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close this book Forestry training manual for the Africa region
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View the document Session 1 : Welcome, expectations, and evaluation criteria
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View the document Session 3 : The forests of the world, peace corps' forestry goals, the individual volunteer's role
View the document Session 4 : Record keeping - group process
View the document Session 5 : Video tapes
View the document Session 6 : Agro-forestry data collection
View the document Session 7 : Feedback
View the document Session 8 : Flowers, seeds, the beginning
View the document Session 9 : Nutrition
View the document Session 10 : Non-verbal communication
View the document Session 11 : Germination
View the document Session 12 : Coping skills
View the document Session 13 : Basic site selection, planning & layout of a nursery
View the document Session 14 : Review of trainees' nursery plan
View the document Session 15 communication through illustration
View the document Session 16 : Soil preparation, seedbed sowing
View the document Session 17 : Individual interviews
View the document Session 18 : Reproduction by clippings and nursery review
View the document Session 19 : Introduction to extension
View the document Session 20 : Protection and record keeping (Insect collection)
View the document Session 20A : Chicken preparation
View the document Session 21 : The volunteers' role as an extensionist
View the document Session 22 : Tropical horticulture: care, tending and disease control
View the document Session 23 : Women in development - part I
View the document Session 24 : Team building
View the document Session 25 : Building and using a rustic transit
View the document Session 26 : Women in development - part II
View the document Session 27 : Working with groups as an extension worker
View the document Session 28 : Trees: identification & planting
View the document Session 29 : Lesson plan and use of visual aids in teaching
View the document Session 30 : The ugly American
View the document Session 31 : Catchments - sowing of seedlings into catchments
View the document Session 32 : Weekly interview
View the document Session 33 : Agro-forestry
View the document Session 34 : Community analysis introduction
View the document Session 35 : Soils
View the document Session 36 : Community analysis
View the document Session 37 : Irrigation
View the document Session 38 : Review of expectations - mid-way
View the document Session 39 : Problem analysis
View the document Session 40 : Soil erosion
View the document Session 41 : Species report - research demonstration
View the document Session 42 : Cultural values
View the document Session 43 : Wellbeing
View the document Session 44 : Field trip overview
View the document Session 45 : Agro-forestry reports
View the document Session 46 : Weekly interview
View the document Session 47 : Leave on week-long field trip
View the document Session 48 : Pesticides
View the document Session 49 : Review of field trips
View the document Session 50 : Resources
View the document Session 51 : Area measurement, pacing, compass use
View the document Session 52 : Compost heap - greenhouse construction - germination percentage
View the document Session 53 : Culture shock
View the document Session 54 : Range management
View the document Session 55 : Grafting and fruit trees
View the document Session 56 : Professional approaches to interaction with host country officials
View the document Session 57 : Project planning: goal setting
View the document Session 58 : Final interviews
View the document Session 59 : Ecology teams presentations
View the document Session 60 : Graduation

Session 35 : Soils

Total time 2 hours


- To introduce varieties of soils found in Africa,

- To explain soil fertility,

- To discuss fertilization of soils,

- To explain the steps for taking soil samples,

- To explore the techniques to be used in soil conservation extension.


The technical trainer introduces the subject of soil in the host country(ies). He/she talks about different types, fertility, and fertilization as a means of improving soil quality. The trainer explains the steps for taking soil samples and discusses the techniques to be used in soil conservation extension work.

Trainer’s Note: It may be possible to get a local soil expert to give a presentation during this session.


1. Soils


Flip charts, magic marker, tape, movies, soil testing kit.

Exercise 1 Soil Lecture

Total time 2 hours


The technical trainer introduces the soil section of training and discusses the varieties of soil found in host country(ies), the fertility of the soils, and the use of fertilizers on the soil. He/she explains the steps for taking soil samples and gives examples of techniques to be used in soil conservation extension work.



1. The technical trainer lectures on soils. This lecture must be country specific, and, if not, the trainees must know how to find specifics on the host country.


