Soil testing is the single most important guide to the profitable use of fertilizer and lime. It is in the best interest of farmers, lawn care professionals, landscapers, gardeners, fertilizer suppliers, and consultants to promote the use of soil testing for several reasons.
The purpose of soil testing is to identify the soil fertility that the plants or crop, in a given area will experience. The soil area and volume could be a large field, a small garden, or simply the root zone of a single tree or shrub. The most difficult step in soil testing is accurately representing the desired area of soil. A laboratory cannot improve the accuracy of a sample that does not represent the area.
In most soils, it takes more than one year to make significant changes to the soil test levels. As the soil improves with better fertility programs, subsequent crops or plant growth should show increasing rates of improvement. Soils are formed over thousands of years, and are not easily changed in a short time.
You should plan how the field is to be sampled before you begin. You should plan how you will divide the field into sampling areas…then always use the same areas in the future. A soil map, available at the county Soil Conservation Service office will often be helpful. Some factors that might cause you to sub-divide the field into separate sampling areas are…
Soil test levels in any soil area will vary both laterally and vertically. This, plus the intended use of the field, dictates how the field or area of land should be sampled (see example in Fig. 1).
Tip: Permanently mark your soil probe at the depths that you expect to pull samples. This will help you maintain constant depth. A probe with a welded foot pedal at the correct depth is also helpful.
Pack the soil sample bags into the boxes available from our lab, or other suitable shipping containers. If you have less than a full box of sample containers, it would be best to include some additional packing to keep the soil sample containers from sliding loose in the box. Be sure to include a completed Soil Sample Information form in the boxes. If you do not use our soil sample information form, be sure to list your full name, address, and telephone. Also, include a list of samples in each box with your sample number included. Finally, tell us which tests we are to conduct, and whether or not you want fertilizer recommendations. If you want fertilizer recommendations, you must tell us the crop and yield goal, or plant species if ornamentals are being grown. While our soil sample bags are extremely water resistant, and we can accurately analyze “mud”, it will be difficult for you to pull representative samples from a muddy field.
Tip: We strongly recommend that you send samples by UPS, or a similarly efficient method. Most of our customers use UPS, and we often get 2 day service from anywhere in the country. Third class mail can take many days, and may cost more for heavier boxes.
Tip: When you have several boxes to send, put a shipping label on each box, then tape several boxes together…you'll get a cheaper shipping rate from most carriers! Check with your carrier for any weight limits that may apply.
We will typically complete a soil analysis on the following work day after we receive the sample. We often work on Saturdays and sometimes Sundays during the peak of the busy fall season. The printed report forms will be returned by first-class mail. Total sample turn-around time will typically range from 6 to 10 days, depending on how you ship samples to the lab. Sample results can also be accessed at www.spectrumanalytic.com, or by e-mail. Contact Spectrum Analytic to set up this service.
Research indicates that it is advisable to take 2 separate soil samples. A 2 inch deep sample to evaluate surface pH, and a normal 7 inch deep sample for nutrient information. Since most fertilizer N is applied to the surface of no-till soils, they tend to develop a much more acid layer in the top 2 inches (see Table 1). This can lead to poor performance of herbicides. No-till practices cause significant nutrient stratification in the soil, which can complicate sampling and interpretation. However, fertilizer recommendations are still best accomplished with a normal 7 inch deep sample.
As you read this, you may decide that neither option is practical. If so, the best approach is to either avoid any known fertilizer bands entirely, or to take at least 15 cores per sample, pulled at random across the area to be sampled. This approach is not perfect, so you may see some unusual results at times. If you want to use a more “scientific” method, read on. These methods are applicable to both “starter fertilizer” bands, and those from pre-plant “field banding”.
The key to getting statistically correct representative samples is to take the correct number of cores of soil from outside a band, in relation to each core taken in the band. The following systems are the best methods that we know to determine the ratio of “in-band” to “out-of band” soil cores.
Since nitrate-N (NO3-N) is the dominate form of N in most soils, most calibration data and interpretation is based on it (even though the crop can use both forms of N). This work assumes a typical background level of ammonium N (NH4-N). This presents several conditions that must be taken into account to get the best use out of soil N testing.
