Plant absorbs moisture from soil through their root system. The method and quantity of water absorption varies with crops and their rooting pattern. The moisture extraction pattern revels about how the moisture is extracted and how much quantity is extracted at different depth level in the root zone. The moisture extraction pattern shows the relative amount of moisture extracted from different depths within the crop root zone.
The moisture extraction pattern of plant growing in a uniform soil without a restrictive layer and with adequate supply of available soil moisture throughout the zone is shown in Figure. It is seen from the following figure that about 40% of the total moisture is extracted from the first quarter of the root zone, 30% from second quarter, 20% from the third quarter and 10% from last fourth quarter.
This indicates that in most of the crops the effective root zone will be available in the 1st quarter and it does not mean that the last quarter will not need any water. Hence, soil moisture measurements at different depths in the root zone have to be taken.
• to estimate the soil moisture status, and
• to work out the irrigation quantity to be applied.
A. Rooting Characteristics and Moisture Extraction Pattern
The root system is extremely variable in different crop plants. The variability exists in rooting depth, root length and horizontal distribution of roots. These are further influenced by environmental factors and the genetic constitution. The roots of cereals apparently occupy more surface area of the soil than other crops.
For example, it has been proved that cereals’ roots extend to 200–400 cm of soil surface area as against 15–200 cm/m2 for most graminaceous plants. The amount of soil moisture that is available to the plant is determined by the moisture characteristics of the soil depth and the density of the roots.
The moisture characteristics of soil like FC and PWP cannot be altered so easily and greater possibilities lie in changing the rooting characteristics of plants system to go deeper and denser and more proliferation to tap water from deeper layer of soil as well as from the larger surface area. Plants vary genetically in their rooting characteristics. (Figures) vegetable crops like onion, potato, carrot etc., have very sparse rooting system and unable to use all the soil water in the root.
Rice, grasses, sorghum, maize, sugarcane have very fibrous dense root system, which can extract much water from soil. Millets, groundnut, grams are moderately deep rooted. Maize, sorghum, lucerne, cotton and other perennial plants have deep root system and can utilize effectively the moisture stored in root zone as well as in the unexploited deeper zones.
Crops, which have dense and deep root system, like cotton, sorghum and red gram tolerate high reduction of soil water content. Shallow rooted crops like rice, potato, tomato tolerate low level of soil water reduction. Moderately deep-rooted crops like millets, groundnut, and grams tolerate medium level of soil water reduction.
Root zone moisture extraction pattern – The root growth of the crop plants is affected by genetic nature, high water table, shallow nature of soil and permeability of soil layer, soil fertility and salt status of soil.
B. Effective Root Zone Depth
It is the depth in which active root proliferation occurs and where maximum water absorption is taking place. It is not necessary that entire root depth should be effective.
Effective Root Zone Depth of some Common Crops
Shallow (60 cm)
Rice, Potato Cabbage Lettuce Onion
Medium to deep (90 cm )
Wheat Ground nut Carrots Soybean Pea
Deep (120 cm)
Maize, Cotton, Sorghum, Pear millet, Sugar beet, Chilies
Very deep (180 cm)
Sugarcane, Citrus, Coffee, Sunflower, Bean.
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