In a trial designed to test nitrogen requirements for potato varieties under furrow irrigation in a N deficient field, the highest potato yield (518 cwt/ac over all varieties tested) was obtained in 1995 by the use of only 84 lb N/ac soon after planting. Additional N produced no additional economic returns. Varieties did not differ significantly in their response to N fertility. The new potato variety A082611-7 had higher total US Number One and marketable tuber yield than Russet Burbank and Shepody in this trial.

The development of new potato varieties has made it possible to achieve good tuber yield and quality with furrow irrigation. These new varieties might differ from each other in their nitrogen requirements. Previous studies under sprinkler irrigation showed that the optimum N rate was less than the rate recommended by either the Oregon or Idaho fertilizer guides (Feibert et al., 1995). This trial compared Russet Burbank, Shepody, Frontier Russet, Ranger Russet, and three experimental varieties AO82611-7 and COO83008-1 (both for processing), and NDTX 8-731-1R (a fresh market, red variety) as to their nitrogen requirements under furrow irrigation.

The 1995 trial was conducted on an Owyhee silt loam previously planted to wheat at the Malheur Experiment Station. The field was bedded into 36-inch hills in the fall of 1994. A soil sample taken from the top foot on May 1, 1995 showed a pH of 7.8, 1.7 percent organic matter, 19 CEC, 8 ppm nitrate-N, 4 ppm ammonium-N, 13 ppm phosphorus, 439 ppm potassium, 2350 ppm calcium, 383 ppm magnesium, 370 ppm sodium, 1.0 ppm zinc, 12.2 ppm iron, 8.8 ppm manganese, 1.0 ppm copper, 19 ppm sulfate-S and 0.7 ppm boron.

Two-ounce seed pieces were planted April 27 at 9-inch spacing. On May 19, Thimet 20G at 3 lbs ai/ac was shanked-in with the urea for the nitrogen treatments. The urea was applied after planting and before emergence to both sides of the hill (Figure 1). The shanks were adjusted to place the urea in bands located at the same depth as the seed piece and offset 9 inches from the hill center. The hills were remade with a Lilliston cultivator. Prowl at 1 lb ai/ac and Dual at 2 lbs ai/ac were broadcast on the entire soil surface on May 23 and incorporated with the Lilliston. A late blight and insect control program consisting of weekly aerial applications of fungicide and insecticide mixes was initiated on July 14 and continued through August 26.

The experimental design had four N treatments as main plots and the seven potato varieties as split-plots within the main plots (Table 3). The main plots were 9 rows wide and 50 feet long. The four nitrogen treatments were replicated six times.

Nitrogen fertilizer rates were 0, 84, 144, and 204 lb N/ac (Table 1). Pre-emergence urea was applied on May 19. The second nitrogen application consisted of urea applied to the furrow bottom immediately before an irrigation to simulate water-run nitrogen.

Table 1. Nitrogen rates applied to seven potato varieties. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.

* Does not include mineralized nitrogen during the season.

Twenty four granular matrix sensors (GMS, Watermark Soil Moisture Sensors Model 200, Irrometer Co., Riverside, CA) were installed in the top foot of soil and six GMS were placed in the second foot of soil. The daily sensor readings were used to schedule irrigations. The GMS in the top foot of soil were offset 6 inches from the hill top and centered 8 inches below the hill surface. The second foot GMS were placed in the hill center and centered 20 inches below the hill surface. Half of the first foot sensors were located on the wheel traffic side of the potato hill and the other half were located on the non-wheel traffic side of the hill. Sensors were read five times per week from June 10 to September 4 at 8 AM. Irrigations were started when the average soil water potential in the first foot of soil dried to -50 kPa.

At each irrigation, every other furrow was irrigated, with the irrigated furrows alternating from irrigation to irrigation. Seventeen irrigations were applied from June 12 to September 1. Irrigation durations were 24 hours from June 12 through July 17 and 12 hours from July 17 through September 1.

Petiole samples were collected every two weeks from June 21 to August 16, and analyzed for nitrate.

Russet Burbank, Shepody, and Frontier Russet plants in each plot were sampled. Plant available-N contributed from organic matter mineralization was determined by the buried bag method (Westermann and Crothers, 1980).

Tubers from 40 feet in each plot were harvested on September 26 and evaluated for yield and grade. A subsample was stored and analyzed for tuber specific gravity and stem-end fry color in early November.

The soil remained approximately within the range of soil moisture (Figure 2). Soil much drier than -60 kPa at the 8-inch depth is associated with an increase in US Number Two tubers.

The maximum total potato yield, over all varieties, was 518 cwt/ac obtained at 84 lb N/ac (Table 3). This yield was achieved with substantially less N fertilizer than the university fertilizer guides would have recommended for this field (Table 2). The N in this trial was shanked in after planting, which would improve the use efficiency compared to pre-plant broadcast applications. When broadcast N applications are used, substantial amounts of N can be lost to leaching, lost to volatilization, or be located in tops of the potato hill that are inaccessible to the roots. The average total potato yield for Malheur County in 1995 was 405 cwt/ac, using substantially more N fertilizer than used in this trial (225 lb N/ac).

