|Amaranth to Zai Holes, Ideas for Growing Food under Difficult Conditions (ECHO, 1996, 397 p.)|
|28 additional technical notes about tropical agriculture|
Echo technical note a-8
DEFINITION:Dry Farming is the profitable production of crops, without irrigation, of land with a low average or highly variable rainfall.
1. Farm practices must conserve and utilize the available
rainfall to the fullest extent.
2. Quick maturing, drought resistant crops must be grown.
1. Rainfall must be greater than 10 inches per year
2. Wind and heat must not cause excessive evaporation at critical stages of plant growth.
3. Soil should be deep (preferably 10 feet - 3 meters) with no clay, sand, or gravel seams to interfere with capillary movement of water. The minimum feasible soil depth is 18 inches (450mm) but water storage capability and drought resistance increases with increasing soil depth.
To obtain maximum storage of moisture under any rainfall condition, the soil must absorb as much water as possible when it rains and losses by evaporation or transpiration must be kept to a minimum.
A. PREVENT A WATER SEAL AT SURFACE. Probably the greatest deterrent to a high rate of water absorbtion is the tendency for soils to puddle at the surface and form a seal against water intake. The beating action of raindrops tends to break down cloddiness and disperse the soil.
1. By tillage, create a rough, cloddy surface which lengthens the time necessary for the rain to break down the clods and seal the surface. For seed bed preparation in general, small seeds should have a finer, mellower bed than large seeds.
2. After harvest, create a stubble mulch on the surface. Such material not only prevents raindrops from inpinging directly on the soil, but impedes the flow of water down the slope, increasing absorbtion time.
B. REDUCE THE RUNOFF OF WATER. To the extent that waterlogging is not a problem, the runoff of water and its attendant erosion must be stopped.
1. Cropland should be as level as possible.
2. All tillage and plantings must run across (or perpendicular to) the slope of the land. Such ridges will impede the downward movement of water.
3. For every two feet of vertical drop or 250 feet of horizontal run, the field should either have bunds or contour strips (details of these practices are discussed later).
A. REDUCING SOIL EVAPORATION. Water in the soil exists as a continuous film surrounding each grain. As water near the surface evaporates, water is drawn up from below to replace it, thinning the film. When it becomes too thin for plant roots to absorb, wilting occurs.
1. Shelter belts of trees or shrubs reduce wind speeds and cast shadows which can reduce evaporation 10 to 30 percent by itself and also reduce wind erosion.
2. Mulching reduces the surface speeds of wind and reduces soil temperatures.
3. Shallow tilling can create a dirt mulch 2 to 3 inches deep which dries out easily but is discontinuous from the subsurface water, preventing further loss. Tillage must be repeated after each rain to restore the discontinuity. This is most workable where rainfall occurs in a few major rainfalls with relatively long intervals in between.
B. REDUCING TRANSPIRATION. All growing plants extract water form the soil and evaporate it from their leaves and stems in a process known as transpiration.
1. Weeds compete not only for soil nutrients, but water as well and so their control is critical.
2. Selection of crop is significant as well. Dwarf varieties have less surface and so lose less water. Some plants close their stomae when it is hot, reducing their water loss. Others, like corn, curl their leaves during hot afternoon and open them at night, effectively changing their surface area in response to conditions.
3. In dry farming, the number and spacing of plants is reduced so that fewer plants compete for soil moisture. The exception to this occurs when allowances for insect, bird, and rodent loss must be made at planting.
4. Where rainfall is frequently marginal to insufficient, drought "insurance" can be obtained by clear fallowing a sufficient area. An area clear of growing vegetation with a properly maintained stubble and soil mulch can retain 20 to 70 percent of the precipitation received until the next year. Where 5 to 6 acres each year per family have been so set aside in India, the specter of famine due to drought has been eliminated.
