| Soils, Crops and Fertilizer Use |
|Chapter 3: Basic soil conservation practices|
THE EFFECTS OF RAINFALL EROSION
On those sloping soils where soil conservation methods aren't used to combat rainfall erosion, farmland and yields will be adversely affected for several reasons:
• Soil depth decreases due to loss of topsoil.
• Soil fertility declines. Rainfall erosion carries away mainly the smallest soil particles - the nutrient-laden humus and clay particles that contain most of a soil's fertility. Studies have shown that eroded material found at the bottom of a slope contains 2-5 times more plant nutrients than what's left behind.
• Soil filth deteriorates due to the loss of topsoil and humus.
• Soil moisture decreases due to increased water runoff and less infiltration into the soil.
In addition, the surrounding environment is harmed:
• Floods increase due to more water runoff into rivers and streams. This often causes offfarm erosion as well.
• Canals, rivers, and dams can become silted up as eroded soil accumulates.
THE MECHANICS OF SOIL EROSION BY RAINFALL
It helps to know your enemy. Rainfall erosion is caused by the ''dynamic duo " of raindrop splash and surface flow on sloping soils. Raindrops act much like tiny hammer blows that do several nasty things, given their overwhelming numbers:
• They compact the soil, reducing water infiltration and increasing runoff.
• They break down clods into their component particles which are then more vulnerable to being carried away by moving water.
• They keep soil particles in suspension, which greatly aids their transport and also exerts a scouring action against the soil surface, detaching even more particles.
Raindrops hitting a level field aren't a problem, because soil is moved equally in all directions. However, when rainfall strikes sloping soils (and it takes surprisingly little slope, as we'll see), serious erosion can occur due to 2 types of surface flow:
• Laminar flow is a uniform flow of water most common on gentle slopes and causes sheet erosion.
• Channelized flow is more common on steeper slopes where water tends to collect and move downhill in channels. At first, this produces small furrows only a few centimeters deep called rills. If allowed to continue, however, gulleys 30-100 cm deep are the inevitable result.
The Magnitude of Rainfall Erosion Losses: An unprotected field with only a 4-5 percent slope (4-5 meters drop per 100 meters horizontal distance or about a 2° incline) can easily lose 100 metric tons per hectare of soil a year, which equals about 1 centimeter of depth.
FACTORS THAT INFLUENCE RAINFALL EROSION
Human interference, slope, rainfall, soil condition, and ground cover are the main factors affecting the amount of rainfall erosion that occurs.
Although erosion often occurs in the untouched natural environment, several types of human interference have led to accelerated rainfall erosion:
• Forest fires and grassland burning.
• Logging and firewood cutting.
• Farming on unprotected slopes: Even the practice of shifting cultivation (see Chapter 8), which can be ecologically sound under low population pressure, is now a prime cause of erosion in countries such as the Philippines where pressure on the land has forced an everdecreasing length in the rejuvenating vegetative fallow period.
• Overcultivation of farmland due to land pressure and excessive production of cash crops like peanuts and cotton. Such crops are likely to encourage erosion, because they're slow to produce protective ground cover and leave relatively little post-harvest residue for maintaining soil organic matter.
• Overgrazing by livestock on slopes is a main cause of the serious rainfall erosion damage in countries such as Lesotho.
Both the steepness of slope and its length are important:
• Doubling the steepness of slope increases erosion losses by 150 percent, due to increased water flow speed. The soil-carrying ability of water is proportional to the 6th power of its velocity. (For example, doubling the speed of flowing water increases its capacity to transport soil by 64-fold.)
• Doubling the length of slope increases soil losses by 50 percent by allowing water to build up more volume and speed.
Measuring slope: Two methods are described in the upcoming section on conservation practices.
Both the amount and intensity of rainfall are important, especially the latter. An annual rainfall of 250 mm concentrated over just 3 months can cause more erosion than 2000 mm spread out over 10-12 months. In fact, rainfall erosion is very common in semi-arid areas such as the African Sahel whose short rainy season consists largely of brief, high-intensity showers; in addition, such regions usually lack the protective vegetative cover of wetter areas.
The condition of the soil has a big effect on erosion losses:
• A healthy level of humus (at least 3-4 percent by weight) acts as a beneficial "glue" that binds together soil particles to produce a crumb-like structure. These miniature clods (called aggregates) are heavy enough to avoid being moved downslope by water and also resist being broken apart by raindrop splash.
