Saline Water Can Be Used to Irrigate Sugarbeets

 Stephen Kaffka, Dong Daxue, Gary Peterson
 
The reuse of saline tile drainage water or saline shallow well water for crop production is one of a number of in-valley salt management solutions that are objectives of the San Joaquin Valley Drainage Implementation Program.  Sugarbeet is among the most salt-tolerant crops and can be used as part of a cyclic reuse program to reduce drainage water volume and economize on water from high quality sources.  It is very deep rooted and can use water stored in the soil profile missed by other crops.  Because sugar percentage declines with excess nitrogen, the presence of nitrogen in drainage water or large amounts of residual soil N makes irrigating sugarbeet with these alternative water sources more complex.  Nitrogen accounting in a cyclic reuse program is necessary to assure adequate crop quality for sugarbeet and for good environmental stewardship for all cropping systems.
 
 

The management of salts in the San Joaquin Valley

The amount of salt transported into the San Joaquin Valley in rivers and with irrigation water is estimated as 1,860,000 tons per day (Alemi, 1998).  The amount transported out of the valley in the San Joaquin River is approximately 30% of the amount brought in or 600,000 tons per day.  Over time, this negative balance will cause increasing amounts of productive agricultural land to become saline.  To maintain the productivity of one of the world’s best farming regions, some means must be found to manage the surplus salts accumulating in valley soils. Otherwise, in time hundreds of millions of dollars will be lost to the state’s economy, and the world as a whole will loose one of its most valuable and efficient food-producing regions.
Simply draining soils and returning the salts in drainage water to the ocean via the San Joaquin River is made difficult by the presence of ecologically significant amounts of selenium and other trace elements that may cause harm in estuarine systems in the Delta and San Francisco Bay.  While ocean disposal likely will be necessary in the long term, until the disposal problem is better understood, a number of in-valley solutions have been proposed including the reuse of subsurface tile drain water from farm fields and the use of water from shallow wells.  Reusing tile drain water reduces the volume of water containing salts that must be disposed, while using shallow well water lowers local water tables, keeping the crop root zone from becoming too saline for crop production.  Both types of water have elevated salt contents and may also contain nitrogen.  If drain and shallow well water is moderately to very saline (> 4.0 dS m-1), salt-tolerant crops may be grown.   Sometimes, saline water is blended with low salinity water and then reused.  Depending on the amount of blending, the resulting water may be used for a wider range of crops. None of these alternatives, however, significantly reduce the amount of salt accumulating in the San Joaquin Valley.
 

The use of salt tolerant crops

Several field crops commonly grown in the San Joaquin Valley are moderately to very tolerant of salinity.  These include safflower, sugarbeet, wheat, barley, and cotton. Sugarbeet is also highly tolerant of boron, a common trace element that can be toxic to some crop species when present in amounts greater than 2 to 4 mg kg-1 (Hanson et al., 1993).  Sugarbeet also is efficient at taking up soil N (Hills et al., 1983).  Wheat, barley and sugarbeet are produced during the winter when evaporative demand is low.  Salt in soils and water acts as a water stress factor by reducing the amount of available water in the profile.  The adverse effects of salt on crops growing during the winter period is less, because crop water requirements are much less.
Studies have been conducted in the Imperial Valley and San Joaquin Valley evaluating the growth of diverse crops, including sugarbeet, when irrigated in part with saline water (Ayars et al., 1993; Rhoades et al., 1988).  In general, these studies have shown that sugarbeet tolerates moderate salinity in irrigation water (4 to 8 dS m-1).  The sugar content of the roots was affected by nitrogen also present in the water, but results were not consistent.  Previous trials were limited to the conditions occurring on the farms where they took place, so a wider range of treatments could not be evaluated, making it difficult to formulate general recommendations for crop management.
At the U.C. Westside Research and Extension Center (WSREC), a field site has been used over an eleven year period for diverse projects investigating salt tolerance, particularly involving cotton and processing tomatoes. These plots had been differentially irrigated with high quality Central Valley Project (CVP) water or saline water from a nearby shallow well for an eleven year period, representing a range of irrigation treatments from 0 % to approximately 70 % saline water.  As a result, average soil profile salinity contents (0 to 9 feet) vary from approximately 1.5 dS m-1 to 7.0 dS m-1.  This change and other correlated changes in soil quality reflect the outcome of different cyclic reuse intensities over time on a productive San Joaquin Valley soil.  The WSREC site offered an opportunity to investigate the growth and development of a sugarbeet crop under a range of moderate soil and water salinity levels in one location.  During the 1996-97 growing season, these plots were used:

