Cover Image
close this book Soils, Crops and Fertilizer Use
close this folder Chapter 7: Evaluating a soil's fertility
View the document Soil testing
View the document Plant tissue testing
View the document Fertilizer trials
View the document Using visual "hunger signs"

Soil testing

Soil testing by a reliable lab is the most accurate and convenient method for evaluating a soil's fertility. Most labs will also make a fertilizer recommendation, too. This service is often free or very low-cost, yet is often underutilized by farmers and development workers.

Some Factors Affecting the Usefulness of Soil Testing

• Improper sampling procedures by farmers and extension workers are common and produce inaccurate results.

• The test for available soil nitrogen (in the form of nitrate N) isn't very accurate. That's because a soil's available N fluctuates, because it depends a lot on the kind and amount of crop residues present and the rate that bacteria will break them down to release nitrate N (this rate varies a lot with temperature and moisture). If the sample sits at the lab for a week or two, a falsely high nitrate reading is likely.

• Most labs don't routinely test for sulfur or micronutrients. Some of these tests aren't very accurate, anyway.

• The reliability of soil labs varies, not only in terms their accuracy in evaluating a soil's fertility, but also as far as the resulting fertilizer recommendations. More on this below:

Soil Lab's Aren't Necessarily Reliable

When different soil test labs analyze the same soil sample, there can be some large discrepancies in both the fertility evaluations and the fertilizer recommendations. In fact, recent studies of public and private (commercial) labs in the U.S. have brought to light some surprising differences. For example, in one of the studies conducted by the Rodale Research Institute (Emmaus, Pennsylvania), 4 soil labs were sent an identical sample and asked to make a fertilizer recommendation for lettuce. Their recommendations varied as follows: N: 65, 130, 130, 220 kg/ha; P2O5 0, 0, 90, 220 kg/ ha; K2O: 0, 0, 35, 220 kg/ha. The resulting fertilizer cost ranged from $75-$235/ha. In another study, 5 labs were asked to analyze 4 sets of identical soil samples for 4 fields. The total cost of the fertilizer recommended by them ranged from a low of $168 to a high of $320.

How to Find a Reliable Lab Despite these variations, soil testing is still a very useful tool if you can locate a reliable soil testing lab. The Ministry of Agriculture, agricultural schools, ag research stations, or fertilizer companies may maintain labs in your country. You can inquire among technicians and farmers as to their opinion of the labs and do some additional investigation as well. Here are some useful indicators of reliability.

• Adequate equipment and well trained technicians.

• Enough greenhouse/field trials with the erect's soils to correlate soil test results with actual crop response to different rates of fertilizer. Soils vary in their response, so such correlation data is vital.

• Lack of bias: Labs run by fertilizer companies may be more interested in selling fertilizer than in accuracy, though not always. In some cases, labs don't consider the special circumstances of limited-resource farmers but gear their rates toward large farmers; this factor, in itself, can produce wide variations in fertilizer recommendations among labs testing the same soil sample.

NOTE: Even if the lab is biased, soil testing can at least provide you with valuable baseline data for "customizing" the recommendation to suit a farmer's actual circumstances.

• The lab should give credit for the farmer' A intended use of manure, compost, or green manure crops since this can substantially lower fertilizer needs.

• Farmer input: Good labs supply a detailed questionaire for the farmer to fill out concerning farm size, past and future crops, past yields, yield expectations, past fertilizer applications, intended use of manure or green green manure crops, limiting soil factors, etc.

• Sampling instructions: Reliable labs are likely to provide detailed written instructions on how to take and collect soil samples.

How to Evaluate a Lab's Fertilizer Recommendations

• Compare them with Table 9-4 in Chapter 9.

• Send an identical sample to two or more labs.

• Compare the lab's recommendation with trial strips using higher and lower rates.

• Plant tissue testing can be a useful supplement to soil testing since it can monitor N-P-K levels in the plant itself. (The section following soil sampling discusses tissue testing.)

