Cover Image
close this bookSoils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)
View the document(introduction...)
View the documentAbout this manual
View the documentAcknowledgements
close this folderChapter 1: Down to earth - Some Important Soil Basics
View the documentWhat is soil, anyway?
View the documentWhy do soils vary so much?
View the documentTopsoil vs. subsoil
View the documentThe mineral side of soil: sand, silt, and clay
View the documentDistinguishing ''tropical'' soils from ''temperate'' soils
View the documentOrganic matter - a soil's best friend
View the documentThe role of soil microorganisms
close this folderChapter 2: Trouble-shooting soil physical problems
View the document(introduction...)
View the documentGetting to know the soils in your area
View the documentSoil color
View the documentSoil texture
View the documentSoil tilth
View the documentSoil water-holding capacity
View the documentSoil drainage
View the documentSoil depth
View the documentSoil slope
close this folderChapter 3: Basic soil conservation practices
View the document(introduction...)
View the documentRainfall erosion
View the documentWind erosion
close this folderChapter 4: Seedbed preparation
View the document(introduction...)
View the documentThe what and why of tillage
View the documentCommon tillage equipment
View the documentThe abuses of tillage and how to avoid them
View the documentMaking the right seedbed for the crop, soil, and climate
View the documentHow deep should land be tilled?
View the documentHow fine a seedbed?
View the documentSome handy seedbed skills for intensive vegetable production
close this folderChapter 5: Watering vegetables: When? How Often? How Much?
View the document(introduction...)
View the documentIt pays to use water wisely
View the documentSome common watering mistakes and their effects
View the documentFactors influencing plant water needs
View the documentOk, so get to the point! how much water do plants need and how often?
View the documentSome methods for improving water use efficiency
close this folderChapter 6: Soil fertility and plant nutrition simplified
View the document(introduction...)
View the documentLet's Make a Deal
View the documentHow plants grow
View the documentAvailable vs. unavailable forms of mineral nutrients
View the documentSoil negative charge and nutrient holding ability
View the documentSoil pH and how it affects crops growth
View the documentImportant facts on the plant nutrients
close this folderChapter 7: Evaluating a soil's fertility
View the document(introduction...)
View the documentSoil testing
View the documentPlant tissue testing
View the documentFertilizer trials
View the documentUsing visual ''hunger signs''
close this folderChapter 8: Using organic fertilizers and soil conditioners
View the documentWhat are organic fertilizers?
View the documentOrganic vs. chemical fertilizers: which are best?
View the documentSome examples of successful farming using organic fertilizers
View the documentHow to use organic fertilizers and soil conditioners
close this folderChapter 9: Using chemical fertilizers
View the document(introduction...)
View the documentWhat are chemical fertilizers?
View the documentAre chemical fertilizers appropriate for limited-resource farmers?
View the documentAn introduction to chemical fertilizers
View the documentCommon chemical fertilizers and their characteristics
View the documentThe effect of fertilizers on soil pH
View the documentFertilizer salt index and ''burn'' potential
View the documentBasic application principles for N, P, and K
View the documentFertilizer application methods explained and compared
View the documentTroubleshooting faulty fertilizer practices
View the documentGetting the most out of fertilizer use: crop management as an integrated system
View the documentUnderstanding fertilizer math
close this folderChapter 10: Fertilizer guidelines for specific crops
View the document(introduction...)
View the documentCereals
View the documentPulses (grain legumes)
View the documentRoot crops
View the documentVegetables
View the documentTropical fruit crops
View the documentTropical pastures
close this folderChapter 11: Liming soils
View the document(introduction...)
View the documentThe purpose of liming
View the documentWhen is liming needed?
View the documentHow to measure soil pH
View the documentHow to calculate the actual amount of lime needed
View the documentHow and when to lime
View the documentDon't overlime!
close this folderChapter 12: Salinity and alkalinity problems
View the document(introduction...)
View the documentHow salinity and alkalinity harm crop growth
View the documentLab diagnosis of salinity and alkalinity
close this folderAppendixes
View the documentAppendix A: Useful measurements and conversions
View the documentAppendix B: How to determine soil moisture content
View the documentAppendix C: Spacing guide for contour ditches and other erosion barriers*
View the documentAppendix D: Composition of common chemical fertilizers
View the documentAppendix E: Hunger signs in common crops
View the documentAppendix F: Legumes for green manuring and cover-cropping in tropical and subtropical regions
View the documentAppendix G: Some sources of technical support
View the documentAppendix H: A bibliography of useful references

