Soil Sampling in Fertilizer-Banded Fields

Chapter 6 – Fertility, No. 14, Fall 1990

Roger Veseth

Band application of fertilizer at or before seeding of cereals and other crops in the Northwest has become a common practice under conservation tillage systems as well as under conventional systems. The practice of broadcast applying fertilizer is generally being restricted to irrigated production areas and to spring topdress applications of nitrogen (N) in areas of relatively high precipitation where overwinter loss of N is a problem.

Fertilizer banding for e~fective early root access can improve fertilizer use efficiency and enable the crop to be more competitive against weeds. This can provide potential increases in yield and/or reductions in fertilizer costs compared to broadcasting. However, crop response to banded fertilizer can vary significantly with soil nutrient levels, the crop being grown, precipitation level and timing, soil temperature and other factors. Fertilizer banding is often particularly beneficial in crop production under conservation tillage systems.

Sampling Considerations

Along with this new fertilizer placement technology, producers can have an additional challenge in accurately determining the soil nutrient level in fields which have received band applications of fertilizer, particularly when the following crop is to be seeded under a no-till or minimum tillage system. How do you collect soil samples to accurately determine nutrient availability and fertilizer needs when there may be some concentrations of fertilizer remaining in bands? This is primarily a concern for determining the levels of non-mobile nutrients, such as phosphorus (P), potassium (K) and some micronutrients. Mobile nutrients, such as N and sulfur (S), move readily with soil water. Any N or S remaining from the fertilizer band application, not used by the crop, is probably well dispersed out of the initial concentrated band.

To address this question on soil sampling in fields with fertilizer bands, Robert Mahler, University of Idaho soil scientist at Moscow, recently conducted some field research to evaluate different sampling approaches. He is one of more than 100 Northwest researchers involved in conservation farming research related to the STEEP (Solutions To Environmental and Economic Problems) program underway in Idaho, Oregon and Washington.

Mahler explains that if a field has been moldboard plowed or extensively tilled with other implements below the depth of the fertilizer band, then any fertilizer remaining in the band has probably been dispersed in the tillage zone of the soil, In this case, soil sampling in respect to fertilizer bands should not be a concern. On the other hand, when the following crop will be seeded under no-till or reduced tillage conditions, the fertilizer band can remain largely undisturbed.

Sampling Methods

Little research has been conducted to determine the best method of sampling in fertilizer-banded fields. Mahler points out that an ideal sample would be a continuous soil slice 1 to 2 inches wide and 12 inches deep extending from the center of one band to the center of the next band (Note: university fertilizer recommendations in the Northwest for immobile nutrients are based on a sampling depth of O to 12 inches). Currently, three different sampling approaches are widely used to sample fields which have undisturbed fertilizer bands. He delineates them as systematic, controlled and random.

Systematic Sampling Method

To use this method you must know the direction, depth and spacing of the fertilizer band to obtain a representative soil sample. An example could be where a no-till drill placed the fertilizer band directly below the seed row in the previous crop and the crop stubble remains undisturbed. He suggests taking 5 to 10 soil samples perpendicular to the band rows beginning at (and including) one band row and ending at the edge of the next band (Fig. 1). Follow this procedure on at least 20 sampling sites in each field or portion of a field being sampled. Thoroughly mix the samples from 20 or more sites together and take a composite subsample for analysis.

Controlled Sampling Method

As with the systematic sampling method, you also need to know the direction, depth and spacing of the fertilizer bands to obtain a representative soil sample with the controlled method. He suggests taking 20 to 30 soil cores from random sites throughout the field or uniform portion of the field, but avoid sampling directly in a fertilizer band.

Thoroughly mix the soil cores and take a composite subsample. Mahler points out that this method of sampling may result in slightly lower soil test values of non-mobile nutrients than the systematic and random sampling methods because the fertilizer bands are not sampled.

Random Sampling Method

In some situations the locations of the previous crop’s fertilizer bands are not known. An example of this is when the fertilizer was banded in a separate operation from seeding. Because of the presence of the fertilizer bands, Mahler suggests that more samples be taken for a composite subsample from the area than would be necessary if the field did not contain fertilizer bands. A total of 40 to 60 cores are suggested, instead of 20 to 30 cores used in the systematic or controlled sampling methods.

Field Research Results

To compare the three sampling methods, Mahler selected three growers fields in northern Idaho in 1988. All three fields had been no-till seeded to winter wheat with the fertilizer banded directly below the seed rows. Both N and P fertilizers had been applied together in the bands. Application rates ranged from 60 to 90 pounds N/acre and from 20 to 40 pounds P20Jacre. Each field was divided into six units for sampling replications.

A l-inch diameter soil sampling probe was used to collect the cores to a depth of 12 inches. In the systematic sampling method, eight cores were taken between the fertilizer bands for composite samples from 20 locations. The first core sample was in a band and the other seven cores extended to the edge of the next band. The controlled sample method included 25 cores and the random method included 50 cores. Samples were analyzed for NO3-N and P (NaOAc extractable).

Results for N

Mahler found no differences between the three sampling methods in determining soil N content (Table 1). Apparently, fertilizer N was sufficiently utilized by the previous crop and dispersed from the band so that all three sampling methods provided similar results.

