The California sugar beet industry has declined from more than 300,000 acres and 10 processing plants 40 years ago to about 50,000 acres and 2 processors today. Increasing sugar on the world market, static prices to growers and the high cost of farming in California have whittled this once Goliath of an industry down to a David-sized group of only the best growers. The folks left in the game have to aim their slingshots with increasing precision to maintain profitable yields and no area is more critical than nitrogen management.
Dozens of California sugar beet trials going back for more than 30 years have proven that maximum sugar yields are dependant on knowing the nitrogen (N) and water dynamics of the particular field you're farming. A “one-size-fits-all” approach for every field does not work. In these past studies, the highest sugar yields were obtained with N fertilizer rates ranging from 0 to 240 pounds per acre. Maximum profit (not always maximum sugar) was obtained with irrigation cutoffs of 30 to 50 days. In the absence of any other field specific information, a 160 pounds per acre level of N fertilizer and a 30 day cutoff gave the best average performance.
It takes about 8.5 pounds of N to make a ton of sugar beets — 350 pounds per acre for a 40-ton crop. Most of this usually comes from residual N in the soil, but the trick is to know how much additional N fertilizer to apply. Too little fertilizer and you lose tonnage; too much and you have excessive nitrates at harvest which depress sugar percentage and make sugar crystallization more difficult.
Kern field sampling
In 2001 and 2002 six Kern County, Calif., grower production fields were intensively sampled for soil and plant N levels and instrumented with soil moisture monitors to look at optimizing irrigation and N management on clay to sandy loam soils. Beets were planted and watered in October and harvested in June and July. Over the total of 24 sites monitored in these fields root tonnage varied from 32 to 64 tons per acre with 12.9 percent to 16.9 percent sugar for final sugar yields of 10,000 to 19,000 pounds per acre.
Soil nitrate-N reserves in the top 3 feet of soil ran from 50 to 450 pounds per acre. Soil samples at the end of harvest showed that total N (soil residual plus applied fertilizer) down to a depth of 9 feet had been depleted by 230 to 585 pounds per acre. Fertilizer applications ranged from 125 to 240 pounds per acre.
In general, root tonnage increased and percent sugar content decreased with increasing soil nitrate-N. The highest total field yield was 47 tons per acre roots at 15.1 percent sugar for a total of 14,179 pounds per acre sugar.
A 15 percent sugar content is the unspoken criteria of the processor. Below this, the grower receives no premium and net profits start to fall. Percent sugar content was significantly correlated with mid-February soil nitrate-N and fell below 15 percent when the concentration was more than 20 ppm (240 pounds per acre) in the top 3 feet. Four to six in-season irrigations were optimal, depending on soil type.
2003 soil sample
In mid-February 2003 a composite soil sample to 3 feet was taken from every sugar beet field in Kern County in order to determine the district-wide diversity of residual soil nitrate-N and its impact on yield and sugar content. Out of 63 contract fields, 43 were in the lower residual nitrate-N category below 20 ppm, 14 were between 20 and 30 ppm and 16 were greater than 30 ppm. Seven fields, from the lowest soil nitrate-N at 9 ppm to the highest at 99 ppm were chosen for additional petiole monitoring over the season. Only one field (with a 48 ppm soil nitrate-N) maintained plant petiole nitrate-N above 1,000 ppm after mid-May — usually a cause for low sugar percentage for a July harvest. But this field had a 14.1 percent sugar content and better tonnage than another field that had a mid-February soil nitrate-N of only 16 ppm and ended up with a 13.7 percent sugar.
District-wide beet tonnage ranged from 19 to 59 tons per acre and 12.2 to 17.4 percent sugar. Unlike the 2001 and 2002 trials, the sugar percentage was only weakly correlated to the mid-February soil nitrate-N content. However, the threshold was the same where sugar percentage sugar was less than 15 percent, on average, when this value was greater than 20 ppm for the 7 intensively sampled fields. For the entire district this threshold was 29 ppm.
Only two statistically significant relationships were apparent in this regional sampling effort:
An average plant population of 45,000 plants per acre yielded 40 tons per acre of roots.
Sugar percentage decreased rapidly when root pulp nitrate at harvest increased; dropping below 15 percent with concentrations more than 126 ppm.
Final recommendations: For fall planted beets, apply no more than 50 pounds per acre N fertilizer at planting. Sample the soil to a depth of 3 feet in February making one composite sample from 10 to 12 locations per field.
Where soil nitrate-N is greater than 30 ppm do not sidedress fertilizer, for 20 to 30 ppm sidedress 50 pounds per acre, 15 to 20 ppm sidedress 80 pounds per acre and less than 15 ppm sidedress 120 pounds per acre.
Watch beet petiole nitrate-N and water-run 30 to 40 pounds per acre N if it falls below 1,500 before mid-April. Do not water-run after this time or you will depress sugar content for a July harvest.