumference is covered with plastic to prevent further ash and lahar deposits from choking the healthy soil.
Ash fallouts are expected to lower fertility and raise requirements for water and fertilizer, but areas covered with fine, sandy materials in the ash fallouts will have improved water-holding capacity. It is useless to irrigate land covered with lahar or ash deposits more than 30 cm deep until the cover is scraped off. Volcanic ash by itself is unlikely to provide a suitable growth medium for many years because it may be acidic and contain toxic elements.
The Soilsearch Centre recommends a 50:50 ash-soil ratio. Draft animals can turn ash up to about six cm deep into the soil, and tractors about 10 cm.
The major problem with the land around Mount Pinatubo is its high sulphur content, which has toxic effects on young rice plants and other upland crops like eggplants, mung beans, and peanuts on farms with poor soils and poor land drainage, especially irrigated rice lands. The roots of young seedlings cannot penetrate the 2- to 4-cm deep black soil layer where soil and ash mix. In order to avoid damage from sulphide toxicity, planting and transplanting of rice plants must be done two to four weeks after the area has been submerged in irrigation or rainwater. Alternate irrigation and drainage is recommended to flush out toxic sulphides, but the farm must not be drained beyond field capacity or to the point where the soil surface starts to crack.
On lowland rice farms and farms with poorly drained soils, organic fertilizers can be applied about two to four weeks ahead of actual planting/transplanting to speed up the release of sulphides and improve organic matter. On upland farms with good land drainage, two or three applications of organic fertilizer can improve the fertility of areas with ash deposits.