2 hours


2. Slide chow.

3. Field trips - visit to poor and good soil management and the affects on crops.

Soils: Principles and Definitions

Prepared by: Oliver A. Chadwick

Dept. of Soils, Water and Engineering

University of Arizona

During plant and bacterial growth phophorus, sulfur, calcium, iron and other minerals are removed from the soil and nitrogen fixation takes place. Nutrient cycles consist of the incorporation of chemical elements during the process of growth followed by their release during respiration, excretion, and decay. These processes are carried out by plants, the herbivorous animals that eat them, the carnivores, and lastly countless microorganisms which return a portion of the minerals to the soil for reuse (Morowitz, 1983). Plant nutrients come from either the atmosphere or the lithosphere and their release in usable form is critically important for man's agricultural efforts. Where minerals are missing from the geologic substrate or atmospheric inputs are insufficient, minerals must be added. In other cases, physical characteristics may restrict air or water movement thus inhibiting root growth. Man has developed remedies for many of these agricultural problems.

This discussion introduces the complex interactions between biological, chemical and physical processes in soil and their relevance to agriculture. Specific management procedures are not recommended because it takes a detailed series of laboratory and field trials before relevant crop-soil management procedures can be developed. These recommendations must be developed by country or regional agricultural specialists. It will be your responsibility as volunteers to find these experts and interpret their recommendations. Therefore, I will introduce you to some of the important definitions and principles of soil science.


Cation Exchange Capacity (CEC) - Nearly all soils have a net negative charge an are able to retain positively charged ions on clay and organic matter surfaces. These ions can also be exchanged with ions in the soil solution and thus are available for plant uptake. Most plant nutrients are cations. CEC is an important soil parameter because it indicates how many nutrients can be retained by the soil and readily lost by leaching. Clay content and mineralogy is the primary determinant of the CEC level. In general, the greater the clay content the greater the CEC, but it varies from 10 meq/100g to nearly 180 meq/100g for kaolinite and montmorillonite respectively. These variations in mineralogy are primarily due to surface area characteristics, with montmorillonite having a much higher surface area than kaolinite. Two main factors control which cations will be preferentially held on the negatively charged surface: one is the valence charge on the cation and the other is mass action. Mass action simply means that if there are enough certain cations they will be absorbed on the exchange complex regardless of their valence compared to other cations. Based on valence, aluminum (Al3+) is held preferentially to calcium (Ca2+) and magnesium (Mg2+), while sodium (Na+) and potassium (K+) are held least strongly. Many factors influence cation availability for mass action exchange. The amount of water leaching through the soil is a primary factor. High leaching conditions favor aluminum over the monovalent ions because the latter are held less strongly. Cations are available for plant uptake when they are in the soil solution (i.e. dissolved in water held loosely in soil pores). When a plant takes up a cation from the soil solution, there are less of that species to hold their own in the mass action process and therefore more of the cation will be released to the soil solution and to plants.

Base saturation - The primary plant nutrient cations are ammonium (NH4+), calcium, potassium, and to a lesser extent magnesium and sodium. They are called bases and their total percentage on the cation exchange complex is base saturation. This is an important measure of soil fertility status because these bases can be released to soil solutions for plant uptake as described above.

Exchange acidity - In addition to the bases mentioned above, the exchange complex contains hydrogen (H+) and aluminum, primarily the latter. These elements are not plant nutrients and aluminum can be toxic in high concentrations. Their total percentage on the exchange is termed exchange acidity. It is an important measure because it indicates potential aluminum toxicity and indirectly the nutrient status of the soil.

pH - pH is the negative logarithm of hydrogen ion activity in the soil solution. This means that a pH of 8 indicates less hydrogen activity than one of 6. The primary importance of soil pH measurements is to indicate which of several chemical reactions are dominant. The chemical reactions in turn control plant nutrient availability. At pH values below 5.5, the soil is primarily controlled by chemical reactions associated with aluminum. The exchange is dominated by aluminum and most of the nutrient bases have been leached from the soil. Phosphate, calcium, and molybdenum are held in insoluble compounds limiting their availability. Aluminum, zinc and manganese are soluble and may cause plant toxicities. At pH 5.5 - 7.5, most of the necessary nutrient elements are available for plant uptake. Because the exchange complex and soil solution contain moderate amounts of many different nutrients, there are few toxicities and deficiencies. At pH values from 7.5 - 8.5 calcium is the dominant cation on the exchange complex. High calcium levels cause insoluble precipitates with phosphate. Iron, maganese and zinc may also be deficient at these pH levels. At pH values above 8.5, the exchange complex is dominated by sodium. Nutrient deficiencies occur because of insoluble compound formation but the primary concern is a physical effect on soil clays. The clays are dispersed into very fine particles which plug pores and lower soil permeability causing poor soil-water relationships.

