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The objectives of this study were to examine the amount and importance of N mineralized from soil organic matter in commercial sugar beet fields and to make observations of sugar beet plant N uptake in N fertilizer trials conducted by beet fieldmen in commercial fields to improve our understanding of beet responses to N fertilizer.
Sugar beet nitrogen fertilizer guidelines are based on either fall or spring nitrate-N levels to 3-foot depth in the soil and total plant needs estimated at 8 lb N/ton of beets. These assumptions provide estimates for N fertilization which have been useful for many years. To make beet production and processing as efficient as possible, it is important to use only the N fertilizer needed to grow the crop. In the cases where soil organic matter mineralization is large compared with residual soil nitrate or fertilization, the current guidelines could overestimate crop fertilizer needs.
When sugar beets receive N in excess of their needs, total beet yield and leaf growth are high, but both beet sugar content and total sugar yields can be depressed. Extra nitrate and ammonium in the beet pulp reduce sugar factory efficiency.
Sugar beet fieldmen have conducting N fertilizer rate trials in growers fields during the last few years. These trials have the basic structure of large plots with several N rates and one to three replicates. In general the recommended N fertilizer rate is based on the soil nitrate present in the soil profile to a depth of 3 feet in each field and a yield goal for that field. Other large field plots are treated with twice the recommended N rate, half the recommended N rate, and no fertilizer as the check. In a some of the fields with high residual nitrate, the fieldman and grower opted to leave all plots unfertilized. Ten fields were studied between Parma, Idaho, and Vale, Oregon, in 1996. Twelve growers' fields were studied through harvest in 1994 and an additional eight fields in 1995 between Brogan, Oregon, and Burley, Idaho (Table 1). Several other fields were set up each year, but their experimental value was lost in cultivation or harvesting errors.
The growers took care of all of the cultural practices, and the fertilizers were applied in cooperation with fertilizer industry representatives. Sugar beet fieldmen kept track of crop progress and coordinated the harvest, collecting data on yield and quality based on the entire field plots. Beets were evaluated for total yield, tare, sucrose content, conductivity, and beet pulp nitrate by the Amalgamated Sugar Co.
Additional work was done by the Malheur Experiment Station. The percent sugar extraction and recoverable sugar were calculated based on empirical formulas. Soil samples were collected in the spring for estimates of N mineralization via three methods: anaerobic incubation, aerobic incubation, and the buried-bag method. Twelve to fourteen representative beets with their leaves and crowns were harvested from each plot just before harvest then taken to the Malheur Experiment Station. Leaves and crowns were dried, weighed, ground, and analyzed for total N content. The beets were weighed fresh after the leaves and crown were removed, ground, and a subsample of the beet pulp was weighed wet, oven-dried to determine dry matter content, then analyzed for total N content. Beet N uptake per acre in the leaves, crowns, and beets was calculated based on the clean beet yield of each plot, the ratio of beet dry weight to fresh weight, the proportion of dry crown and leaf tissue to dry beet in the tissue samples, and the tissue sample N contents.
Total available soil N supply was calculated based on the sum of spring available nitrate and ammonium, any applied fertilizer N, and N mineralization (estimated by anaerobic incubation or seasonal N balance). Nitrogen use efficiency was calculated for each plot by dividing the total plant N uptake by the total available N supply for each plot and multiplying by 100. Nitrogen mineralization was also estimated by aerobic incubation and buried bag methods.
Results and Discussion
Spring soil nitrate N ranged from 61 to 399 lb N/acre, depending on the field (Table 2). Optimistic yield goals ranging from 25 to 40 ton/acre of beets implied N fertilizer needs of 0 to 216 lb N/acre. The lowest applied N rates were 0 lb N/acre at twenty two of the thirty sites.
The 1994 season was favorable for high yields (Table 3), and all cooperating growers kept weeds and diseases under control. The 1995 season was far less favorable, with lower temperatures and cloud cover during the growing season. Repeated rainfall events in 1995 made efficient N use difficult and reduced residual nitrate and ammonium in the fields at harvest. Repeated hail during 1995 in the Treasure Valley was damaging at certain locations. The 1996 season was more favorable.
The highest-yielding N fertilizer rates ranged from 0 to 205 lb N/acre depending on the field studied. Sugar beet response to N fertilizer varied substantially. Reasonable yields were routinely obtained near Vale, Ontario, and Nyssa with low rates of N fertilizer (Tables 2 and 3). Clear increases in beet yield and sugar production occurred on shallow soils with irrigation apparently in excess of evapotranspiration. Beet pulp nitrate at 0 applied N suggested that there was excessive N supplies in many fields, even without any fertilizer nitrogen (Table 3). Beet petiole nitrate levels were consistent with high yields at low N fertilizer inputs in these fields (data not shown).