Total tuber yield, total US Number Ones, large US Number Ones and marketable tuber yield increased with 84 lb N/ac, over all varieties (Table 3). The proportion of large US Number One tubers increased with 84 lb N/ac (Table 4).

Varieties AO82611-7 and COO83008-1 had among the highest marketable yield and AO82611-7 had the highest total US Number One yield. Ranger Russet had the highest tuber specific gravity (Table 5). Shepody, AO82611-7 and COO83008-1 had among the lightest tuber stem-end fry color.

Organic matter mineralization in the top foot of soil released 10 lbs N/ac between May 1 and May 18 and 46 lb N/ac between May 18 and July 5, based on analysis of the soil in the buried bags (Figure 3). The nitrogen in the buried bags represents residual soil available nitrogen plus nitrogen released from organic matter mineralization without the effects of crop uptake, leaching, and other losses.

The positive response of potato yields to N fertilization in 1995 compared to 1994 was probably due to the lower pre-plant soil nitrate and ammonium N in 1995 (75 lb N/ac on May 1) compared to 1994 (108 lb N/ac on March 24). Nitrogen mineralization released about the same amount of N by June 30 both years (90 lb N/ac from March 24 to June 30 in 1994 and 98 lb N/ac from May 1 to July 5 in 1995). By July 5 the buried bags in the top two feet of soil contained amounts of nitrate and ammonium corresponding to 173 lb N/ac in 1995 compared to 198 lb N/ac on June 30 in 1994.

Only the 204 lb N/ac treatment for Russet Burbank and Shepody and the 144 lb N/ac and 204 lb N/ac treatments for Frontier Russet resulted in petiole nitrate in the sufficiency range (Jones and Painter, 1974); levels that proved to be unnecessary for maximum yields (Figures 4-6).

Sidedressed N fertilization beyond 84 lb N/ac did not increase potato yields in 1995, over all varieties. The 84 lb N/ac resulting in the maximum total yield of 518 cwt/ac is substantially less than the university fertilizer recommendations for this field.

Averaged over all N rates, the experimental processing varieties AO82611-7 and COO83008-1, performed as well as, or better than Shepody and Ranger Russet in US Number One and marketable yield. Russet Burbank and Frontier Russet had among the darkest stem-end fry color and lowest tuber specific gravity.

Table 2. University N fertilizer recommendations compared to actual sidedressed N fertilizer needed to maximize furrow irrigated potato yield. Malheur Experiment Station, Ontario, Oregon, 1995.

Anonymous. 1985. Fertilizer Guide: Irrigated Potatoes (Columbia Basin-Malheur County). FG 57. Oregon State University Extension Service.

Feibert, E.B.G., C.C. Shock, and M. Saunders, 1995. Nitrogen fertilization for potato varieties grown under sprinkler irrigation, 1994 trial. Oregon State University Agricultural Experiment Station Special Report 947.

Jones, J.P. and Painter, C.G., 1974. Tissue analysis: A guide to nitrogen fertilization of Idaho Russet Burbank Potatoes. University of Idaho, College of Agriculture, Cooperative Extension Service, Agricultural Experiment Station, Current information series # 240, June 1974.

McDole, R.E., D.T. Westermann, G.D. Kleinschmidt, G.E. Kleinkopf, and J.C. Ojala. 1987. Idaho Fertilizer Guide: Potatoes. Current Information Series No. 261. University of Idaho, College of Agriculture.

Westermann, D.T., and S.E. Crothers. 1980. Measuring soil nitrogen mineralization under field conditions. Agronomy Journal 72: 1009-1012.

Table 4. Tuber market grade response of seven potato cultivars to four nitrogen fertilizer treatments. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.