It is the aim of soil management to attempt to keep soil pH within the range of 6 - 7.5. The pH is usually lowered using gypsum (CaSO4.2H2O) or sulfuric acid (H2SO4). It is usually raised using lime (CaCO3). Information on specific amounts and methods of application are available from local agricultural research centers.


Nitrogen - In moot cases nitrogen is the limiting nutrient for plant growth. This is partly because it is required to a greater extent than any other soil supplied element and partly because there are limited inputs of nitrogen to soil. Most nitrogen in soil is in organic form which may be released as ammonium by microbial respiration. Under favorable conditions ammonium may be oxidized to nitrate (NO3-). Most plants can utilize both ammonium and nitrate.

The amount of nitrogen in the form of soluble ammonium and nitrate is seldom more than 1-2 percent of the total nitrogen present, except where large applications of inorganic fertilizers have been made. This is fortunate because inorganic nitrogen is subject to lose from soils by leaching and volatilization. Only enough is needed to supply the daily requirements of growing plants.

In the course of a year nitrogen undergoes many complex transformations, some of which may be partly controlled by man while others are beyond his command. This succession of largely biochemical reactions is known as the nitrogen cycle.

External inputs of nitrogen are from commercial fertilizers, crop residues, green and farm manures, and ammonium and nitrate salts brought down by precipitation. There is also fixation of atmospheric nitrogen accomplished by certain microorganisms. Nitrogen depletion is primarily due to crop removal, leaching, erosion and denitrification (i.e. loss in gaseous form).

The process of incorporating inorganic nitrogen into organic form is called immobilization while its release is called mineralization. Immobilization can be accomplished by plants or microorganisms. Mineralization occurs when microorganisms break down nitrogen rich organic compounds releasing ammonium.

One of the most important factors in practical soil management is the maintenance of a proper balance between the soil's organic matter content and nitrogen immobilization and mineralization processes. Ample amounts of organic matter are needed in soils to maintain favorable physical properties such as aeration and permeability. If, however, organic matter is added which has very little nitrogen (e.g. straw or sawdust), microorganisms will immobilize much of the available inorganic nitrogen as they break down the carbon rich organic additions. If, on the other hand, succulent green plant materials (green manure) are incorporated into the soil, microorganisms will release excess nitrogen as they break down the nitrogen rich plant material. Addition of low nitrogen organic materials to improve soil physical characteristics must be accompanied by inorganic nitrogen fertilizers to stimulate microbial activity and provide enough nitrogen for plant growth as well.

Phosphorus - Phosphorus is critically important to plant growth. It is needed for cell division, flowering, seed production, and root development. Legumes cannot fix nitrogen without adequate amounts of phosphorus. Therefore low phosphorus levels impinge on nitrogen availability as well.

Phosphorus may be unavailable because of a small total amount available in soils, the unavailability of this native phosphorus, or a marked precipitation of added soluble phosphates. Since crops remove relatively small amounts of phosphorus and world supplies are large, supplying sufficient total phosphorus is not difficult. Increasing the availability of the native soil phosphorus and retarding precipitation of added phosphates are important problems.

Phosphorus occurs in soil in inorganic and organic forms and both are nutrient sources for plants. The inorganic compounds fall into two groups: those containing calcium and those containing iron and aluminum. The simpler calcium compounds are relatively soluble while the more complex ones are insoluble especially at high pH values. Iron and aluminum phosphate compounds are least soluble at low pH values.

Other than pH, phosphorus availability is influenced by the amount and decomposition state of organic matter as well as microbial activity. As with nitrogen, the rapid decomposition of organic matter and consequent high microbial populations result in temporarily tying up of inorganic phosphates in microbial tissue. As organic matter is decomposed by microorganisms, phytin and nucleic acids are produced. These are sources of phosphorus. Phytin is absorbed directly by plants while nucleic acids are broken down by enzymes at the root surfaces and the phosphorus is then absorbed in either organic or inorganic form. Plants commonly suffer from phosphorus difficiency even in the presence of large quantities of organic forms of the nutrient. Just as with inorganic phosphates, the problem is one of availability.

Phytin is similar to inorganic phosphates, forming iron and aluminum phytates and calcium phytates at low and high pH values respectively. Under acid conditions nucleic acids are strongly absorbed by clays, especially montmorillonite.

