- Author: Richard Smith
- Author: Michael D Cahn
Background
Testing the soil for nitrate is critical for managing fertilizer in crop production. Soil nitrate levels are continually in flux due to inputs of nitrate from fertilizer, mineralization of soil organic matter and crop residues, and irrigation water, as well as from losses of soil nitrate from leaching, crop uptake and denitrification. As a result, we advise to measure soil nitrate as close to a fertilization event as possible to make your decision based on current soil nitrate levels. Soil samples can also be sent to a laboratory for nitrate analysis, but there can be a lag in getting the results back which can reduce the usefulness of the analysis for making fertilizer decisions.
The soil nitrate quick test has the advantage over laboratory tests of analyzing soil nitrate in a timely fashion so that accurate fertilizer application decisions can be made. The soil nitrate quick test is often used just before a fertilizer application. If soil nitrate values are high enough, it is possible to reduce the fertilizer rate or even skip the fertilizer application without jeopardizing crop yield because the existing nitrate in the soil supplies the plant with nitrogen in the same way as applied fertilizer.
Procedure
Soil cores are taken to a 12-inch depth for lettuce and spinach; however, for deeper rooted crops such as broccoli and cauliflower, soil cores of the second foot of soil during the last half of the crop cycle provide additional information on residual soil nitrate available for crop growth. Scrape away the soil from the top 2 inches of soil as it may be high in nitrate (due to upward movement of salts), but too dry for the plants to access. We have found that on some soil types (e.g. clays, silty clays) it is important to angle the soil probe in the direction of the fertilizer bead or drip tape (in fertigated situations) (See Figures 1 & 2). The reason for this is that in these soils, the fertilizer sometimes does not move far with the irrigation water and by angling the probe, you collect a more representative sample. As a matter of habit, we angle the probe on all soil types to keep our sampling method uniform.
Sample the field in a pattern that goes from one end of the field to the other, both sides of the field and through the middle – generally an “X” or “N” shaped pattern is fine. For a representative sample, it is important to collect enough samples, generally, 15 to 20 soil cores.
After collection, the sample needs to be thoroughly homogenized. Sandy soils are easily homogenized, but sticky clays or even wet loams are too gummy to mix. In these situations, we do the “pinch” method by laying out the soil cores and pinching off small, uniform amounts of soil from up and down each core. We then mix the pinches and use them for placing in the calcium chloride solution (see below). The strips are read with colorimetric test strips (see photo). The cheapest are the MQuant nitrate test strips described below. They can also be read with the Reflectoquant reader which provides a more accurate reading of the color development using Reflectoquant test strips.
Figures 1 & 2. Insert the probe in the seedline, but angle it to go beneath the bead of fertilizer or beneath the drip tape.
Procedure for conducting the nitrate quick test:
Equipment Needed
- MQuant 1.10020.0001 nitrate and nitrite test strips (0 to 500 ppm nitrate). They are available from MilliporeSigma or Amazon and contain 100 strips. They should be stored in a refrigerator when not being used for field testing. Also because color development may change as the strips age, it is advisable to store solutions of known nitrate concentration in a refrigerator to test if the strips are still accurate.
- 50 ml polyethylene centrifuge tubes (Figure 3) and a rack to hold sample tubes. These can be ordered from scientific supply companies, but they want to sell large batches that cost more. Amazon will sell smaller batches that are cheaper.
- Calcium chloride dihydrate. Can be ordered from scientific supply companies, but aquarium or food grade (e.g. canning supply companies or bulkfoods.com) calcium chloride is also fine for conducting the test.
- 1 gallon of distilled water
- Add 5.6 grams of calcium chloride to 1 gallon of distilled water to make up the 0.01 M calcium chloride solution
Note: Nitrate test strips should be stored in a refrigerator when not being used for field testing. Also because color development may change as the strips age, it is advisable to store solutions of known nitrate concentration in a refrigerator to test if the strips are still accurate. Using water samples from several wells with different concentrations of nitrate could be used to test if the strips continue to provide consistent readings.
Procedure
- Collect a composite soil sample as described above.
- Fill a volumetrically marked tube or cylinder to the 30 ml level with 0.01 M Calcium Chloride (CaC12) solution.