  1. To evaluate the growth, development and yield of fall-planted sugarbeet exposed to different combinations of soil and irrigation water salinity,
  2. To relate yield, crop growth and quality characteristics to irrigation practices, water quality, soil profile salinity, and soil residual N content, and
  3. To measure the amount of water used and the uptake pattern of sugarbeet throughout the season, and relate water use to soil and water salinity.

Methods

Sugarbeets (SSNB7) were planted in field 41 at the WSREC on October 16, 1996 and harvested on July 19, 1997.  There were 40 plots 32.5 wide by 140 feet long.  Soils were sampled shortly after planting in early November and again immediately after harvest in late July.  Cores nine feet deep were collected in one foot increments in all the plots and analyzed for electrical conductivity of the saturation paste extracts (ECe). Twelve plots were equipped with neutron access tubes, two per plot.  In plots with neutron access tubes, two cores per plot were collected.  These samples were analyzed for ECe, nitrate nitrogen (NO3-N), and boron.  Root depth was determined indirectly by inferring root activity from soil water uptake data measured with a sealed source neutron probe.   Water samples were collected at each irrigation for analysis (Table 1).  Approximately 7.5 tons of salt and 68 lb of N was applied per acre foot of saline irrigation water.
All plots received 100 lbs of N at planting.  Sugarbeets were established and irrigated with non-saline water in October and November, and irrigated with either non-saline or moderately saline water drawn from a nearby shallow well, depending on the treatment, in the April-June period prior to harvest.  All plots were irrigated at the same frequency.  Furrow irrigation was used and the rate of water application was estimated by checking the time required to fill a container of known volume. Irrigation was cut off in late June, approximately four weeks before harvest.
Starting in spring, different irrigation (salinity) treatments were applied to the variably salinized plots.  Irrigation treatments were chosen to maintain or enhance differences in plot salinity and to reflect differences in the availability or application of saline water under farm conditions.  Some plots were irrigated only with low salinity water from the Central Valley Project (average ECe = 0.4 dS m-1), some with saline water only  (average ECe = 6.7 dS m-1), some with saline water in early spring, followed by low salinity water in late spring (average ECe = 4.2 dS m-1), and the last set of treatments used low salinity water in early spring, followed by saline water in late spring  (average ECe = 3.3 dS m-1).  Different average ECe reflect different proportions of saline water applied  (0 %, 100 %, 62 %, and 49 % saline water).  Well water also contained nitrates (Table 1).  The amounts of N applied with irrigation water were  2.2 , 132, 84, and 63 pounds of N per acre, respectively.  A few plots that had received large amounts of saline water in previous years were irrigated with CVP water and a few that had been irrigated previously primarily with CVP water were irrigated with saline water to observe the effects of reclaiming salinized soils or the effects of first using saline water on previously non-salinized plots.  Results from these plots were compared to adjacent ones receiving opposite irrigation treatments.
Plant population was determined during the growing season and at harvest.   At harvest, total biomass, root biomass, percent sucrose, and root Na, K, amino N, and NO3-N concentrations were analyzed, as well as total plant N in tops and roots.  These were used in calculating recoverable sugar.  The Spreckels Sugar Company in Mendota carried out root quality analyses. Yields were determined by harvesting three 70 foot rows from the center of each plot.
Consumptive water use with depth in the profile was monitored throughout the growing season using a sealed source neutron device.  Crop water use was determined using a soil-water balance equation based on neutron probe data:

ETc = P + I + SWD - D

Where ETc= crop evapotranspiration, P = precipitation, I = irrigation, SWD = soil water depletion and D = drainage.  Volumteric soil moisture content in the deepest soil layers changed very little during the growing season (data not shown), except near the end of the season when plant roots penetrated to their deepest extent.   In calculating crop water use based on the mass balance equation, drainage was set equal to zero.
 

