Chapter 4.1.1

4.1.1 - Soil testing.

Dylan P. Harding, University of Guelph, Canada

Suggested citation for this chapter.

Harding,DP (2022) Soil testing.In Farmpedia, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. http://www.farmpedia.org

Introduction

The purpose of applying fertilizer to a field is to replace the nutrients that are removed by crops. On most soils, crops will benefit from application of the macronutrients every year. Micronutrients on the other hand are generally removed by crops at a low enough rate that application is not necessary in every season. To apply fertilizers most efficiently, a farmer should ideally know the approximate level of each nutrient in the soil. Soil nutrient levels are most commonly determined through nutrient extraction tests that require specialized equipment and training to perform. Although many of these testing methods are highly accurate, they are generally impractical for poor farmers. Unfortunately, the need for more accessible methods of accurately determining soil nutrient levels has gone largely unanswered. When proactively testing soil nutrient levels is not possible, there are simple ways to characterize the soil and plan its management. Several of these low-tech methods for soil characterization are described below. It should be kept in mind that these techniques, while useful, cannot take the place of conventional soil testing, which should be taken advantage of if available. “On the spot” soil test kits are available that employ colour changing indicators to show soil fertility levels. Although these kits are inexpensive and do not require specialized training or equipment to use, the accuracy of these kits varies widely. In a comparison of the results of 5 different home soil test kits sold in the US to laboratory analysis of the same soil, agreement between the home soil test kit results and laboratory results ranged from 33% to 94% (Faber, Downer, Holstege, & Mochizuki, 2007). The accuracy of nutrient indication also varied between minerals with the accuracy of potassium levels being most consistent and the accuracy of phosphorus levels being the least consistent (Faber et al., 2007). Only pH and macronutrient levels were indicated by the kits tested. It should be noted also that most home test kits indicate soil nutrient levels to be simply “low”, “medium”, or “high”, rather than providing specific values from which the most efficient fertilizer rate could be calculated. For further information on important characteristics to look for in an accurate soil test kit, please see the work of Faber et al., (2007) entitled “Accuracy Varies for Commercially Available Soil Test Kits Analyzing Nitrate–Nitrogen, Phosphorus, Potassium, and pH”. It is important to keep in mind that home soil test kits are not as accurate as laboratory analyses, and should only be employed where more accurate laboratory testing is not available.

To determine if laboratory soil testing is available in a given area, the best course of action is generally to contact the regional or national ministry of agriculture, or local agricultural universities where they are present. These institutions often have internal soil testing labs or may be able to provide contact information to local soil testing labs.

The timing of soil testing can be very important. Some forms of mineral nitrogen (e.g. nitrate) is extremely mobile in the soil and for this reason soil testing (if possible) should be done within a few days of fertilizer application to ensure the test results are still accurate. Mineralization of organic nitrogen and loss of nitrogen to the environment will quickly cause variation from the test levels. Mineral nitrogen can easily be lost within a few days if there is no vegetation to absorb it (Principals of Plant Nutrition, 2001). Nitrogen loss is promoted following heavy rainfall causing leaching, and through conversion to gas when oxygen is unavailable (e.g. when a clay soil is waterlogged) (Principals of Plant Nutrition, 2001).

Areas such as Africa and South Asia that have not (on a geological time scale) been recently glaciated tend to have soils with lower nutrient holding capacities and overall lower nutrient contents (Lotter, 2010). Although there is variation between areas, this general trend will pervade in most soils within these regions. Global maps of soil characteristics such as depth, pH, dominant soil type, etc. are available from the FAO at http://www.fao.org/nr/land/soils/en/. It should be noted that these maps indicate the general trends in soil types within an area, but one should expect variation within these trends when individual fields are investigated.

The underlying bedrock material will have a strong influence on a soil’s mineral content, particularly for potassium, phosphorus, and calcium and this should be considered when planning the management of a soil. Local departments of geology, where established, will often be able to provide information on local bedrock composition.

Soil Nutrient Diagnosis Using Plants

The most effective method of determining whether or not a given fertilizer will have a beneficial effect on crop growth is through doing a small split-plot trial. To do this, a small test plot should be established in which half receives a consistent application of the test-fertilizer, and the other half does not. The management of the entire plot should otherwise be as consistent as possible, and ideally the farmer should not know which half of the plot is which to encourage consistent management. If the application of fertilizer is found to cost-effectively improve crop yields, then wider use of the fertilizer can be considered

Alternatively, the easiest way to recognize nutrient deficiencies in crops without specialized equipment is through observing symptoms of deficiency as they develop in crops. However, this is not the most ideal method of recognizing nutrient deficiency as by the time a shortage becomes observable in a crop it may be too late for fertilizer application to improve growth in that season. Also, many plant nutrient deficiencies have similar symptoms and thus can be confused with one another. However, where soil testing is not practical this method may be the most effective way to learn which nutrients are most needed in a given field. Links to image galleries of nutrient deficiencies as they appear in common crops are provided at the end of this chapter.