The optimal N rate was determined independently for each field based on the highest yield of recoverable sugar. Beet plants at the optimal applied N levels contained 126 to 439 lb N/acre at harvest depending on the field (Table 4). Anaerobic incubation estimates of N mineralization ranged from 88 to 285 lb N/acre, depending on the field (Table 5). Mineralized N appears to be a large N source averaging 163 lb N/acre over the 20 fields (Table 4). Fields with high spring residual nitrate are not necessarily going to have high rates of N mineralization; fields with low spring residual nitrate are not necessarily going to have to low rates of N mineralization , r2 = 0.029, based on 1994 and 1995 data.
In 1994, the anaerobic incubation estimates of mineralized N ranged in the same order of magnitude as field method using N balance. The N balance method was based on measuring residual soil nitrate and ammonium at harvest and plant N content at harvest, then subtracting all known available N sources. The N balance was not comparable in 1995, as would be expected after a season with untimely rainfall events. In 1996, fields with suspected heavy irrigation showed low available N balances. The buried bag method of N mineralization was laborious and provided numbers similar and less than the anaerobic method. The aerobic method of soil incubation provided estimates of N mineralization in the same range as the anaerobic method, but the numerical values were more erratic.
At the most productive N level tested at each site, sugar beets were able to recover between 35.4 and 90.4 percent of the estimated total N supply (based on the sum of soil nitrate and ammonium to the 3-foot depth, fertilizer N, and N mineralization in Table 4). The one efficiency of 125.7 percent occurred in a field irrigated with considerable nitrate in the irrigation water. Efficiencies less than 75 percent appear to be related to very high N supply at 0 N applied, irrigations and rainfall in excess of evapotranspiration, or sugar beet cyst nematode.
1. The mineralization of organic matter provided on average 163 lb N/acre per year, but the fertilizer guides assume that only 30 to 50 lb N/acre will be mineralized.
2. Nitrogen fertilizer guides overestimated crop fertilizer needs. Nitrogen fertilization was of marginal benefit for sugar beets when nitrogen mineralization is high on deep soils without excessive irrigation. In these studies, N fertilization was often counter productive.
3. The anaerobic incubation method of estimating N mineralization was useful.
These trials depended of the work of many growers and fertilizer fieldmen, without which the effort would have been impossible. We extend special appreciation to Del Traveler, Terry Tindall, Stacy Camp, Bill Walhert, and Bob Huffaker, Don Bowers, Dave Elison, Steve Lund, Clark Millard, Bob Komoto, Monty Saunders, Saud Hafez, Ray Winegar, Rod Faham, Al Scott, Lou Wettstein, and Terry Miller.
The financial support of the beet growers associations, Amalgamated Sugar, and the Oregon Department of Agriculture is gratefully acknowledged.
Table 1. Characteristics of 20 sugar beet fields used for soil N mineralization studies in 1994, 1995 and 1996, Malheur Experiment Station, Oregon State University, Ontario, Oregon.