• Soils in good filth (physical condition) allow water to penetrate much more readily, which lessens the amount of downslope runoff.
• Some very old and weathered tropical soils with a high proportion of hydrous oxide clays (see Chapter 1) have a natural crumb-like structure unrelated to humus content, which makes them less erosion-prone, much like soils with good organic matter levels.
Effect of vegetative cover on soil erosion losses on a 40% slope under 2000 mm annual rainfall in Puerto Rico.
Soil Loss per Year (metric tons per hectare)
Bare, exposed soil
Crop rotation of maize, sweet potatoes, etc.
Amount of Ground Cover
Any type of ground cover, such as mulch, crops, or natural vegetation, is very effective at reducing raindrop splash and surface flow velocity. Plant roots also help hold soil together. The value of plant cover depends a lot on the crop, its spacing, and the amount and rate of leaf growth. As shown by Table 3-1, nearly any type of vegetative cover is far superior to soil.
TECHNIQUES FOR COMBATTING RAINFALL EROSION
This section is designed to give you some basic entry-level skills in soil conservation and to lay the groundwork for further investigation and intelligent discussion with farmers and conservation specialists.
As with many other farming practices, soil conservation is a location-specific endeavor, meaning that methods must be adapted to the particular soil, climate, and farming practices. If you plan to promote soil conservation in your work area, you'll want to do several things first:
• Seek assistance from the local ag extension service and also the forestry service which usually is engaged in soil conservation too. Whenever possible, plug yourself into any ongoing programs rather than working on your own.
• Visit farms where successful conservation measures are in effect. Bring along farmers who are considering conservation.
• Confer with farmers likely to benefit from conservation practices to sound out their ideas, needs, and expectations.
• Read up on the subject; some helpful references are listed in Appendix H.
Motivating Farmers to Adopt Soil Conservation Practices
It's a fact - soil conservation also means moisture conservation: It's not always easy to convince farmers anywhere in the world to undertake soil conservation practices. Some methods, such as terracing and contour ditching, require much labor, and the immediate benefits may appear vague. Approaching soil conservation solely from the standpoint of saving soil isn't likely to motivate farmers enough. However, an important immediate benefit of any soil-saving method is increased moisture retention which usually results in higher yields. Unprotected slopes are usually droughty due to high water runoff and reduced infiltration. Such a yield boost may, in itself, provide enough motivation for farmers to adopt soil conservation practices; this has been the case in El Salvador for many small farmers who plant maize, sorghum, and beans on steep slopes.
A Summary of Some Common Rainfall Erosion Control Methods
Soil conservation is a specialty too broad to be fully covered here. Instead, we'll first briefly summarize the common techniques listed below and then cover a few in more detail (those marked with an asterisk). You'll also learn how to determine slope and lay out contour lines - two essential initial steps in most conservation methods.
Common Soil Conservation Methods
• Mulching * (discussed in Chapter 8)
• Contour plowing and planting
• Contour strip cropping
• Contour ditching and banking
• Rock walls
• Various types of terracing *
• Reversion to permanent pasture or forest
It's helpful to remember that any practice that reduces raindrop splash or surface flow will help combat rainfall erosion.
Mulching: Even a small amount of mulch cover can markedly cut erosion losses. One study on a moderate slope showed that soil losses were cut 75 percent by spreading straw over the soil surface at a rate equal to 850 kg/hectare (85 grams per sq. meter). In some cases, erosion scars on steep slopes can be stabilized by covering the area with brush, working from the bottom up, and anchoring it with stakes. Soil runoff from above gradually covers the brush, forming an area where plant cover can be established.
Contour Plowing and Planting: In this method, plowing is done following the contour of the slope (i.e. at right angles to the direction of the slope) instead of straight up and down it. The crop rows also run along the contour. When used as the sole conservation measure, it may prove adequate on slopes up to 8 percent and not more than 100 meters long, depending on soil, cropping, and rainfall factors. At best, this practice cuts erosion losses about 50 percent.
Contour Stripcropping: This method carries contour plowing and planting a step further, and some versions may be suited to slopes as high as 40-50 percent. The main features are:
• Strips of close-sown or otherwise protective crops (i.e. leucaena, grass, sisal, pineapples, small grains) are alternated between strips of row crops such as maize and beans, all following the contour.