In practical phosphorus management, the small amount of control that can be exerted over phosphate availability seems to be associated with pH control, fertilizer placement, and organic matter maintenance. Phosphate precipitation is kept to a minimum if soil pH is kept between 6 and 7. Phosphate fertilizers are applied in concentrated bands to prevent rapid precipitation reactions. They may be pelleted to retard soil contact and slow phosphate diffusion.

A major portion of added phosphates will be precipitated regardless of what precautions are followed. These precipitated forms are not totally lost since they are slightly soluble and release small amounts of phosphates. This may be significant in areas with high amounts of precipitated phophorus compounds. In summary, maintaining sufficient phosphorus in a soil requires the addition of phosphorus containing fertilizer and the partial regulation of phosphate precipitation.

Potassium - Potassium la needed by plants for photosynthesis, starch formation, and translocation of sugars. It encourages strong root systems and is necessary for tuber development. All root crops respond to liberal applications of potassium.

In contrast to phosphorus, moat mineral soils, except sandy ones, are relatively high in total potassium. The amount held in easily exchangeable form is, however, usually very small. Moat potassium is part of the primary soil minerals especially mica and feldspar. It must be released by geologic weathering processed which are very slow compared to crop growth cycles. A smaller but significant portion is held in slowly soluble form in the crystal lattice of minerals. Soluble potassium is in the soil solution and on the exchange complex. It is not held tightly on the exchange complex and la readily susceptible to leaching losses. Potassium supplying power la the term used to describe the supply of potassium being released from the slowly available forma. Soluble potassium comes from the inherent potassium supplying power, organic matter decomposition, and added fertilizer. Competition by microorganisms contributes to its unavailabilty to growing plants. Thus, potassium is similar to both phosphorus and nitrogen because a large proportion of all three is insoluble and unavailable to growing plants.

Potassium management has three complications: a very large proportion is relatively unavailable to higher plants because its available forms are soluble and not strongly held on the exchange complex, they are easily leached, and, the removal of potassium by crops is sometimes excessive because of luxury consumption.

Practical fertility management should take full advantage of the potassium supplying power of the soil (it is difficult to measure and not known in most places) in calculating fertilizer needs, Fertilizer applications should be light and frequent to minimize leaching and luxury consumption losses.

Fertilizers - A fertilizer is any substance which is added to soil to supply plant nutrient elements. It can be an organic material which releases inorganic elements when broken down by microorganisms, or it can already be in inorganic form.

Organic materials are usually produced locally by green manuring, return of plant residues, or use of animal manures. They are bulky, difficult to ship, and have a lower nutrient supplying power compared to inorganic fertilizers. Their varied carbon compounds have beneficial physical and chemical effects on the soil. Organic materials should be used to supply part of the necessary plant nutrients. Animal manures must always be composted before use to avoid plant damage.

Inorganic fertilizers are fairly concentrated and easy to ship and apply. Because the nutrient elements are usually rapidly available for plant uptake, it is important not to over fertilize and burn plants. Immediate availability also implies a potential for large nutrient losses through leaching. Inorganic fertilizers are the simplest way to add plant nutrients. Continual reliance upon them with no organic additions, however, will lead to degradation of soil porosity and aeration.

A complete fertilizer is one which supplies the three major plant nutrients. Nitrogen fertilizer carriers are usually one or several of the followings sodium nitrate (NaNO3), ammonium sulfate ((NH4)2SO4), ammonium nitrate (NH4NO3), or diammonium phosphate ((NH4)2HPO4). The primary sources of phosphorus in inorganic fertilizers are superphosphates (CaHPO4 and Ca(H2PO4)) which have higher phosphate availability than rock phosphate. Potassium is usually supplied by one of the following salts: potassium chloride (KCl), potassium sulfate (K2SO4), or potassium nitrate (KNO3).

Fertilizer grade refers to the minimum guarantee of the plant nutrient content in terms of total N. available P2O5, and water soluble K2O in a complete fertilizer (e.g. 5-10-10). Fertilizer ratio refers to the relative percentages of N. P2O5, and K2O in a complete fertilizer. For instance, a 5-10-10, and 816-16, a 10-20-20 and a 15-30-30 all have a 1-2-2 ration. These fertilizers should give essentially the same results when applied in equivalent amounts. Thus, 1,000 pounds of 10-20-20 provides the same amounts of nitrogen, phosphorus, and potassium as does a ton of 5-10-10.

It is best to fertilize using many light applications rather than one heavy application. This ideal is seldom possible because the specific site conditions and economics usually dictate procedures. Slow release fertilizers are available to mitigate these problems but they are more expensive.


