Nitrogen Use Management

Attempts to "predict" how much N might be available in season have been made by various researchers throughout various sugarbeet production areas. None have been totally successful due in part to the unpredictability of the affects which weather, crop rotation, soil type, etc. can have on the outcome of the mineralization process.

For over three decades, Amalgamated consultants and producers have used the constant that eight lbs. of N should be required to produce each ton of beets. With improvements in seed genetics in general and the effects of Round-Up Ready culture in specific it has become apparent that further refining of this approach should be undertaken. Because soil type has an inherent effect upon the potential mineralization of N, we determined to discover if the amount of N required could be further refined by whatever varying amount the soil type might dictate. In taking this approach it was hoped that at least a general degree of effect could be determined and not necessarily specific mineralized amounts of N. Where we approach N input requirement as lbs. required per projected tons to be yielded (basis of previous yield and quality history) we were hoping to refine the input down from eight lbs. per ton by whatever amount would improve sugar percentage and other quality factors without compromising tonnage, (basis of soil type).

Over a three-year period, 2008-2010, we selected sites in our production areas where we could look at the response of varying lbs. of N per projected tons to be produced on varying soil types. The first year, because silt loam comprised the greater balance of soil in our production areas, two sites of silt loam were chosen. The second year, we chose another site of silt loam and two sites of sandy loam soil. The third year we looked at one site with silt loam soil, one with sandy loam and two sites with clay loam soils.

All plot sites were located inside a commercial field. Attempts were made to choose an area inside each fields' boundaries that was uniform. The grower-cooperators' five-year average for tons per acre production was used as treatment yield goal. All plots were arranged in randomized complete block design. Each treatment for projected N-usage was replicated six times. Plot size was six rows (22-inches), 11-feet wide x 30-40-feet in length.

Soil samples of first-, second- and third-foot depth were pulled and analyzed separately for each replication area within the study. The number of treatments were determined by the various lbs. input which we looked at in the given year. In 2008-09, N treatments of 5, 6, 7 & 8 lbs. per historical ton per acre were used. In 2010, we used N treatments of 4, 6, 8 and 11 lbs. per historical ton. The change in treatments came from the desire to look at the effects beyond top and bottom of potential yield and quality ranges to further define if we were "in the zone" for potential yield, (top end on N input), as well as for quality, (low N at harvest for maximizing quality factors).

Soil sample results from each foot increment were added together and multiplied by the factor of 4 to convert ppm N to lbs. N. The result was considered as the carry-over, available N contained in each replication area. Each plot had an assigned lbs. N per projected ton to be produced, (as per the above mentioned treatments) which was multiplied by the historical five-year average T/A yield goal. This determined the overall lbs. N required per treatment which also would be the overall lbs. N available. The carried-over lbs. N figure was then subtracted from each overall lbs. N required figure to determine how many lbs. of N should be applied to each plot. In regard to overall lbs. N available it should be explained that the variable of mineralizable N potential is considered as the unknown, which in part we were seeking to further refine basis of soil type and the crops response to varying lbs. N input on a ton produced basis and the quality factors of sugar percent, brei N and conductivity.

The prescribed amounts of additional N were applied in the form of Urea. Since each of the study sites was irrigated with over-head irrigation, the urea was then incorporated via irrigation water. The application timing of N varied some, dependent on weather conditions etc. but was between 2-6 leaf stage of growth overall, which was considered adequately ahead of when there would be demand for it.

Table 1 (Statistical difference is noted between different letters as well as color of box.)

Observations

In-season observations of the response to the varied rate of N input were as follows: Plots with the lowest rates of input started showing slower growth in size and lighter coloration of tissues, i.e. (smaller petioles and leaf blades as well as fewer leaves formed), at about the end of June to the first 10 days in July. These observations continued until harvest. At harvest, the least fertilized were much yellowed and had noticeable less foliage as well as root size as indicated by the lower yield rates for these plots. The plots which were heavier fertilized showed continued growth and more green coloration into harvest. The petiole and leaf sizing as well as numbers of leaves were much more extensive than that of the lower rates of N fertility. The sizing of roots and of course weights was greater, indicating the increase in yield.

The degree of variation for these observations also changed with the soil type for the most part. In sandy loam soil the variation was more pronounced as would be expected. In the silt loam soil the observations varied but at a lesser degree and in the clay loam soil the sizing of leave blade and petioles decreased with lower N input but the coloration was less diminished, yet observable.