• The strips' downslope widths vary with soil conditions but range from about 25-40 meters wide for 2-7 percent slopes to 4 meters wide for 40-50 percent slopes.
• The close-sown strips slow down water runoff speed (which builds up on the row-crop strips), increase water infiltration, and reduce runoff water volume.
Later in this chapter, we'll cover a contour strip-cropping technique called S.A.L.T. (Sloping Agricultural Land Technology) that's proven successful in the Philippines.
Contour Ditch and Barrier System: It can be reasonably effective on slopes as high as 45 percent (about 24 degrees). The system's main features are:
• It combines contour plowing/planting with the placement of small ditches that follow the contour and that are spaced at intervals down the elope.
• The soil excavated during ditch building is placed along the downhill side of the ditches to prevent water from overflowing them.
• A 30-50 cm wide live barrier of dense grass (elephant, guinea, etc.), pineapple, or sisal is planted along the upper bank of the ditches to hold the soil together and to trap any soil that might be carried downslope into the ditches by runoff water.
• Row crops can be planted on the contour between the ditches which are spaced anywhere from 4-20 meters apart, depending on conditions.
• If upright crops like maize, sorghum, and bush beans have soil thrown into the crop row (called "hilling up") for weed control and support, this will also act as a further barrier to downslope water movement.
• The ditches are designed to have a gentle 0.5% slope so that they can slowly conduct excess water off the field.
• The ditches lead into a grass- or rock-lined waterway which carries the water to the bottom of the slope without causing gulleying. (NOTE: A protected waterway is critical, or the field may suffer serious erosion, no matter how well made the rest of the system is).
• Where the subsoil is clayey and holds water well, catchment basins can be dug (either inside the ditches or at their ends) to collect and save water for using on the crops during dry spells.
The contour ditch and bank system has been used with success in Central America, the Caribbean, and other areas. We'll cover it in more detail later on in the chapter.
Rock Walls: On rocky, hilly land in El Salvador, many farmers have controlled erosion successfully using low, drystone (uncemented rock) walls built at intervals along the contour. Here are the basic features:
• The walls help check erosion by trapping downward-moving soil and slowing the speed of runoff water. (Remember that the soil-carrying ability of moving water is proportional to the 6th power of its velocity!)
• For stability, rock walls shouldn't exceed a height of 60 cm above the soil surface and should have a tapering profile (about 40 cm thick at the top to about 60 cm at the base). In addition, the base should begin about 30 cm below ground.
• Over time, rock walls and other barrier systems will often result in a natural terracing and reduction in slope as soil carried downslope by runoff piles up in front of the walls.
Terraces: There are several different types of terraces and all involve reshaping of the land to a greater extent than the other methods above. Below are 3 types of terraces and their features. All can be built with lots of hand labor, but the first 2 can also be made with animal-or tractor-drawn plows and scrapers (V-drags).
• Broad channel terraces are designed mainly to intercept and then divert excess water safely from the field and are usually most suited for wetter regions and slopes not exceeding 15-20 percent. As with the contour ditch and barrier system (see above), a grass or rock-lined waterway is essential for safely conveying excess water off the field without erosion.
• Ridge terraces are designed to intercept and retain excess water by spreading it out over wide areas of the field between the ridges. Unlike channel terraces, they're better suited to drier regions on soils that absorb water readily. Note that ridge terraces are best used on gentle slopes (not much over 3 percent), since the area over which the water is spread can be large without having to make an unreasonably high ridge.
• Bench (step) terraces can be used on slopes of 20-50 percent and convert the land into a series of "steps" separated by nearly vertical risers lined with rocks or vegetation for protection. Though labor requirements are very high, well constructed bench terraces give excellent erosion control and allow a wide variety of crops to be grown. We'll cover bench terraces in more detail later on in this chapter.
Reversion to Permanent Pasture or Forest: On steep, badly eroded land, this may be the sole feasible alternative, but will succeed only where overgrazing, unrestricted tree cutting, or burning can be prevented.
Some Useful Skills for Rainfall Erosion Control
Now that we've summarized the main control methods, we'll cover some of the useful skills and practices in more detail:
• How to measure soil slope with homemade devices.
