Cover Image
close this book Soils, Crops and Fertilizer Use
View the document About this manual
View the document Acknowledgements
close this folder Chapter 1: Down to earth - Some Important Soil Basics
View the document What is soil, anyway?
View the document Why do soils vary so much?
View the document Topsoil vs. subsoil
View the document The mineral side of soil: sand, silt, and clay
View the document Distinguishing "tropical" soils from "temperate" soils
View the document Organic matter - a soil's best friend
View the document The role of soil microorganisms
close this folder Chapter 2: Trouble-shooting soil physical problems
View the document Getting to know the soils in your area
View the document Soil color
View the document Soil texture
View the document Soil tilth
View the document Soil water-holding capacity
View the document Soil drainage
View the document Soil depth
View the document Soil slope
close this folder Chapter 3: Basic soil conservation practices
View the document Rainfall erosion
View the document Wind erosion
close this folder Chapter 4: Seedbed preparation
View the document The what and why of tillage
View the document Common tillage equipment
View the document The abuses of tillage and how to avoid them
View the document Making the right seedbed for the crop, soil, and climate
View the document How deep should land be tilled?
View the document How fine a seedbed?
View the document Some handy seedbed skills for intensive vegetable production
close this folder Chapter 5: Watering vegetables: When? How Often? How Much?
View the document It pays to use water wisely
View the document Some common watering mistakes and their effects
View the document Factors influencing plant water needs
View the document Ok, so get to the point! how much water do plants need and how often?
View the document Some methods for improving water use efficiency
close this folder Chapter 6: Soil fertility and plant nutrition simplified
View the document Let's Make a Deal
View the document How plants grow
View the document Available vs. unavailable forms of mineral nutrients
View the document Soil negative charge and nutrient holding ability
View the document Soil pH and how it affects crops growth
View the document Important facts on the plant nutrients
close this folder Chapter 7: Evaluating a soil's fertility
View the document Soil testing
View the document Plant tissue testing
View the document Fertilizer trials
View the document Using visual "hunger signs"
close this folder Chapter 8: Using organic fertilizers and soil conditioners
View the document What are organic fertilizers?
View the document Organic vs. chemical fertilizers: which are best?
View the document Some examples of successful farming using organic fertilizers
View the document How to use organic fertilizers and soil conditioners
close this folder Chapter 9: Using chemical fertilizers
View the document What are chemical fertilizers?
View the document Are chemical fertilizers appropriate for limited-resource farmers?
View the document An introduction to chemical fertilizers
View the document Common chemical fertilizers and their characteristics
View the document The effect of fertilizers on soil pH
View the document Fertilizer salt index and "burn" potential
View the document Basic application principles for N, P, and K
View the document Fertilizer application methods explained and compared
View the document Troubleshooting faulty fertilizer practices
View the document Getting the most out of fertilizer use: crop management as an integrated system
View the document Understanding fertilizer math
close this folder Chapter 10: Fertilizer guidelines for specific crops
View the document Cereals
View the document Pulses (grain legumes)
View the document Root crops
View the document Vegetables
View the document Tropical fruit crops
View the document Tropical pastures
close this folder Chapter 11: Liming soils
View the document The purpose of liming
View the document When is liming needed?
View the document How to measure soil pH
View the document How to calculate the actual amount of lime needed
View the document How and when to lime
View the document Don't overlime!
close this folder Chapter 12: Salinity and alkalinity problems
View the document How salinity and alkalinity harm crop growth
View the document Lab diagnosis of salinity and alkalinity
close this folder Appendixes
View the document Appendix A: Useful measurements and conversions
View the document Appendix B: How to determine soil moisture content
View the document Appendix C: Spacing guide for contour ditches and other erosion barriers*
View the document Appendix D: Composition of common chemical fertilizers
View the document Appendix E: Hunger signs in common crops
View the document Appendix F: Legumes for green manuring and cover-cropping in tropical and subtropical regions
View the document Appendix G: Some sources of technical support
View the document Appendix H: A bibliography of useful references

Soil drainage

Drainage refers to the soil's ability to get rid of excess water (water in the macropores) through downward movement by gravity. It is affected by topography, texture, filth, depth, and the presence of pans (compacted or cemented zones). Nearly all major crops need fairly good drainage so that their roots can obtain enough oxygen; some exceptions are rice and most varieties of taro (Colocasia esculenta).

Poorly drained soils adversely affect crop yields in several ways:

• Roots lack adequate oxygen, since the macropores are largely filled with water.

• Soil-borne fungal and bacterial diseases are encouraged.

• Nitrate nitrogen (a nutrient) is subject to loss by a process called denitrification (see Chapter 6).

• Manganese and iron may become soluble enough to injure plant roots.

Although clayey soils are more likely to have drainage problems, they also may occur on sandy soils in cases where the water table is close to the soil surface. (The water table is the upper surface of the ground water, below which the soil is completely saturated with water.)