- Add soil to the tube until the liquid level rises to 40 ml; cap tightly and shake vigorously until soil is thoroughly dispersed. Let sit until soil particles settle out.
- When solution is reasonable clear, dip the nitrate test strip into the solution, shake off excess solution, and wait 60 seconds. Compare color with the color chart provided (Figure 4)
- To minimize variability inherent in soil sampling, run duplicate samples for each field soil evaluated.
Figures 3. An empty polyethylene centrifuge tube. Figure 4. Dip the test strip in the clear supernatant and allow it to develop color for 1 minute
Figure 5. Use the scale indicated by red arrow for calculating soil nitrate concentration for mQuant test strips.
Interpretation
The MQuant test strips are calibrated both in parts per million ppm NO3 and ppm NO3-N. Reading the ppm NO3 value (Figure 5), use the table and equation below to convert the reading to ppm NO3-N in dry soil:
Strip reading (ppm NO3) ÷ correction factor = ppm NO3-N in dry soil
|
Correction Factor |
||
|
Soil Texture |
Moist Soil |
Dry Soil |
|
Sand |
2.3 |
2.6 |
|
Loam |
2.0 |
2.4 |
|
Clay |
1.7 |
2.2 |
For instance, a reading from the test strips of 25 ppm NO3 from a moist loam would have 12.5 ppm NO3-N in the soil (25 ÷ 2 = 12.5). The ppm NO3-N values can be converted to pounds of nitrogen per acre in the top foot of soil by multiplying by 3.7, and in this example, that would equal 46 pounds of nitrogen per acre.
In general, soils with less than 10 ppm NO3-N are considered low for fast growing vegetable crops and soils with levels above 20 ppm NO3-N may have enough available N to supply crop needs for a limited period. Intermediate concentrations between 12 and 15 ppm NO3-N may warrant a half rate of fertilizer. However, it is important to get familiar with the nitrate quick test by doing small trials on your farm. As you gain more confidence in using the test to adjust fertilizer applications, you can do larger trials. Keep in mind that nitrate is very mobile, and in light textured soils, heavy irrigation/rainfall events can reduce the amount of available nitrogen in the soil. That is why it is always good to be cautious in reducing fertilizer applications based on the soil nitrate test. Feel free to contact either of us if you have any questions.
Additional information resources on soil nitrate testing and nitrate in irrigation water:
Using the Pre-Sidedressing Soil Nitrate ‘Quick Test' to Guide N Fertilizer Management
Accuracy of Test Strips for Assessing Nitrate Concentration in Soil and Water
Sampling for Soil Nitrate Determination
RQflex reader can improve the accuracy of nitrate test strips
Presidedress nitrate quick test reduces nitrate leaching hazard in lettuce
Estimating N contribution from irrigation water containing nitrate
- Author: Michael D Cahn
- Author: Richard Smith
Monterey County Agricultural Center
1432 Abbott Street, Salinas, CA
Tuesday, February 26
7:45 a.m. to 12:30 p.m.
7:45 Registration (free)
8:00 Production of organic baby spinach using buried drip irrigation
Ali Montazar, Irrigation Advisor UCCE, Imperial and Riverside Counties
8:30 Mitigating pesticides and sediments in tail water using polyacrylamide (PAM): a new approach
Michael Cahn, Irrigation and Water Resources Advisor, Monterey County
9:00 Full season nitrogen management of vegetables
Richard Smith, Vegetable Crops and Weed Science Adivsor, Monterey County
9:30 Optimizing water management in celery using ET weather-based scheduling
Michael Cahn, Irrigation and Water Resources Advisor, Monterey County
10:00 Break
10:30 Region 5 water quality coalitions – demonstrating achievements in water quality
Parry Klassen, Executive Director, Coalition for Urban Rural Environmental Stewardship (CURES)
11:00 Evaluation of salinity effects on strawberry production
Andre Biscaro, Irrigation Water Resources Advisor, Ventura County
11:30 Water and nitrogen management of Asian vegetables/SWEEP and Healthy soils programs
Aparna Gazula, Small Farms Advisor, Santa Clara County
12:00 Grower Panel: Nitrogen Management in Practice
Saul Lopez, D'Arrigo Brothers; Mark Mason, Huntington Farms; and Sal Montes, Christiansen Giannini
12:30 Pizza Lunch
CCA & DPR continuing education credits have been requested
- Author: Michael D Cahn
CropManage Workshop: Hands-on training
Monterey County Agricultural Center Conference Room
1432 Abbott St, Salinas CA 93901
Thursday, April 2nd 2015
(8:30 am – 12 pm)
We will offer a hands-on training to learn in depth about the features of CropManage, a free online decision support tool for water and nutrient management of coastal crops. In addition to head and romaine lettuce, CropManage now supports broccoli, cauliflower, cabbage, and strawberries.
