Sustainable Agriculture Research and Education - Publications - Building Soils for Better Crops- Chapter 19

the popular mind is still fixed on the idea that
a fertilizer is the panacea.
J.L. Hills, C.H. Jones, and C. Cutler, 1908

Although fertilizers and other amendments purchased from off the farm are not a panacea to cure all soil problems, they play an important role in maintaining soil productivity. Soil testing is the farmer's best means for determining which amendments or fertilizers are needed and how much should be used.

The soil test report provides the soil's nutrient and pH levels and, in arid climates, the salt and sodium levels. Recommendations for application of nutrients and amendments accompany most reports. They are based on soil nutrient levels, past cropping and manure management, and should be a customized recommendation based on the crop you plan to grow.

Soil tests and proper interpretation of results are a very important management tool for developing a farm nutrient management program. However, deciding how much fertilizer to apply or the total amount of nutrients needed from various sources is part science, part philosophy, and part art. Understanding soil tests and how to interpret them can help farmers better customize the test's recommendations. In this chapter, we'll go over sources of confusion about soil tests, discuss N and P soil tests, and then examine a number of soil tests to see how the information they provide can help you make decisions about fertilizer application.

The usual time to take soil samples for general fertility evaluation is in the fall or in the spring, before the growing season has begun. These samples are analyzed for pH and lime requirement as well as phosphorus, potassium, and magnesium. Some labs also routinely analyze for selected micronutrients, such as boron, zinc, and manganese.

Soil tests and their recommendations, although a critical component of fertility management, are not 100 percent accurate. Soil tests are an important tool, but need to be used by farmers and farm-advisors along with other information to make the best decision regarding amounts of fertilizers or amendments to apply.

Soil tests are an estimate of a limited number of plant nutrients, based on a small sample, which is supposed to represent many acres in a field. With soil testing, the answers aren't quite as certain as we might like them. A low potassium soil test indicates that you will probably increase yield by adding the nutrient. However, adding fertilizer may not increase crop yields in a field with a low soil test level. The higher yields may be prevented because the soil test is not calibrated for that particular soil (and the soil had sufficient potassium for the crop despite the low test level) or because of harm caused by poor drainage or compaction. Occasionally, using extra nutrients on a high-testing soil increases crop yields. Weather conditions may have made the nutrient less available than indicated by the soil test. So, it's important to use common sense when interpreting soil test results.

Guidelines for Taking Soil Samples

1. Don't wait until the last minute. The best time to sample for a general soil test is usually in the fall. Spring samples should be taken early enough to have results in time to properly plan nutrient management for the crop season.

2. Take cores from at least 15 to 20 spots randomly over the field to obtain a representative sample. One sample should not represent more than 10 to 20 acres.

3. Sample between rows. Avoid old fence rows, dead furrows, and other spots that are not representative of the whole field.

4. Take separate samples from problem areas, if they can be treated separately.

5. In cultivated fields, sample to plow depth.

6. Take two samples from no-till fields: one to a 6-inch depth for lime and fertilizer recom mendations, and one to a 2-inch depth to monitor surface acidity.

7. Sample permanent pastures to a 3- to 4-inch depth.

8. Collect the samples in a clean container.

9. Mix the core samplings, remove roots and stones, and allow to air dry.

10. Fill the soil-test mailing container.

11. Complete the information sheet, giving all of the information requested. Remember, the recommendations are only as good as the information supplied.

12. Sample fields at least every three years. Annual soil tests will allow you to fine-tune nutrient management and may allow you to cut down on fertilizer use.

Modified from The PennState Agronomy Guide, 1999.

People may be easily confused about the details of soil tests, especially if they have seen results from more than one soil testing laboratory. There are a number of reasons for this, including:

Labs Use Varied Procedures
One of the complications with using soil tests to help determine nutrient needs is that testing labs across the country use a wide range of procedures. The main difference among labs is the solutions they use to extract the soil nutrients. Some use one solution for all nutrients, while others will use one solution to extract potassium, magnesium and calcium; another for P; and yet another for micronutrients. The various extracting solutions have different chemical compositions, so the amount of a particular nutrient that lab A extracts may be very different from the amount extracted by lab B. However, there are frequently good reasons to use a particular solution. For example, the Olsen test for phosphorus (see below) is more accurate for high-pH soils in arid and semi-arid regions than are the various acid-extracting solutions commonly used in more humid regions. Whatever procedure the lab uses, soil test levels must be calibrated with crop yield response to added nutrients. For example, do the yields really increase when you add phosphorus to a soil that tests low in P? In general, university or state labs in a given region use the same or similar procedures that have been calibrated for local soils and climate.