• How to lay out contour lines using simple, homemade devices.
• How to lay out and construct a contour ditch and bank system.
• The basics of the S.A.L.T. (Sloping Agric. Land Technology) strip cropping method being used in the Philippines.
• How to lay out and build a contour ditch and bank system.
• Some guidelines for bench terrace construction.
Be sure to check the local ag extension and forestry service for further information, as well as the bibliography in Appendix H.
HOW TO MEASURE SOIL SLOPE
One of the first steps in undertaking soil conservation practices on a farm is to measure soil slope, since it's a main factor influencing erosion susceptibility as well as the type of conservation methods best suited.
Soil Slope Basics
Soil slope is almost always measured in percent rather than degrees, and you'll see why this is more convenient as we move along. Land with a 4 percent slope has a vertical drop of 4 meters for every 100 meters of horizontal distance. A slope of 100 percent is not a vertical cliff but equal to 45 degrees (1 meter vertical drop per 1 meter horizontal distance).
How to Use a String Level to Measure Soil Slope
Perhaps you can borrow an inclinometer (Abney level) from your extension or forestry service. However, farmers can easily measure soil slope accurately using a simple device called a string level ("nivel de pita" in Spanish) that's used by masons and carpenters worldwide (Figs. 3-5 and 3-6). It should be available at any hardware store for about $1-$3 (U.S. ).
Items Needed: String level
Length of string or twine 4-10 meters long; nylon twine works best.
Measuring tape or homemade ruler about 1-1.5 meters long with centimeter markings.
Two people (one will do).
STEP 1: Hang the string level at one end of a length of string. Ten meters (1000 cm) is a good length for moderate slopes and makes the math work a snap. As explained below, a 5 meter length is more practical for steeper slopes.
STEP 2: Choose a fairly uniform section of the slope. Hold that end of the string that has the level near it, and have your partner grasp the other end. Your partner should now proceed directly upslope (not laterally) until the string is taut.
STEP 3: Have your partner anchor her end of the string to the ground with her fingers; if alone, use a small stake.
STEP 4: Now raise your end of the string slowly upwards until the string level gives a level reading (the bubble will be centered in the middle right between the two lines). For accuracy, be sure to clear away any vegetation or clods along the string's path that may interfere with it.
STEP 5: Now measure the vertical distance between the end of the level string and the ground. This will give you the slope's vertical drop over 10 meters. (NOTE: Use a shorter length of string on steeper slopes or you'll have trouble raising your end high enough. Also, if the slope varies over the field, take and record readings at several representative locations.
STEP 6: Calculate the percentage slope using this simple formula:
% slope = Vertical drop in centimeters / String length (1000 cm in this case)
Examples: 1. Suppose you measure a vertical drop of of 60 cm on a 10 meter string. The slope would be 6 percent:
60 cm / 1000 cm = 6 / 100 = 6%
2. Suppose you use a 4 meter length of string on a steeper slope and measure a vertical drop of 140 cm. The slope would be 35 percent:
140 cm / 400 cm = 35 / 100 = 35%
HOW TO LAY OUT CONTOUR LINES
A Homemade Device for Laying out Contour Lines
Nearly all rainfall erosion control methods, including strip cropping, the ditch and bank system, and terracing, are done on the contour (i.e. across the slope). Laying out contour lines that mark the path for plowing, crop rows, and barriers is an important skill to master. You can show farmers how to do this easily and accurately using a device called an A-frame.
NOTE: A water level is another simple device for laying out contour lines and can also be used to measure the percentage of slope. Its main component is a water-filled length of clear plastic tubing about 5-15 meters long. It functions on the principle that the water level at each end of the tube denotes 2 points of equal elevation. However, since the water level costs more, this manual is describing the A-frame in detail. The local ag extension or forestry office may have water levels and can show you how to make and use them.
Making and Calibrating an A-frame
Items Needed: Two wood or bamboo poles about 2 meters long
One wood or bamboo pole about 100-120 cm long A hammer and nails
A small carpenter's level (about 20-40 cm long) or a homemade plumb-bob and 1 meter of string.
STEP 1: Build the A-frame as in the drawing above. Poles cut from brush or bamboo will do fine. Twine or nails can be used to secure the 3 pieces together. The crossarm is placed about 1/2-2/3 of the way up and should be mounted reasonably level. Make the feet somewhat pointed at the bottom.