Considering that the drought is continuing into a 4th year, and nutrient management continues to be linked to water quality regulations, efficiently using water and nitrogen fertilizer is a high priority for Central Coast growers. CropManage can play an important role in providing quick decision support on water and nutrient management on a field-by-field basis.
This training will provide an opportunity to learn how to use CropManage for improving the efficiency of your farming operations or for adding value to your consulting services. We will provide in depth hands-on training so that you can learn step-by-step how to navigate and use CropManage for assisting with fertilizer and water management decisions and record keeping. Wi-Fi internet access is available at our conference room so please bring a laptop or tablet computer so that you can follow along as we tour through the features of the software. There should be sufficient time to answer questions as we cover the following topics:
Agenda:
8:30 – 9:00 Registration and Refreshments
9:00 – 9:30 Introduction and update on CropManage
9:30-10:15 Getting started with CropManage
10:15 (Break)
10:30 – 11:15 Strategies for using CropManage for decision support and record keeping
11:15- 11:45 Advanced features and interfacing sensors with CropManage
11:45-12:00 Discussion of new features or changes needed.
To keep the group size manageable so that we can provide individual help, we would like to limit the workshop to 30 participants. If you have attended previous workshops and or feel proficient in using the on-line tool, then you are welcome to just attend the second half of the workshop (10:30-12 pm). Whether or not you plan to attend the entire or part of the workshop, please RSVP in advance by sending an email to larriaga@ucdavis.edu or mdcahn@ucdavis.edu with the subject heading “CropManage workshop” and let us know the number of participants in your group. We will email you a confirmation. Thank you, and I hope to see you soon.
Respectfully,
Michael Cahn, Irrigation and Water Resources Advisor
Certified Crop Adviser CEU hours requested (1.5 hrs irrigation management, 1.5 hrs nutrient management)
- Author: Richard Smith
- Author: Michael D Cahn
Conversion between nitrate (NO3) and nitrate-nitrogen (NO3-N):
|
To convert |
To |
Multiply by |
|
Nitrate (NO3) |
Nitrate-nitrogen (NO3-N) |
0.22 |
|
Nitrate-nitrogen (NO3-N) |
Nitrate (NO3) |
4.43 |
The reason for this conversion is that nitrate molecule weighs 62 grams per mole; the nitrogen content of nitrate is 22.5% of the total weight of the molecule.
Nitrogen content of irrigation water*
|
Water content of |
Multiply by |
To determine |
|
PPM NO3 |
0.052 |
Pounds N/acre inch |
|
PPM NO3 |
0.62 |
Pounds N/acre foot |
|
PPM N03-N |
0.23 |
Pounds N/acre inch |
|
PPM N03-N |
2.74 |
Pounds N/acre foot |
* water analyses from most labs report NO3 in units of ppm, but it is very important to pay attention to which units the results are reported.
How much of the nitrogen in water should be credited to your crop is debatable. Consider that lettuce transpires 5 to 8 inches of water between germination and maturity in the Salinas Valley during the summer. Extra water applied beyond crop ET would be lost as drainage and therefore would not contribute N to the crop. The extra water also would likely leach plant available soil nitrate below the root zone. In addition, some ground water that is high in nitrate is also high in salts and may require a leaching fraction (extra water applied to leach salts below root zone) to attain maximum production. The good news is that you can account for the N contribution from the nitrate in the irrigation water using the quick nitrate soil test for previous irrigations. However, this test will not estimate the contribution of N from the irrigation water for future waterings.