5. Post harvest tillage will create stubble and dirt mulches and destroy weeds before the onset of the dry season.
Dry farming builds upon a knowledge of general agriculture but carries out its practices in the light of the significant probability that this year or next will be a drought. The following agriculture practices are discussed with this back-ground.
A. BUNDING. The first essential step in dry farming is bunding. The land is surveyed and level contours determined every hundred feet. For unusual slopes, it is recommended that for every fall of two feet, a bund 18 to 24 inches in height be constructed. Even when land is fairly flat, a 12 inch high bund every 250 feet is still found useful. Excess storm water is released by constructing periodic waste weirs with a sill of one-half bund height. This will retain water and minimize the loss of topsoil.
In order to make the bunds, land must be marked by the surveyor with bund lines. A few feet on either side of it, the land should be plowed and harrowed. The bund former should be worked along the bund twice, side by side, leaving a furrow in between. This furrow in the middle should be filled in with soil from the plowed portions on both sides, by means of a scraper. The outlets or "waste weirs" should be constructed of stones.
The natural drainage of the area must not be completely stopped but should be controlled by providing suitable outlets for excess storm water to pass gradually, without carrying much silt with it, and after fully saturating the soil and subsoil. The major natural drains in each village area or watershed must be properly maintained so that all fields have some outlet for the extra storm water.
B. STRIP CROPPING. Strip cropping is a technique that serves to control erosion and increase water absorbtion thereby maintaining soil fertility and plant response. In effect, it employs several good farming practices such as crop rotation, contour cultivation, stubble mulching, etc.
By growing in alternating strips crops that permit erosion and exposure of soil soil and crops that inhibit these actions, several functions are performed:
1. Slope length is maintained.
2. Movement of runoff water is checked.
3. Runoff water is desilted.
4. Absorption of rainwater by soil is increased.
5. Dense foliage of the erosion resisting crop prevents rain from beating directly on the soil surface.
Strips are, of course, planted perpendicular to either the slope of the land or the prevailing wind direction, according to whether water or wind presents the more serious erosion potential. Additionally, crops which do not resist erosion should be rotated with crops which do. Research has shown that:
1. A normal seed rate of groundnut (peanut) is an efficient and suitable crop for checking erosion.
2. The normal seed rate of leguminous crops other than goundnut does not provide sufficiently dense canopy to prevent raindrops from beating the soil surface; is should be raised to three times the normal seed rate.
3. On the average, the most effective width of contour strips for cereals such as sorghum and millet is 72 feet and for the intervening legume, 24 feet. As slopes vary, so do the optimum strip widths, as shown below:
C. SUMMER FALLOW. All of the principles of water conservation and utilization pertaining to dry-farming will not make a crop grow if sufficient rain does not fall. Where the soil depth exceeds 18 inches (450mm), however, it has been shown that it is possible to store water as soil moisture from one year to the next by the use of proper summer fallow techniques. With a soil depth of 10 to 15 feet, up to 75% of the incident water may be retained though 20% to 40% is more normal. Thus, in an area that averages sufficient rainfall for crop growth, it will be rare that the sum of the stored water and incident water will not be sufficient for crop production. Where families in India have faithfully set aside 5 to 6 acres for summer fallow each year, drought-induced famine has been virtually eliminated.
The partial loss of a crop in the year of fallow is offset to a great extent by a very much increased yield in the year of cropping. Such increased yield in a year of failure of the general crop in the surrounding areas, has a far greater value than a normal crop of a good season.
In order to accomplish this objective, the soil must be loose and permeable to soak up the rainfall and the dirt/stubble and mulch must be maintained to minimize evaporation. The land is worked with a tine-cultivator followed by occasional harrowing, particularly after rainfall, and weeds (which use as much or more water as crops) must not be allowed to grow. Though this expenditure on cultivation is relatively small, neglecting to provide the surface mulch at any time may cause more moisture to evaporate in a few, hot days than would fall during the whole season.
