Posts Tagged: soil nitrate
Fate of surface applied nitrogen in season-long drip irrigated fields
During the past two years acreage of season-long drip in lettuce has increased rapidly in the Salinas and adjacent valleys. Using drip for the entire crop cycle allows growers to germinate seeded crops with buried tape (Photo 1), and eliminates labor needed for installing and removing sprinklers. The rapid expansion of this irrigation practice is due to 1) reliable thin-walled single-use drip tape which assures high application uniformity for less cost than thick walled tape; 2) Better injection equipment that can uniformly place drip tape 2-3 inches below the soil surface allowing cultivation without damaging the tape (Photo 2), and 3) development of tape removal equipment that saves labor and efficiently bundles the tape for recycling (Photo 3). The use of drip for germinating lettuce often can improve the uniformity of stands and save water by eliminating common problems associated with using sprinklers such as emergence patterns caused by wind and crusting of the soil surface. Drip germination works best on light to medium textured soil types such as sandy loams, gravelly sandy loams, loams, and silt loams (e.g. along the river and on the eastside of the Salinas Valley).
Unlike sprinklers which infiltrate water at the soil surface, water applied by buried drip wicks upward keeping herbicides and fertilizers sprayed on the bed tops close to the soil surface. The upward movement of moisture from buried drip tape and subsequent evaporation of water from the bed top yields a net accumulation of salts (including nitrate) near the soil surface (Photo 4). This upward movement of applied materials benefits the preemergent herbicide, Kerb, which is often pushed too deep in the soil by sprinkler applied water at germination. The amount of wetting of the soil surface provided by drip germination is sufficient to set Kerb and keep it in the zone where weed seeds germinate which improves its effectiveness (for more information on this subject go to: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=30847).
With buried drip, surface-applied fertilizers used as anticrustants or for thinning by automated thinners are not moved deep enough into the soil to be taken up by the lettuce crop during the early season. In one evaluation, we observed that the fertilizers were strongly stratified in the top inch of soil. Soil samples of the top six inches of soil by one-inch increments indicate that the nitrate levels in the top inch of soil are commonly higher than the deeper in the profile due to the upward movement of salts by evaporation mentioned above (Table 1). However, following the application of 20 gallons of 28-0-0-5 for thinning, the levels of mineral nitrogen (ammonium-N and nitrate-N) became extremely high (> 200 ppm NO3-N) and did not decline for the two weeks of the evaluation. In another field evaluation in which nitrogen fertilizer was applied to the soil surface (as an anticrustant (5-20-0) and for thinning (14-0-0-5)) the nitrogen remained in the upper two inches of soil for more than 5 weeks. This nitrogen just below the surface would be unavailable for crop growth because the soil is dry and root growth is minimal.
Table 1. Total mineral nitrogen (ammonium-N + nitrate-N) in the top 6 inches of soil. May 22 – prior to thinning; three subsequent sampling dates following application of 28-0-0-5 fertilizer by an autothinner
Since growers must report the total nitrogen applied to vegetables to the Regional Water Quality Control Board (RWQCB), the nitrogen remaining on the soil surface creates a problem. For instance, a typical application of 20 gallons of 14-0-0-5 contains 29 lbs of nitrogen/acre. This nitrogen is reported to the RWQCB but does not necessarily provide nitrogen for crop growth. More nitrogen would need to be added to keep up with the N demand of the crop. It would be advantageous to use materials in the autothinners that contain no or low amounts of nitrogen.
High levels of nitrogen on the surface also creates a challenge for collecting an accurate soil sample for determining plant-available nitrogen using the nitrate quick test or laboratory analysis. Photo 5 shows the results of three measurements: 1) high levels of nitrate-nitrogen found in the top 2 inches of soil (test strip on the left); 2) moderate amount of nitrate-nitrogen found in the 2 to12 inch layer (top 2 inches scraped off, test strip in the middle); and 3) high levels of nitrate-nitrogen found in the top 12 inches of soil (top 2-inches of soil is not scraped away, test strip on the right). We have always recommended scraping the dry surface soil away before collecting a soil core (see https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=4406), however, the extremely high amounts of nitrate-nitrogen in the top 2 inches of drip irrigated fields that are autothinned with a nitrogen fertilizer makes this practices particularly critical in order to not over estimate the amount of plant-available nitrogen in the soil when making critical fertilizer application decisions.
Summary:
- Season-long use of buried drip keeps herbicides and soil applied fertilizers (from anti-crustants and automated thinners) close to the surface due to the water wicking upward.
- Surface applied fertilizers remain in the top 2 inches of soil and are not plant available for much of the season. Ideally, a zero or low nitrogen containing thinning chemical would avoid this issue.
- It is important to scrape away the top two inches of soil when collecting samples for nitrate testing in order to not over estimate the amount of plant-available nitrogen when making fertilizer application decisions.
Details on the Soil Nitrate Quick Test
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
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