Introduction
Under continual flooding, swamp soils develop characteristics which are fundamentally different from those of upland soils. Although it is not essential that the farmer have a complete technical understanding of the differences between the two types of soils, it is important to know about some basic properties of flooded soils so that management practices will be appropriate for local conditions. This chapter describes basic properties of flooded soils and recommends measures that may be adopted to help overcome the sorts of soil problems most often encountered in swamp farming systems. In addition, the problem of iron toxicity is discussed.
I. Characteristics of Flooded Soils
Three major changes - physical, biological, chemical - occur when a soil is flooded. A brief review of these changes will help lead to a better understanding of soil management practices which will maximize yields in irrigated rice.
1. Physical Changes
Upon flooding, the pore spaces (air spaces) in the soil become saturated with water. As a result, the soil swells, and hard clods soften and break into small aggregates. Puddling completely destroys the remaining structural aggregates (clods and clumps) and transforms the soil into a sludge, or soupy mixture. This slows the drying of the soil, since the exchange of air between the atmosphere and the soil is impeded, and since the water particles are held by soil particles and prevented from percolating downward and escaping.
2. Biological Changes
The absence of soil air (and particularly oxygen) in flooded, puddled soils causes a change in the varieties of microbes, or microscopic organisms which live in the soil. Microbes existing in the absence of oxygen are known as anaerobic microbes, and they tend to be much slower, less efficient decomposers of organic matter than their aerobic cousins. Consequently, the rate of decay of organic matter tends to be slow in flooded soils. Also, the end products produced by anaerobic decomposition differ' some are toxic to rice, particularly those released during the first two weeks after decomposition begins. This can be important in timing organic matter incorporation between plantings: if the farmer plants too early into a flooded plot containing plowed-under stubble and/or chaff, the toxicities produced during normal decomposition may stunt the growth of the rice.
3. Chemical Changes
Flooded soils develop two distinct chemical zones. (see figure below) The upper zone, a thin 1-10 mm, absorbs oxygen from the water, turns brown in color, and reacts to nitrogen like an unfolded soil. This zone is called the oxidized zone, in reference to its chemical condition of being oxidized. The lower zone, which extends down as far as the water, is extremely low in available oxygen, turns dark blue or gray in color, and takes on chemical properties quite different from those of the oxidized layer above. This lower zone is known as the reduced zone.
When a soil is flooded, the nitrogen in the incorporated plant (and animal) residues is changed to the ammonium form (NH4), which is stable under flooded conditions and will later be used by growing rice plants. If the soil is allowed to dry thoroughly (e.g., when it is drained for plowing), a micro bacteriological change takes place during which the ammonium form of nitrogen is changed to the nitrate form (NO3).
When the soil is later re-flooded, part of the nitrogen held in the nitrate form is changed into nitrogen gases (N2, NO3) ) and escapes into the air. Between 20-700 kg/ha of nitrogen can be lost through this process, known as denitrification, so it is extremely important to keep the Plot thoroughly flooded at all times after initial irrigation has taken place.
Figure: Flooded Soils and De-nitrification
II. Management of Problem Soils
All swamp farming systems are not created equal. Although swamps tend to look the same from the ground, often they vary considerably below the surface. Some swamps are shallow and sandy, others deep and peaty, still others filled with rock or clay deposits. Because soil characteristics significantly affect plant growth, the farmer should know what steps should be taken to minimize soil-related problems.
The key to management of problem soils lies in anticipating potential problems before they occur and taking the steps necessary to head them off. Problem soils can be attacked three ways' 1) through the selection of an appropriate variety, 2) through proper swamp development, and 3) through effective management practices. Never wait until after the crop is in the ground to begin thinking about soil problems, since by then it will be too late to change varieties or modify the water control system.
Always begin thinking about the soil early in the development process. The very first time you visit a farmer's swamp, take the time to dig a few scattered pits to reveal local soil characteristics. Question the farmer about the depth of the topsoil, about the type of underlying material, about the presence of undecomposed organic material, etc. With a little practice, you will learn to identify problem soils long before planting time, and consequently you and the farmer will be able to devise an effective strategy involving development, varietal selection, and crop management.
A. Sandy Soils
Many swamp systems in Sierra Leone are extremely shallow and sandy. The main problem with sandy soils is leaching: water percolates through sand very easily, carrying away nutrients and bringing in toxic materials. Effective water control is difficult, because sandy soils do not retain water well' as a result, drought stress is common among crops grown in sandy swamps. Nutrient deficiencies occur regularly, as evidenced by stunting, yellowing of the leaves, and low tillering.
