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

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