|Soils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)|
|Chapter 5: Watering vegetables: When? How Often? How Much?|
· Watering too shallowly: Roots won't grow downward into dry soil, so shallow watering leads to shallow rooting. Deep rooting not only enables plants to absorb more nutrients, but also permits longer intervals between waterings. It's important to understand that both frequent, light watering and less frequent, heavier watering can result in insufficient water penetration if the total amount applied per week is too small.
· Watering too deeply: This is very easy to do with shallow rooted crops or with sandy soils because of their low water-holding capacity. The excess water moves down beyond the root zone, not only wasting it but carrying away mobile nutrients like nitrate nitrogen and sulfur. (Such nutrient loss is called leaching.) On dense clayey soils, overwatering may cause poor drainage, depriving roots of needed oxygen and favoring soil-borne fungal and bacterial diseases. (To appreciate the difference in the depth of water penetration between a clayey and sandy soil, refer to Figure 2-2 in Chapter 2.)
· Inconsistent watering: "Feast or famine" watering not only stresses the plants and lowers yield, but may cause physiological problems like blossom end rot in tomatoes and peppers, fruit cracking in tomatoes, splitting heads in cabbage, and bitterness in squash.
· Watering too late in the day: This promotes development of damping-off fungal disease in young seedlings by keeping the soil surface moist overnight which helps the fungi proliferate. In older plants, it may favor leaf fungal diseases if the foliage remains wet overnight.
A note on watering in the heat of the day: Contrary to popular belief, hand watering in the full sun at mid-day will seldom injure plants, except in the case of some very tender ornamentals. There is no evidence that water droplets injure leaves by acting like tiny magnifying glasses. However, high soil moisture (after a heavy rain or watering) may sometimes cause a mild "burning" effect (known as sunscald) on the edges of leaves if it occurs in conjunction with high temperatures, intense sunlight, and wind. This is caused by guttation which is the evaporation of excess water from leaf cells, leaving salt deposits behind; these deposits draw water from nearby cells, causing "burning".
TAKE TIME TO LEARN THE BASIC PRINCIPLES OF WATERING
There are no quick and easy methods for determining how much water plants need and how often it should be applied. The so called "shiny layer" method popularized in some garden books isn't reliable enough. (With this method, the soil has supposedly received enough water when a shiny layer of water remains on the soil surface for a certain number of seconds after watering stops). You can greatly improve on such "ball park" methods and do much to help farmers, if you'll take the time to learn some fairly straightforward concepts and figures. So, here goes:
HOW WATER IS USED OR LOST
· Actual Plant Usage (called transpiration)) is very small when plants are tiny but rapidly increases with plant size and then becomes the main source of water loss. In fact, something like 220-660 liters of water are transpired into the air for every kilogram of dry matter produced.
· Evaporation from the soil surface is the main source of water loss when plants are small and much of the soil surface is unshaded. Still, evaporation losses are much less than transpiration losses of older plants and decrease as plant leaves shade the soil surface more completely.
· Downward drainage through the soil beyond the depth of rooting causes water loss when too much is applied at once. Remember that soils can hold water in their smaller pore spaces (micropores; see Chapter 2) without losing it to drainage (i.e. gravity). Only the water in the larger pore spaces (macropores) isn't held and keeps moving downward.
· Surface runoff usually doesn't amount to much except on sloping soils that lack conservation measures such as contouring.