*Sources include FAO, 1979; FAO, 1975: FAO, 1976; and Black, 1968.

Particle Size Distribution - Texture influences infiltration, permeability, moisture and nutrient retention, and susceptibility to erosion. Its effect on these qualities may be modified by soil structure, nature of the clay minerals, organic matter, and calcium carbonate content. Soils of all textural classes, except perhaps coarse sand, are arable by an appropriate method. In arid regions fine textured soils may be high in soluble salts, while in humid regions they may be water-logged during part of the year. Coarse textured soils are subject to rapid permeability, low water-holding capacity, and low nutrient holding and supplying capacity. Thus, medium textured soils are generally favored for agriculture.

Available Water Capacity - Readily available water is that portion of the water in the soil that can be readily absorbed by plant roots (about 50-75 percent of the total available moisture). The "total available moisture" is usually defined as the difference between the soil moisture content at "field capacity" and "wilting point". The capacity of a soil to retain water available to plants has a direct bearing on required rooting depth and frequency of precipitation or irrigation. It is important, therefore, in judging the suitability of a soil for agriculture.

Permeability - The permeability of a soil profile is used to determine subsurface drainage and to evaluate the possibility of perched water table conditions developing, which may injure crop roots. Soil morphological features that influence or reflect permeability include texture, structure and structure stability, color and mottling, visible pores, and depth to impermeable strata such as bedrock or hardpan. A moderate to rapid permeability allows the greatest flexibility on water, fertility and salinity management.

Effective Rooting Depth - Root penetration is inhibited by physical actors such as bedrock or cemented pans, by chemical factors such as high calcium carbonate or gypsum concentrations, or by poor drainage. Effective rooting depth greater than 90 cm is considered optimal.

Restricted rooting inhibits proper root formation, decreases plant stability, and limits the amount of soil that the plant can utilize thus restricting its intake of nutrients and moisture.

Water Table - Perched or regional high water tables can act as root inhibiting layers by restricting aeration. In arid areas soils having high water tables should not be developed for irrigation because the groundwater will be degraded by leaching of natural salts and fertilizer salts. The concentrated salts can then rise into the root zone by capillary action. This will lower the agriculture' productivity of the soil (see salinity section below).

For irrigated agriculture, good quality water should not be higher than about 90 em for any 24-hour period after irrigation has begun. Pre-irrigation water tables must be lower than that. Saline water tables should never be higher than three meters below the surface (FAO, 1979).

Topography - Slope and local relief influences the type and scale of agriculture which can be developed. Level to nearly lover land can be utilized for large scale monoculture cropping while steep dissected terrain is more suited to intensive agro-forestry techniques. Table 1 presents some choices of irrigation method e and crops for different topographical conditions. For extensive tree planting, small rainfall runoff catchments can be designed for each tree. In arid regions, upper elopes may be left bare as a collection area for tree belts lower on the slope.

Salinity - An excess of soluble salts is probably the most widespread soil quality adverse to crop growth in arid irrigated araes. It is fortunate that, owing to their solubilty, such salts are mobile and can be removed by leaching where drainage conditions are satisifactory.

The primary deleterious effect of excessive salinity is to raise the concentration of the soil solution. The flow of water into the plant by osmosis is reduced or reversed and the plant is starved of water even though the soil is moist. Some ions, particularly sodium, chloride, and sulfate, have specific toxicity for certain crops. The variation among plants in their tolerance to salinity (Table 2) affects the choice of cropping pattern when evaluating the possible effects of salinity.

Inadequate drainage and a rising water table after a few years of irrigation may lead to the entry of saline water into the root zone. The salinity level and sodic conditions at the time of sampling are not stable characteristics of the soils, and both can be changed with irrigation, salinity being the easiest and cheapest to correct. Important considerations in the evaluation of saline or sodic soils include: water quality to be used for irrigation, infiltration and permeability rate of the soil, leveling required to provide a suitable surface for leaching' ability of substrata to transmit the necessary leaching water; the level of salinity or sodic conditions; and availability or absence of gypsum to replace sodium in sodic soils.