Labs Report Soil Test Levels Differently
Different labs may report their results in different ways. Some use part per million (10,000 ppm = 1 percent); others use lbs./acre (they do this usually by using part per two million, which is twice the part per million level); and others use an index (for example, all nutrients are expressed on a scale of 1 to 100). In addition, some labs report phosphorus and potassium in the elemental form, while others use the oxide forms, P₂O₅ and K₂O.

Most testing labs report results as both a number and a category, such as low, medium, optimum, high, very high. However, although most labs consider high to be above the amount needed (the amount needed is called optimum), some labs use optimum and high interchangeably. If the significance of the various categories is not clear on your report, be sure to ask. Labs should be able to furnish you with the probability of getting a response to added fertilizer for each soil test category.

Different Recommendation Systems
Even when labs use the same procedures, as is the case in most of the Midwest, different approaches to making recommendations lead to different amounts of recommended fertilizer. Three different philosophies are used to make fertilizer recommendations based on soil tests. One approach the sufficiency level system suggests there is a point, the sufficiency or critical soil test value, above which there is little likelihood of response to an added nutrient. Its goal is not to produce the highest yield every year, but, rather, to produce the highest average return over time from using fertilizers. Experiments that relate yield increases with added fertilizer to soil test level provide much of the evidence supporting this approach. As the soil test level increases from optimum to high, yields without adding fertilizer are closer to the maximum obtained by adding fertilizer (figure 19.1). Of course, farmers should be shooting for the maximum economic yields, which are slightly below the highest possible yields.

Another approach used by soil test labs the build-up and maintenance system calls for building up soils to high levels of fertility and then keeping them there by applying enough fertilizer to replace nutrients removed in harvested crops. It is used mainly for phosphorus, potassium, and magnesium recommendations.

The basic cation saturation ratio system, a method of estimating calcium, magnesium, and potassium needs, is based on the belief that crops yield best when calcium (Ca⁺⁺), magnesium (Mg⁺⁺), and potassium (K⁺) usually the dominant cations on the CEC are in a particular balance. Although there are different versions of this system, most call for calcium to occupy about 60 to 80 percent of the CEC, whereas magnesium should be from 10 to 20 percent and potassium from 2 to 5 percent of the CEC. Great care is needed when using the base cation saturation ratio system. For example, the ratios of the nutrients can be within the guidelines, but there may be such a low CEC (such as with a sandy soil that is very low in organic matter), that the amounts present are insufficient for crops. In addition, when there is a high CEC, there may be plenty of the nutrient, but the cation ratio system will call for adding more. This can be a problem with soils that are naturally high in magnesium, because the recommendations may call for high amounts of calcium and potassium to be added when none are really needed.

Research indicates that plants do well over a broad range of cation ratios, as long as there are sufficient supplies of potassium, calcium, and magnesium. However, there are occasions when the calcium-magnesium-potassium ratios are very out of balance. For example, when magnesium occupies more than 50 percent of the CEC in soils with low aggregate stability, using calcium sulfate may help restore aggregation. As mentioned previously, liming very acidic soils sometimes results in decreased potassium availability and this would be apparent when using the cation ratio system.

The sufficiency system would also call for adding potassium because of the low potassium levels in these very acid soils. The sufficiency level approach is used by most fertility recommendation systems for potassium, magnesium, and calcium. It generally calls for lower application rates and is more consistent with the scientific data than the cation ratio system. The cation ratio system can be used successfully, if interpreted with care and common sense not ignoring the total amounts present.

Labs sometimes use a combination of these systems, something like a hybrid approach. Some laboratories that use the sufficiency system will have a target for magnesium, but then suggest adding more if the potassium level is high. Others suggest that higher potassium levels are needed as the soil CEC increases. These are really hybrids of the sufficiency and cation ratio systems. At least one lab uses the sufficiency system for potassium and a cation ratio system for calcium and magnesium. Also, some labs assume that soils will not be tested annually. The recommendation that they give is, therefore, a combination of the sufficiency system (what is needed for this crop) with a certain amount added for maintenance. This is done to be sure there is enough fertility in the following year.

Crop Value, Fertilizer Rates, and Recommendation System
The value of your crop can have a major impact on the economics of over-applying fertilizer. As a general rule, the lower the per acre value of your crop, the greater the economic penalty for applying extra fertilizer (see box on p. 183). Farmers growing agronomic crops should take special care not to over-apply fertilizer.