Table 1, below, provides descriptions of nutrient deficiencies as they appear in crops. The relative speed with which a given nutrient is transported through the plant affects where the deficiency will be exhibited. In general, deficiency of nutrients the move through plants relatively slowly will appear in new leaves and shoots, and deficiency of highly plant-mobile nutrients will appear in older leaves and the lower stem. Recognizing the relative mobility of the deficient nutrient can help narrow down the possible range of nutrients that are limiting crop growth.

Programs for cell-phones and other mobile devices that provide photo galleries of crop nutrient deficiency symptoms are also useful tools. The International Plant Nutrition Institute (IPNI) offers a Crop Nutrient Deficiency Photo Library that can be downloaded for about $5 (see below). Programs such as this are convenient because they enable on the spot comparison of field conditions to reference photos. The application “Learning Plant Language” from Agronomic Acumen serves a similar purpose, and is available for about $30. The International Potash Institute offers a free App that provides a photo gallery of potassium deficiency symptoms for various crops. The “Nutrient Removal Application” from Ag-PhD is another useful tool that provides estimates of nutrient removal rates of different crops based on yield. Links to these tools are provided below.

Soil nutrient concentrations can significantly vary over a small area, and for this reason symptoms of nutrient deficiency may appear in patches throughout a stand of crops. If patchy symptoms are observed in a field, subsequent soil testing should be performed to compare nutrient levels between areas where deficiency is exhibited and areas where it is not, if possible. When patches of deficiency symptoms are observed, these areas should be prioritized for subsequent application of the apparently deficient nutrient. Applying fertilizer only to areas that exhibit crop deficiency symptoms can also be considered if additional methods of soil testing are not available, or if the nutrient considered to be deficient is in short supply.

It should also be kept in mind that non-nutrient related challenges to the growth of a crop can create the impression of a mineral deficiency. For example, pests such as nematodes can damage a crop’s root system, limiting its nutrient uptake capacity and thus creating the appearance of a mineral deficiency despite potentially sufficient levels of that nutrient in the soil. Many crop diseases can also cause symptoms that are similar in appearance to nutrient deficiencies. For this reason, field testing with a reliable soil test kit or ideally through the establishment of a trial plot (described above) are important tests to perform when nutrient deficiency is suspected.

There is likely potential to understand soil nutrient balances through observing which wild species tend to do well on a given field. The available literature suggests that indicator species are generally used to estimate overall soil fertility, a measure which is obscured by the influence of many different factors. Unfortunately, there is little available information that associates indicator species with deficiencies or abundances of specific nutrients. Indicator species could perhaps be used as indicators of less variable soil characteristics such as cation exchange capacity, pH, and structure, which are each explained in greater detail below. Further investigation, ideally combining indigenous knowledge with modern nutrient testing, may help illuminate the use of wild plant populations to estimate soil nutrient levels.

Texture Diagnosis

Soils can be broadly classified according to texture. Texture refers to the average size of the individual particles that make up a soil. Coarse, sandy soils are mostly made up of relatively large particles and heavy clay soils are mostly made up of relatively small particles. The particular mixture of particle sizes will affect the behaviour of water, nutrients, and plant roots within a given soil.

Knowing the texture of a soil is important in determining an appropriate fertilization rate. In general, sandier soils (sometimes referred to as “red soils”) have lower nutrient holding capacities, because their larger size results in a lower surface area to bind nutrients for a given volume (Principals of Plant Nutrition, 2001). This means that nutrients are more likely to drain from these soils and be lost, and for this reason less fertilizer should be applied to sandy soils at a time. Conversely, soils that are heavy in clay (sometimes referred to as “brown” or “black” soil) can hold more nutrients at a time, and are less prone to nutrient loss, because their small particle size translates into a large surface area for binding of nutrients (Principals of Plant Nutrition, 2001).

The texture and related water holding characteristics of a soil will often dictate which crops will be produced. For example, rice paddy production, which requires saturated conditions, generally takes place on clay soils. On the other hand, crops with underground edible organs such as peanut and cassava are typically grown on sandy soils as the quicker drainage in these soils discourages root rot. Soils near the equator tend to be coarsely textured although there are exceptions to this trend. For many crops, the ideal soil will be a mixture of both sand and clay commonly referred to as loam.