Differences in water use

Plots were grouped by irrigation treatment and water use and yields compared.  There was no significant difference among the four irrigation treatments in average seasonal water use (fig. 1).  While increasing salinity in some treatments did not reduce overall water use, more water may have been  taken up from deeper in the soil profile by plants in saline irrigation plots than in plots receiving CVP water.  Differences in seasonal soil water depletion (SWD) with depth, while not a large portion of total water use, indicate that roots were more active deeper in the soil profile in saline irrigation plots (fig. 2).   As plots received larger amounts of salts with irrigation, surface soil layers became more saline (fig. 3),  perhaps restricting water uptake.  A smaller proportion of the available water in the upper soil layers was used by plants in the more saline plots throughout the season, though differences were not always significant (fig. 4).  Higher salinity appeared to force plants to recover water from a larger soil volume and from areas that were less saline.
 

Crop growth and yield responses

Plant populations were stable throughout the trial and were not affected by irrigation treatments (Table 2).   Root rots were not observed.  There were no measurable differences in leaf area index among irrigation treatments (data not shown).  Sugarbeet root yields, when grouped by the irrigation treatments,  were not significantly different from one another (Table 2).  Sugar percent and gross and recoverable sugar yield, however, declined when the crop was irrigated with saline water.
Even though the sugar concentration in roots was higher in plots receiving only CVP water, sugar percent was low on average compared to levels thought necessary for good economic returns in California.  For sucrose concentrations in roots to be as high as possible, sugarbeet crops are managed to become N deficient six to eight weeks prior to harvest.  Crop N status is determined by the analysis of leaf petioles, which should decline to 1,000 ppm NO3 -N or less on a dry matter basis.  At harvest, the petiole N level was 4,000 ppm in the CVP irrigated plots, and a few thousand ppm higher in all the other irrigation treatments, suggesting that in all plots, large amounts of residual soil N were present.  The analysis of soil core samples collected in fall after planting and again after harvest confirmed that large amounts of residual soil N were present at 4 to 6 feet in depth in all the plots (fig. 3).  This N had accumulated particularly in plots that had received large amounts of saline water over time, but also accumulated from normal fertilization and irrigation practices as well, though in lesser amounts.
In a few plots, irrigation treatments were reversed, with saline water applied to plots with a prior history of irrigation with CVP water primarily, and CVP water was applied to plots that had been irrigated predominantly with saline water.  Four representative plots are compared in Table 3.  Plots that had received little saline water had significantly higher sugar percentages in roots and larger sugar yields.  Lower root nitrate content was correlated with higher sugar percentage in roots (data not shown).  This result is commonly reported.  Applying low nitrate CVP water to previously salinized plots (with large amounts of residual N) did not significantly raise sugar percent from the overall plot average, while applying saline, high nitrate water to unsalinized plots reduced sugar percent compared to the unsalinized treatment (Table 3).  Sugar yields declined in plots that received saline irrigation water in proportion to the amount of water applied.  Plots that were initially lower in salinity declined at a greater rate than those that were higher (fig. 5).
 

Manage both salts and nitrogen when using shallow well and tile drain water

Results from this experiment in some ways reflect the outcome of a longer term cyclic reuse strategy for the disposal of saline drainage or shallow well water.  Because of the multi-year use of these plots with saline irrigation, both salts and nitrates had accumulated at depth in the soil profile.  These accumulations adversely affected sugar yields in beets, which are sensitive to excess soil N late in the growing season.  Root yields (and total biomass accumulation) were unaffected by the combinations of soil and water salinity experienced by beets in this trial.  By harvest, some plots had reached an average salinity (0 to 9 feet), of 8 dS m-1, with even higher higher levels in some upper soil layers (data not shown).  Sugarbeet was able to compensate for this increased salinity, we believe, by taking up water from deeper in the soil profile where soils were less saline.   But because of large amounts of N present in both irrigation water and in soils as residual N (from prior saline irrigation treatments and management trials with cotton and tomatoes), salinity alone had little effect on sugar yield.
Nitrogen present in drainage and shallow well water confounds the effects of salinity and makes the cyclic reuse of drainage water on sugarbeet or other N sensitive crops more complex.  The prudent use of drainage or shallow well water for sugarbeet will depend on the amount of N present in the water and the amount of residual N present in the crop root zone.  This may be deeper than traditionally reported, and deeper than the 3 foot depth used for preplant soil N testing.  Sugarbeet is a deep rooted crop that grows for extended periods of time.  In this trial and others conducted previously at WSREC, some water was taken up by beets from nine feet deep.
This capacity must be taken into account in managing beet crops. If a large amount of N is present in drainage or well water, it should be used for irrigation as early in the growing season as possible, followed by the use of better quality water, allowing for the depletion of N by the growing crop six weeks prior to harvest.  The N applied with the water also should be used to discount the amount needed as fertilizer.  If  a large amount of N is present in the soil profile, the addition of N in irrigation water may not affect sugar yields if the N supply meets or exceeds the crop’s requirements, especially late in the growing season.  Under such conditions, there may be little the grower can do to achieve a high sugar percentage.
In developing a cyclic reuse strategy for subsurface tile drain or shallow well water, an accounting of both the salts and the nitrogen applied is necessary.   For crops like sugarbeet, N accounting is necessary to assure larger sugar percentages.  Other crops like wheat may be affected by excessive N through lodging or increased susceptiblity to disease.  For crops that do not respond adversely to excess N, good environmental stewardship still requires that the N applied with irrigation water be considered a part of the grower’s fertilization program.
 