Experience has shown that where rainfall is 10 to 15 inches per year (250 to 375 mm/yr.) a clear fallow every other year is necessary and, at 15 to 20 inches per year (375 to 500mm/yr.), every third year.
1. MECHANISM OF SOIL DRYING. Water easily enters porous soil and, as it seeps downward, becomes absorbed as films of water around the soil grains. These films form a continuous column of water to the surface of the soil. The film tends to remain the same thickness around all the soil grains with which it is in contact. This film of water in the soil is known as the capillary water and is the source of water for the plants.
The sun, wind, and dry air will cause evaporation at the surface, thus reducing the thickness of the film at the surface. The thicker films in the subsoil will rise to equalize the distribution again. This will continue until the films are so thin that the plant roots can draw no further moisture from them. The result is drought.
2. STUBBLE MULCH. Stubble mulching aims at disrupting the soil drying process by protecting the soil surface at all times, either with a growing crop or with crop residues left on the surface during fallows. To be effective, at least one ton per hectacre must cover the surface, and the maximum benefit per unit residue is obtained at about two tons per hectacre. Benefit may still be obtained at 8 tons per hectacre.
The first benefit of a stubble mulch is that wind speed is reduced at the surface by up to 99%, significantly reducing losses by evaporation. In addition, crop and weed residues can improve water penetration and decrease water runoff losses by a factor of 2 to 6 times and reduce wind and water erosion by factors of 4 to 8 relative to a bare fallow field.
There are two limitations to the advantages of stubble mulch farming:
a. Dead surface vegetative matter can provide a home/breeding ground for plant diseases, insects or rodents. Use of a mulch not related to the succeeding crops will minimize much of the disease and insect effects. Use of stubble mulch only in the dry season will minimize all biological activity.
b. For decomposition, the ideal carbon to nitrogen ratio (C/N) is 25 to 30. Dry, woody, or non-green straw, stalks, etc. have a C/N of 50 to 100. This tends to slow decomposition and deplete soil nitrogen temporarily. Nitrogen is a major requirement for protein synthesis by plants. A stubble mulch during a biologically active period such as the rainy season, should only be used when either:
1. Soil nitrogen is very high.
2. Plant nitrogen needs are very low (such as cassava).
3. A nitrogen-containing fertilizer is used.
To obtain the benefit of mulching on soil structure without causing temporary de-nitrification, the mulch can be composted before adding it to the soil. Rapid bacterial action in the tropics makes composting less beneficial than in temperate climates but may still be worthwhile.
3. DIRT MULCH. Dirt mulching aims at disrupting the soil drying process with tillage techniques that separate the upper layer of the soil from the lower layers, making the soil moisture film discontinuous. In addition the soil surface is made more receptive to water intake.
Principles of dirt mulching:
a. Effectiveness increases with increasing depth to a limit of to 4 inches (75 to 100mm).
b. Increasing the dirt mulch depth decreases the available fertile soil.
c. The effectiveness of dirt mulches decrease with age. Consequently it must be recreated by shallow tillage of harrowing after each rain or each month (whichever is more frequent).
d. The crumb form of dirt mulch (particles greater than 1mm) is more effective and resists wind erosion more than the dust form.
e. Dirt mulches can only be properly made when the soil is moist.
f. For a climate with a "rainy" growing season and a hot, windy, dry season, dirt mulching should only be performed during the rainy season and with a growing crop to slow the wind and water and hold the soil.
The improper use of a dirt mulch presents serious erosion potential. The "dust bowl" condition in the great plains of the U.S. that destroyed or damaged millions of acres of prime cropland was a direct consequence of the abuse of the dirt mulch.
E. PLOWING/TILLAGE PRACTICES. Plowing, when the soil is in the proper condition, wears the soil into thin layers, and forces the layers past each other. If the soil is too wet when plowed (especially if it is heavy), the soil crumbs or granules are destroyed, thus puddling or compacting the soil. When the soil is too dry, the soil tends to pulverize and form dust. Plows with steep moldboards have the greatest pulverizing action upon the soil. The plow with the less steep moldboard has less tendency to puddle the soil and is of less draft.