There are several methods commonly employed to determine soil texture. The most common method is the hand texture test. Hand-texturing is performed by adding a small amount of water to a soil sample and observing how well the soil mass stays together when rolled between the thumb and fingers. If the soil clumps, it is high in clay, whereas sandy soils will not clump together and are coarse to the touch. Specific directions for diagnosing a soil through this method are available from the USDA at: http://soils.usda.gov/education/resources/lessons/texture/

Soil texture can also be determined by submerging a soil sample in water, mixing it thoroughly, and observing the period it takes for the soil to settle. Sand particles will settle in a few minutes, whereas clay particles can remain suspended in water for up to 24 hours before sinking, because of their finer size and hence weight. By observing how much of the soil sample has settled at several points over a 24 hour period, the approximate texture of the soil can be determined. The procedure for this method is explained in more detail at: http://www.nce-mstl.ie/files/Ag_Sc_posters/Agricultural%20Science%20poster%20Soil%20texture%20by%20sedimentation.pdf.

Drainage

Soils can become compacted over time due to many factors including the absence of roots in the soil, the frequency and method with which a soil is tilled, and overall traffic overtop the soil. Although tillage will reduce compaction temporarily, it can also encourage future compaction of the soil by disrupting its internal structure. Compaction creates several challenges to the growth of crops, particularly through limiting the passage of water and air into lower levels of the soil. In extreme cases, compaction can create waterlogged soil conditions under which crop root systems will become deprived of oxygen and eventually die. The risk of soil compaction becoming problematic is higher with finely textured clay (brown or black) soils than with sandy soils.

The drainage rate of a soil can generally be estimated with reasonable accuracy through direct observation. To do this, several holes of varying depths (up to the approximate maximum rooting depth of the crop) should be dug throughout the field. Information on the maximum rooting depths of various crops is available from the FAO at http://www.fao.org/docrep/X0490E/x0490e0e.htm. When a hole is complete, a small amount of water (at least a cup) should be poured into the hole. If the water is quickly absorbed into the soil, it is reasonably safe to assume that no significant compaction of the soil has occurred at that soil depth. If water is observed to pool in the hole before being absorbed however, then compaction has likely occurred at that depth. It is important to note that compaction at one level will inhibit the entry of water and air into lower levels of the soil, even if the lower levels are not compacted.

An effective way of reversing compaction is through establishing deep rooted cover crops that can loosen the soil and provide organic matter through a range of soil depths. Tillage can also be a temporary solution to compaction however it is important that root systems are established within the soil shortly after tillage in order to rebuild soil structure before compaction re-occurs.

It should be noted that in some systems such as paddy rice production, compaction and stagnant water atop the soil may be desirable. The specific goals of local production practices should always be considered before suggesting new crop management strategies.

Salinity and Conductivity

“Salinity” refers to the degree to which salts have accumulated in a soil. Saline soil is generally undesirable for the growth of most crop species because it interferes with water absorption by roots and can also cause biological imbalances when salts are taken up in soil water. The concentration of salts within a soil is generally measured by passing an electrical current between two conductors placed within a soil sample and measuring how effectively that current is transferred. As the presence of salts will facilitate electrical conduction within a soil, more salinized soil will have higher conductivity values. Easy to use tools known as conductivity meters (costing $85 and up) can be purchased to estimate this soil property (see below). The supplier Grainger offers several different models of conductivity meter of varying costs and can ship their products internationally. Their catalogue can be accessed online at http://www.grainger.com/Grainger/wwg/search.shtmlsearchQuery=conductivity+meters&op=search&Ntt=conductivity+meters&N=0&GlobalSearch=true&sst=subset. The presence of saline soil conditions will also be indicated by a white crust forming on the soil surface during dry conditions. If salinity is apparent within a soil, the management regime should be redesigned to minimize or eliminate potential sources of salt addition and to facilitate flushing out salts that are already present in the soil. .

Coastal areas are at especially high risk of soil salinization because of the entry of salts from seawater into adjacent ground water. Additionally, areas that have been heavily irrigated or fertilized (e.g. China) often face issues with salinity because of salt-impurities in the various additives (including slightly saline irrigation water) that have been used.

Salt tolerant cultivars of various crops have been developed, and variation in salinity tolerance between different cultivars of the same species has been widely observed (El-Akhal et al., 2013; Garg & Chandel, 2011; D. L. N. Rao, Giller, Yeo, & Flowers, 2002; Rashid, Qureshi, Hollington, & Wyn Jones, 1999). Cultivars that are salt tolerant can be obtained from national breeding programs and CGIAR institutes.

Soil Acidity and pH

The acidity of a soil will affect the availability of the plant nutrients it contains as well as have a direct effect of the growth of plants. pH can be approximately measured using litmus paper.

Organic Matter

The organic matter content of a soil refers to the fraction of the soil that has been derived from the breakdown of once living (organic) material. The presence of organic matter in a soil increases its resistance to compaction, its ability to retain water and nutrients, and provides a highly beneficial bank of nutrients that release slowly over time, reducing the need for synthetic fertilizers. The approximate level of organic matter in a soil can be assessed through observing the smell and colour of a soil. Soil that has higher levels of organic matter will have a more distinctive “earthy” smell than soils low in organic matter, which tend to be odourless. Most organic matter is dark in colour and thus a darker soil can often indicate higher levels of organic matter. When using colour as an indicator, the colour change within a field over time will be more accurate than comparisons between different fields because other soil characteristics (such as texture) that will vary between fields can also influence their colour.