Factors influencing plant water needs

There are 3 sets of factors that largely determine the frequency and amount of watering that plants need:

Soil Factors: Water-holding capacity and usable depth

Weather Factors: Temperature, wind, humidity, and rainfall

Crop Factors: Type, depth of roots, stage of growth

Let's look at these more closely:

Soil Factors Affecting Plant Water Needs

· A soil's water-holding capacity depends mainly on its texture (see Chapter 2). Compared with clayey soils, sandy soils have more macropores (large pore spaces) which don't retain water as well as micropores. In fact, sandy soils can hold only about half as much usable water per unit of depth as clayey soils which means that:


· A soil's humus content also affects its water-holding capacity, but only on sandier soils. Adding compost or manure to clayey soils won't improve their already high water-holding ability. (Humus is partly decomposed organic matter that has become dark and crumbly.)

· Usable soil depth is another important factor. Shallow soils or those with hardpans or very compacted subsoils that restrict root depth will require lighter and more frequent waterings than usual. Very acid subsoils (below a pH of 5.0-5.5.) can also restrict normal rooting depth.

Weather Factors Affecting Plant Water Needs

An easy way of understanding this is to realize that any weather condition that speeds up drying your clothes on the line also increases plant water usage. For example:

· Temperature: Plants use more water on hot days, plus evaporation losses from the soil surface are also higher.

· Sunlight: Plants use more water on sunny days than cloudy days. Unshaded soil will lose more water on sunny days too.

· Relative humidity: Water use by plants increases as humidity decreases; the same is true with evaporation.

· Wind: It increases plant water usage as well as evaporation losses.

NOTE: In some areas such as the Sahel region of Africa, a combination of low humidity, high temperatures and persistent wind is common during much of the the dry season and can dramatically increase water needs.

Crop Factors Affecting Plant Water Needs

· Type of Crop: Among field crops, the millets are the most drought-tolerant, followed by grain sorghum and peanuts. Cowpeas, while not as drought-tolerant as sorghum and peanuts, do better than common beans and especially maize when moisture is low.

Among the root crops, manioc (cassava) is very drought-hardy, and sweet potatoes have some resistance. Tropical yams (Dioscorea sup.) can tolerate short dry spells, but most types of true taro (Colocasia spp.) require high soil moisture. However, the taro-like Xanthosoma sagittifolium (tannia, yautia) tolerates drier conditions.

NOTE: In all cases, even drought-resistant crops like millet will yield much less under low moisture. However, non-tolerant crops will often fail.

In general, vegetable crops don't have good drought tolerance, and both yield and quality can be severely affected by moisture stress. However, watermelon and other deeperrooted veggies like okra, eggplant, and tomatoes are more resistant to dry spells than the shallow-rooted veggies like lettuce, onion, and the Crucifer family (cabbage, cauliflower, collards, broccoli, radish, turnip, Brussels sprouts). These shallow-rooted crops need more frequent and lighter waterings than deeper-rooted ones. (See Table 5-1).

· Depth of roots: Aside from differences in final root depth just discussed, all plants will need more frequent and lighter waterings when young. As their roots grow deeper, watering intervals can be spread out and larger amounts applied per application.

· Stage of growth: Plant water needs increase with growth and reach a peak around flowering, fruiting, or heading time. With most vegies, this peak use period continues until harvest time. However, for field crops like maize, sorghum, and dry beans that are harvested at the fully mature, dry stage, water needs taper off as maturity nears.

NOTE: While young plants can often fully recover from a period of moisture stress, a water shortage during flowering, fruiting, or heading can severely affect yield and quality.

TABLE 5-1 Rooting Depth of Crops When There's no Barrier to Penetration

(45-60 cm)

(90-120 cm)

(More than 120 cm)


Moderately Deep





Brussels sprouts


Bean, lima









Field Maize

Chinese cabbage




Muskmelon (cantaloupe)

Squash, winter





Pea, garden

Sweet potato








Squash, summer



Sweet maize