Nutrient Management – Soil Test Recommendations – Commercial Fertilizers

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Intro Soils – Lab 7 Nutrient Management – Soil Test Recommendations – Commercial Fertilizers

Lecture Materials: Nutrient Management (Chapter 16)

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Lab 7 – Nutrient Management – Soil Test Recommendations – Fertilizer Application

Nutrient Management and Soil Testing

Nutrient management hopefully is a central goal of any producer, large or small, to maintain and

balance yield goals with the maintenance and enhancement of soil health and environmental quality.

The goals, though lofty, are relatively simple in nature; produce a high quality, high volume crop with

the least amount of inputs and in doing so build and maintain the natural resources which helped to

produce that crop. For nutrient inputs, especially manures and commercial fertilizers, the over

application and subsequent movement of those nutrients off site into surface waters has caused

substantial negative environmental impacts. Non-point source pollution from agricultural runoff has

been linked to severe eutrophication issues worldwide.

Environmental concerns, significantly increased input costs, research knowledge, and precision ag

technology has helped greatly shifted the focus of nutrient application from fertilizing the plant to

fertilizing the soil. Soil testing to determine recommended rates of nutrient application is now the

standard for applying nutrients in most arenas including row crop agriculture, forestry, and turf. The

goal is to maintain levels of nutrients so they are not yield-limiting while taking into consideration what

levels of nutrients the soil can provide as well.

Soil testing and subsequent recommendations classify nutrient levels into categories based on crop

response to nutrient addition. Soil test vary in the range of items to analyze, but most include pH (water

and buffer), phosphorus, potassium, calcium, magnesium, sodium, manganese, zinc, sulfur, iron, copper,

and boron. These are the main macro and micronutrients required for crop production. Nitrate is

generally not included due to the extreme variability in available nitrate throughout the year. Nitrogen

recommendations are based on previous cropping systems and target yield of the crop to be planted in

the upcoming season. The presidedress soil nitrate test can be utilized to identify nitrate levels in soil

when a corn crop is approximately 12 inches tall prior to when the corn needs the most nitrogen to

determine if additional nitrogen is need to maximize yields.

A first, vital step even before testing is taking the soil sample. It is extremely important to take a

representative soil sample. Depending on your operation and nutrient management goals, this may

mean taking a significant number of samples for analysis. There are numerous strategies for taking soil

samples, but the most prudent approach is to utilize the sampling method recommended by your

professional soil testing facility. These methods should also include guidance on when to collect

samples, depth at which to sample, and how the samples should be handled between sampling and

analysis. Many producers, consultants, and soils labs have their preferred method of sampling. There

is no one set method that is considered the standard, but the main goal always is to collect

representative samples of the specific size unit of characterization. Depending on the operation and use

of precision variable rate technologies, these units may range in size from 2.5 acre grids, specific

management zones, or a field hundreds of acres in size. A very small amount of soil is ultimately

analyzed to characterize a very large quantity and area of soil even on small grid scales.

Once the soil samples have been collected, analysis can begin. Laboratories and universities across the

country use a variety of extractants to remove the nutrients from soil for analysis depending on soil

types in their region. Labs in the southern US and Midwest regions vary between Meilich I and Meilich

III. The goal is to characterize the extractable the nutrients from soil that are expected to be plant

available during the growing season. These values are not total extractable nutrients as those values are

not useful indicators of plant available nutrients. Soil testing facilities routinely utilize inductively

coupled plasma spectrometry (ICP) coupled with atomic adsorption (AA) spectroscopy to determine a

wide range of elemental concentrations along with other analytical methods specific to nutrients of

interests. Recommendations for nutrient additions (or lack thereof) are based the amount of

extractable nutrients in the soil and how that crop will respond to an amendment with that particular

nutrient. The categories and crop responses are based on calibration data gathered from state and

regional from professional laboratories and universities based on specific crops and their needs, various

soil types, and various weather conditions, and crop uptake from the previous harvest. The philosophy

behind the recommendations is if test values are low, chances of a positive yield increase are very good,

but if your soil test values are high, then adding additional nutrients is not likely to produce an increase

in yield (Figure 1). Most recommendations in this region are in an effort to maintain optimal levels of

the nutrients in the soil profile.

Figure 1. Soil Test Value vs Probability of Response. As soil test values increase along the x-axis, the

probability of an increase in yield is low (y-axis). Conversely, when soil test values are low, the

probability of adding additional nutrients will increase yields is high. Where the curve levels off, at

maximum yield values is the window where recommendations are generally made.


Below are links to soil testing reports from the University of Tennessee Soil Plant and Pest Center in

Nashville (, as well as a report and explanatory

documents from a private testing facility, A&L Laboratories, in Memphis, TN ).

