6.3.1: Plant Root Zone

Established perennial plants such as alfalfa, grasses, trees, and shrubs have relatively constant root zone depths. The maximum effective root depth depends on several environmental, crop, and soil factors. The range of maximum effective root zone depths for various crops is summarized in Table 6.2. The maximum effective depth used for scheduling, which is usually less than the maximum depth where roots are found, represents the depth of the soil profile that has enough rooting density for extraction of available water. Values in Table 6.2 should be used cautiously and adjusted for local soil and climatic conditions.For annual crops the root depth prior to the date of maximum rooting is described by:

R_d = R_dMIN + (R_dMAX – R_dMIN)R_f (6.6)

where: R_d = root depth

R_dMIN = minimum root depth for young plants,

R_dMAX = maximum effective root depth, and

R_f = root growth factor.

Table 6.2. Range of maximum effective rooting depths for fully grown plants (from Martin et al., 1990).

Crop	Maximum Effective Depth (ft)
Alfalfa	3.3-10
Banana	1.3-2.6
Barley	3.3-4.3
Beans	1.3-2.6
Cabbage	2.0-3.3
Carrots	1.6-3.3
Celery	1.0-1.7
Citrus	3.3-5.9
Clover	2.0-3.0
Corn	3.3-6.6
Cotton	3.3-6.6
Cucumber	2.3-4.0
Dates	5.0-8.3
Flax	3.3-5.0
Grapes	3.3-6.6
Grass	1.7-5.0
Groundnuts	1.7-3.3
Lettuce	1.0-1.7
Maize	3.3-6.6
Melons	3.3-5.0
Olives	2.6-6.6
Onions	2.6-6.6
Palm trees	2.3-3.6
Peas	2.0-3.3
Peppers	1.7-3.3
Pineapple	1.0-2.0
Potatoes	1.3-2.6
Safflower	3.3-6.6
Sorghum	3.3-6.6
Soybeans	2.6-5.0
Spinach	1.0-1.7
Strawberries	0.7-1.0
Sugar beet	2.6-6.6
Sugarcane	4.0-6.6
Sunflower	3.3-8.3
Sweet potatoes	3.3-5.0
Tobacco	1.7-3.3
Tomatoes	2.3-5.0
Wheat	3.3-5.0
[a] The maximum values given represent the full expression of the genetic potential for root growth and are only found in uniform, fertile soils of low resistance to root penetration.

The development of a corn root zone during the season is illustrated in Figure 6.3. The minimum root depth for seedlings is normally considered to be 4 to 6 inches. The actual initial depth may deviate slightly from this value, but an error on the minimum root depth will have very little effect on the soil water balance or irrigation scheduling. The root growth factor, which describes the rate of root zone expansion during the season, can be computed as:

\(R_f = \frac{D_{ag}}{D_{tm}}\) (6.7)

where: D_ag = days after germination and

D_tm = days from germination to maximum effective depth.

The time required for roots to reach the maximum effective depth varies considerably for different environments, crops, and varieties. Local values and individual experience must be used to determine these values. Root zone depths for various stages of crop development are given in Table 6.3. Plants do not extract water uniformly throughout their rooting depth. Usually there is more water extracted from shallow depths and less from deeper depths. An approximation of the extraction pattern is the 4-3-2-1 rule, i.e., 40% of the water comes from the top 25% of the root zone, 30% from the second 25%, and so forth. This conceptual approximation is illustrated in Figure 6.4. If for example, the root zone depth is 4 feet, and the plants extract 2 inches of water between irrigations, 0.8 inches would be obtained from the first foot, 0.6 inches from the second foot, 0.4 inches from the third foot, and 0.2 inches from the fourth foot. This concept applies only when the root zone is refilled, or nearly refilled, following irrigation. If the root zone is not completely refilled to field capacity during irrigation, then more water will be obtained from the shallower depths. Under these conditions, there is usually a sandwiched layer of dryer soil between the upper part of the root zone and the lower part.

Figure 6.3. Development of a corn plant’s root zone.

Figure 6.4. Average water extraction pattern from the plant root zone, the 4-3-2-1 rule.

Example 6.2

Determine the root zone depth for corn at early tassel assuming that depth at germination is 6 in, maximum rootding depth is 4 ft, full depth occurs 90 d after germination, and early tassel occurs 50 d after germination.

Given:: D_ag = 50 d

D_tm = 90 d

R_dMIN = 0.5 ft

R_dMAX = 4.0 ft

Find: R_d at early tassel

Solution

\(R_f = \frac{D_{ag}}{D_{im}}\) (Eq. 6.7)

\(R_f = \frac{50 \text{ days}}{90 \text{ days}} = 0.56\)

\(R_d = R_{dMIN} + (R_{dMAX} - R_{dMIN})R_f\) (Eq. 6.6)

\(R_d = 0.5 \text{ ft} + (4.0 \text{ ft} - 0.5 \text{ ft}) 0.56 = 2.5 \text{ ft}\)

Table 6.3. Example root zone information for various annual crops grown in Nebraska (adapted from Melvin and Yonts, 2009)

Assumed Root Depth (ft)	Corn 3) ^[a]	Grain Sorghum (3)^[a]	Soybean (3)^[a]	D ry Beans (2.5)^[a]	Sugar Beets (3)^[a]	Winter Wheat (4)^[a]	Alfalfa (4)^[a]
1.0	vegetative	vegetative	vegetative	vegetative
1.5				initial flowering pod set
2.0	12 leaf		early bloom	beginning pod fill	June 1	fall growth
2.5	16 leaf	flag leaf	full bloom	full seed fill	July 1	spring growth
3.0	silking	boot	pod elongation		July 15	joint
3.5	blister	bloom			August 1	boot
4.0	beginning dent	dough	full seed fill		Sept. 1	dough
5.0
6.0							established sand

Even though root zone depths exceed 3 feet by midseason for many crops, to be on the safe side, many managers use a 3-foot root zone until late in the season. As maturity approaches, the plants are allowed to extract water from the entire root zone.