Conclusions

1.  Sugarbeet grew well in response to the moderate salinity levels experience in this trial.  Based on estimates from the literature, it is likely to tolerate higher levels of salinity than those observed in this trial, particularly if soils are deep and hold residual soil water at depths not used by shallower rooted crops, and the soils are not sodic.

2.  Nitrogen present in subsurface tile drainage and shallow well water confounds the effects of salinity and makes the cyclic reuse of saline drainage water on sugarbeet more complex.  The prudent use of saline water will depend on the amount of N present in the water and the amount of residual N present in the soil profile to the maximum likely rooted depth.   To the degree possible, both sources should be taken into account in fertilizing the crop.  On deeper soils, sugarbeet seems able to take up water and nitrogen as deep as 9 feet.

3.  A cyclic reuse strategy must account for the presence of N in water and soils, as well as for the accumulation of salt in the soil profile.

References

Alemi, M. (1998).  Drainage Management in the San Joaquin Valley.  A Status Report.  Calif. Dept. of Water Resources.  San Joaquin Valley Drainage Improvement Program.  Sacramento, California. 65p

Ayars, J.E., Hutmacher, R.B., Hoffman, G.J., Ben-Asher, J., and Solomon, K.H. (1990).  Response of sugar beet to non-uniform irrigation.  Irrig. Sci. 11:101-109.

Hanson, B., Grattan, S.R., and A. Fulton  (1993).  Agricultural Salinity and Drainage. Water Management Series pub. No. 93-01.  Department of Land, Air, and Water Resources, Univ. Calif. Davis.

Hills, F.J., Broadbent, F.E., and Lorenz, O.A. (1983).  Fertilizer nitrogen utilization by corn, tomato, and sugarbeet.  Agron. J. (75):423-426.

Rhoades, J. D., Bingham, F.T., Letey, J., Dedrick, A.R., Bean, M., Hoffman, G.J., Alves, W.J., Swain, R.V., Pacheco, P.G., and Le Mert, R. D.  (1988).  Re-use of drainage water for irrigation:  Results of Imperial Valley study.  Hilgardia 56(5)1-45.

Figures

Figure 1.  Water use by sugarbeets averaged by irrigation treatments.

Figure 2.  Soil water depletion (SWD) by sugarbeet plants by depth in the soil profile and cumulative water use by depth.   A.  SWD for 8 plots receiving a mixture of CVP and saline water, or CVP water only.  B.  SWD for 4 plots receiving only saline water (S).

Figure 3.  Soil NO3-N and ECe values in plots receiving only saline water (S) or CVP water (F).  The saline irrigation treatments were applied primarily to plots that had been received the most saline water in prior years.

Figure 4.  Average volumetric soil water content (0 to 6 feet) in saline (S) and non-saline (F) irrigated plots.

Figure 5.  The decline in sugar yield in relation to the amount of nitrate N applied with irrigation water.  Plots are grouped according to their average profile salinity levels near planting.  The application of N in irrigation water to plots with smaller amounts of residual N caused a greater decline in less saline plots than in plots with higher amounts of residual N.
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