Crop residue and manure are good sources of organic matter that will increase the fertility of a soil over time. Most organic matter will eventually be broken down to carbon dioxide as it is consumed by micro-organisms within the soil. For this reason, some form of new organic matter should ideally be added to a soil every season to replace that which is being lost. Organic matter usually contains nitrogen and phosphorus as a part of larger carbon-based molecules that are not immediately available to plants. As organic matter is broken down over time, these nutrients are released in the form of simple molecules that can be used by plants. Thus, regular addition of organic matter and minimizing removal of crop leaves and stems from the field can help to prevent nutrient deficiencies. However, it must be kept in mind that even when organic matter is being added to a soil on a regular basis, there is potential for nutrient deficiencies to develop if the organic matter being added to a soil contains less of a given nutrient than a crop removes. For this reason, the regular inspection of crops for symptoms of nutrient deficiency is recommended even when organic matter is being regularly applied.

Useful Links

The FAO report “Simple soil, water, and plant testing techniques for soil resource management” provides a detailed overview of different soil testing methods (both laboratory and field techniques) and their applicability to use in Africa. This report is available online at:

ftp://ftp.fao.org/agl/agll/docs/misc28.pdf

FAO information on rooting depths of common crops: http://www.fao.org/docrep/X0490E/x0490e0e.htm

Images and Descriptions of Nutrient Deficiency Symptoms for Different Crops

Rice

Wheat

Corn/ Maize.

Banana.

Soybeans.

Sorghum.

iPad iPhone nutrient apps: Note that a compatible mobile device is required to use these applications as they will not run on most personal computers.

“Crop Nutrient Deficiency Photo Library” from the International Plant Nutrition Institute.

“Learning Plant Language” from Agronomic Acumen.

“K gallery” (potassium deficiency image guide) from The International Potash Institute (somewhat limited, but free).

The “Nutrient Removal Application” from Ag-PhD provides estimates of nutrient removal rates of different crops based on yield: http://www.agphd.com/resources/ag-phd-mobile-apps/ag-phd-nutrient-removal-by-crop-app/

References

1.El-Akhal, M. R., Rincon, A., Coba de la Pena, T., Lucas, M. M., El Mourabit, N., Barrijal, S., & Pueyo, J. J. (2013). Effects of salt stress and rhizobial inoculation on growth and nitrogen fixation of three peanut cultivars. Plant biology (Stuttgart, Germany), 15(2), 415-421. doi: 10.1111/j.1438-8677.2012.00634.x.

2.Faber, B. A., Downer, A. J., Holstege, D., & Mochizuki, M. J. (2007). Accuracy Varies for Commercially Available Soil Test Kits Analyzing Nitrate-Nitrogen, Phosphorus, Potassium, and pH. HortTechnology., 17(3), 358-362.

3.Garg, N., & Chandel, S. (2011). The Effects of Salinity on Nitrogen Fixation and Trehalose Metabolism in Mycorrhizal Cajanus cajan (L.) Millsp Plants. Journal of Plant Growth Regulation, 30(4), 490-503. doi: 10.1007/s00344-011-9211-2.

4.Lotter, Don. (2010). Making Connections To Healthy Soils, Sustainable Agriculture in East Africa. BioCycle, 51(10), 44-45.

5.Mahmood, A., & Quarrie, S. A. (1993). Effects of salinity on growth, ionic relations and physiological traits of wheat, disomic addition lines from Thinopyrum bessarabicum, and two amphiploids. Plant breeding = Zeitschrift fur Pflanzenzuchtung., 110(4), 265-276.

6.Martin, P. K., Taeb, M., & Koebner, R. M. D. (1993). The effect of photoperiod insensitivity on the salt tolerance of amphiploids between bread wheat (Triticum aestivum) and sand couch grass (Thinopyrum bessarabicum). Plant breeding = Zeitschrift für Pflanzenzüchtung., 111(4), 283-289.

7.Rao, D. L. N., Giller, K. E., Yeo, A. R., & Flowers, T. J. (2002). The effects of salinity and sodicity upon nodulation and nitrogen fixation in chickpea (Cicer arietinum). Annals of Botany, 89(5), 563-570. doi: 10.1093/aob/mcf097.

8.Rashid, A., Qureshi, R. H., Hollington, P. A., & Wyn Jones, R. G. (1999). Comparative responses of wheat (Triticum aestivum L.) cultivars to salinity at the seedling stage. Journal of Agronomy and Crop Science, 182 (3) pp. 199-207.