Silt loam soils, which were assessed showed results (Table 1) which indicated that there is significant difference between using 5-6 lbs. N per ton, rate of fertilizing vs. 7-8 lbs. per ton.

The root yield, even though higher by one ton at the 8 lb/ton rate was not significant. This would indicate that the lower rate of N input did not compromise any T/A yield potential. The quality factors were all enhanced by lowering the rate of N input. Though gross dollar per acre did not show significantly greater at any N treatment level, when the additional costs of fertilizer and hauling expense are applied for the additional tonnage, the net result favors lower N rates by $72 per acre increase. This fact of course is the most important to a producers' bottom economic line.

Sandy loam soil was studied in 2009 at two locations and at one in 2010. The location in 2010 showed no significant differences between rates of input, largely due to unexpected high mineralization experienced late in the season. I mention this site, mainly to reference the fact that what we are trying to better manage is resultant from the very thing illustrated by what happened with this trial site. Mineralization is one of the hardest variables of N management with which we work.

One of the main considerations in trying to manage N use in sandy soils has always been the concern of losing use of N in season to excessive leaching through the profile, as well as not accomplishing adequate mineralization level of N. This relates to our approach or method of basing management of N on projecting what tonnage may be produced and how many lbs. per ton it might take to do that. Some field situations make it hard to adequately gauge what the tonnage outcome might be (even basis of past years production), and then the weather factors complete "the disconnect" from your best management projections and the reality of what actually happens in the season.

We had one site (K. Bowen), out of the three which were sand that showed statistically what we had supposed. In sand, the main concern is to supply adequate N so as to maintain adequate tons production without over-fertilizing and damaging quality. Because of leaching and lower mineralization rate, N availability does usually drop low enough later in-season so as to not interfere with sugar formation. If levels drop too low too early, however, they can short tonnage potential.

The results from this trial are shown in Table 2. There was 7.92 t/a difference between the low rate of 5 lbs/ton input and the 8 lbs/ton rate. There was a difference of 2282 lbs/acre of sugar produced between the 5 lbs. and 8 lbs. N rates. To a producer this would equate to $287/acre difference in income. The results indicate that for sandy soils the higher rates of 7-8 lbs. N per ton would be more correct for N management.

Clay loam soil sites were trialed in 2010, one site in western Idaho and one in south-central Idaho. Whereas clays have higher organic matter content and greater nutrient holding capacities, it was expected that a lower input rate of N would show more favorable without diminishing yields, yet potentially increasing quality factors and hence overall sugar per acre production. Table 3 shows the results from the south-central Idaho site which supports the above expectation.

Root yield was statistically not significantly different which counters the concern that reducing N input levels might cost a grower potential tonnage which could be produced. The sugar percent was greatly enhanced, by 1.28 percent more from the 11 lbs. rate to the 4 lbs. rate of input. It was increased by .62 percent from the 8 lbs. to the 4 lbs. rate. Even though the nitrate N levels would be considered adequately low at the 4-8 lb. input levels, there was still a statistical break between the 6 and 8 lb. rates suggesting that not going over 6 lbs. N per ton is more favorable. This break in statistical significance applied to all the quality factors as well as the gross dollar per ton valuation. Though the gross dollar per acre and net dollar per acre determinations did not show significantly different, largely due to the overriding effect of tonnage, at the break between 6 and 8 lbs. N input, there is a $80.50 per acre difference favoring the input of 6 lbs. or less of N per ton produced when the additional grower expenses of freight and fertilizer cost are subtracted.

Summary:

It appears that rate of N per projected ton to be produced should be lower than 8 lbs. depending on soil type.

In silt loam soils the rate of 4-6 lbs. per projected ton proved most favorable by increasing quality factors, without compromising tonnage production. A rate of 5 lbs. per ton produced was shown to be the most favorable. For sandy soils, the higher rate of 7-8 lbs. per ton should be maintained in order to retain optimal tonnage production. The variation in rates made little significant difference on quality, most probably due to other mechanical features relevant to sandy soils which facilitate decrease in availability of N later in season. This should be qualified on basis of organic matter content and hence mineralization potential for N basis of past cropping history. Clay soils showed significant increase in quality and productivity of sugar produced by lowering the input rates of N to 4-6 lbs. N per ton produced, leaning more toward 4 than 6. Decreasing N input made no difference in tons produced potential.

Editor's note: David M. Elison 1*, and Greg A. Dean2, 1The Amalgamated Sugar Co., P.O. Box 700, Paul, ID 83347 and 2The Amalgamated Sugar Co., P.O. Box 8787, Nampa, ID 83653