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

Ok, so get to the point! how much water do plants need and how often?

Well, as you can see by all the variables above, there's no quick answer. BUT, we can give you some definite parameters; you'll need to adjust the figures to suit conditions. First, let's see how much water plants need per week and then deal with how often Per week.

Amount Needed per Week

Crop watering recommendations are often given in terms of inches or millimeters (mm) of water per week. One inch (or one mm) of water is equal to filling a flat-bottom tub with 1 inch (or 1 mm) of water. Note that these measurements refer only to the actual thickness of the water layer and say nothing about the size of the tub (or field), nor how deep the water will penetrate in a soil. In terms of actual water volume needed per area, here are some very useful conversions:

1 INCH OF WATER = 7 GALLONS (25 liters) PER SQUARE METER

1 MILLIMETER OF WATER = 1 LITER PER SQUARE METER

TABLE 5-2 TOTAL WEEKLY WATER NEEDS (1)

(Includes both plant usage and evaporation from the soil)

 

Inches of Water

Millimeters of Water

Liters Needed (2) per Sq. Meter

VERY YOUNG PLANTS IN WARM WEATHER

0.75-1.0"

19-25 mm

19-25 liters

PEAK USE RATES FOR VEGETABLES IN WARM WEATHER (during flowering, fruiting, or heading)

1.4-2.0"

35-50 mm

35-50 liters 3

PEAK USE RATES FOR FIELD CROPS (from pollination through first 3-4 weeks of grain fill)

1.75-2.75"

45-70 mm

45-70 liters

*1. If the root zone is very dry, it should be watered before planting to "recharge" it. (See the section on pre-irrigation further along in this chapter.)

*2. Refers to sq. meters of actual planted area. Where the bed-and-alley system is used, only the bed area itself should be watered at these rates; don't water the alleyways, because little or no root growth occurs there.

$3. Severe weather conditions (high heat + hot/dry winds) can increase these rates up to 20% above the maximums given.

How to Use Table 5-2: You'll need to consider weather conditions and crop stage of growth. As a crop grows larger, you'll want to gradually increase the weekly total of water (barring any sudden change in the weather), rather than suddenly increasing it from 25 liters to 45 liters per sq. meter.

Note also that weekly water needs are the same whether a crop is grown on a sandy or a clayey soil. The difference is that clayey soils can tolerate longer intervals between waterings than sandy soils. An exception might be those clayey soils prone to severe cracking when they begin drying out; in this case, considerable extra water could be lost by evaporation from the cracks, unless the soil were mulched or heavily shaded by the crop's leaves. (Heavy additions of sand or organic matter will lessen cracking.)

How Often to Water

There are basically two approaches you can take:

• Lighter, but more frequent waterings.

• Heavier, but less frequent waterings.

Both approaches will satisfy crop water needs, as long as the total amount applied per week is adequate. (Refer to the water dosage table above.) Both methods will achieve the same depth of water penetration, given equal amounts of water per week. It's possible that frequent watering may result in somewhat higher evaporation losses on unmulched (or unshaded) soils; on the other hand, frequent watering may help prevent soil cracking on certain clayey soils, thus reducing evaporation losses.

In deciding which of the approaches to take, you'll need to consider 4 factors: soil waterholding capacity, root depth, water supply, and labor considerations.

• Soil water-holding capacity: Sandy soils need more frequent (about twice as often) but lighter waterings than clayey soils, because they can hold only about half as much usable water per unit of depth.

• Root depth: The shallower the root system, the more often watering is needed. Young plants need more frequent watering, because their roots are shallow and the water around them more quickly exhausted. Naturally shallow-rooted crops like lettuce and cabbage need more frequent watering than deeper-rooted crops like eggplant and tomato.

NOTE: During the first few days following transplanting, seedlings will often need more frequent watering than their size would indicate. In hot weather, twice-daily watering may be needed for up to a week after setting. Seedlings that have been container-grown suffer less root damage during transplanting and are less susceptible to drying out. Likewise, proper hardening by restricting watering for 7-10 days prior to setting out the seedlings will lessen initial water needs.

• Water supply: If you're hand-watering from a well that has a limited daily output, it may be necessary to make light applications once or twice a day instead of heavier, less frequent ones which might exhaust a hand-dug well's daily capacity. (An alternative would be to water only a portion of the garden each day.). Farmers using furrow-irrigation from a cooperative system may receive water only once every several days.

• Labor considerations: In some cases, farmers/gardeners will prefer to even out the watering labor by watering as often as once or twice a day (using light applications), even though crop/soil factors might allow one heavier watering every 2-4 days of more.

How often to water before seedling emergence: Most seeds must be surrounded by constantly moist soil to be able to sprout. Large seeds such as maize and beans can usually be planted deep enough so that they will require no additional water after planting in order to sprout. However, most smaller seeds, especially the tiny ones like such as lettuce and amaranth, need to be planted very shallow (5-15 mm). In this case, the soil surface should be kept continually moist until seedling emergence. Unless a pre-emergence mulch is used, this may require watering up to 3 times a day on sandy soils in hot, sunny weather. (Pre-emergence mulching is explained in Chapter 8).