(This is not a recommendation for use of either lab, but are mentioned here as they are two labs

frequently utilized by producers in the state of TN.)




Additional nutrients are often recommended in many situations ranging from production row crops to

pastures to home gardens and lawns. Recommendation amounts again are based on the crop, target

crop yield, and probability of a positive response to additional nutrients. The producers is given a series

of recommendations from the soil test that may or may not include lime as well as macro and


The various nutrients come in an absolutely variety of forms, formations, and formulations. Producers

can make a choice on what they utilize based several factors including but not limited to availability,

cost, formulation, amount of acidification created, and ease of application. Many formulations are

granular salts containing specific amounts of the nutrient. A large portion of the fertilizer in the region is

surface applied in granular form in the spring. Nitrogen can also be applied in a liquid formulation or as

anhydrous ammonia which is a compressed gas and required specialized equipment to apply. Some

crops like corn and wheat use split applications of nutrients, a second application to supply, typically

additional nitrogen, at the time the plants need it the most. For corn, this application is generally known

as side dressing.

Nutrient contents or fertilizer grades are expressed as the percentage of nitrogen (N), phosphorus

(P2O5), and potassium (K2O). Potash is the general term used for the application of potassium and

includes several different actual formulations of salts containing potassium. The traditional convention

for a fertilizer grade is to list the percentage of the oxide forms of phosphate and potassium.

Recommendations though may vary whether they are reported as the oxide or actual elemental units of

P and K respectively. The following simple conversions can be used to easily convert back and forth

between the two units to best fit computational needs.

Equation 1: Phosphorus P x 2.3 = P2O5 P2O5 x 0.44 = P

Intro Soils – Lab 7 Nutrient Management – Soil Test Recommendations – Commercial Fertilizers

Equation 2: Potassium K x 1.2 = K2O K2O x 0.83 = K The fertilizer grade includes values for all three of the macronutrients in a three digit code which is the percentage by weight for N – P2O5 – K2O, respectively. For example, a fertilizer with a grade of 13-13-13, is 13% nitrogen, 13% phosphate (P2O5) and 13% potash (K2O).

Another way to describe the fertilizer grade, is that 100 lbs. of 13-13-13- would contain 13 lbs. each of N, P2O5, and K2O. Some fertilizers come as a complete fertilizer and contain all three nutrients like the one just described (e.g. 13-13-13), but others are mixed and contain just two of the three (e.g. 18-46-0), or just one of the three, called a straight fertilizer (e.g. 46-0-0).

The text (Table 16.13) lists several of the more common inorganic fertilizer materials, their percentages of N,P,K, acid formation values, and some other comments. For nitrogen, common amendments include urea (45% N) and UAN (32% N) which is a liquid formulation of urea and ammonium nitrate, as well as anhydrous ammonia which is a gas. Many producers are utilizing liquid formulations of nitrogen due to its relative ease and uniformity of application and more rapid potential for plant uptake. Further, safety and liability concerns for storing and handling ammonium nitrate have curtailed its use recently as it is hazardous material routinely used to make explosives.

For phosphate, common amendments are diammonium phosphate (DAP) or triple super phosphate (TSP). And finally for potassium or potash, muriate of potash (MOP) is the most commonly used amendment. Sulfur is another routine recommendation regionally and is often applied as ammonium sulfate. Many of the micronutrients like zinc, boron, and others are typically needed in very small amounts and are often included as a foliar feed alongside herbicide application. Soil test recommendations for row crops are typically reported in lbs./acre and for turf and garden varieties often in lbs./square feet.

Depending on the grade of the fertilizer, one must calculate how many pounds of product are needed to deliver the recommended rate. Large agronomic providers utilize computer programs to readily generate blends of fertilizers to meet the recommendations often times with mixing several different fertilizers. For instance, the recommendation calls for phosphorus and nitrogen, so DAP (18-46-0) might be used to supply the P and at the same time some, but not all of the nitrogen.

Urea (45% N) or ammonium nitrate (33% N) might be blended in with the mixture to fulfill the N recommendation; another route might also be to apply liquid UAN (32% N) in a split application for wheat or corn. The calculations for supplying recommended rates of nutrients are the same whether calculating for your garden or large row crop operation. For instance, if the recommendation calls for 150 lbs. per acre of nitrogen and you have ammonium nitrate readily available (32-0-0). Rate to add is 150 lbs./acre, there are 32 lbs. of N in every 100 lbs. of ammonium nitrate, so you will need to add 441 lbs./acre (Equation 3).