What about Portable Soil Test Kits?

You can buy portable soil test kits for measuring NPK levels, but they definitely aren't accurate enough for several reasons:

• They're unlikely to be correlated with local soil conditions.

• Their reagents break down with time and may be difficult to replace.

• The color bar plates used to measure readings are often of poor quality and standardization in the cheaper kits.

Portable soil pH test kits are also available, and the better quality ones are accurate to within 0.1-0.3 pH units. They can be useful for troubleshooting, but a lab test will still usually be needed to determine how much lime is needed to raise the pH of an overly acidic soil. (Liming is covered in Chapter 11.)

What Useful Information Does a Soil Lab Provide?

• Most labs routinely test for:

Soil pH

C.E.C. (negative charge)

Available N, P, K, Ca, Mg

• Most labs don't routinely test for micronutrients or sulfur.

• The lab will give a fertilizer recommendation for NPK either in terms of kg/ha of N, P, and K or in terms of the actual kinds and amounts of fertilizers needed. The recommendation is based on the amount of nutrients already present in the soil and should also take into account the farmer's yield expectations.

• Liming: If the soil tests out overly acidic, the lab will give a liming recommendation.

• In dry regions where salt buildup is a problem, the lab will test both the soil and the irrigation water for salt content and make recommendations; there is usually an extra charge for this. (Chapter 12 covers salt problems.)

How Often is Soil Sampling Needed?

Where low to moderate rates of fertilizer are being used, once every 3-5 years is sufficient. That's because such rates feed the crop itself rather than also building up the soil's residual fertility.

What About Soil Testing and Organic Fertilizers??

Unlike chemical fertilizers which have an analysis label, the exact nutrient content of organic fertilizers, like manure and compost, is highly variable and difficult to judge. If this is so, is it worthwhile to test the soil in cases where organic fertilizers are used? Soil testing may still be a good idea for several reasons:

• If the soil is severely deficient in a nutrient like P or a micronutrient, some chemical fertilizer may have to be used to supplement the organics.

• Knowing the fertility status of the soil will allow you to determine which organic materials can best provide what is needed.

• The lab's pH test may indicate liming is needed. The lab can best determine how much lime is needed.


When to Sample

• At least 2 months before the results are needed. If farmers wait until a few weeks before planting, the lab is likely to become overloaded and unable to provide the results in time.

• Sampling may be easier to do in the wet season when the soil isn't as hard. In the case of flooded rice soils, check with the lab for the best sampling time. (While flooded, soils have different chemical properties then when unflooded.)

Avoid Improper Sampling

Improper sampling is a very common cause of inaccurate lab results. Each 200-500 gram sample sent to the lab may represent up to 15,000 metric tons of soil. One sign of a good lab is that it will provide sampling containers along with detailed instructions of how to take samples.

Involving Farmers in the Sampling Process

Avoid taking the samples on your own; instead, be sure to involve the farmer in the process. After all, extension should aim to "enable" farmers rather than create dependency. Also, the farmer's input is essential when it cones to mapping the farm and filling out information on past management, yield history, cropping plans, and expected yields.

The Sampling Procedure

The steps below provide general guidelines for sampling. Always consult the lab's instructions,too.


What is a sampling unit?: An area of soil that is likely to be uniform in its fertility. Even small farms usually have several sampling units.

How to Distinguish Sampling Units

Each of the following factors indicate likely differences in soil fertility:

• Soil color

• Soil texture

• Topography (slope vs. flat vs. depression)

• Past management (Use of manure, fertilizer, lime; type of crops grown. For example, new ground that has been in pasture for years will have a different fertility status than land that's been cropped steadily.)