To estimate the percentages of the various cations on the CEC, the amounts need to be expressed in terms of quantity of charge. Some labs give both concentration by weight (ppm) and by charge (me/100g). If you want to convert from ppm to milliequivalent per 100 grams (me/100g), you can do it as follows:
(Ca in ppm)/200 = Ca in me/100g
(Mg in ppm)/120 = Mg in me/100g
(K in ppm)/390 = K in me/100g
As discussed previously (chapter 18), adding up the amount of charge due to calcium, magnesium, and potassium gives a very good estimate of the CEC for most soils for most soils above pH 5.5.

As the soil test level of a particular nutrient increases, there is less chance that adding the nutrient will result in a greater yield. However, it may be worth adding fertilizer to high-value crops grown on soils with the same test levels that call for no fertilizer use low-value crops (figure 19.2). This difference should be reflected in the recommendations provided by soil testing laboratories.

Recommendation System Comparison

Most university testing laboratories use the sufficiency level system, but some make potassium or magnesium recommendations by modifying the sufficiency system to take into account the portion of the CEC occupied by the nutrient. The build-up and maintenance system is used by some state university labs and many commercial labs. An extensive evaluation of different approaches to fertilizer recommendations for agronomic crops in Nebraska found that using the sufficiency level system resulted in using less fertilizer and gave higher economic returns than the build-up and maintenance system. Other studies in Kentucky, Ohio, and Wisconsin indicate that the sufficiency system is superior to the build-up and maintenance or cation ratio systems.

Plant Tissue Tests
Soil tests are the most common means of assessing fertility needs of crops, but plant tissue tests are especially useful for nutrient management of perennial crops, such as apple, citrus and peach orchards, and vineyards. For most annuals, including agronomic and vegetable crops, tissue testing is not widely used, but can help diagnose problems. The small sampling window available for most annuals and an inability to effectively fertilize them once they are well established, except for N during early growth stages, limits the usefulness of tissue analysis for annual crops. However, leaf petiole nitrate tests are sometimes done on potato and sugar beets to help fine-tune in-season N fertilization. Petiole nitrate is also helpful for N management of cotton and for help managing irrigated vegetables, especially during the transition from vegetative to reproductive growth. With irrigated crops, in particular when the drip system is used, fertilizer can be effectively delivered to the rooting zone during crop growth.

What Should You Do?
After reading the discussion above you may be somewhat bewildered by the different procedures and ways of expressing results, as well as the different recommendation approaches. The fact is that it is bewildering! Our general suggestions of how to deal with these complex issues are: 1. Send your soil samples to a lab that uses tests evaluated for the soils and crops of your state or region. Continue using the same lab or another that uses the same procedures and recommendation system.

2. If you're growing low value-per-acre crops (wheat, corn, soybeans, etc.), be sure that the recommendation system used is based on the sufficiency approach. This system usually results in lower fertilizer rates and higher economic returns for low value crops. [It is not easy to find out what system a lab uses. Be persistent and you will get to a person that can answer your question.]

3. Dividing the same sample in two and sending it to two labs may result in confusion. You will probably get different recommendations and it won't be easy to figure out which is better for you, unless you are willing to do a comparison of recommendations. In most cases you are better off staying with the same lab and learning how to fine-tune recommendations for your farm. However, if you are willing to experiment, you can send duplicate samples to two different labs, with one going to your state-testing laboratory. In general, the recommendations from these labs call for less, but enough, fertilizer. If growing crops over large acreage, set up a demonstration or experiment in one field where you apply the fertilizer recommended by each lab over long strips and see if there is any yield difference. A yield monitor for grain crops would be very useful for this purpose. If you've never set up a field experiment before, you should ask your extension agent for help, and might find the brochure How to Conduct Research on Your Farm or Ranch of use (see Sources at end of chapter).

4. Keep a record of soil tests for each field, so that you can track changes over the years (figure 19.3). If records show a build up of nutrients to high levels, reduce nutrient applications. If you're drawing nutrient levels down too low, start applying fertilizers or off-farm organic nutrient sources. In some rotations, such as the corn-corn-4 years of hay shown at the bottom of figure 19.3, it makes sense to build up nutrient levels during the corn phase and draw them down during the hay phase.