Exchangeable Sodium Percentage (ESP) - The exchangeable sodium percentage is the degree of saturation of the soil exchange complex with sodium. ESP is usually a good indication of the structural stability of a soil and of its physical response to irrigation water. Moat soil containing expanding type clay minerals exhibit unfavorable physical properties at EST greater than 15 percent. In general, physical properties become increasingly unfavorable with increasing ESP. Expanding 2:1 clay minerals are more strongly affected than non-expanding clays. In sandy soils, sodium-induced clay dispersion may favorably increase water-holding capacity.


Irrigation method




Widely spaced borders

Land slopes capable of being graded to less than 1% slope and preferably 0.2%

Alfalfa and other deep rooted close-growing crops sad orchards

The most desirable surface method for irrigating close-growing crops where topographical conditions are favourable. Even grade in the direction of irrigation is required on flat land and is desirable but not essential on slopes of more than 0.5%. Grade changes should be alight and reverse grades must be avoided. Cross slope is permissible when confined to differences in elevation between border strips of 6-9 cm.

Closely spaced borders

Land slopes capable of being graded to 4% slope or less and preferably less than 1%


Especially adapted to shallow soils underlain by clay pan or soils that have a low water intake rate. Even grade in the direction of irrigation is desirable but not essential. Sharp Grade changes and reverse grades should be smoothed out. Cross slope is permissible when confined to differences in elevation between borders of 6-9 ¢m. Since the border strips may have less width, a greater total cross slope is permissible than for border irrigated alfalfa.

Check back and cross furrows

Land slopes capable of being graded to 0.2% slope or less


This method is especially designed to obtain adequate distribution and penetration of moisture in soils with low water intake rates.


Land slopes capable of being graded to slopes between 0.5% and 12%

Alfalfa pasture and grain

This method is especially adapted to steep land and small irrigation streams. An even grade in the direction of irrigation is desirable but not essential. Sharp grade changes and reverse grades should at least be smoothed out. Due to the tendency of corrugations to clog and overflow and cause serious erosion, cross slopes should be avoided as much as possible.

Graded contour furrows

Variable land slopes of 2-25% but preferably less

Row crops and fruit

Especially adapted to row crops on steep land, though hazardous due to possible erosion from heavy rainfall. Unsuitable for rodent infested fields or soils that crack excessively. Actual grade in the direction of irrigation 0.5-1.5%. No grading required beyond filling gullies and removal of abrupt ridges.

Rectangular checks

Land slopes capable of being graded so single or multiple tree basing will be level within 6 cm


Especially adapted to soils that have either a relatively high or low water intake rate. May require considerably grading.

Countour checks

Slightly irregular land slopes of less than 1%

Fruit, rice, grain and forage crops

Reduces the need to grade land. Frequently employed to avoid altogether the necessity of grading. Adapted beat to soils that have either a high or low water intake rate.

Contour ditches

Irregular slopes up to 12%

Hay, pasture and grain

Especially adapted to foothill conditions. Requires little or no surface grading.

Portable pipes

Irregular land surface

Hay, pasture on small scale

Minimum preparation of land surface required



Shallow rooted crops such as potatoes or grass

Requires a water table, very permeable subsoil conditions and precise levelling. Very few areas adapted to this method.


Undulating 1-> 35% slope

All crops

High operation and maintenance costs. Good for rough or very sandy lands in areas of high production and good markets. Good method where power costs are low. Ha, be the on 4 practical method in areas of steep or rough topography. Good for high rainfall areas where oaf, a small supplemental water supply is needed.

Contour bench terraces

Sloping land - best for slopes under 3% but useful to 6%

Any crop, but particularly well rutted to cultivated crops

Considerable loss of productive land due to berms. Require expensive drop structures for water erosion control.

Subirrigation(installed pipes)

Flat to uniform slopes up to 1% surface should be smooth

Any crop, row crops or high value crops usually used

Requires installation of perforated plastic pipe in root zone at narrow spacings. Some difficulties in roots plugging the perforations. Also a problem as to correct spacing. Field trials on different soils are needed. This is still in the development stage.


Any topographic condition suitable for row crop farming

Row crops or fruit

Perforated pipe on the soil surface drips water at base of individual vegetable plants or around fruit trees. Has been successfully used in Israel with saline irrigation water Still in development stage.

*Source: FAO (1979). from Richards et al. (1954).