Our best estimate of how much N the irrigation water would contribute to future waterings is to divide the crop evapotranspiration by the irrigation efficiency. For example, for 7 inches of crop ET and an 80% irrigation efficiency, the following values would approximate the N contribution of irrigation water for the indicated range of nitrate concentrations:
|
Nitrate (NO3) concentration in irrigation water |
Nitrate (NO3-N) concentration in irrigation water |
Lbs nitrogen/A in seven inches of irrigation water taken up by lettuce* |
|
45 |
10 |
13 |
|
89 |
20 |
25 |
|
177 |
40 |
51 |
|
266 |
60 |
76 |
* multiplied by 0.8 to account for the irrigation system efficiency
As can be seen, waters containing less than 45 ppm NO3 generally do not contribute a significant amount of nitrogen for crop growth. However, if well waters contain more than that amount they begin to contribute greater amounts of water for crop growth.
- Author: Richard Smith
The lack of rain in the Salinas Valley brings many concerns. The lack of runoff into lakes San Antonio and Nacimiento of course is a concern for the availability of water to run down the river to recharge the ground water for irrigation purposes. In addition, the lack of rain will affect the levels of salts that remain up in the root zone of the crops. Soil nitrate (NO3-) is one of the anions that will remain in the soil if leaching by winter rains does not occur. Nitrate is highly mobile and can be easily leached with just one or more significant rain storms; figure 1 illustrates nitrate leached from the top foot of soil by a series of storms that delivered 2.0 inches of water over the course of one week in the winter of 2010. High residual soil nitrates may come from several sources: 1) unused fertilizer from the previous crops or fall preplant nitrogen applications; 2) mineralization of crop residues from the previous crop; and 3) mineralization of soil organic matter over the winter (mineralization of soil organic matter is much slower during the winter but will still occur to a minimal degree).
We recently surveyed several soils looking for a site to conduct a fertilizer trial and observed that residual soil nitrate levels were routinely over 20 ppm nitrate-nitrogen. These levels were in contrast with levels that we observed last year following a wet spring where, in general, residual soil nitrate levels were in the 5 – 10 ppm nitrate-nitrogen range. The difference in conditions between a dry winter like this and a wet winter like last year is that it has implications for planning nitrogen fertilizer programs; with soil residual nitrate levels this high, the nitrogen fertilizer needs of the first crop fields will behave like second crop fields in that the robust amounts of residual soil nitrate in the soil that can provide for the crop needs and allow you to reduce nitrogen fertilizer programs.
To illustrate this point, we observed a great difference in the fertilizer needs of first vs second crop spinach during the 2011 growing season. In a first crop spinach planting, residual soil nitrate levels were at 5 ppm at the beginning of the trial. Spinach responded to at-planting applications of nitrogen up to 40 lbs nitrogen/A (Figure 2). The second crop spinach planting had initial levels of residual soil nitrate of 28 ppm nitrate-nitrogen which allowed the grower to skip the at-planting nitrogen application; he made one top-dress nitrogen application two weeks after planting to bring the crop to harvest. The results of a top-dress nitrogen evaluation indicated that there was no improvement in yield beyond 25 lbs nitrogen per acre (Figure 3).
These results indicate the importance of deep percolation of water on residual levels of soil nitrate. Winter rains have the beneficial effect of leaching salts from the soil. It is very unfortunate that nitrate is one of the salts that is leached with the water, but that is the case. In many of the discussions that we have had over the last several years regarding managing nitrogen fertilization more efficiently, we have emphasized that testing for residual soil nitrate is generally most effective for the second crop of the season. However, given the extreme lack of leaching rain events this winter, residual soil nitrate levels are also high at the beginning of the first crop in many areas in the valley and can be taken into consideration when planning nitrogen fertilization.
Figure 2. Yield response of first crop spinach under five
application rates of at-planting nitrogen (0 – 80 lbs N/A)
Figure 3. Yield response of second crop spinach under five application rates of top-dressed nitrogen (0 – 105 lbs N/A); no at-planting nitrogen was applied to this planting.