Mapping the Farm

The map isn't needed by the lab, but is used to delineate the different sampling units and serve as a record of which sample came from where. It's also a useful management tool for the farmer. If the farmer is agreeable, make an extra copy for yourself - it will be useful in further extension work with him. Here's how to map a farm:

• Start by drawing in the boundaries and the location of buildings, wells, and the fields along with their dimensions .

• Indicate variations in topography, slope, soil color, soil texture, and past use of fertilizers and lime.

• Indicate past, present, and future crops and their locations.

• Once the sampling units have been determined, draw their boundaries and number them.

FIGURE 7-1: Farm map denoting different sampling units.


It's important to realize that each sample sent to the lab is really a composite sample composed of 10-20 subsamples taken at random within that sampling unit.

Guidelines for Extracting Subsamples

Tools: A shovel, machete or knife (for paring down the samples), and a pail or sturdy sack (for placing and mixing subsamples). Special soil sampling tubes or augers may be available on loan from the ag extension office but aren't essential.

Depth to sample: Most labs want a uniform slice of the top 15-20 cm of soil, the normal depth of topsoil. Some labs may also request separate subsoil samples. If the field is severely eroded, the normal sample will also include some of the subsoil, but that's OK.

Extraction method: If using a shovel, you can use several methods. The important thing is to end up with a uniform slice of soil from the surface to a depth of 15-20 cm. One way is to make a hole with about 45 degree sides to the right depth. Then use the shovel to trim off a 3-4 cm thick slice 15-20 cm deep. If a second person holds the face of the slice with one hand, it won't crumble apart. Scrape off any surface debris like stones or stalks before sampling.

Use a random Pattern: A zig-zag pattern is fine, but don't sample along fence lines, in fertilizer bands, under animal droppings, or in transition zones between sampling units.

Uniform size: Each subsample should be of equal size. Use a knife or machete to pare down each one to a similar width and depth (see Fig. 7-2).

If a zinc test will be done, don't collect subsamples in a galvanized pail.

FIGURE 7-2: (Left) Extracting a subsample using a shovel. (Right) Subsample after trimming with a machete.


• NEVER mix together subsamples from different sampling units!

• Drying: The soil can be slightly moist. If overly wet, it can be sun dried. Do not oven dry, as the heat will alter the soil's potassium, resulting in an erroneously high reading.

• Be sure to number each composite sample so it corresponds with the sampling unit from which it came.


Another sign of a good soils lab is its provision of a detailed information sheet requesting data on the farmer's situation. One purpose of the form is to provide extra information on the soil that's not revealed by the samples themselves (i.e. depth). In addition, it should attempt to evaluate the farmer's management level and the likely limiting factors affecting the intended crop's yield potential. The data requested should include most of the following:

• Farm size

• Soil depth

• Soil slope

• Soil drainage

• Past applications of fertilizer and lime

• Past crops

• Past yields

• Crop to be grown

• Variety to be used

• Intended use of compost, manure, or green manure crops

• Yield goal

• Capital available for fertilizer

• Limiting factors like insects, diseases, nematodes

A lab that requests little information isn't as likely to tailor its fertilizer recommendations to varying farmer circumstances.

Getting the Information: Depending on the culture and the farmer, there may well be hesitancy in providing some of the above data, especially regarding past yields and available capital. A lot may depend on the familiarity, trust, and credibility you have established with the farmer. It's important to explain the purpose the information will serve and yet not be overly insistent about obtaining it.


Depending on your situation, the samples can be mailed in, taken to the local extension office, or delivered personally to the lab. Visiting the lab is a worthwhile experience; it will give a better idea of what goes on there, and the personnel can often provide very useful information on soils and fertilizer use.


The form in which soil test results and fertilizer recommendations are given varies a lot with the lab and with the role played by the extension service. Often, there will be two parts to the results, the first being the fertility analysis of soil, and the second being the actual fertilizer recommendation. Even if a farmer or family member can read, the recommendation sheet may be too complex. In some cases, the lab mails the results to the local extension office where they are then put into a more readily understandable form.