Crop Value and Care with Fertilizer Rates

Most agronomic crops grown on large acreages are worth around $200 to $400 per acre and the fertilizer used may represent 30 to 40 percent of out-of-pocket growing costs. So, if you use 100 lbs. of N you don't need, that's about $30/acre and represents a major economic loss. Some years ago, one of the authors worked with two brothers who operated a dairy farm in northern Vermont that had high soil test levels of N, P, and K. Despite his recommendation that no fertilizer was needed, the normal practice was followed and $70 per acre worth of fertilizer N, P, and K was applied to their 200 acres of corn. The yields on 40 feet wide, no-fertilizer strips that they left in each field were the same as where fertilizer had been applied, so the $14,000 for fertilizer was wasted.
When growing fruit or vegetable crops worth thousands of dollars per acre fertilizers represent about 1 percent of the value of the crop and 2 percent of the costs. But when growing specialty crops (medicinal herbs, certain organic vegetables for direct marketing) worth over $10,000 per acre, the cost of fertilizer is dwarfed by other costs, such as hand labor. A waste of $30/acre in unneeded nutrients for these crops would cause a minimal economic penalty assuming you maintain a reasonable balance between nutrients but there may be environmental reasons against applying too much fertilizer. However, there may be a justification for using the build-up and maintenance approach for phosphorus and potassium on high-value crops because: a) the extra costs are such a small percent of total costs and b) there may occasionally be a higher yield because of this approach that would more than cover the extra expense of the fertilizer.

Soil samples for nitrogen tests are usually taken at a different time and using a different method than for the other nutrients (which are typically sampled to plow depth in the fall or spring). Before the mid-1980s, there was no reliable soil test for N availability in the humid regions of the US.

fig 19.3 soil test phosphorus and potassium trends under different fertility management regimes

Figure 19.3 Soil test phosphorus and potassium trends under different fertility management regimes. Modified from The PennState Agronomy Guide, 1999.

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 also goes under other names (see box). All of these names refer to the same test a soil sample is taken to 1 foot depth, 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 both field corn and sweet corn and research is underway in the northeastern U.S. to extend its use to pumpkins and cabbage. Although the PSNT is widely used, there are some situations, such as the sandy coastal plains soils of the deep south, where it is not very accurate.

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 follow:

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 to your advantage is to apply only the amount of manure necessary to provide sufficient N to the plant. The PSNT will indicate whether or not you need to side-dress any additional N fertilizer. It will also show you whether you've 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 test 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.

Other nitrogen soil tests. In the drier parts of the country, a nitrate soil test, requiring samples to 2 feet or more, 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 also be taken at the same time for analysis for other nutrients and pH. A few states in the upper Midwest offer a preplant nitrate test, which calls for sampling to 2 feet in the spring.

Names Used For The PSNT

Pre-Sidedress Nitrate Test (PSNT)
Magdoff PSNT
Late Spring Nitrate Test (LSNT)
June Nitrate Test
Vermont N Soil Test

Soil test procedures for phosphorus are different than for nitrogen. When testing for phosphorus, the soil is usually sampled to plow depth at a different time in the fall or in the early spring before tillage and the sample is usually analyzed for phosphorus, potassium, sometimes other nutrients (such as calcium, magnesium, and micronutrients) and pH. The methods used to estimate available P vary from region to region, and sometimes, from state to state within a region (Table 19.1). Although the relative test value for a given soil is usually similar when using different soil tests (for example, a high P-testing soil by one procedure is generally also high by another procedure), the actual numbers can be different (table 19.2).

The various soil tests for P take into account a large portion of the available P contained in recently applied manures and the amount that will become available from the soil minerals. However, if there is a large amount of active organic matter in your soils from crop residues or manure additions made in previous years, there may well be more available P for plants than indicated by soil test. (On the other hand, the PSNT reflects the amount of N that may become available from decomposing organic matter.)

A word of caution when comparing your soil test organic matter levels with those discussed in this book. If your laboratory reports organic matter as "weight loss" at high temperature, the numbers may be higher than if the lab uses the traditional wet chemistry method. A soil with 3 percent organic matter by wet chemistry might have a weight-loss value of between 4 and 5 percent. Most labs use a correction factor to approximate the value you would get by using the wet chemistry procedure. Although either method can be used to follow changes in your soil, when you compare soil organic matter of samples run in different laboratories, it's best to make sure the same methods were used.

There is now a laboratory that will determine various forms of living organisms in your soil. Although it costs quite a bit more than traditional testing for nutrients or organic matter, you can find out the amount (weight) of fungi and bacteria in a soil, as well as analysis for other organisms. (See the Resources section at the back of the book for laboratories that run tests in addition to basic soil fertility analysis.)

Unusual Soil Tests?

From time to time we've come across unusual soil test results. A few examples and their typical causes are given below.

Very high phosphorus levels. High poultry or other manure application over many years.

Very high salt concentration in humid region. Recent application of large amounts of poultry manure or immediately adjacent to road where de-icing salt was used.

Very high pH and high calcium levels, relative to potassium and magnesium. Large amounts of lime-stabilized sewage sludge were used.

Very high calcium levels given the soil's texture and organic matter content. Using an acid solution, such as the Morgan, Mehlich 1, or Mehlich 3, to extract soils containing free limestone causes some of the lime to dissolve, giving artificially high calcium test levels.