How to Spot Drainage Problems

You and farmers can easily spot areas of poor drainage in a field. Here's what to look for:

Topography: Poor drainage is most likely to occur on level fields or in low spots where water tends to collect after a rain or irrigation. Soils with even a gentle slope seldom have drainage problems but are likely to have the opposite problem of excessive water runoff.

Presence of Hardpans or Claypans: A hardpan is a hardened, cemented layer a few centimeters thick, usually located in the lower topsoil or upper subsoil. It remains hard even when wet, and restricts drainage and root growth. A claypan is a thicker, dense clayey layer in the subsoil which will soften somewhat when wet. It still impedes drainage and root growth. Dig a pit to check for such pans.

Crop Appearance: Crops growing in poorly drained areas will be stunted and yellow compared to surrounding portions. Beware, though, that other conditions such as nitrogen deficiency and disease can produce these symptoms. Suspect poor drainage only when stunting and yellowing are associated with low spots or areas of standing water.

Standing Water: Any area where water ponds for a day or two after rainfall or irrigation is likely to be poorly drained.

Subsoil Color: Red, reddish brown, or Yellow subsoil colors indicate very good drainage. That's because the presence of sufficient air allows the soil's iron and manganese to remain in the oxidized form, indicated by bright colors. Dull greys and blues indicate a reduced state (little oxygen) which means poor drainage. Some soils in wet-dry climates have subsoils with alternate streaks of bright and dull colors. This color pattern is called mottling and indicates fluctuations in soil drainage (i.e. good in the dry season, poor in the wet season) caused by the seasonal variation in the height of the water table.

How to Test Soil for Poor Drainage

The hole test: Dig a hole 60-90 cm deep and fill it with water; allow it to drain, and refill it again. In a well drained soil, the water level should fall by 2-3 cm an hour and disappear in 24 hours. However, if poor drainage is being caused by a hardpan or claypan, this test won't be valid, as you will have overcome the problem by digging through them.

Checking for a high water table: Poor drainage in low spots is often caused by a high water table. Ideally, the water table should be at least 100 cm below the soil surface, at least during the cropping season. When digging, you can easily tell when the water table has been reached, since water will begin to pond in the hole.

NOTE: In some cases, a high water table can actually benefit crop growth by supplying water to the roots during long dry spells by upward capillary movement (as long as it's not high enough to affect drainage in the major root zone area). However, there's always a risk of poor drainage in wet years on such land.

Dealing with Drainage Problems

First, determine what is causing the drainage problem before deciding which of the methods below will be effective.

• Seedbed styles for poorly drained areas: Growing crops on raised beds or ridges can alleviate drainage problems that aren't serious. See Chapter 4.

• Breaking up pans: This can be done with digging hoes and picks or with tractor-drawn sub-soilers (narrow shanks that penetrate 40-50 cm deep). Hardpans can sometimes be permanently fractured and loosened. Claypans, however, tend to reconsolidate after being loosened, especially since they're usually moist and don't tend to fracture. On small plots, the best way to permanently loosen a claypan is by double-digging the soil and adding an organic soil conditioner to the pan area. (Double-digging is covered in Chapter 4.)

• Drainage ditches for surface water: These are shallow, wide ditches that follow the natural depressions in the field to conduct water away. Make sure the outlet is satisfactory, so one farmer's drainage problems won't be passed on to another.

• Drainage ditches for subsurface water: Shallow ditches remove only surface water. To remove excess subsurface water, deeper and more numerous ditches can be used. They will "pull" (attract) this excess water from the soil between them. These ditches are usually spaced about 15-45 meters apart, depending on the soil (the closer spacings for clayey soils) and are dug 30-60 cm deep. Top widths range from about 2-5 meters and bottom widths about 1.5-2 meters. Ditches with V-shaped sides permit the passage of farm machinery. Of course, the ditches must be designed to convey the water off the field and eventually into a natural drainage way such as a stream.

• Drainage tile or Plastic pipe: Drain pipe can be rayed 80-100 cm underground to drain saturated subsoils and conduct the water off the field. Short sections (30-40 cm long) of 10-12 cm diameter clay pipe can be laid end to end in a trench and covered with straw, building paper, or earth to facilitate water entry and retard plugging. Flexible, perforated plastic pipe may also be available for this purpose in your country. The pipe or tile are laid at a slight slope (about 25-50 cm per 100 meters of length) and lead to an outlet such as a ditch or canal. The distance between the tile or pipe lines varies from about 10-20 meters on clayey soils to 30-90 meters on sandy soils. If the land has a natural drainage way, running such an underground drainage line along this path can speed up the removal of water from these areas of accumulation.

• Land leveling will fill in depressions and lower high spots, although the high spots may end up losing lots of topsoil. Animal-drawn scrapers can be fabricated locally.

Seedboxes have special drainage problems: See Chapter 4.