Some Practical Examples of Watering Frequency

NOTE: These examples are designed to calculate the minimum allowable frequency per week; it's OK to water once or twice a day, as long as the liters per application are reduced proportionally so the same total is applied per week.

EXAMPLE 1: Suppose you're growing cabbages on a clayey soil in warm weather, and they're at the heading stage. You figure that 40 liters per sq. meter are needed weekly. Clayey soil has a good water-holding capacity, but cabbage is shallow-rooted, so you'll probably need 2 waterings a week of 20 liters per sq. meter each.

EXAMPLE 2: Now let's substitute tomatoes at the flowering/ fruiting stage in the above example. Unlike cabbages, they're a deep-rooted crop (barring no barriers to root penetration). Supposing that 50 liters/sq. meter are needed weekly, you could probably water them once every 5 days. Here's how you'd calculate the amount needed per watering:

50 liters/sq. meter per week needed

5/7ths x 50 liters = about 36 liters/sq. meter every 5 days

EXAMPLE 3: Now let's take a nursery seedbed with young tomatoes, peppers, and cabbage on a very sandy soil in warm weather. Weekly water needs will be about 20-25 liters/sq. m. You'd probably have to water once or twice a day during the first week after seedling emergence, applying 3-3.5 liters/sq. m daily. Obviously, if you put on the entire 2025 liters/sq. m all at once, most of the water would end up beyond the root zone and be wasted. By the time the plants are 1-2 weeks old, you could probably reduce the frequency to once every 2 days.

All the above examples are only approximations; you'll have to decide the amount and frequency that bests suits the situation.

How to Tell When Plants Need Water

Initial signs of moisture stress: Wilting, leaf curling (or rolling), and, in some cases, color changes (maize and other plants will often turn bluish-green).

Advanced symptoms of moisture stress Yellowing and eventual browning ("firing") of the leaves, starting at the tips.

NOTE: Most of these symptoms can also be caused by anything else that interferes with water uptake or transport such as nematodes, soil insects, stem borers, fungal and bacterial wilts, fertilizer burn, and even high temperatures. N deficiency can cause yellowing too.

Ideally, plants should never be allowed to reach the advanced stages of moisture stress between waterings. A little wilting won't affect young plants, but even a day of it can lower yields and quality of crops at the flowering, fruiting, and heading stage.

Two Tests to Determine if Watering is Needed

• The "Scratch" Test can be used on young seedlings when their roots are shallow. Scratch into the soil with your finger and see how far down you have to go to reach moist soil. If the soil is dry more than 2-3 cm down, it may be time to water shallow rooted seedlings.

• The "Squeeze" Test: Using the table in Appendix B, you can estimate the percentage of available water left in the root zone. Water should usually be applied before half of the root zone's available water has been used up. Plants take up about 40 percent of their water needs from the the top quarter of the root zone. Once this top top quarter gets down to 0 percent available moisture, it's time to apply more.

Measuring how deep water has penetrated: Use a 10-12 mm (about 0.5") diameter steel rod slightly tapered at end. Wait about 12-24 hours after watering and then push the tapered end into the ground. It should penetrate fairly easily until it strikes dry soil. (Hardpans may affect the accuracy of this method.)

Figuring in Rainfall

Since rainfall will affect the need for watering, it's important to record it. Buy a rain gauge or make one out of a tin can with straight sides. Amounts below 6 mm (1/4 inch) aren't much use to plants, because a lot may be lost to evaporation. Likewise, heavy downpours may result in much wasted runoff or loss from downward drainage beyond the root zone. For example, a 100 mm (4") rainfall may only add the useful equivalent of 20-30 liters/sq. meter for a shallow-rooted crop like cabbage on a sandy soil. (Remember that each millimeter of rainfall is equal to 1 liter of water per sq. meter.)

What about Pre-Irrigation?

It's often advisable to pre-irrigate the soil down to eventual rooting depth before planting, especially if the root zone is very dry. Use the chart in Appendix B to determine the moisture status of the soil. Then use the water-holding capacity chart in Chapter 2 to determine how much pre-irrigation is needed. If the proper amount of water is applied, little will be lost except the small amount that will evaporate from the soil surface and the first few centimeters. The rest will be safely held in the micropores for future use. (Clayey soils prone to cracking while drying out may have have higher evaporation losses). Pre-irrigation also has some other possible benefits:

• Improving filth (workability) of very clayey soils: Very hard, dry, clayey soils can be much more easily worked if given a good soaking (at least 25 liters/sq. meter 1-2 days before land preparation).

• Pre-planting weed control in vegetable plots: Watering a prepared vegetable bed 7-10 days before planting will encourage many weed seeds to sprout. A very shallow weeding with a sharp hoe blade (using a scraping action) will kilt these weeds without moving more weed seeds closer to the soil surface where they can more easily germinate. This can substantially reduce weed problems on soils with high populations of annual weeds (i.e. those that reproduce by seed).