Fruit Crops

High salt tolerance

Medium salt tolerance

Low salt tolerance

ECe x 103 = 8

ECe x 103 = 4

Date Palm

























ECe x 103 = 8

ECe x 103 = 4

ECe x 103 = 2

Vegetable Crops

ECe x 103 = 12

ECe x 103 = 10

ECe x 103 = 4

Garden beets








Green beans


Bell popper






Sweat corn (maize)














ECe x 103 = 10

ECe x 103 = 4

ECe x 103 = 3

Field Crops

ECe x 103 = 16

ECe x 103 = 10

ECe x 103 = 4

Barley (grain)

Rye (grain)

Field beans

Sugar beet

Wheat (grain)

Sugar cane


Oats (grain)



Rica Sorghum (grain)












ECe x 103 = 10

ECe x 103 = 6


Forage Crops

ECe x 103 = 18

ECe x 103 = 12

ECe x 103 = 4

Alkali sacaton

White sweetclover

White Dutch clover


Yellow sweetclover

Meadow foxtail

Nuttal alkali grass

Perennial ryegrass

Alsike clover

Bermuda grass

Mountain brome

Red clover

Rhodes grass

Strawberry clover

Ladino clover

Fescue grass

Dallis areas


Canada wildrye

Sudan grass


Western wheatgrass

Hubam clover


Barley (hay)

Alfalfa (Calif. Common)


Birdsfoot trefoil

Tall fescue


Rye (hay)


Wheat (hay)


Oats (hay)




Blue grama


Meadow fescue


Reed canary


Big trefoil


Smooth brome


Tall meadow oatgrass


Cicer Milkvetch




Sickle milkvetch.


ECe x 103 = 12

ECe x 103 = 4

ECe x 103 = 2

*Source: FAO (1979) from Richards et al. (1954).

In addition to the possible deleterious effects that ESP levels may have on physical properties of a soil, some crops have a low tolerance for exchangeable sodium. Tolerance of various crops to ESP is presented in Table 3 and ESP crop reduction is shown in Table 4.

Calcium Carbonate - Calcium carbonate commonly accumulates in soils developed under arid and semiarid climates. It may be diffused throughout the soil profile, or may take the form of soft concretions, or nodules, or may be concentrated in a continuous horizon of varying hardness and at varying depths below the surface. The amount of carbonate present, the form of its distribution in the profile, and the depth to the carbonate-rich horizons are all important factors in judging the suitability of a calcareous soil for irrigated agriculture.

The presence of calcium carbonate affects both physical and chemical characteristics of a soil. Continuous horizons of carbonate accumulations may not restrict water movement, but may prevent root penetration. The presence of carbonates reduces the ability of calcareous soils to retain moisture, especially at high tensions (FAO, 1979).

Up to 10-15 percent calcium carbonate may assist formation of stable aggregates associated with relatively large pores and rapid water movement. With an increased content of 20 or 25 percent, precipitation of carbonate within capillary tubes tends to increase the proportion of very small pores and reduce diffusivity.

Surface crusting can be a serious problem in newly irrigated calcareous soils, especially those of low organic matter content. Crusts not only affect infiltration and soil aeration, but also impede or prevent the emergence of seedlings. Heavy applications of water on soils with a high content of fine-grained carbonate encourages the formation of thick crusts on drying. Therefore, soils which have a tendency to crust will require a frequency of irrigation sufficient to prevent drying and hardening of the surface.

The physical characteristics of calcareous soils often change when they are irrigated. From a favorable virgin condition the soils become more coherent and resistant to root penetration, especially in the part of the profile subjected to wetting and drying. The effect is likely to be more marked if organic matter content is low. Careful timing of tillage operations and careful seedbed preparation moat be foreseen. The optimum moisture range for plowing calcareous soils is very narrow and occurs within four to five days after irrigation, whereas seven to eight days after irrigation the plowing operation is often rather difficult.

Nutrient deficiencies of phosphorus, iron, and micro-nutrients are common in plants grown on calcareous soils. High lime content usually results in a need for later inputs of fertilizers and is a dilutant factor for roots seeking nutrition.


Tolerance to ESP and range at which affected


Growth responses under field conditions

Extremely sensitive

(ESP = 2 - 10)

Deciduous fruit Nuts, avocado, cassava

Sodium toxicity symptoms even at low ESP values


(ESP . 10 - 20)


Stunted growth at low ESP values even though the physical condition of the soil may be good

Moderately tolerant

(ESP = 20 - 40)

Clover, oats, tall Rescue, rice, dallis grass

Stunted growth due to both nutritional factors and adverse soil conditions


(ESP = 40 - 60)

Wheat, cotton, alfalfa, barley, tomatoes, beets

Stunted growth usually due to adverse physical conditions of soil

Most tolerant

(ESP more than 60)

Crested and fairway wheatgrass, tall wheatgrass, rhodes grass

Stunted growth usually due to adverse physical conditions of soil

*Source: FAO (1979) from Bower (1959).