Tree crops must be sampled much differently than row crops or forages. Also, with tree crops, plant analysis is more important that soil testing, and should be done every year! There are several opinions on the best way to sample tree crops, and they are somewhat dependent on the planting pattern of the trees. We prefer the following; however it requires additional care on your part…
Sample depth should be 4 inches, and should not include accumulated surface organic materials such as thatch, or the blades of grass. It would be desirable to not include the grass roots either, but this may be nearly impossible to avoid. Sample handling procedures at Spectrum Analytic will remove most of the roots and larger pieces of non-soil material. The pattern of taking the cores is similar to that used in sampling crop fields (random and scattered), and avoiding unusual areas, unless the unusual areas are of interest.
We offer analysis for soybean cyst nematodes. The following instructions apply to all nematode samples. However, soybean cyst samples do not require that the nematodes be kept alive in the sample during transit to the lab. If you sample a field for nematodes other than soybean cyst nematodes, you will need to take special steps to insure that the soil moisture and temperature are correct at all times for the nematodes survival.
Nematodes do not normally infest a field in a uniform pattern. You will normally see an oval to irregular pattern of stunted plants (possibly dead ones in the center of the damaged area). It is best to take a nematode sample in the growing crop, so you are sure to sample the infected area of the field. In a growing crop, take from 10 to 20 cores from the perimeter of the affected area (see Fig. 4) to make a sample. You should avoid the interior of a dead area because the nematodes will have either left the area, or died due to lack of a host plant. You want to sample the area of heavily infected plants, so you will also want to take the cores from within the root zone, where the nematodes are. The depth of sampling should be from 6 to 8 inches in all soil except sandy soil, where your cores should be 10 to 12 inches deep.
Make a 1-2 pint composite sample from the cores, place it in a regular soil sample bag and ship it to the lab. We do not have a special nematode information form at this time, so simply write on the regular form that a particular sample is to be tested for soybean cyst nematodes.
As a general rule, you gain most of the needed soil fertility information from proper topsoil samples; however there are times when a sub-soil sample can provide valuable information. Roots must thrive in the entire natural rooting volume of the soil for top yields. Most cultivated plants, except some turf varieties and a few ornamentals, extend their root systems into the subsoil, so problems in the sub-soil can limit yields. This is especially true of deep rooted perennials, annuals with tap roots, and trees. The subsoil can inhibit root growth in several ways such as undesirable pH, deficient or toxic levels of certain elements, severe compaction, excess moisture, and possibly other reasons. In these cases, the roots will be restricted to the topsoil for their entire needs. This can lead to nutrient or water stress and lower yields. Of course, correcting sub-soil problems can be very difficult, so identifying the cause of a problem does not necessarily mean that there is a practical solution.
Subsoil samples should be taken separately from the topsoil sample. To do this, first take the appropriate topsoil sample and place it into a plastic bucket. Then insert the soil probe back into the same hole and take another core to the new depth. Place these cores into another plastic bucket. When you label the bags of soil, and the soil information forms, record the depth of each sample for future reference. The lab analysis will be the same for each of the two samples, and we do not have a special recommendation program for subsoil samples, but the subsoil results can be used to modify the recommendations from the topsoil sample on a case-by-case basis.
Q: When is the best time to take soil samples?
A: Anytime that you need them! In the fall, just after harvest is usually the preferred time in farming because the fields are empty and sampling is quicker, and the crop has had all season to pull nutrients out of the soil. A key to proper soil sampling is to be consistent. It is important to try to always sample a given field at the same time of the year. A field may give different values in the spring vs. the fall.
Q: How frequently should I resample a field?
A: It depends…Every two or three years is OK for most fields. Sandy soils that have very low nutrient reserves or fields producing high value crops such as fruits and vegetables could be sampled every year. Also, sample every year when there is an aggressive soil build-up program, or when there is a significantly low fertilizer rate.
Q: Should I sample an area even if it can not be fertilized separately?
A: Maybe…It may be worth sampling a “poor growth” area in order to find out why it performs as it does. You may get a clue as to how to “fix” a poor area.
Q: Are there areas of a field that should not be sampled?
A: Yes. Avoid the following areas in the field, especially when they cannot be treated separately (see section on grid sampling).
Q: Can I sample frozen soil?
A: Yes…While it is true that a frozen sample will probably show slightly higher levels (no more than 10%) of cations (K, Mg, Ca, etc.), the difference in recommendations will be small.
We are here to help you. Please call us anytime that you have questions about any aspect of soil sampling, understanding the results, or any area of agronomics.