Below are five soil test examples, including discussion about what they tell us and the types of practices that should be followed. Suggestions are provided for conventional farmers and organic producers. These are just suggestions there are other satisfactory ways to meet the needs for crops growing on these soils. The soil tests were run by different procedures, to give examples from around the US. Interpretations for a number of commonly used soil tests relating test levels to general fertility categories are given later in the chapter (tables 19.3 and 19.4). Many labs estimate the cation exchange capacity that would exist at pH 7 (or even higher). Because we feel that the soil's current CEC is of most interest (see chapter 18), the CEC is estimated by summing the exchangeable bases. The more acidic a soil, the greater the difference between its current CEC and the CEC it would have near pH 7.

Following the five soil tests is a section on modifying recommendations for particular situations.

Field name: North
Sample date: September (PSNT sample taken the following June)
Soil type: loamy sand
Manure added: none
Cropping history: mixed vegetables
Crop to be grown: mixed vegetables

It is too acidic for most agricultural crops, so lime is needed.
Phosphorus is low, as are potassium, magnesium, and calcium. All should be applied.
This low organic matter soil is probably also low in active organic matter (indicated by the low PSNT test, see table 19.4a) and will need an application of nitrogen. (The PSNT is done during the growth of the crop, so it is difficult to use manure to supply extra N needs indicated by the test.)
The coarse texture of the soil is indicated by the combination of low organic matter and low CEC.

General recommendations:
1. Apply dolomitic limestone, if available, in the fall at about 2 tons/acre (and work it into the soil and establish a cover crop if possible). This will take care of the calcium and magnesium needs at the same time the soil's pH is increased.
2. Because no manure is to be used after the test was taken, broadcast significant amounts of phosphate (probably around 50 to 70 lbs. phosphate (P₂O₅)/acre) and potash (around 150 to 200 lbs. potash (K₂O)/acre). Some phosphate and potash can also be applied in starter fertilizer (band applied at planting). Usually N is also included in starter fertilizer, so it might be reasonable to use about 300 lbs. of a 10-10-10 fertilizer, which will apply 30 lbs. of N, 30 lbs. of phosphate, and 30 lbs. of potash per acre. If that rate of starter is to be used, then broadcast 400 lbs. per acre of a 0-10-30 bulk blended fertilizer. The broadcast plus the starter will supply 30 lbs. of N, 70 lbs. of phosphate, and 150 lbs. of potash per acre.
3. If only calcitic (low magnesium) limestone is available, use Sul-Po-Mag as the potassium source in the bulk blend to help supply magnesium.
4. Nitrogen should be sidedressed at around 80 to 100 (or more) lbs./acre for N-demanding crops, such as corn or tomatoes. About 300 lbs. of ammonium nitrate or 220 lbs. of urea per acre will supply 100 lbs. of N.
5. Use various medium-to-long-term strategies to build up soil organic matter, including use of cover crops and animal manures.

Most of the nutrient needs of crops on this soil could have been met by using about 20 tons wet weight of solid cow manure/acre or its equivalent. It is best to apply it in the spring, before planting. If the manure had been applied, the PSNT test would probably have been quite a bit higher, perhaps around 25 ppm.

1. Use dolomitic limestone to increase the pH (as recommended for the conventional farmer above).
2. Apply 2 tons/acre of rock phosphate, or about 5 tons of poultry manure for phosphorus, or better yet a combination of 1 ton rock phosphate and 2 ½ tons of poultry manure. If the high level of rock phosphate is applied, it should supply some phosphorus for a long time, perhaps a decade.
3. If the poultry manure is used to raise the phosphorus level, add 2 tons of compost per acre to add some longer lasting nutrients and humus. If rock phosphate is used to supply phosphorus, then use livestock manure and compost (to add N, potassium, magnesium, and some humus).
4. Establish a good rotation with soil-building crops and cover crops.
5. Care is needed with manure use. Although the application of uncomposted manure is allowed by organic certifying organizations, there are restrictions. For example, three to four months may be needed between application of uncomposted manure and either harvest of root crops or planting of crops that accumulate nitrate, such as leafy greens or beets. A two-month period may be needed between uncomposted manure application and harvest of other food crops.

Field name: Smith upper
Sample date: November (no sample for PSNT will be taken)
Soil type: silt loam
Manure added: none this year (some last year)
Cropping history: legume cover crops used routinely
Crop to be grown: corn

The high pH indicates that this soil does not need any lime.
Phosphorus is high, as are potassium, magnesium, and calcium (see table 19.3d).
The organic matter is very good for a silt loam.
There was no test done for nitrogen, but this soil probably supplies a reasonable amount of N for crops, because the farmer uses legume cover crops and allows them to produce a large amount of dry matter.

