Template:Chapter 4.1.1

4.1.1 - Soil testing

Dylan P. Harding, University of Guelph, Canada

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

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

Wearable

Comfortable gloves help farmer's work longer because their hands will not hurt from completing your task. Sizing is very important when finding comfortable gloves (Melco, 2016). Make sure gloves are the proper length and width, as not to restrict movement. There will be less pain from pulling weeds and they will be able to pull more weeds because they would not have to wait a long for the pain to subside between pulling each weed, because there will be no pain if wearing gloves (Food and Agriculture Organization, 2016). If farmer's find they are working hard and their hands start to sweat the gloves should be removed , dry your hands, and put on a new pair. Cloth gloves are more breathable then rubber ones, using them is another way to prevent hands from getting sweaty. The cloth gloves can also be softer and easier to clean, but are more restricting to movement due to their durability and tougher material. Since children will also be farming, smaller glove sizes can be found. Gloves are designed to fit a farmer's hand snugly, so children should not wear adult sized gloves when working.

Constraints To Adoption

Gloves are very useful to farmers, but there can still be some drawbacks. Possible culturable taboos might vary from location to location. Gloves might seem feminine and not easily adopted by men in the community. Gloves act as a second, tougher skin, but they are not a farmer's skin and can slide around while working. This may feel odd and uncomfortable but farmers can get used to the new feeling over time. Gloves can come in many colours and thicknesses, which may make a farmer's hands look funny or larger. Human skin is very stretchy and flexible, while glove materials tend to be tougher than skin and will reduce movement, but not enough to hinder work. Rubber gloves can stretch well, but make hands sweat, while cloth gloves are breathable but reduce dexterity.

Farmers can find gloves to use and get started from local vendors (European Commission For The Control Of Foot-And-Mouth Disease, 2016). Once you have completed your work for the day you can clean them are reuse them, or dispose of them if they were ripped or torn (Kim, et al., 2013). You can get gloves made of rubber and like materials as well as ones made of durable cloths. The thin rubber gloves tend to be made for a single use only. A trick that the European Commission For The Control Foot-And-Mouth Disease mentions that you can wear two pairs of rubber gloves at the same time for extra protection (European Commission For The Control Of Foot-And-Mouth Disease, 2016).

Helpful Links To Get Started

Here are websites to find more information about how to obtain gloves:

Alibaba

Indiamart

Store Nzfarmsource

Adenna

Farmcity

Crazystore

References

Espasandín-Arias, M., & Goossens, A. (2014). Natural rubber gloves might not protect against skin penetration of methylisothiazolinone. Contact Dermatitis, 70(4), 249-251. doi:10.1111/cod.12221

European Commission For The Control Of Foot-And-Mouth Disease. Suggested FMD PPE guidelines - Food and Agriculture, (2016) Food and Agriculture Organization. Rural women in household production: Increasing contributions and persisting drudgery. (2016).

Furlong, M., Tanner, C. M., Goldman, S. M., Bhudhikanok, G. S., Blair, A., Chade, A., . . . Kamel, F. (2015). Protective glove use and hygiene habits modify the associations of specific pesticides with Parkinson's disease. Environment International, 75, 144-150. doi:10.1016/j.envint.2014.11.002

Keeble, V. B., Correll, L., & Ehrich, M. (1996). Effect of Laundering on Ability of Glove Fabrics to Decrease the Penetration of Organophosphate Insecticides Through in vitro Epidermal Systems. J. Appl. Toxicol. Journal of Applied Toxicology, 16(5), 401-406. doi:10.1002/(sici)1099-1263(199609)16:53.3.co;2-6

Kim, J., Kim, J., Cha, E., Ko, Y., Kim, D., & Lee, W. (2013). Work-Related Risk Factors by Severity for Acute Pesticide Poisoning Among Male Farmers in South Korea. International Journal of Environmental Research and Public Health, 10(3), 1100-1112. doi:10.3390/ijerph10031100

Melco, M. (2016). Gardening Gloves. Retrieved from Garden Lovetoknow

Schaffner, A. D. (2013). Minimizing Surgical Skin Incision Scars with a Latex Surgical Glove. Aesthetic Plastic Surgery, 37(2), 463-463. doi:10.1007/s00266-013-0071-y