Soil – plant and plant – water relations

1. SOIL– PLANTAND PLANT –
WATER RELATIONS

2. Soil – Plant and Plant – Water Relations
◦To design a successful irrigation system, it is essential to know
the plant rooting characteristics, effective root zone depth,
moisture extraction pattern and moisture sensitive periods of
crops.

3. Rooting characteristic of plants
◦ The purpose of irrigation is to provide
adequate soil moisture in the immediate
vicinity of the plant roots.
◦ All plants do not have the similar rooting
pattern i.e., root penetration and
proliferation. Some plants have relatively
shallow root system (for example annual
crops), while others develop several meters
under favourable conditions (for example
tree crops). It is obvious, therefore, that if
one is to plan an efficient irrigation schedule
for providing adequate soil moisture to plant
roots, it is necessary to understand rooting
habits of plants.

4. Soil properties influencing root development
 a) Hard pan: Root penetration is seriously affected by presence of a hard pan or compacted layer in the soil
profile. Thus roots cannot penetrate a hard layer except through cracks, if any. Thus, in shallow soils, crop roots
may be confined to a thin layer of soil irrespective of their normal genetic rooting pattern in a soil having uniform
structure and texture.
 b) Soil moisture: Since roots cannot grow in soil that is depleted in moisture down to and below the permanent
wilting point, a layer of dry soil below the surface in the profile can restrict root penetration.

5. ◦ c) Water table: A high water table limits root growth, and a rising water table may
kill roots that have previously grown below the new water level .
◦ d) Toxic substances: Presence of toxic substances in the sub-soil also limits root
growth and development. Saline layers or patches in the soil profile therefore inhibit or
prevent root penetration and proliferation.

6. Effective root zone depth
◦ The soil depth from which the crop extracts most (nearly 90%) of the water needed to meet its
evapotranspiration requirements is known as effective crop root zone depth. It is also referred to as
design moisture extraction depth, the soil depth used taken into account to determine the irrigation water
requirements for scheduling irrigation’s to crops.
◦ For best results, it is the depth in which optimum available soil moisture must be maintained for higher
productivity of crops. It is not necessarily the maximum rooting depth for any given crop, especially for
plants that have a long tap root system.

7. ◦ Since root development for any one crop varies in different agro-climatic zones because of soil and
climatic differences, the design depth should be based on local moisture-extraction data for
locally adapted crops.
◦ If two or more plant species with different rooting characteristics are to be grown together, the
design depth should be that of the plant having the shallower root system. In the absence of any
local moisture extraction pattern data for design, the information given below in Table may be used as a
guide. Design moisture extraction depths for crops grown on very deep, well drained
soils

8. SCHEDULING IRRIGATION
◦ Irrigation Scheduling Concept
◦ Irrigation scheduling is essential for good water management and it deals with two
classical questions related to irrigation. These are
◦ (1) how much to irrigate and
◦ (2) How often to irrigate.

9. ◦ In many cases irrigation scheduling is performed based on the irrigator's personal
experience, plant appearance, watching the neighbor, or just simply irrigating
whenever water is available.
◦ However, over the year a number of irrigation scheduling techniques based on soil
water monitoring, plant monitoring and water balance approach have been
developed.

10. Advantages of Irrigation Scheduling
◦ Irrigation scheduling offers several advantages:
1. It enables the farmer to schedule water rotation among the various fields to
minimize crop water stress and maximize yields.
2. It reduces the farmer's cost of water and labouras it minimizes the number of
irrigations.
3. It lowers fertilizer costs by holding surface runoff and deep percolation
(leaching) to a minimum.
4. It increases net returns by increasing crop yields and crop quality.
5. It minimizes water-logging problems by reducing the drainage requirements.
6. It assists in controlling root zone salinity problems through controlled leaching.
7. It results in additional returns by using the saved water to irrigate non-cash
crops that otherwise would not be irrigated during water-stress periods.

11. Full Irrigation
◦ It provides the enough
water to meet the entire
irrigation requirement and is
aimed at achieving the
maximum production
potential of the crop.
◦ Excess irrigation may
reduce crop yield because
of decreased soil aeration.

12. Deficit Irrigation
◦ It means partially meeting the crop water
requirement. It is practiced when there is water
scarcity or the irrigation system capacity is limited.
With deficit irrigation root zone is not filled to the field
capacity moisture level.
◦ Deficit irrigation is justified in case where reducing
water application below full irrigation causes
production cost to decrease faster than revenue
decline due to reduced yield.
◦ This method allows plant stress during one or more
periods of growing season.
◦ However, adequate water is applied during the critical
growth stages to maximize water use efficiency.
◦ Critical growth stage of some the crops are shown in
the following Table.

13. Critical growth stages for managing water use efficiency
Crop
Growth period Most
sensitive to water Stress
Growth Interval in which irrigation
Produces Greatest Benefits
Sorghum Boot- heading Boot- soft dough
Wheat Boot- flowering Jointing- soft dough
Corn Tassel- pollution 12 leaf- blister kernel
Cotton First bloom- peak bloom First bloom- boils well- formed
Dry beans Flowering –early podfill Axillary bud- podfill
Potatoes Tuberization Tuberization- maturity
Soybean Flowering- early podfill Axillary bud- podfill
Sugar beets No critical stages
WUEa is maximized when water depletion is
limited ton about 50% available water
depletion

14. Irrigation Interval
◦ It is the number of days between two successive irrigations.
◦ It depends on the crop ET, effective rainfall, and available water holding
capacity of the soil in the crop root zone and management allowable
depletion.

15. Methods of Irrigation Scheduling
◦ Over the years, a number of methods have been developed for
irrigation scheduling. These can be broadly classified into following
categories:
1. Soil indicators
2. Climatological
3. Plant indices
4. Water balance

16. When to irrigate
1. Maintenance of soil moisture around field capacity is ideal for many crops.
2. As the soil moisture tension increases crops can’t extract needed moisture for
optimum growth.
3. Crop starts wilting leading to retard growth and permanent wilting.
4. Crop should not experience moisture stress between two irrigations.
5. Moisture in crop root zone and ET demand, irrigation need can be determined.

17. Soil cum sand mini plot technique:
◦ In this method, one cubic meter pit is dug in the middle of field.
◦ About five percent of sand by volume is added to the dug soil, mix well and pit is filled in the
natural order.
◦ Crops are grown as usual in the entire area of the field including the pit area.
◦ The plants in the pit show wilting symptoms earlier than the other plants in the remaining area.
◦ Irrigation is scheduled as soon as wilting symptoms appear on the plants in the pit.