General recommendations:
1. Continue building soil organic matter.
2. No phosphate, potash, or magnesium needs to be applied. The lab that ran this soil test recommended using 38 lbs. potash (K₂O) and 150 lbs. of magnesium (MgO) per acre. However, with a high K level, 180 ppm (about 8 percent of the CEC) and a high Mg, 137 ppm (about 11 percent of the CEC), there is a very low likelihood of any increase in yield or crop quality from adding either element.
3. Nitrogen fertilizer is probably needed in only small to moderate amounts (if at all), but we need to know more about the details of the cropping system or run a nitrogen soil test to make a more accurate recommendation.

Soil Test #2 Report Summary*
	lbs/Acre	PPM	Soil Test Category	Recommendation Summary
P	174	87	high	none
K	360	180	high	none
Mg	274	137	high	none
Ca	3880	1940	high	none

pH	7.2			no lime needed
CEC **	11.7 me/100g
OM	3%			add organic matter: compost, cover crops, animal manures

N	No N soil test			little to no N needed
* Soil sent to a commercial lab. P using Bray-1 solution. This is probably the equivalent of over 20 ppm by using Morgan or Olsen procedures. Other nutrients extracted with pH 7 ammonium acetate (see table 19.3d).

Recommendations for organic producers:
1. A good rotation with legumes will provide nitrogen for other crops.

Field name: #12
Sample date: December (no sample for PSNT will be taken)
Soil type: clay (somewhat poorly drained)
Manure added: none
Cropping history: continuous corn
Crop to be grown: corn

The high pH indicates that this soil does not need any lime.
Phosphorus and potassium are low.
The organic matter is relatively high. However, considering that this is a somewhat poorly drained clay, it probably should be even higher.
About half of the CEC is probably due to the organic matter with the rest probably due to the clay.
Low potassium indicates that this soil has probably not received high levels of manures recently.
There was no test done for nitrogen, but given the field's history of continuous corn and little manure, there is probably a need for nitrogen. A low amount of active organic matter that could have supplied nitrogen for crops is indicated by past history (the lack of rotation to perennial legume forages and lack of manure use) and the moderate percent organic matter (considering that it is a clay soil).

General recommendations:
1. This field should probably be rotated to a perennial forage crop.
2. Phosphorus and potassium are needed. Probably around 30 lbs. of phosphate (P₂O₅) and 200 or more lbs. of potash (K₂O) applied broadcast, preplant, if a forage crop is to be grown. If corn will be grown again, all of the phosphate and 30 to 40 lbs. of the potash can be applied as starter fertilizer at planting. Although magnesium, at about 3 percent of the effective CEC, would be considered low by relying exclusively on a basic cation ratio saturation recommendation system, there is little likelihood of an increase in crop yield or quality by adding magnesium.
3. Nitrogen fertilizer is probably needed in large amounts (100 to 130 lbs./acre) for high N-demanding crops, such as corn. If no in-season soil test (like the PSNT) is done, some preplant N should be applied (around 50 lbs./acre), some in the starter band at planting (about 15 lbs./acre) and some side-dressed (about 50 lbs.).
4. One way to meet the needs of the crop is as follows:
a) broadcast 500 lbs. per acre of an 11-0-44 bulk blended fertilizer;
b) use 300 lbs. per acre of a 5-10-10 starter; and
c) sidedress with 150 lbs. per acre of ammonium nitrate.

This will supply approximately 120 lbs. of N, 30 lbs. of phosphate and 210 lbs. of potash.

Recommendations for organic producers:
1. 2 tons/acre of rock phosphate (to meet P needs) or about 5 to 8 tons of poultry manure (which would meet both phosphorus and nitrogen needs), or a combination of the two (1 ton rock phosphate and 3 to 4 tons of poultry manure).
2. 400 lbs. of potassium sulfate per acre broadcast preplant. (If poultry manure is used to meet phosphorus and nitrogen needs, use only 200 to 300 lbs. of potassium sulfate per acre.)
3. Care is needed with manure use. Although the application of uncomposted manure is allowed by organic certifying organizations, there are restrictions. For example, three to four months may be needed between application of uncomposted manure and either harvest of root crops or planting of crops that accumulate nitrate, such as leafy greens or beets. A two-month period may be needed between uncomposted manure application and harvest of other food crops.

Field name: River A
Sample date: October
Soil type: sandy loam
Manure added: none
Cropping history: continuous cotton
Crop to be grown: cotton

With a pH of 6.5, this soil does not need any lime.
Phosphorus is very high, and potassium and magnesium are sufficient.
Magnesium is high, compared with calcium (Mg occupies over 26 percent of the CEC).
The low CEC at pH 6.5 indicates that the organic matter content is probably around 1 to 1.5 percent.