STEP 2: You can use either a plumb-line or a carpenter's level on the A-frame for determining contour lines.
A plumb-line will be accurate enough when the wind is still but isn't reliable in any type of breeze.
a. Mounting the plumb-line: It should be hung from a nail (centered in the juncture) where the 2 legs meet. Hang a small weight like a fishing sinker or piece of metal scrap at the end as a plumb-bob.
b. Mounting a carpenter's level: In this case, be sure to use a very straight and smooth piece of wood for the crossarm. Tie the carpenter's level firmly to the crossarm with twine as shown above. NOTE: You can try using a small string level instead.
STEP 3: Now you can calibrate the A-frame to make it accurate as follows:
a. Calibrating a plumb-line A-frame
• To find the level mark on the crossarm, choose a fairly level spot of ground. Now pound 2 pointed stakes about 25-30 cm long halfway into the ground at a distance equal to the spread of the A-frame's legs.
• Stand the A-frame on the stakes and make a pencil mark where the plumb-line comes to rest. Now reverse the legs and make a second mark. The true level mark will be located exactly halfway between. Mark it with a pencil.
• To double-check your calibration, adjust the A-frame's position by gradually pounding one of the stakes further into the ground until the plumb-line comes to rest. on the true level mark. Now reverse the A-frame's legs again; if the plumb-line returns to rest on the level mark, the A- frame is properly calibrated.
b. Calibrating a carpenter's level A-frame
• In this case, you want the carpenter's level to read level (i.e. bubble centered between the 2 lines) when the A-frame's legs are sitting on 2 points that are perfectly horizontal to each other. The easiest approach is to make sure that the following measurements in Fig. 3-7 are equal to each other: AB = AC, DB = EC; also make sure that the piece of wood used for the crossarm is perfectly straight.
• To test the A-frame, set its legs on 2 stakes that have been hammered into the ground (as in a. above). If the bubble rests to the right of center, gradually pound the right stake into the ground until the bubble is centered between the lines. If the bubble rests to the left, do the same with the left stake.
• As a final check, rotate the A-frame 180 degrees so that the position of the legs is exactly reversed. The bubble should indicate level again. If not, either the distances DB and EC aren't equal or perhaps the carpenter's level is defective. (When placed on a perfectly horizontal surface, an accurate level will produce no change in the bubble's centered position when rotated 180 degrees.)
Laying Out Contour Lines with an A-frame
Contour lines run at right angles to the slope (across the slope). All points on the line are at the same elevation. Here's how to lay out a contour line with an A-frame:
Items needed: A-frame, stakes, hammer (or rock)
STEP 1: Pound a stake into the ground at the starting point for a contour line (i e. the start of a proposed contour ditch or a strip of a closely-sown crop that will run across the field).
STEP 2: Stand the A-frame in the soil with one leg directly at the base (not on top) of the first stake and the other leg pointing in the approximate direction the contour line will run.
STEP 3: Now keep the leg at the base of the first stake in place and use it as a pivot point to move the other leg up and down the slope until you get a level reading with the plumbline (or carpenter's level). Pound in a second stake where this leg comes to rest.
STEP 4: Now shift the A-frame one stake over so that the leg that rested at the base of the first stake now rests at the second stake. As in Step 3, move the other leg uphill or downhill until you get a level reading, and pound in another stake.
STEP 5: Keep repeating Step 4 until you reach the end of the field.
STEP 6:: Even out irregularities: Even on uniform slopes, small rises and depressions in the terrain may cause some stakes to be out of line. You can take out any abrupt deviations by slightly altering some of the stakes' positions as shown in Figure 3-9.
NOTE: Be sure to recalibrate the A-frame each time it's used after storage.
HOW TO LAY OUT A CONTOUR DITCH AND BARRIER SYSTEM
This system was summarized a few pages ago. Here's how to lay out and construct the ditches and barriers:
STEP 1: The protective waterway must be established first. In a heavy rain, the contour ditches will conduct large volumes of water off the field. This means they must empty into a protected waterway so that the excess water can reach the bottom of the slope without causing erosion. If possible, consult with a soil conservation specialist as to the best location and type of waterway. Some guidelines:
• Given the potential volume of runoff, waterways on either side of the field may be needed.