1. Purposes of Tillage Operations:
a. To produce a rough, cloddy surface that will increase moisture absorption and reduce runoff, as well as erosion from wind and water.
b. To control/destroy weeds that compete with crop for sunlight, nutrients, and water.
c. To destroy or prevent the formation of a hard pan (sole) which can develop after repeated shallow plowing or harrowing. This hard pan can stunt root growth, reduce water storage, and check the capillary rise of water from the subsoil.
d. Promote bacterial activity by aerating soil, encouraging the decay of residues and the release of nutrients.
2. Time of Tillage:
a. Plowing, like planting, is sensitive to moisture and neither should be done when soil is either too wet or dry. In the arid and semiarid tropics, proper moisture conditions are likely to occur only at the beginning of the rainy season and should be done on the same day. If possible, planting should immediately following plowing, with seed rows centered on the furrow slices. A crosswise harrowing will cover seeds and close air spaces, thus creating a dirt mulch and keeping out the drying winds. If the crop is then harrowed/cultivated several times during the season, especially after rains, much moisture will be conserved. The proper soil moisture condition for plowing is indicated by a manual soil test. The usual test is to squeeze a handful of soil. If it sticks together in a ball and does not readily crumble under slight pressure by the thumb and finger, it is too wet for plowing or working. If it does not stick in a ball, it is too dry. When examining soils, samples should be taken both at and a few inches below the surface. Soil that sticks to the plow or to other tools is usually too wet. A shiny, unbroken surface of the turned furrow is another indication of excessive soil moisture. In general, sandy soils and those containing high proportions of organic matter bear plowing and working at higher moisture contents than do heavy clay soils.
b. In semi-arid regions, the soil after harvest time is generally too dry for good plowing. Yet if the field is left uncultivated, this dry condition may become even worse and weeds will also grow and go to seed. The field should be harrowed (or plowed without moldboard) and crop residues left to form a stubble mulch to absorb/retain moisture and soil until the rains return. Stubble should not be immediately covered and incorporated in the soil unless rodent or insect infestation is heavy (and even then burning should be considered). It has been well demonstrated that it is normally impossible to raise the soil organic matter content in areas where temperatures are high for long 0periods. When moisture is present, the rates of oxidation are extremely high and incorporated organic matter is lost quickly. The benefits thus derived from decomposition, as occurs in the more temperate regions, are not normally experienced. When left on the surface, however, organic matter does not decay so rapidly. Incorporation with the soils will tend to depress the levels of available nitrogen, to the detriment of crops if soil nitrogen is low. If soil nitrogen levels are adequate, the incorporation of residues to the soil may be beneficial if done with spring plowing at the start of the rainy season.
3. Depth of Plowing
a. Variation with Soil Type. Generally speaking, heavy clay soils should be plowed deeper than light, sandy soils, in order to promote circulation of the air and bacterial activity. Deep plowing on sandy soils, which are naturally porous and open, tends to disconnect the seed bed from the subsoil and speeds soil drying by too free a circulation of air in the soil.
b. Depth Affects Moisture Reservoir. In semi-arid climates, the greatest advantage to be gained from deep plowing (5-8 inches) is the development of a comparatively large moisture reservoir. When land is not plowed more than 3 or 4 inches deep for a period of years, a hard plow sole is very likely to form, through which roots and rain can only penetrate with difficulty. A shallow plow sole will saturate quickly with rainwater and increase runoff rates. As a rule, tillage below 5-6 inches also causes increased evaporation rates, losing precious water. This deep plowing need not necessarily be done annually. Depending on soil and rainfall, a deep plowing of 5-6 inches every 2 to 5 years is satisfactory. As noted earlier, the soil mulch attains maximum effectiveness at a depth of 3-4 inches which can be maintained with a hand harrow/cultivator.
c. Exposure of Acidic Subsoil. Deep plowing in some clay and loam soils will reduce yields for one or two seasons afterward as a result of an acidic subsoil. This may be dealt with by liming the soil (neutralizing the acidity) or by varying the depth of the plowing slowly so that the acidic subsoil is exposed a little at a time. This practice will also eliminate the plow sole.
