21.5: Soil Testing for Nitrogen

As we discussed in Chapter 19, nitrogen management poses exceptional challenges because gains and losses of this nutrient are affected by its complex interactions in soil, crop management decisions and weather factors. The highly dynamic nature of nitrogen availability makes it difficult to estimate how much of the N that crops need can come from the soil. Soil samples for nitrogen tests are therefore usually taken at a different time using a different method than samples for the other nutrients (which are typically sampled to plow depth in the fall or spring).

Figure 21.2. Soil test phosphorus and potassium trends under different fertility management regimes. Modified from The Penn State Agronomy Guide (2019–2020).

In the humid regions of the United States there was no reliable soil test for N availability before the mid-1980s. The nitrate test commonly used for corn in humid regions was developed during the 1980s in Vermont. It is usually called the pre-sidedress nitrate test (PSNT) but is also called the late spring nitrate test (LSNT) in parts of the Midwest. In this test a soil sample is taken to a depth of 1 foot when corn is between 6 inches and 1 foot tall. The original idea behind the test was to wait as long as possible before sampling because soil and weather conditions in the early growing season may reduce or increase N availability for the crop later in the season. After the corn is 1 foot tall, it is difficult to get samples to a lab and back in time to apply any needed sidedress N fertilizer. The PSNT is now used on field corn, sweet corn, pumpkins and cabbage. Although it is widely used, it is not very accurate in some situations, such as the sandy coastal plains soils of the southeastern United States.

Different approaches to using the PSNT work for different farms. In general, using the soil test allows a farmer to avoid adding excess amounts of “insurance fertilizer.”

Two contrasting examples:

• For farms using rotations with legume forages and applying animal manures regularly (so there’s a lot of active soil organic matter), the best way to use the test is to apply only the amount of manure necessary to provide sufficient N to the plant. The PSNT will indicate whether the farmer needs to side dress any additional N fertilizer. It will also indicate whether the farmer has done a good job of estimating N availability from manures.

• For farms growing cash grains without using legume cover crops, it’s best to apply a conservative amount of fertilizer N before planting and then use the test to see if more is needed. This is especially important in regions where rainfall cannot always be relied upon to quickly bring fertilizer into contact with roots. The PSNT provides a backup and allows the farmer to be more conservative with preplant applications, knowing that there is a way to make up any possible deficit. Be aware that if the field receives a lot of banded fertilizer before planting (like injected anhydrous ammonia), test results may be very variable depending on whether cores are collected from the injection band or not.

Table 21.1 Phosphorus Soil Tests Used in Different Regions

Region	Soil test solutions used for P
Arid and semiarid Midwest, west, and northwest	Olsen AB-DTPA
Humid Midwest, mid-Atlantic, Southeast, and eastern Canada	Mehlich 3 Bray 1 (also called Bray P-1 or Bray-Kurtz P)
North central and Midwest	Bray 1 (also called Bray P-1 or Bray-Kurtz P)
Washington and Oregon	Bray 1 for acidic soils Olsen for alkaline soils
Southeast and mid-Atlantic	Mehlich 1
Northeast (New York and parts of New England), some labs in Idaho and Washington	Morgan or modified Morgan Mehlich 3
Source: Modified from Allen, Johnson and Unruh et al. (1994)

Table 21.2 Interpretation Ranges for Different P Soil Tests*

	Low and medium	Optimum	High
Olsen	0–11	11–16**	>16
Morgan	0–4	4–10	>10
Bray 1 (Bray P-1)	0–25	25–45	>45
Mehlich 1	0–20	20–40	>40
Mehlich 3	0–30	30–50	>50
AB-DTPA (for irrigated crops)	0–8	8–11	>12
*Individual laboratories may use somewhat different ranges for these categories or use different category names. Also note: Units are in parts per million phosphorus (ppm P), and ranges used for recommendations may vary from state to state; Low and Medium indicates a high to moderate probability to increase yield by adding P fertilizer; Optimum indicates that there is a low probability for increasing yield with added P fertilizer; High soil test levels indicate increasing potential for P pollution in runoff. Some labs also have a Very High category. **If the soil is calcareous (has free calcium carbonate in the soil), the Olsen soil test “optimum” range would be higher, with over 25 ppm soil test P for a zero P fertilizer recommendation.