• Check over the area just beyond where ditches will end and look for natural depressions that could serve as a waterway path.
• Allow for sufficient waterway width (5-10 meters, depending on the area of the field).
• Erosion protection: The waterway area must be either lined with rocks or planted to a dense, low-growing grass such as bermuda grass, kikuyu (Pennisetum clandestinum), carpet grass (Axonopus compressus, bahia grass (Paspalum notatum), star grass (Cynodon plectostachyus), and molasses grass (Melinis minutiflora). The vegetative cover must be well established before the ditches are built and the rains come. This takes at least several months and may not be feasible during the dry season. Establishing the vegetation during the previous wet season may be the best solution. The same is true for the vegetative barriers needed along the uphill bank of the ditches (Step 7).
• Before planting the cover, check the soil pH, and apply lime, if needed. Apply fertilizer to encourage rapid growth. (Refer to the section on pastures in Chapter 10.)
• Keep livestock out of the waterway, and don't use it as a roadway.
STEP 2: Measure the field's slope. The spacing of the ditches depends on the field's slope (the steeper it is, the closer they need to be spaced). First measure the slope and then refer to the table in Appendix C to find the correct horizontal interval.
STEP 3: Determine the ditches' locations. Begin at the top of the field. Suppose the field's slope is 25 percent. The table gives a horizontal interval of about 7 meters. This means the first ditch should be 7 meters directly downslope from the top. The succeeding ditches will also be located at 7 meter intervals. Proceed downslope and place a stake at each of these intervals. These are master stakes which serve as guides for laying out the contour lines along which the ditches will run (see Fig. 3-10). The stakes can be placed down one side of the field or down the middle. If the slope noticeably changes as move downhill, you'll need to measure these variations and adjust the ditch spacings accordingly (use the table in Appendix C).
STEP 4: Use the A-frame to mark out the ditch paths with stakes. You'll need lots of stakes since they'll be placed at intervals equal to the A-frame's leg span. Begin at the master stake for either the top bottom ditch and lay out the contour path for each ditch following the guidelines given in the last section.
NOTE: Need for a ditch grade: In order to prevent excess water from overflowing the ditches, they should be laid out to have a 0.5% slope (i.e. a 50 cm drop every 100 meters of length) so that excess water can be safely conducted off the field to a grass - or rocklined waterway where it can flow to the bottom without causing erosion. Don't exceed a 0.5% slope or the water's speed may erode the ditches. You'll learn how to "build in" a 0.5% slope into an A-frame shortly.
NOTE: Don't worry if the slope changes as you move across the field laying out the lines. You'll see that the lines are "self-correcting" in that they'll move apart as the slope decreases and move closer as it increases. That's because, being contour lines, the vertical interval between them won't change even when the slope changes (although the horizontal distance will)
STEP 5: Even out any irregularities in the stake lines as shown in Figure 3-9.
STEP 6: Dig the ditches. If available, have an animal- or tractor-drawn plow loosen up and dig out soil right along the line of stakes (see Fig. 3-11). (It's OK for the plow to knock out the stakes, since the furrow it makes will indicate the ditch path.) If using a moldboard (turning plow), run it so the soil is thrown to the downhill side of the ditch, as that's where the soil barrier will be built.
Continue digging out the ditch with shovels, piling the excavated soil along the downhill bank. Begin by making the ditch about 30 cm wide and 30 cm deep as in Fig. 3-12.
To help prevent the ditches from caving in, taper the sides like so:
STEP 7: Make a vegetative or rock barrier on the uphill bank of the ditches. This will prevent soil from being carried into the ditches and should be about 30-40 cm wide. Some possible plants: elephant grass, jaragua grass, kikuyu grass, pineapple, sisal. Check locally to find out what's best suited. Some grasses like kikuyu spread rapidly but may begin overrunning the field and become weeds.
STEP 8: Make dikes in the ditches if appropriate. In some cases, partial dikes can be built in the ditches every 5 meters or so to slow down water flow and encourage infiltration. This will depend on the soil and rainfall conditions.
Using the system: Once installed, the ditches also serve as the master contour lines for the plow and the crop rows to follow. Where subsoils are clayey and hold water well, catchment basins can be dug at intervals along the ditches or at their ends for collecting water for use during dry spells.