General recommendations:
1. No phosphate, potash, magnesium, or lime is needed.
2. Nitrogen should be applied, probably in a split application totaling about 70 to 100 lbs. N/acre.
3. This field should be rotated to other crops and cover crops used regularly.

Recommendations for organic producers:
1. Although poultry or dairy manure can meet the crop's needs, that means applying phosphorus on an already high-P soil. If there is no possibility of growing an overwinter legume cover crop (see below), then about 15 to 20 tons of bedded dairy manure (wet weight) should be sufficient.
2. If time permits, this soil can use a high-N producing legume cover crop, such as hairy vetch or crimson clover, to provide nitrogen to cash crops.
3. Develop a good rotation so that all the needed nitrogen will be supplied to non-legumes between the rotation crops and cover crops.
4. Although the application of uncomposted manure is allowed by organic certifying organizations, there are restrictions when growing food crops. Check with the person doing your certification to find out what restrictions apply to cotton.

Soil Test #4 Report Summary*
	lbs/Acre	PPM	Soil Test Category	Recommendation Summary
P	102	51	very high	none
K	166	83	high	none
Mg	264	132	high	none
Ca	1158	579		none

pH	6.5		moderate	no lime needed
CEC **	4.2 me/100g
OM	not requested			use legume cover crops, consider crop rotation

N	No N soil test			70-100 lbs. N/acre
*all nutrients determined using Mehlich 1 solution (see table 19.3b).

Field name: Hill
Sample date: April
Soil type: silt loam
Manure added: none indicated
Cropping history: not indicated
Crop to be grown: corn

The pH of 8.1 indicates that this soil is most likely calcareous.
Phosphorus is low, there is sufficient magnesium, and potassium is very high.
Although calcium was not determined, there will be plenty in a calcareous soil.
The organic matter at 1.8 percent is low for a silt loam soil.
The nitrogen test indicates a low amount of residual nitrate (table 19.4b) and, given the low organic matter level, a low amount of N mineralization is expected.

Soil Test #5 Report Summary*
	lbs/Acre	PPM	Soil Test Category	Recommendation Summary
P	14	7	low	20-40 lbs. P₂O₅
K	716	358	very high	none
Mg	340	170	high	none
Ca	not determined			none

pH	8.1			no lime needed
CEC **	not determined
OM	1.8%			use legume cover crops, consider rotation to other crops that produce large amounts of residues

N	5.8 ppm			170 lbs. N/acre
*K and Mg extracted by neutral ammonium acetate, P by Olsen solution (see table 19.3d).

General recommendations:
1. No potash, magnesium, or lime is needed.
2. About 170 lbs. of N/acre should be applied. Because of the low amount of leaching in this region, most can be applied pre-plant, with perhaps 30 lbs. as starter (applied at planting). Using 300 lbs. per acre of a 10-10-0 starter would supply all P needs (see below) as well as give some N near the developing seedling. Broadcasting and incorporating 300 lbs. of urea or 420 lbs. of ammonium nitrate will provide 140 lbs. of N.
3. About 20 to 40 lbs. of phosphate (P₂O₅) is needed per acre. Apply the lower rate as starter, because localized placement results in more efficient use by the plant. If phosphate is broadcast, apply at the 40 lb rate.
4. The organic matter level of this soil should be increased. This field should be rotated to other crops and cover crops used regularly.

Recommendations for organic producers:
1. Because rock phosphate is so insoluble in high pH soils, it would be a poor choice for adding P. Poultry (about 6 tons per acre) or dairy (about 25 tons wet weight per acre) manure can be used to meet the crop's needs for both N and P. However, that means applying more P than is needed, plus a lot of potash (which is already at very high levels).
2. A long-term strategy needs to be developed to build soil organic matter better rotations, use of cover crops, and importing organic residues onto the farm.
3. Care is needed with manure use. Although the application of uncomposted manure is allowed by organic certifying organizations, there are restrictions. For example, three months may be needed between application of uncomposted manure and either harvest of root crops or planting of crops that accumulate nitrate, such as leafy greens or beets. A two-month period may be needed between uncomposted manure application and harvest of other food crops.

Specific recommendations must be tailored to the crops you want to grow, as well as other characteristics of the particular soil, climate, and cropping system. Most soil test reports use information that you supply about manure use and previous crop to adapt a general recommendation for your situation. However, once you feel comfortable with interpreting soil tests, you may also want to adjust the recommendations for a particular need. What happens if you decide to apply manure after you sent in the form along with the soil sample? Also, you usually don't get credit for the nitrogen produced by legume cover crops because most forms don't even ask about their use. The amount of available nutrients from legume cover crops and from manures is indicated in table 19.5. Another common situation occurs because most farmers don't test their soil annually and the recommendations they receive are only for the current year. Under these circumstances, you need to figure out what to apply the next year or two, until the soil is tested again.

