Soil Nitrate Nitrogen "Quick Test" For Drip Irrigation Systems

By: Richard Phillips

Soils 620 Instrumentation

 

 

Background:

Chile growers that fertigate (the practice of applying water and fertilizer at the same time through the drip system) can save money by conducting on-farm soil nitrate nitrogen (NO3-N) "Quick Tests". Best management practices dictate that growers apply fertilizer only when appropriate. By doing so, they can increase profitable yields, reduce their costs, and minimize groundwater contamination.

However, it is imperative that chile plants have an adequate supply of nitrate nitrogen during critical growth stages. Dr. Tim Hartz, Extension Vegetable Crop Specialist at the University of California – Davis, recommends (Hartz, T. K. .2000) that drip irrigation growers maintain 20 parts per million (ppm) of nitrate nitrogen in the top 12 inches (30 cm) of root-zone. In order to accomplish this, growers must periodically check their soil nitrate nitrogen levels. The "Quick Test" is reliable and inexpensive soil test that can be perform on-farm to check soil nitrate nitrogen levels.

 

The "Quick Test" method described below will work on soils within a pH range of 1-12. The test strip use in the test is a Meckoquant® Nitrate Test Strips, manufactured by MERCK KgaA, Darmstadt, Germany. The chemistry involved in this testing procedure, as described by the manufacturer, is the following: Nitrate is reduced to nitrite by a reducing agent. In the presence of an acidic buffer, the nitrite is converted to nitrous acid which diazontizes an aromaticamine, this couples with N-{1-napthyl}ethylenediamine to form a red-violet azo dye.

Basically, this is the same testing procedure used in many soil labs. The test kit allows you to visually make a color comparison reading in the field. This is in contrast to the laboratory procedures, which utilize colorimetric measure the wavelength produced by the chemical reaction with nitrogen.

 

 

Materials: (note: The sources listed below are not the only sources of these materials. They are listed for the connivance of the user. It is not a specific endorsement of these businesses).

To conduct the nitrate nitrogen "Quick Test" a grower or crop consultant will need the following items (Figure 1.):

  1. Merckquant® 0-500 ppm nitrate-nitrogen test strips (available from Ben Meadows Company at: http://www.benmeadows.com/)
  2. 50 ml volumetrically marked tube, with cap, and with 10 ml graduation marks (available from VWR Scientific Products, at: http://www.vwrsp.com/catalog/product/index.cgi?object_id=1120502
  3. .01 M calcium chloride solution (available from VRW Scientific Products at: http://www.vwrsp.com/catalog/product/index.cgi?object_id=1152787)
  4. Soil sampler/probe (available from Ben Meadows Company at: http://www.benmeadows.com/cgi-bin/SoftCart.exe/scstore/p-220920.html?L+scstore+gpxi1629+988140377
  5. Soil mixing bucket

Testing Procedures and Interpretations:

Sampling Decisions

  1. Prior to the first fertigation a grower can use the "Quick Test" to check the level of residual nitrate nitrogen in the soil. Possible sources of nitrogen include pre-plant applications of fertilization, carry-over from previous crops, or mineralization of organic matter.
  2. Because part of the test will require you to let the samples settle and separate into solids and liquid it is recommended that you conduct the "Quick Test" first. Then leave the samples to settle, scout the field for other management decisions, and then return to take the soil nitrate nitrogen reading from the sample.
  3. If there are large soil type differences in the field, and they can be managed separately, then they should also be sampled and tested separately for best results.

 

 

Sampling Techniques:

  1. Collect 12- 15 soil samples from the top 12 inches of moist soil from throughout field. Remove the first few inches of dry soil that will not support root growth. Using a soil probe sample the top 12 inches of soil (Figure 2.).
  2. Put samples in a bucket and mix thoroughly. Make sure you break-up all the clods and make a granular soil mix.

 

 

 

 

 

 

 

Sample Preparation:

  1. Take three sub-samples. File three volumetrically marked tubes with 30 ml of the 0.01 M calcium chloride solution each. The calcium chloride is used to flocculate the soil. That is, it helps separate and settle the soil particles.
  2. Gradually add the granular soil to each of the cylinders that contains the 30 ml of calcium chloride until the liquid rises to the 40 ml mark.
  3. Secure the caps and shake vigorously until all the soil is dispersed. Shake the samples until all the clods are dissolved.

Sample Separation:

  1. At this point the samples need to settle. Wait until the solids and liquid have separated and the liquid has cleared. The amount of time required for the sample to settle will depend upon the texture of the soil. Sandy soils will settle quickly, clay soils will require an hour or more.

 

Sample Testing:

  1. Once the sample has settled (Figure 3.) and a relatively-clear solution is present, dip a Meckoquant® Nitrate Test Strips into the solution. Use a separate test strip for each sample. Shake off the excess solution and wait 60 seconds.

 

 

 

 

 

 

 

Sample Evaluation:

  1. Compare the test strip color with that of the color chart attached to the test strip container. Each color represents a NO3 level. The NO3 level is reported in mg/l (note: mg/l = ppm). These results compare directly to laboratory reported test results that are based on the dry-weight basis (Figure 4.).

















  2. The test strip results must be converted to nitrate nitrogen.
  3. This conversion is based on the following factors: 1) soil texture; and 2) soil moisture. Soil texture is divided into sand, loam, or clay. Dr. Tim Jones, NMSU, Department of Agronomy and Horticulture has developed a procedure for determining soil texture in the field. See Texture by Feel at: http://soil-physics.nmsu.edu/sp/classes/gl/soilclass/tbf.html .
  4. Next decide if the soil is moist or dry. Note that both texture (particle size) and moisture impacts the density per unit volume of soil. Both of these physical characteristics are factored into the conversion table listed below (Table 1.).
  5. Table 1 lists the appropriate correction factors by texture and moisture. The field nitrate nitrogen level is estimated by dividing the test strip reading by the correction factor for your given soil texture and moisture conditions (test strip reading ÷ correction factor = ppm of nitrate nitrogen).

 

Table 1. Quick Test Correction Factors by soil texture and soil moisture.

Soil Texture

Moist Soil

Dry Soil

Sand

2.3

2.6

Loam

2.0

2.4

Clay

1.7

2.2

 

 

  1. Table 2 lists the general recommendations for fertigation with nitrate nitrogen based on the corrected test reading scores.

 

Table 2. Fertigation recommendation based on Quick Test nitrate nitrogen results.

Nitrate Nitrogen Level (ppm)

Interpretation of Nitrate- Nitrogen Availability

Management Recommendation

Less than 10 ppm

Limited

fertigate immediately

10-20 ppm

Questionable

Moderate fertigation warranted

Greater than 20 ppm

Adequate

No immediate action

 

 

 

References:

Hartz, T. K. .2000. Don’t throw money away. The Grower. Vol 33, No. 9 , October 2000. pgs. 20 22. Vance Publishing Corportation, Lenexa KS.

Jones, T.L. 2000. Determining Texture by Feel. http://soil-physics.nmsu.edu/sp/classes/gl/soilclass/tbf.html