How to Build a 0.5% Slope into an A-frame
This is necessary in order to lay out the ditches with a gentle slope so excess water is encouraged to move off the field rather than overflowing the ditches. Here's how to build in a 0.5% slope into an A-frame:
For A-frame with Plumb-line
STEP 1: Calibrate the A-frame and place it on a level surface so that the plumb-line indicates level.
STEP 2: Measure the leg span in cm and multiply this by 0.5%.
Example: 140 cm leg span X 0.5% = 0.7 cm or 7 mm
STEP 3: Now raise one leg up a distance equal to the figure derived in Step 2, and make a mark where the plumb-line comes to rest. Now draw a short arrow from the mark so that it points toward the direction of the unraised leg.
STEP 4: Place the A-frame back on level ground, and repeat Step 3 using the other leg.
Using the adjusted A-frame: Use it just as you would to lay out normal level contour lines. However, in this case, don't use the level mark but one of the 0.5 percent marks you made in Steps 3 and 4. The arrow next to the mark you use indicates the downhill direction the ditch will run. When laying out a ditch line across a field, always use the same mark from start to finish, and don't rotate the A-frame 180 degrees. Otherwise, the ditch may change direction from uphill to downhill. However, if you're using 2 waterways on either side of the field, half the ditches (or half of each ditch) should lead to one side and the rest to the other side.
For A-frame with Carpenter's Level
In this case, there are 2 ways of building in a 0.5% slope:
METHOD 1: Make one of the A-frame's legs shorter than the other by an amount equal to 0.5% of the leg span. Draw an arrow on the cross arm pointing toward the longer leg. The arrow indicates the downhill direction the ditch will run when the ditch line is laid out with the bubble centered on carpenter's level. When laying out the ditch line, don't rotate the A-frame 180 degrees or the ditch will change its gradual slope from downhill to uphill.
METHOD 2: Calibrate the A-frame and place it on a level surface so the bubble is centered. Now raise up one leg of the A-frame an amount equal to 0.5 percent of the leg span. Adjust the carpenter's level so that it reads level even though the A-frame isn't. Now draw an arrow on the crossarm pointing toward the unraised leg. When laying out ditches with a 0.5 percent slope, the arrow indicates the downhill direction of the ditch when the bubble is centered. Don't rotate the A-frame as you proceed across the field or the ditch's direction will reverse.
THE S.A.L.T. SYSTEM FOR CONTROLLING RAINFALL EROSION
The S.A.L.T. system (Sloping Agricultural Land Technology) was briefly described earlier in this chapter under strip-cropping methods. It was developed by the Mindanao Baptist Rural Life Center in Davao del Sur, Philippines in 1979 and has become increasingly popular in that country since then. Although it can be effective on very steep slopes, it's best suited to regions with a long rainy season which allows the growing of soil-stabilizing perennial crops like cacso, banana, and pineapple without irrigation. It incorporates the use of a leguminous tree/shrub called leucaena or ipil-ipil (Leucaena leucocephala), although other perennial legumes could be subsituted. Here are the main features and practices of S.A.L.T:
• Contour lines are laid out across the slope at intervals of 4-6 meters, depending on steepness and soil conditions.
• A one meter-wide strip along each contour line is plowed up following the marker stakes. Two planting rows 50 cm apart are prepared, and leucaena seeds are sown 2-5 cm apart. Though slow-growing at first, the leucaena will form a thick, erosion-resistant hedgerow in 4-6 months and may reach 5-6 meters in a year. Aside from erosion protection, its leaves are used for mulching and fertilizer (being a legume, they're high in N), and the plants provide firewood, poles, and animal feed (leaves and pods).
• While the leucaena is becoming established, the land is best left in its natural vegetation instead of being plowed. However, tree crops like citrus and banana can be planted and weeded by working only the immediate area around the plant, thus not inviting erosion.
• Once the leucaena is established, other strips between these perennial crops can be plowed and planted to more erosion susceptible annuals like maize and vegetables.
• The leucaena is cut once a month down to 1 meter height and the leaves are placed around the base of the crops for fertilizer and mulch.
• Natural terrace formation: Barriers are formed by piling rocks, stalks, leaves, and branches at the base of the leucaena rows. Over the years, any soil carried downslope will collect at the barriers, and a natural terracing will take place.