No single recommendation, based only on the soil test, makes sense for all situations. For example, your gut feeling might tell you that a test is too low (and fertilizer recommendations are too high). Let's say that you broadcast 100 lbs. N/acre before planting, but a high rate of N fertilizer is still recommended by the in-season nitrate test (PSNT), even though there wasn't enough rainfall to leach out nitrate or cause much loss by denitrification. In this case, you may not want to apply the full amount recommended.

Another example: a low potassium level in a soil test (let's say around 40 ppm) will certainly mean that you should apply potassium. But, how much should you use? When/how should you apply it? The answer to these two questions might be quite different on a low-organic matter, sandy soil where high amounts of rainfall normally occur during the growing season (in which case potassium may leach out if applied the previous fall, or early spring) versus a high-organic matter clay loam soil that has a higher CEC and will hold onto potassium added in the fall. This is the type of situation that dictates using labs whose recommendations are developed for soils and cropping systems in your home state or region. It also is an indication that you may need to modify a recommendation for your specific situation.

Making Adjustments to Fertilizer Application Rates

If information about cropping history, cover crops, or manure use is not provided to the soil testing laboratory, the report containing the fertilizer recommendation cannot take these factors into account. Below is an example of how you can modify the report's recommendations.

Past crop = corn
Cover crop = crimson clover, but small to medium amount of growth.
Manure = 10 tons of dairy manure that tested at 10 lbs. N, 3 lbs. of P₂O₅, and 9 lbs. of K₂O per ton. (A decision to apply manure was made after the soil sample was sent, so the recommendation could not take those nutrients into account.)

Making Adjustments to Fertilizer Application Rates
	N	P₂O₅	K₂O
Soil Test Recommendation Accounts for contributions from the soil. Accounts for nutrients contributed from manure and previous crop only if information is included on form sent with soil sample.	120	40	140
Credits (Use only if not taken into account in recommendation received from lab.)
Previous crop (already taken into account)	-0
Manure (10 tons @ 6 lbs. N2.4 lbs. P₂O₅9 lbs. K₂O per ton, assuming that 60% of the nitrogen, 80% of the phosphorus and 100% of the potassium in the manure will be available this year.)	-60	-24	-90
Cover Crop (medium growth crimson clover)	-50
TOTAL NUTRIENTS NEEDED FROM FERTILIZER	10	16	50

Sources
Allen, E.R., G.V. Johnson, and L.G. Unruh. 1994. Current approaches to soil testing methods: Problems and solutions. pp. 203220. In Soil Testing: Prospects for Improving Nutrient Recommendations (J.L. Havlin et al., eds). Soil Science Society of America. Madison, WI.

Cornell Cooperative Extension. 2000. Cornell Recommendations for Integrated Field Crop Production. Cornell Cooperative Extension, Ithaca, NY.

Hanlon, E. (ed.). 1998. Procedures Used by State Soil Testing Laboratories in the Southern Region of the United States. Southern Cooperative Series Bulletin No. 190Revision B. University of Florida. Immokalee, FL.

Herget, G.W., and E. J. Penas. 1993. New Nitrogen Recommendations for Corn. NebFacts NF 93111, University of Nebraska Extension. Lincoln, NE.

How to Conduct Research on Your Farm or Ranch. 1999. Available from SARE regional offices. Also available at: www.sare.org/publications

Jokela, B., F. Magdoff, R. Bartlett, S. Bosworth, and D. Ross. 1998. Nutrient Recommendations for Field Crops in Vermont. University of Vermont Extension. Brochure 1390. Burlington, VT.

Penas, E.J., and R.A. Wiese. 1987. Fertilizer Suggestions for Soybeans. NebGuide G87-859-A. University of Nebraska Cooperative Extension. Lincoln, NE.
Recommended Chemical Soil Test Procedures for the North Central Region. 1998. North Central Regional Research Publication No. 221 (revised). Missouri Agricultural Experiment Station SB1001. Columbia, MO.

Rehm, G., 1994. Soil Cation Ratios for Crop Production. North Central Regional Extension Publication 533. University of Minnesota Extension. St. Paul, MN.

Rehm, G., M. Schmitt, and R. Munter. 1994. Fertilizer Recommendations for Agronomic Crops in Minnesota. University of Minnesota Extension. BU-6240-E. St Paul, MN.

The PennState Agronomy Guide. 1999. The Pennsylvania State University. University Station, PA.