Variety	Nitrogen fertilizer rate	Potato market grade distribution
		US Number One				US Number Two				Marketable	Undersize
		4-6 oz	6-10 oz	>10 oz	total	4-6 oz	6-10 oz	>10 oz	total
	lb N/ac	-------------------- % --------------------
R. Burbank	0	22.8	31.5	22.8	77.1	0.5	1.6	3.8	5.9	83.0	17.0
	84	21.0	30.8	24.3	76.1	1.0	2.2	5.4	8.6	84.7	15.3
	144	19.3	30.4	26.5	76.2	1.2	2.6	5.4	9.2	85.4	14.6
	204	17.5	28.3	29.5	75.3	0.6	2.6	5.6	8.8	84.0	16.0
	Average	20.2	30.2	25.7	76.2	0.8	2.3	5.1	8.1	84.3	15.7
Shepody	0	9.0	22.8	62.6	94.4	0.1	0.4	1.2	1.6	96.0	4.0
	84	6.9	17.3	67.3	91.4	0.3	0.6	3.2	4.0	95.4	4.6
	144	6.8	18.8	63.7	89.3	0.4	0.6	5.8	6.7	96.0	4.0
	204	5.1	15.4	68.3	88.9	0.2	0.8	6.9	8.0	96.9	3.1
	Average	6.9	18.6	65.5	91.0	0.3	0.6	4.3	5.1	96.1	3.9
F. Russet	0	17.3	34.3	33.0	84.6	0.2	0.6	3.4	4.2	88.9	11.1
	84	14.9	26.3	42.3	83.5	0.4	0.4	3.5	4.3	87.8	12.2
	144	15.0	28.4	39.1	82.5	0.4	1.2	5.0	6.6	89.1	10.9
	204	15.0	23.9	41.6	80.5	0.6	1.5	4.9	6.9	87.5	12.5
	Average	15.6	28.3	39.0	82.8	0.4	0.9	4.2	5.5	88.3	11.7
R. Russet	0	11.4	33.3	41.6	86.3	0.7	1.5	4.4	6.7	93.0	7.0
	84	8.0	24.4	56.0	88.4	0.5	1.1	4.8	6.4	94.8	5.2
	144	8.9	22.7	55.8	87.3	0.2	1.5	5.2	7.0	94.3	5.7
	204	8.6	17.8	56.9	83.4	0.6	2.1	7.0	9.7	93.1	6.9
	Average	9.2	24.5	52.6	86.3	0.5	1.6	5.4	7.5	93.8	6.2
AO 82611-7	0	16.3	37.9	32.6	86.8	0.2	0.7	2.5	3.4	90.2	9.8
	84	11.7	30.1	45.4	87.2	0.3	1.2	4.0	5.4	92.6	7.4
	144	11.4	26.9	48.2	86.5	0.2	1.1	4.3	5.6	92.1	7.9
	204	10.6	25.7	51.3	87.5	0.4	1.0	3.3	4.7	92.3	7.7
	Average	12.5	30.1	44.4	87.0	0.3	1.0	3.5	4.8	91.8	8.2
COO 83008-1	0	9.2	28.9	50.1	88.2	0.2	1.6	5.5	7.3	95.5	4.5
	84	6.1	18.5	62.7	87.4	0.3	1.6	8.0	9.9	97.3	2.7
	144	8.0	27.2	54.2	89.3	0.3	1.8	4.4	6.5	95.8	4.2
	204	5.4	22.1	60.6	88.1	0.2	0.9	6.6	7.7	95.8	4.2
	Average	7.2	24.2	56.9	88.3	0.3	1.5	6.1	7.9	96.1	3.9
NDTX 8-731-1R	0	13.2	33.3	43.9	90.4	0.0	0.0	0.0	0.0	90.4	9.6
	84	13.8	29.3	47.8	90.9	0.0	0.0	0.0	0.0	90.9	9.1
	144	15.0	31.9	44.3	91.1	0.0	0.0	0.0	0.0	91.1	8.9
	204	13.4	32.2	44.8	90.4	0.0	0.0	0.0	0.0	90.4	9.6
	Average	13.9	31.7	45.2	90.7	0.0	0.0	0.0	0.0	90.7	9.3
All varieties	0	14.2	31.7	40.9	86.8	0.3	0.9	3.0	0.2	91.0	9.0
	84	11.8	25.2	49.4	86.4	0.4	1.0	4.1	5.5	91.9	8.1
	144	12.1	26.6	47.4	86.0	0.4	1.3	4.3	6.0	92.0	8.0
	204	10.8	23.6	50.4	84.9	0.4	1.3	4.9	6.6	91.4	8.6
LSD (0.05) Trt		1.7	1.9	3.8	ns	ns	ns	1.3	1.6	ns	ns
LSD (0.05) Variety		1.7	2.9	4.6	2.2	0.2	0.6	5.6	1.9	1.2	1.2
LSD (0.05) Trt X Var.		ns	5.8	ns	ns	ns	ns	ns	ns	ns	ns

Table 5. Tuber stem-end fry color and specific gravity response of six potato cultivars to four nitrogen fertilizer treatments. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.

Variety	Nitrogen fertilizer rate	Stem-end fry color	Specific gravity	Variety	Nitrogen fertilizer rate	Stem-end fry color	Specific gravity
								lb N/ac	% reflectance		lb N/ac	% reflectance
								R. Burbank	0	32.5	1.094	R. Russet	0	44.7	1.159
	84	33.8	1.092		84	45.2	1.102
144	33.2	1.088	144	45.5	1.100
204	33.4	1.087	204	44.4	1.098
Average	33.2	1.090	Average	44.9	1.115
Shepody	0	46.5	1.094	AO 82611-7	0	45.6	1.095
	84	47.6	1.093		84	46.5	1.094
	144	46.4	1.087		144	46.3	1.093
	204	44.9	1.089		204	45.9	1.094
	Average	46.3	1.091		Average	46.1	1.094
F. Russet	0	35.6	1.097	COO 83008-1	0	48.3	1.097
	84	34.0	1.088		84	47.2	1.095
	144	32.9	1.088		144	50.3	1.096
	204	30.1	1.083		204	49.1	1.092
	Average	33.1	1.089		Average	48.7	1.095
All varieties	0	42.2	1.106
	84	42.4	1.094
	144	42.4	1.092
	204	41.3			ns	ns
LSD (0.05) Variety		19.7	0.016
LSD (0.05) Trt X Var		ns	ns