Developing Sap Nitrate Tests for Wheat and Barley, Maricopa, 1998

E.A. Riley, T.L. Thompson, S.A. White, and M.J. Ottman



The standard procedure for determining nitrogen (N) status in small grains is to sample lower stem tissue for nitrate (NO3) ana-lysis. The tissues are then submitted to a laboratory for analysis. Sap nitrate (NO3) can be analyzed in the field, immediately after collecting the sample, using a Cardy meter. Guidelines for sap analysis have not yet been determined. The objectives of this study were to: (i) correlate NO3-N in dried stem tissue with sap NO3-N , and (ii) develop sap NO3 test guidelines for N management in durum and feed barley. In November 1997 one variety of durum (Kronos) and one variety of feed barley (Gustoe) were planted at the Maricopa Agricultural Center. Three N rates (80 , 200, and 400 lbs N/acre) were applied in four split applications. Each treatment was replicated five times in a randomized complete block design. Samples were collected from lower stems at the 3-4 leaf, 2 node, flag leaf visible, and heading growth stages. Grain yields ranged from 5185 lbs/A to 7156 lbs/A for Kronos and 6314 lbs/A to 7517 lbs/A for Gustoe. Maximum yields were achieved at 200 lbs N/A for both varieties. Correlation coefficients between stem NO3-N and sap NO3-N were 0.79 for Kronos and 0.84 for Gustoe. Sap NO3-N analysis can be used to determine N status during the season for both Kronos and Gustoe.



Tissue analysis is an accurate method for determining plant N status. In wheat and barley, NO3 concentration in the lower stem is an indicator of plant N status. Therefore, the NO3-N concentration in lower stem tissue can be used to formulate an N management strategy. 

Conventional plant tissue analysis requires the grower to collect representative samples in the field, store the samples properly, and then send them to a commercial lab for analysis. The turnaround time for the entire process is typically 3 or more days. This can delay timely application of N fertilizer. Rapid tests, such as a sap NO3 test using the Cardy meter, are being developed to enable growers to determine crop N status almost instantaneously. This information can save time, increase yield, and result in wise fertilizer use. 

Current tissue test guidelines apply to NO3 measured in dried lower stem tissue. New guidelines are needed to correlate results obtained from the Cardy meter with those found in dried tissue analysis. The objectives of this study were to: (i) correlate NO3-N in dried stem tissue with sap NO3-N , and (ii) develop sap NO3 test guidelines for N management in durum and feed barley.

Materials and Methods

One variety of durum wheat, Kronos, and one variety of feed barley, Gustoe, were planted at the Maricopa Agricultural Center Field 109 on 21 Nov., 1997. The experiment was a randomized complete block design with three N rates (80, 200, 400 lbs N/A) and five replications. The soil at this site is of the Casa Grande series and the dominant surface texture is sandy loam. Sudangrass was grown the previous season to remove residual N. Soil samples collected before planting contained 11 ppm NH4-N plus NO3-N and 9 ppm HCO3-available P. Before planting, phosphate was broadcast at a rate of 50 lbs P205/A as 0-45-0 and incorporated. All N fertilizer was applied as urea 46-0-0 by hand in four split applications (Table 1). Plots were 13 by 20 ft. 

Kronos durum was planted at a seeding rate of 120 lbs/A and Gustoe barley was planted at a seeding rate of 100 lbs/A using a grain drill with a 6 inch spacing. Plots were border-flood irrigated. The irrigation dates were 22 Nov., 11 Dec., 22 Jan., 18 Feb., 11 Mar., and 2 Apr. 

Lower stem tissue was sampled from each plot on 6 Jan., 11 Feb., 4 Mar., and 18 Mar. at Feekes GS 3, 7, 10, and 10.4 (Large, 1954) for stem and sap NO3-N analysis. Approximately 30 to 50 stems were collected in each plot. The stem samples were kept refrigerated for 24 hours and then split. Half of each sample was used for sap extraction and the other half was dried in an oven at 65B C for 48 hours. The dried samples were ground, extracted and analyzed for NO3 using an ion-specific electrode. The sap extraction was accomplished by cutting the halved stem into small pieces and then removing the sap with an arbor press. The sap was then collected and placed on the sensing module of a calibrated Cardy meter. 

Heading, anthesis, and physiological maturity dates were noted for each plot. Plots were harvested on 27 May using a small plot combine. The harvest area was 5 ft x 14 ft. Grain yield was adjusted to a 12% moisture basis. Test weight, kernel weight, grain protein content, hard vitreous amber count (for Kronos only), plant height and lodging percent were determined at harvest (Table 2). 

Data were analyzed using analysis of variance (Table 3). Linear regressions were used to determine correlation coefficients for sap versus stem NO3-N (Figures 1).

Results and Discussion 

Grain yields were responsive to N applications up to 200 lbs N/A (Tables 2 and 3). Grain yield was maximized at 200 lbs N/A in both Kronos (7805 lbs/A) and Gustoe (7517 lbs/A). For both varieties higher amounts of N resulted in significant losses of yield. This reflects the > 50 % lodging observed at the highest N rate for Kronos. Grain protein was maximized at 400 lbs N/A for Kronos (12.5%) and Gustoe (10.2%). Grain yield and grain protein were significantly affected by N rate for Kronos. For Gustoe grain yield and plant height were significantly affected by N rate, but grain protein was not. For both varieties, grain protein was lower than expected. Grain protein content is affected by N rate and other factors such as water availability and temperature. 

Sap NO3-N and stem NO3-N were highly correlated for both Kronos and Gustoe. Correlation coefficients (r2) for sap NO3-N versus stem NO3-N were 0.79 for Kronos and 0.84 for Gustoe. Regression equations relating these two measurements are: Y = -123.8 + 6.29*X for Kronos and Y = -786.8 + 14.2*X for Gustoe, where X is sap NO3-N concentration and Y is stem NO3-N. These results suggest that rapid measurements of sap NO3-N using the Cardy meter can be converted for use with existing stem NO3-N guidelines.

Sap NO3-N was highly correlated with N rate for both Kronos and Gustoe throughout the season (Figure 2). This relationship support a conclusion that sap NO3-N can be used as an indicator of plant N status. These preliminary results suggest that sap analysis can be substituted for conventional stem tissue analysis at the 3-4 leaf, 2 node, flag leaf visible, and heading growth stages. Subsequent studies will allow development of sap NO3-N test guidelines for optimizing yield and grain protein. 


  1. Correlations between sap NO3-N and stem NO3-N are high.
  2. Sap NO3-N concentration can be converted to stem NO3-N values using the regression equations above, for use with established tissue testing guidelines for small grains.


  1. Large, E.C. 1954. Growth stages in cereals. Illustrations of the Feekes scale. Plant Physiology. 3, 128-129.


The assistance of Mark Rogers was greatly appreciated. Financial support for this project was received from the Arizona Grain Research and Promotion Council.

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This is a part of publication AZ1059: "1998 Forage and Grain Agriculture Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721. 
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