|Field||Location||Soil texture||Soil pH||Soil organic matter||Soil depth||Variety||Planting
|Irrigation system||Comments||Previous crop|
|1||Burley||sandy loam||8.4||1.25||> 6||MH 9455||April 11||side roll||cyst nematode||Beets|
|2||Minidoka||silt loam||8||1.5||2.5||PM-9||March 19||side roll||rock at 2-3||Wheat|
|3||Minidoka||silt loam||8||2.35||2.5||WS 91||March 20||side roll||rock at 2-3||Wheat|
|4||Jerome||loam||7.55||1.05||2.5||WS 91||April 25||side roll||rock at 2-3||Potatoes|
|5||Nyssa||silt loam||7.65||1.4||>6||PM-9||last week March||furrow||Onions|
|6||Ontario-Vale||fine sandy loam||7.5||1.6||>6||PM-9||March 7||furrow||Onions|
|7||Ontario-Vale||silt loam||7.75||2.2||>6||PM-9||March 12||furrow||Onions|
|8||Ontario-Nyssa||fine sandy loam||7.4||1.75||>6||PM-9||2nd week March||furrow||Onions|
|9||Nyssa||silt loam||7.9||2.1||>6||PM-9||March 26||furrow||B deficient||Potatoes|
|10||Vale||silt loam||7.6||2.05||>6||PM-9||March 18||furrow||Onions|
|11||Brogan||silt loam||7.6||1.6||>6||RSW-81||March 14||furrow||Onions|
|12||Ontario||silt loam||7.6||1.5||>6||PM-9||April 5||furrow||Beans|
|1||Buhl||silt loam||8.1||1.2||>6||PM-9||May 10||furrow||beans|
|2||Burley||silt loam||8.1||1.49||2.5||Beta 8422||April 10||side roll||wheat|
|3||Rupert||sandy loam||7.8||1.28||>6||WS 62||April 20||furrow||beans|
|4||Minidoka||silt loam||8.2||1.65||2.5||PM-9||April 4||side roll||potatoes|
|5||Nyssa||silt loam||7.6||1.45||>6||PM-9||March 30||furrow||rhizoctonia||onions|
|7||Ontario||silt loam||7.7||1.43||>6||PM-9||March 29||sideroll||potatoes|
|8||Nyssa||silt loam||7.4||1.47||>6||PM-9||March 27||furrow||rhizoctonia, flooding||beans|
|3||Parma||silt loam||8.0||2.2||>6'||PM-9||April 11||solid||radish|
|5||Vale||silt loam||8.0||1.4||>6'||WS 91||April 8||furrow||onions|
|6||Vale||silt loam||8.0||1.4||>6'||WS 91||April 8||furrow||onions|
|7||Vale||silt loam||8.0||1.3||>6'||WS 91||April 8||furrow||onions|
|8||Ontario||silt loam||7.8||1.2||>6'||WS 62||March 19||furrow||onions|
|9||Ontario||silt loam||7.8||1.5||>6'||WS 62||March 25||furrow||onions|
|10||Ontario||silt loam||7.8||1.5||>6'||WS 91||March 26||furrow||onions|
Table 2. Optimistic yield goals, soil nitrate, recommended N fertilizer rates, grower's preferred N fertilizer rates, and best fertilizer N rates for 1994, 1995 and 1996, Malheur Experiment Station, Oregon State University, Ontario, Oregon.
nitrate 0-3 ft
|Total N needed for optimistic yield goal||Recommended N fertilizer for optimistic yield goal||Growers' preferred N rate||Lowest N rate used in trial||Highest yielding N rate for the trial|
|1994||ton/acre||- - - - - - - - - - - - - - - - - - - - - - - lb N/acre - - - - - - - - - - - - - - - - - - - - - - -|
part of field.
1NA: data not available.
Table 3. Beet yield and quality at the best N rate for each of 20 growers' fields, Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1994, 1995 and 1996.
|Summary of characteristics||Highest yielding plant performance|
|Field||Location||Optimistic yield goal||Soil nitrate 0-3'||Most productive N rate for trial||Clean beet yield||Sucrose||Conductivity||Extraction||Recoverable sugar||Pulp nitrate|
|1994||ton/acre||lb N/acre||lb N/acre||ton/acre||%||%||lb/acre||ppm|
Table 4. Comparison of soil nitrogen supply, beet plant nitrogen content, and N use efficiency at harvest for beets grown at the highest-yielding N level in 20 fields. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1994, 1995 and 1996.
|Best plant performance||N supply||Plant N content|
|Field||Location||Beet yield||Recoverable sugar||Most productive N rate for 1994 trial||Spring soil nitrate -N 0-3 ft||Spring soil ammonium -N 0-3 ft||Estimate of N-mineralization (anaerobic)||Total available N supply||Leaves||Crown||Beets||Total||Total plant N content at harvest per ton of beets||N use efficiency1|
|1994||ton/acre||lb N/acre||lb N/ton||%|
|11||Brogan||29.4||7,280||0||165||48||224 2||437 2||100||37||164||301||10.08||68.9 2|
|12||Ontario||45.6||12,732||0||104||49||292 2||445 2||123||46||226||395||8.65||88.8 2|
plant N content as a percent of the total available N supply.
2N mineralization estimate by season-long N balance.
3NA: data not available.
Table 5. Estimates of N mineralization made in 30 growers' sugar beet fields by two different methods, Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1994, 1995, and 1996.
|Grower||Location||Soil texture||Organic matter||Anaerobic incubation||Available nitrogen balance|
|1994||%||- - lb N/acre - -|
|6||Ontario-Vale||fine sandy loam||1.6||149||238|
|8||Ontario-Nyssa||fine sandy loam||1.75||115||125|