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:

SANDY SOILS NEED MORE FREQUENT BUT LIGHTER WATERINGS THAN CLAYEY SOILS.

• 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)
Shallow	Moderately Deep	Deep
Broccoli	Beans	Asparagus
Brussels sprouts	Beet	Bean, lima
Cabbage	Carrot	Parsnip
Cauliflower	Chard	Pumpkin
Celery	Cucumber	Field Maize
Chinese cabbage	Eggplant	Sorghum
Garlic	Muskmelon (cantaloupe)	Squash, winter
Lettuce	Mustard	Sugarcane
Onion	Pea, garden	Sweet potato
Parsley	Pepper	Tomato
Potato	Rice	Watermelon
Radish	Squash, summer
Spinach	Turnip
Sweet maize