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SOIL, WATER AND PLANT RELATIONSHIPS
The water held by the soil between field capacity and permanent wilting point is termed the "available water holding capacity" of the soil. It is water that is "available" for the plant to use. Water added to the soil in excess of field capacity will drain down, below the active root system. Water held by the soil that is below the permanent wilting point is of no use, the plant has died.
As a crop manager you are concerned with the soil moisture throughout the depth of the plant's active root system, the "effective root zone". The effective root zone is that depth of soil where you want to control soil moisture (just as you control fertility and weed/pest pressures). The effective root zone may or may not be the actual depth of all active roots. It may be shallower because of concerns for crop quality or development (as with many vegetable crops). For a preirrigation though, you may want to consider the maximum potential root zone as the effective root zone for that irrigation.
For example, with cotton you may estimate the effective root zone as 6 feet for a preirrigation, 2 feet for the first seasonal irrigation, 4 feet for the second seasonal, and 6 feet thereafter. For an almond orchard, you may estimate the effective root zone as four feet for the entire season. With onions, the major concern is with the top 2 feet.
Table SWP-1 shows some estimates for effective root zones healthy crops grown on deep well-drained soils.
TABLE SWP-1. Normal Crop Rooting Depths (feet).
| Alfalfa | 5-10 | Grain | 3-4 |
| Asparagus | 6-10 | Grass | 2-4 |
| Beans | 3-4 | Lettuce | 1-1.5 |
| Beets(sugar) | 4-6 | Melons | 6 |
| Broccoli | 2 | Onions | 1 |
| Cabbage | 2 | Peas | 3-4 |
| Carrots | 2-3 | Peppers | 2-3 |
| Cauliflower | 2 | Potatoes | 3-4 |
| Celery | 3 | Spinach | 2 |
| Citrus | 4-6 | Squash | 3 |
| Corn(sweet) | 3 | Strawberry | 3-4 |
| Corn(field) | 4-5 | Tomatoes | 6-10 |
| Cotton | 6-8 | Walnuts | 12 |
| Deciduous tree | 6-8 | ||
| Grapes | 4-6 |
Note the wide range in some of the root zones in the table above. This is an example of irrigation as both an art and a science. There are scientific ideas like "effective root zones" and "field capacity" that provide a way to think about water management. But it is up to you, the grower, to apply these ideas, to pick the effective root zone, to estimate the field capacity and permanent wilting points
To make use of the ideas of available soil moisture and effective root zones, we need standards of measurement (as the "foot" is a standard of measurement for length or a "gallon" is a standard for volume). The standard of measurement for effective root zones is depth, either inches or more commonly, feet. But how do we measure soil moisture?
First consider a cubic foot of soil that has just been taken from a field. Soil is not completely solid. It has mineral solids held in a matrix-type structure intermixed with open spaces, pores. Assume we were somehow able to compress the soil so that all the solids were together. We would have a depth of solids (mineral and organic particles), a depth of water that had been held by the soil, and a depth of air that had also been within the soil pores. See figure SWP-2. We measure soil moisture in terms of that depth of water held by a depth of soil.
Commonly this is inches of water per foot of soil, or just "inches/foot". For example, we might say . . . "the field capacity of this soil is 2.0 inches/ft". This means that the most water this soil will hold is 2.0 inches per foot. We might say . . . "the current soil moisture is 1.7 inches/foot".
FIGURE SWP-2
TABLE SWP-2. Representative Available Water Holding Capacities
| Soil Class | Range/Average in/ft |
| VeryCoarse to Coarse Sand | 0.50-1.25/.90 |
| Moderately Coarse Sandy Loams | 1.25-1.75/1.50 |
| Medium Loams | 1.50-2.30/1.90 |
| Clays and Clay Loams | 1.60-2.50/2.10 |
| Peats and Mucks | 2.00-3.00/2.50 |
The numbers in Table SWP-2 are ranges of normal available water holding capacities. The Reference chapter of this handbook contains specific numbers for common soils of the District. The maximum depth of water available to the plant in the effective root zone can be determined by adding the available water holding capacity for each foot of soil in the effective root zone.
For example, assume you have an effective root zone that is four feet deep. The first two feet are Medium Loam, the third foot is a Sandy Loam, and the last foot is a Coarse Sand. The estimated maximum available water (using the example numbers from Table SWP-2) is . . .
Medium Loam - 1-2 ft (2 x 1.9) = 3.8 inches
Sandy Loam - 2-3
ft
= 1.5
Coarse Sand - 3-4
ft
= .9
Maximum
AWHC
= 6.2 inches
If the soil in the entire four-foot effective root zone was at field capacity, there would be 6.2 inches of water available to the crop to use. It's as if there was a 6 inch deep pan of water that the crop was growing in.
Last updated September 2000