Results for P

Different sampling methods resulted in different soil test P values (Table 2). Mahler explains that since fertilizer bands were not included in the controlled sampling method, available P level in the soil is underestimated with this method. The P soil test values with the controlled method were significantly lower than with the other two methods. The systematic method consistently resulted in the highest soil test P values at each site. Intermediate test values resulted from the random method since fewer fertilizer bands were sampled than with the systematic method.

In an actual field production situation, the differences in soil test P with the three sampling methods at these three sites could potentially have resulted in a different P fertilizer recommendation. For example, in the UI Current Information Series 453, Northern Idaho Fertilizer Guide for Wheat, different P fertilizer rates are recommended at the soil test P levels of O to 2 ppm P compared to at 2 to 4 ppm P. No P fertilizer is recommended when the soil test P is greater than 4 ppm. Site 1 has soil test P values in both the O to 2 ppm and 2 to 4 ppm P soil test ranges. Sites 2 and 3 each have a soil test P value which is on the border of one of these soil test P ranges.

It is important to note here that in soils with a high potential to immobilize P, differences in soil test P may not be apparent with these different soil sampling techniques. Also, the increments of ppm soil test P used for determining P fertilizer rates will vary with state fertilizer guides. This can be a result of differences in the laboratory chemicals being used to extract P from the soil sample (e.g. NaHCO~ with alkaline pH soils instead of NaOAc for acid pH soils), depth of soil sample required, yield potentials, soil test correlation results and other factors. Appropriate Extension fertilizer guides from your respective state land-grant university should most accurately address your soil and production conditions.

Implications for Soil Sampling

Mahler concluded that for mobile nutrients such as N, all three sampling methods provided satisfactory results. The random sample method would be preferred because it is easiest to use. More samples were taken in the random method than would ordinarily be suggested for the same size of field where fertilizer had not been banded. Mahler points out that it is not clear from the research results whether the larger number of soil samples for N are necessary or whether sampling for soil test N in fields where N fertilizer was previously banded should not be a concern. Since results with the controlled and systematic methods were not different in this study, he feels that a larger sample number with the random method is probably not necessary for mobile nutrients.

Table 1. Summary of soil test NO3-N to a depth of 12 inches using three sampling methods at three northern Idaho field sites where N fertilizer was banded at seeding of the previous crop, 1988 (Mahler, Ul, Moscow).

Table 2. Summary of soil test P (NaOAc extractable) to a depth of 12 Inches using three sampling methods at three northern Idaho field sites where P fertilizer was banded at seeding of the previous crop, 1988 (Mahler, Ul, Moscow).

For immobile nutrients, such as P, Mahler feels that the systematic sampling method provides the most accurate soil test result. However, he feels that the random sampling method with a larger number of subsamples would probably be the most acceptable method because it is least complicated and provides a soil test P value which was relatively close to the value for the systematic method.

The application of more immobile nutrient fertilizer, possibly more than necessary, might occasionally occur when using soil test values from the random sampling method because it slightly underestimates nutrient availability compared to the systematic method. This would most likely occur when the soil test values from random sampling are borderline between different fertilizer rate recommendations on crop fertilizer guides.

Mahler is concerned that there is also some uncertainty on how the soil test results for immobile nutrients should be interpreted in fertilizer requirements. He explains that most Extension crop fertilizer guides developed by land-grant universities in the Northwest are based on the correlation between soil test values (from sampled fields not receiving banded fertilizer applications) and the crop response to a range of broadcast fertilizer rates. He points out that new correlation research is needed to be certain that present fertilizer guides are accurate for the new fertilizer placement technology. This new information is needed both for soil test values from fields where fertilizer has been banded and for fertilizer recommendations with band applications. Unfortunately, adequate funding for this type of applied research to update fertilizer guides for the new technology is not currently available in the Northwest.

Special Considerations In Sampling No-till Fields

In fields which have an extended history of minimum soil disturbance under continuous no-till and reduced tillage systems, there can be some additional fertility management considerations. Mahler points out that if immobile nutrients have been surface broadcast, there can be an accumulation of nutrients at or near the surface. Fertilizer nutrients in the surface 1-inch of soil will probably not be available to the growing crop unless the surface soil remains moist for extended periods of time during the growing season. This is a relatively uncommon situation in the Northwest. For this reason, Mahler suggests that the surface l-inch of soil be removed before sampling where there is a long history of minimal soil disturbance. Fortunately, broadcast applications of immobile nutrients under no-till and other conservation tillage systems are rarely used now because of widespread availability of new equipment to band fertilizer under conservation tillage systems.

A second concern is the reduction in pH of the surface soil with continuous no-till systems after extended periods of time, particularly in the higher precipitation areas. Increasing soil acidity can affect the availability of fertilizer nutrients as well as the activity of some commonly used herbicides, insecticides and fungicides. He recommends that the pH of the surface foot be determined at 3-inch intervals (O to 3, 3 to 6, 6 to 9, 9 to 12 inches) every 3 to 5 years.

Soil Sampling Publication

A UI Cooperative Extension System publication, EXT 704 Soil Sampling, was recently prepared by Mahler and Robert McDole, UI Extension soil specialist. The bulletin provides an extensive review of soil sampling procedures and considerations, in addition to some of those pointed out here, The price of the publication is 50 cents. To obtain a copy, contact the Extension agricultural agent in your county or order it from the Ag Publications Building, Building J40, Idaho St., University of Idaho, Moscow, ID 83843-4199.