50% Crop reduction at ESP of 15 or less

50% Crop reduction at ESP of 15-25

50% Crop reduction at ESP 35





Dwarf kidney bean


Green beans

Red clover





Tall fescue





Dallis grass

Sweet orange



*Source: FAO (1979) from Lunt (1963).

Accordingly, a highly calcareous soil can be expected to be less productive than slightly calcareous soils if all other factors are equal.

Gypsum - Soils containing gypsum (CaSO4 . 2H2O) are widespread in arid and semiarid areas. A small amount of gypsum is favorable to crop growth in that it serves as a relatively soluble source of calcium to replace sodium on the exchange complex and thus acts to preserve soil structure. Sodic soils containing gypsum are relatively easy and inexpensive to reclaim. High percentages of gypsum in the soil, however, can cause serious problems especially in irrigated agriculture and, in some areas, the content of gypsum must be regarded as an important criterion in judging the suitability of soils for irrigation.


Agriculture is applied ecological management. It is human intervention in natural ecological cycles to maximize our harvest at the expense of other herbivores and carnivores. Some of the controlling mechanisms for these cycles are easily manupulated while others such as weather are beyond our influence.

Soil is one of the major substrates from which plant nutrients are derived. Soil supplied nutrient availability is controlled by biogeochemical cycles. Agricultural science attempts to understand and manipulate these cycles. This understanding becomes critical when applied to extremely dry or wet geographic regions. In the former, there is little or no leaching and soluble salts may accumulate while in the latter almost all nutrients are rapidly leached from plant root zones.

Agricultural researchers are solving management problems at local and regional levels. This basic introduction to soils will allow you to interact with these researchers and utilize their localized information. It will become second nature as it is applied in the field.

Your own powers of observation will help you to become familiar with new geographic areas. Notice the distribution of cultivated fields in relation to topography and to native vegetation. These relationships usually indicate water collection areas and good agricultural soils while certain plants will indicate abnormal soil conditions.

The books listed as references should be consulted for further information on specific problems. N. C. Brady' e book is an excellent introduction to all aspects of soil science. The United Nations Food and Agriculture Organization (FAO) has published a number of Soils Bulletins which are expressly concerned with agricultural and land management problems in developing countries. These can be very useful when specific problems have been identified.


Black, C. A. 1968. Soil-Plant Relationships. John Wiley and Sons, Inc. New York.

Bohn, H. L., B. L. McNeal, and G. A. O'Connor. 1979. Soil Chemistry . John Wiley and Sons, Inc., New York.

Bower, C. A. 1959. Chemical Amendments for Improving Sodium Soils. Agriculture Information Bulletin 195 United States Department of Agriculture, Washington, D.C.

Brady, N. C. 1974. The Nature and Properties of Soils. 8th edition. MacMillan Publishing ., Inc. New York.

Food and Agriculture Organization (FAO). 1973. Sandy Soils. Soils Bulletin 25. Food and Agriculture Organization of the United Nations. Rome.

Food and Agriculture Organization (FAO). 1976. Prognosis of Salinity and Alkalinity. Soils Bulletin 31. Food and Agriculture Organization of the United Nations. Rome.

Food and Agriculture Organization (FAO). 1977. Organic Materials and Soils Productivity. Soils Bulletin 35. Food and Agriculture Organization of the United Nations. Rome.

Food and Agriculture Organization ( FAO). 1979. Soil Survey Investigations for Irrigation. Soils Bulletin 42. Food and Agriculture Organization of the United Nations. Rome.

Lovelock, J. E. 1979. GAIA - A New Look at Life on Earth. Oxford University Press. Oxford.

Lunt, O. R. 1963. Sensitivity of Plants to Exchangeable Sodium Percentage. University of California Report No. 5, Agricultural Water Quality Research Conference, Appendix B.

Morrowitz, H. 1983. Two Views of Life. Science 83. Volume 4, No. 1.

Richards, L. A. et. al. 1954. Diagnosis and Improvement of Saline and Alkali Soils. Agriculture Handbook 60 United States Department of Agriculture Salinity Laboratory, Riverside, California.