Other Nitrogen Soil Tests

In humid regions there is no other widely used soil test for N availability. A few states in the upper Midwest offer a pre-plant nitrate test, which calls for sampling to 2 feet in the spring. For a number of years there was considerable interest in the Illinois Soil Nitrogen Test (ISNT). The ISNT, which measures the amino-sugar portion of soil N, has unfortunately been found to be an inconsistent predictor of whether the plant needs extra N. An evaluation in six Midwestern states concluded that it is not sufficiently precise for making N fertilizer recommendations. Another proposed test involves combining soluble organic N and carbon together with the amount of CO₂ that is evolved when the soil is rewetted. These tests, individually or in combination, have not yet been widely evaluated for predicting N needs under field conditions.

In the drier parts of the country, in the absence of a soil test, many land grant university laboratories use organic matter content to help adjust a fertilizer recommendation for N. But there is also a soil nitrate soil test used in some drier states that requires samples to 2 feet or more, and it has been used with success since the 1960s. The deep-soil samples can be taken in the fall or early spring, before the growing season, because of low leaching and denitrification losses and low levels of active organic matter (so hardly any nitrate is mineralized from organic matter). Soil samples can be taken at the same time for analysis for other nutrients and pH.

Sensing and Modeling Nitrogen Deficiencies

Since nitrogen management is a challenge for many of the common crops (corn, wheat, rice, oilseed rape, etc.) and is also an expensive input, there has been a significant amount of research into new technologies that allow a farmer or consultant to assess a crop’s N status during the season. Generally four types of approaches are used:

• Chlorophyll meters are handheld devices that indirectly estimate chlorophyll content in a crop leaf, which is an indicator of its N status (Figure 21.3, left). It requires field visits and adequate leaf sampling to represent different zones in the field. They are primarily used for final fertilizer applications in cereals, especially when aiming for certain protein contents.
• Canopy reflectance sensors can be handheld or equipment-mounted devices that measure reflectance without contacting the leaf (Figure 21.3, right). Both sense the light reflectance properties of a crop canopy in the near infrared and red (or red-edge) bands, which can be related to crop growth and N uptake. When equipment mounted, it allows for on-the-go adjustment of N rates throughout a field, which in most cases also requires the establishment of a high-N reference strip in the field for use in calibrating the sensor. These sensors are not imaging; in other words, they don’t create pixel maps, but they can be linked with GPS signals to chart patterns in a field.
• Satellite, aircraft or drone imagery can be used to extract reflectance information that can be related to a crop’s N status (Figure 21.4, left), usually also using near infrared and red/red-edge bands, with resolutions in the 30–90 foot (10–30 meter) range.
• Computer models simulate a field’s N dynamics and allow for daily estimation of the soil and crop N status (Figure 21.4, right).

These tools are actively being advanced as part of the drive towards digital technologies in crop production. Each technology has its strengths and weaknesses, and has proven different levels of precision. Use of computer models is relatively inexpensive and scalable. It allows for daily monitoring and is good at integrating other data sources into recommendations, but it does not involve direct field observations. The satellite-derived images are generally available every few days but are highly impacted by cloud cover, which can obstruct fields during critical decision times. Aircraft and drone imaging can avoid cloud issues but are more expensive. The chlorophyll meter is an in-field measurement that is, like the PSNT, relatively labor intensive and costly to repeat for large fields (but more attractive with smaller fields with high-value crops). Canopy reflectance sensors are also generally used once or twice during the season when N fertilizer is applied, but they are not used for continuous monitoring.

Figure 21.3. In-field sensors used to measure nitrogen status of leaves and canopies. Left: leaf chlorophyll (SPAD) sensor. Photo by Konica Minolta. Right: proximal canopy sensors. Photo by Trimble Agriculture.