Equation 3: lbs. product to apply = recommendation lbs. x 100 lbs. product acre acre amount nutrient per 100 lbs. product lbs. NH4NO3 to apply = 150 lbs. N x 100 lbs. NH4NO3 = {(150 x 100)/32} = 441 lbs./acre acre acre 32 lbs. N If utilizing another fertilizer like DAP (18-46-0), both N and P2O5 will be applied, but at different rates, so it is important to include all of the nutrients when calculating fertilizer application prescriptions. When making blends, it is extremely important to note all of the nutrient sources and not over apply nutrients needed in lesser quantities (typically P and K) to meet the larger needs (typically N).

Lawn and garden recommendations are generally much, much lower overall additions and are typically recommended at rates at equal than to less than a pound per 1000 square feet. These recommendations tend to be met with complete fertilizers readily available at lawn and garden centers rather than potentially complex mixtures utilized in large ag production operations. In these cases, it may be more difficult to meet the exact recommendations for all three nutrients with a complete fertilizer (one with N,P, and K).

It is important to get it as close as possible to the recommended rate, and if you over/under apply P and K just adjust during your next application. Another approach, also utilized on large scales, is to utilize the complete fertilizers (fertilizers with more than one nutrient) for your lowest needed nutrients (typically P and K) and straight fertilizer to fill the remaining requirement for your larger needs (typically N).

Variable rate technologies utilize GPS and grid sampling to variably apply nutrients across a field rather than just applying one rate to the entire field. Soil test recommendations from the individual grid or zone samples are integrated with GPS on the fertilizer application unit and prescription rates are applied to each of those grids or zones individually.

It is not a perfect science by any means, but it can greatly reduce the overall amount of fertilizer added to a particular field and also identify grids or zones that may need additional nutrients to be successful. Variable rate and precision ag technologies have dramatically changed how fertilizers are applied and made a huge contribution to nutrient management goals overall. Careful consideration of commercial fertilizer needs is an important part of nutrient management.

Soil testing and utilizing recommendations based on crop response rather than set amounts based on crop needs and uptake have significantly changed how nutrients are managed for the positive. Comprehensive nutrient management can and should be a win-win situation were yields are maximized, input costs are minimized, and natural resources are maintained or even enhanced. References:

Intro Soils – Lab 7 Assignment Questions Nutrient Management – Soil Test Recommendations – Commercial Fertilizers

Utilize Lab, Lecture and Text Materials: Nutrient Management (Ch. 16)

1.) Farmer Johnson’s soil test recommendation calls for 80 lbs./acre of P and 25 lbs./acre of K. His consultant though has provided him with calculations that include P2O5 and K2O, the oxides, instead of the actual elemental units. For ease of comparison how many lbs. of P2O5 and K2O were recommended? (4 pts)

2.) Farmer Smith soil test recommendation calls for 150 lbs./acre of N, 90 lbs./acre of P2O5 and 80 lbs./acre of K2O. DAP (diammonium phosphate) and potash (KCL) are both readily available at her local supplier. Farmer Smith plans on applying DAP (18-46-0) for N and P2O5 needs and potash for K2O needs (0-0-60). (8 pts)

a.) Scenario A: Farmer Smith uses the recommended N rate to calculate how much DAP (18-46-

0) to apply. Calculate how many lbs./acre of DAP to apply to meet the recommended N rate of 150 lbs./acre?

b.) Scenario B: Farmer Smith used the recommended P rate to calculate how much DAP (18-46-

  1. O) to apply. Calculate how many lbs./acre of DAP to apply to meet the recommended P2O5 rate of 90 lbs./acre.

c.) Why do these values differ? What nutrient management issues might arise if Farmer Smith

utilizes scenario A? What crop management issue might arise if Farmer Smith utilizes only Scenario B?

d.) Famer Smith also needs to add some potash (0-0-60). Calculate how much potash to apply

to meet the K2O recommended rate of 80 lbs./acre.

3.) Explain the general idea behind a soil test. Why should we soil test, what type of results might I expect to receive, what do the ppm or lbs/acre values of the various nutrients on the data sheet actually mean, and then how were these recommendations for nutrient additions made? (8 pts)

4.) What are the four main goals of nutrient management? (4 pts)

5.) What is IPM? (4 pts)

6.) What are some pros and cons of utilizing animal manures in a farming operation? What are some measures to take to ensure you are meeting both nutrient and environmental goals? (4 pts)

7.) Why is Liebig’s Law of the Minimum an important concept to keep in mind when thinking about comprehensive nutrient management? How does the philosophy for soil testing also ring true for this law? (4 pts)

8.) What is the P Index? Why was it necessary and how is it utilized by producers? (4 pts)

Lab 6 Soil Nitrogen – Use of Colorimetric Assays

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