To treat sandy soils:
1) Development practices
Develop the swamp so as to discourage leaching. There are several ways to slow movement of water through the soil'
- keep the main drain shallow (so it will not "suck" water from the plots).
- dig deep peripheral gutters (to intercept groundwaters percolating down from adjacent hillsides).
2) Varietal Selection
Select a variety which does well under relatively difficult conditions, i.e. a variety which can withstand an irregular water regime, nutrient deficiencies, and soil toxicities.
3) Management Practices
To minimize leaching and the damage it cases:
- keep a constant, slow-moving or non-moving flood on the plots.
- spread out fertilization by applying top dressings in many small splits.
To improve the structure and fertility of the soil, as well as to improve its ability to retain water, incorporate large amounts of organic material before and, if necessary, after the growing season.
B. Peaty Soils
Swamps which remain permanently flooded often contain large amounts of undecomposed organic matter (because the soil never gets a chance to dry, which means that aerobic decomposition never occurs). Peaty soils are characterized by high acidity and the presence of numerous toxic materials, both of which severely affect crop growth. Peaty swamps are easily recognized by matted clumps of undecomposed organic material, noxious gases, and mineral slicks. Rice grown in peaty soil is usually stunted, browning, and low tillering.
To treat peaty soils:
1) Development Practices
When developing the swamp, be sure to dig an effective drain. The main drain should be extremely deep to draw water not only from the surface of the plots, but from beneath the surface as well.
2) Varietal Selection
Select a variety which does well under conditions of high acidity. An iron toxicity-resistant variety is usually a good bet, or a native variety.
3) Management Practices
To enable aerobic decomposition of organic material in the soil:
- continually drain the plots.
- till the topsoil during the dry season to a depth of several feet, if possible (encourage the farmers to construct vegetable mounds).
To prevent incorporation of additional organic material into the soil, burn all grasses, weeds, and crop residues (stubble, chaff, etc.).
C. Clays
Clays have a high natural fertility, but often they resist giving up their nutrients to plant root systems. Furthermore, because of their dense structure (they are composed of extremely small, tightly-packed particles), they tend to be difficult to work. Therefore, improving the structure of clays should be the farmer's primary goal. When the clay is fairly dry (just before it starts to crack), begin tilling to increase aeration. Incorporate large amounts of organic material, and practice crop rotation to help maintain fertility.
III. Iron Toxicity
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| Iron toxicity |
Iron toxicity is a soil-related condition caused by the presence of too much iron in lowland rice paddies. Occurring in virtually all regions in Sierra Leone, iron toxicity results from a complex set of chemical imbalances tin the soil and in the plant) which in many instances severely inhibits yields. The condition occurs most commonly in sandy areas of swamps adjoining upland slopes, or in peaty areas where drainage is poor.
The symptoms of iron toxicity are numerous and varied, but the most typical indication is a marked bronzing of leaves and stems, generally occurring 4-7 weeks after transplanting. In addition, roots are stunted, coarse, and reddish brown or dark brown in color ( a coating of iron oxides reduces root surface and decreases capacity to absorb soil nutrients). Plants are stunted and tiller poorly. In extreme cases, black or brown spots appear on leaves and stems, and leaf edges turn dark brown and roll in toward the midrib.
The causes of iron toxicity are technically complex and hence difficult to describe succinctly. Very simply, the presence in the soil of excessive quantities of iron inhibits nutrient absorption by the plant and leads to severe nutritional imbalances which manifest themselves in the typical symptoms. The intake of phosphorus and potassium is especially inhibited, and it is the relative lack of these two primary nutrients (in conjunction with the relative abundance of nitrogen) which causes many of the symptoms.
Treatment of iron toxicity can take several forms. Swamp development measures which can help prevent development of a situation favorable to iron toxic conditions include deepening peripheral gutters in sandy swamps (to decrease percolation of iron compounds down from the uplands) and improving drainage in peaty swamps (the mineral slicks in peaty swamps are often associated with iron toxicity). Fertilization with large amounts of phospherus and potassium can be effective, although application of nitrogenous fertilizer generally aggravates the symptoms (since nitrogen is relatively available to the plant already, and the presence of nitrogen in the absence of phosphorus induces the symptoms). The most effective treatment in any case is selection of iron-toxicity-resistant varieties. Certain varieties (often traditional varieties) are able to flourish in iron-toxic soils, and they offer the most practical and economical solution to the problem.