4. Seed Bed Preparation. In general, smaller seeds require a finer, mellower seed bed than larger seeds. Seeds germinate and plants grow more readily on a reasonably fine, well prepared soil than on a coarse, lumpy one, and thorough preparation reduces the work of planting and caring for the crops. It is possible to overdo the preparation of soils. They should be brought to a granular rather than a powder-fine condition for planting.
1. In rows: Planting of crops should be in rows to permit inter-tillage as described later.
2. Planting density: Limited moisture dictates the necessity for wider row spacing and lower rates of seeding (by one-half to two-thirds) than are used in moisture abundant areas. The resulting reduced plant population provides more moisture and nutrients per plant and thus enhances the possibility of the crop reaching maturity before the supplies are exhausted. Cereals should be planted 7 to 14 inches (18 to 35 cm) apart and crops such as millet, sorghum, sesame, safflower, etc. in rows 28 to 42 inches (70 to 105 cm) apart. In some cases, the practice of planting 2 or 4 rows and skipping one is successful in further increasing the efficiency of moisture utilization. In general, with limited rain, higher seed rates produce more straw/stubble at the expense of grain production. (See Table II, below)
G. INTERTILLAGE/INTERCULTIVATION. Crops sown in rows can take advantage of intertillage practices which serve three basic functions:
1. Easy weeding without meticulous hand labor. Weeds compete for moisture and nutrients, thus they should be destroyed while small, before they have grown more than 2 or 3 leaves. If seeds are broadcast, or thickly sown, they can at best only be cultivated manually, a back-breaking task.
2. Increase the formation of nitrates by bacteria. Intercultivation aerates the soil and forms a mulch of dead weeds and stubble on which bacteria operate and form nitrates. Cultivation for this purpose should be undertaken during the early period of plant growth, and should be relatively deep, on the order of 2-3 inches.
3. Intertillage conserves moisture by the formation of a dirt mulch as described earlier. It is imperative that cultivation be performed after rainfalls. Even a light rain can re-form capillary connections between the stored soil moisture and the surface of the ground. After a few drying days like that, it is possible for soil moisture to be lower than before the rainfall.
H. CROP ROTATION AND VARIETIES.
1. UNIQUE ASPECTS OF CROP ROTATION FOR DRY FARMING. One of the first principles of dry farming with regard to cropping practices is that crop rotation as practiced in more humid regions is not necessarily recommended in semiarid lands. The following constitute the chief differences:
a. Only a limited number of crops are adapted to the climatic
conditions and the farmer must sow the crop best suited to the moisture
conditions encountered at that time.
b. Moisture is so dominantly limiting, that "soil improving" crops are much less effective than in more humid areas.
c. Success with rigid or complex sequences is difficult in the face of widely varying rainfall.
2. REASONS FOR CROP ROTATION. There are five basic reasons why crop rotation should be practiced:
a. Moisture Conservation: Any system of crop rotation should be planned with moisture requirements as the main consideration. For a given set of climatic conditions, a crop may be described as either moisture dissipating or conserving. After harvest of a moisture conserving crop, the soil contains more moisture than at planting. This reserve of moisture can help guarantee the succeeding crop. (see paper on Determining the Water Needs of Plants). Crops which are sown in rows so that intertillage and dirt mulching can be practiced tend to be moisture conserving. Under sowing may also assist in conservation. Moisture may be insufficient to both grow a crop and conserve enough water to ensure the succeeding crop. In such a case it is necessary to utilize the dirt and stubble mulched fallow in the rotation. If annual rainfall is 10 to 15 inches (250 to 375 mm) this will be needed at least every other year; if rainfall is 15 to 20 inches (375-500mm) at least one in every three. In the West African sahel drought may be expected one year in four. Between 1968-1973 the rate was one year in two. In a situation like this, setting aside mulched fallow each year for moisture conservation will significantly aid survival. Where this has been faithfully practiced in similar areas in India, the specter of famine by drought has been virtually eliminated.