NOTE: Large blocks of leucaena aren't recommended for steep, unstable slopes, unless they pruned regularly to allow sunlight to reach the soil so that undergrowth will develop.
Some Limitations of leucaena
Like most "wonder crops", leucaena has its strengths and weaknesses. On the one hand, it's fast-growing (although not initially), has a deep taproot, fixes nitrogen, and regrows readily after cutting. It can be used for erosion control, reforestation, fuel, poles, forage, mulch, and fertilizer. However, it does poorly on very acid soils (below pH 5.0), is frost-sensitive, and may not grow well above 500 meters elevation, even in the tropics. Its slow initial growth makes it prone to weeds and insects during the early weeks. Leucaena leaves contain excessive mimosine (an amino acid) which can be toxic to non-ruminants like pigs, chickens, and horses if fed at anything but low levels. (However, some low-mimosine strains of leucaena have been identified.)
In 1985, the Philippines, Tonga, and Fiji reported serious outbreaks of psyllid insects (jumping plant lice of the genus Heteropsylla) in leucaena plantings. These small sucking insects have several natural predators in Mexico and the Caribbean where leucaena is native. It may be possible to introduce these predators to the affected areas.
Where to find out more about S.A.L.T.: Mindanao Baptist Rural Life Center, Kinuskusan, Bansalan, Davao del Sur, PHILIPPINES.
SOME GUIDELINES FOR BENCH (STEP) TERRACES
Bench terraces were briefly described earlier in this chapter and can be used on land with slopes as great as 50 percent (and higher, in some cases). Below 15-20 percent slope, channel or ridge terraces are better suited than bench terraces.
CAUTION: Before trying to put steep slopes into crop production, make sure that this can be justified environmentally.
Natural Bench Terrace Formation
Excavated bench terraces require high amounts of hand labor which may not be feasible. It's often more practical to encourage their natural development instead, as is done with the S.A.L.T. method above or with rock walls. Not only is labor saved, but land can be used for cropping (at least for perennials) during this period. Here are some guidelines for natural terrace formation:
• Natural bench terrace development is best adapted to slopes of about 20-50 percent.
• Rock or vegetative barriers about 1 meter wide are planted on the contour at vertical intervals of about 1.8-2.0 meters. (The horizontal interval can be found by multiplying this by 100/X slope; i.e. 2 m X 100%/40% = 5 meters). Species like elephant grass, jaragua grass, molasses grass, tropical kodzu, and ipil-ipil have worked well in some areas.
• Once the barriers are established, the land can be plowed and cropped. When plowing or hoeing to prepare the ground, the soil should be moved towards the barriers to encourage terrace formation. Cropping and plowing are done on the contour, however.
• Water can usually be allowed to move down the slope through the barriers if they're dense enough.
Some Guidelines for Excavating Bench Terraces
Note these features of the bench terrace in Fig. 3-15:
• It is built by digging soil out from the eventual "heel" (D) and using it to fill out the eventual "toe" area (A). You can see that the bench's surface soil will end up being mainly inferior subsoil (with the topsoil buried below) unless a special technique is used which we'll cover below.
• The riser is not vertical but has a backslope of 1/2:1 (i.e. 5 units horizontal distance for every 10 units vertical distance). This is done to make it less likely to cave in.
• The bench has a gentle backslope of about 8% to prevent water from overflowing the "toe" and to promote infiltration.
In addition, here are some other important factors in bench terrace construction:
• As with the contour ditch and barrier system, the terraces should be laid out with a gentle lateral slope (about 0.25%) so that excess water can be moved off the field in a small ditch made along the "heel" area to a protected waterway
• To avoid excessive erosion of the risers, they should be protected with rocks or with dense vegetation like grass or tropical kudzu.
• CAUTION: Don't attempt excavation of bench terraces during the rainy season. A partially completed terrace system can be very vulnerable to rainfall erosion or mudslides.
• Whenever possible, consult a conservation specialist before beginning a terracing project.
How to Keep the Topsoil on Top When Building Bench Terraces
Refer to Figure 3-15 above and look at the original ground line. You can avoid burying much of the topsoil with subsoil by first removing the topsoil layer and piling it in what will be the middle of the bench (point B). When cutting and filling to form the bench, leave the topsoil pile undisturbed, and transfer subsoil from area C to area A to form the bench. Now you can spread the topsoil out over the bench.