b. Pest Control. Where related crops are successively planted in the same place, viruses, molds, blights, and selective insect pests tend to build up in the soil. Crop rotation that leaves at least two years in between subject plants in the same location will eliminate the abnormal buildup of most such pests for most crops.
c. Erosion Control. Plants which are thickly planted or which produce a thick ground cover tend to resist erosion much better than those which are intertilled or tend to be moisture conserving. Loss of soil due to erosion is a significant dry farming problem and erosion controlling crops should be included in a rotation, preferably in a strip cropping mode.
d. Soil Nutrients and Structure. When related crops are successively planted, specific soil minerals and nutrients are withdrawn faster than they can be replaced by decay or subsoil movement. This selective depletion causes a soil to be "worn out" quickly. Simple rotation of crops makes depletion more uniform so that soils "wear out" more slowly. The planting of legumes (such as gram or groundnut or alfalfa) with their nitrogen fixing capabilities tends to restore soil fertility. The use of green manures (plowing under of a green crop, such as alfalfa, rather than harvesting) can also aid soil nutrients and texture but benefits may be short lived in the tropics and difficult for Third World farmers. The planting of any deep or thickly rooted plants (such as grasses, alfalfa, etc.) tends to improve soil structure and draw subsoil nutrients to the surface like a natural fallow and can increase pasturage during dry periods. Crops like cassava which require relatively little soil nutrients may also be grown for rotation or when soil is almost worn out.
e. Distribution of Labor and Risk. It is generally advisable for the subsistence farmer to grow all crops in the rotation scheme simultaneously, apportioning to each crop the fraction of fields that it requires. This helps the scheduling and distribution of labor at the bottlenecks (planting, harvesting, etc.) so that the entire crop need not be done simultaneously. There is also a reduced risk of total crop failure and increased variety/nutrition in the diet.
3. CROPS AND VARIETIES. Choice of varieties is important. Varieties which have proven excellent in irrigated or high rainfall areas are generally unsuited for dryland conditions. Many attempts at dryland farming have failed, largely due to lack of recognition of the requirements for the variety selection.
a. Variety Requirements For Dry Farming
1. Short-stemmed varieties with limited leaf surface minimize transpiration.
2. Deep, prolific root systems enhance moisture utilization.
3. Quick-maturing varieties are important in order that the crop may develop prior to the hottest and driest part of the year and mature before moisture supplies are completely exhausted.
b. Climatic Requirements of Crops in Brief
1. The TABLES below list favorable conditions for various annual crops.
HIGH TEMPERATURE TOLERANCE
Cotton, Groundnut, Chilies, (favor jute & yams only in humid tropics)
Common Millet, Barley, Chickpeas, Safflower (lower temperatures), Sorghum, Bullrush Millet, [Phaseolus] crops, [radiatus] (gram mung bean), Cassava, Castorbean, Sesame, Groundnut (Spanish variety), Pigeon peas, Sunflower
LOWER TEMPERATURES FAVOR
Wheat, Potato, Sugar, Tomato, Safflower
VERY HIGH RAINFALL
Rice, Cassava, Yam tolerance
WIDE CLIMATIC TOLERANCE
Size, Soybean, Groundnut (Valencia & Virginia type), [Phaseolus lunatis,] Kenaf, Hemp, Sweet Potato, Sugar Cane, Tobacco