Editing Soil test (section)

== Agricultural soil tests ==
In [[agriculture]], a '''soil test''' commonly refers to the [[analysis]] of a [[soil]] sample to determine [[nutrient]] content, composition, and other characteristics such as the [[acidity]] or [[pH level]]. A soil test can determine [[fertility]], or the expected growth potential of the soil which indicates nutrient deficiencies, potential toxicities from excessive fertility and inhibitions from the presence of non-essential [[trace mineral]]s. The test is used to mimic the function of roots to assimilate minerals. The expected rate of growth is modeled by the [[Law of the Maximum]].<ref>{{cite book|url=https://books.google.com/books?id=iP7LWpn5oJ0C&q=%22law+of+maximum%22+wallace&pg=SL4-PA89 |title=Soil Science |date= 1999-08-31 |access-date=2012-11-08 |isbn=9780849331367 |last=Sumner |first=Malcolm E. }}</ref>

Labs, such as those at [[Iowa State]] and [[Colorado State University]], recommend that a soil test contains 10-20 sample points for every {{convert|40|acre|m2}} of field. [[Tap water]] or chemicals can change the composition of the soil, and may need to be tested separately. As soil nutrients vary with depth and soil components change with time, the depth and timing of a sample may also affect results.

Composite sampling can be performed by combining soil from several locations prior to analysis. This is a common procedure, but should be used judiciously to avoid skewing results. This procedure must be done so that government sampling requirements are met. A reference map should be created to record the location and quantity of field samples in order to properly interpret test results.

=== Geographic distribution of samples for precision agriculture ===
In [[precision agriculture]], soil samples may be geolocated using GPS technology in order to estimate the geospatial distribution of nutrients in the sampled area. The geolocated samples are collected using a distribution and resolution that allows for the estimation of the geospatial variability of the soil area where the crop will be grown. Many different distributions and resolutions are used, depending upon many factors including the goals of the geospatial nutrient analysis and cost of sample collection and analysis.<ref>{{Cite web|url=https://cropwatch.unl.edu/ssm/soilsampling|title=Soil Sampling for Precision Agriculture|date=2015-09-17|website=CropWatch|language=en|access-date=2019-05-22}}</ref><ref>{{Cite web|url=https://crops.extension.iastate.edu/using-precision-agriculture-improve-soil-fertility-management-and-farm-research|title=Using precision agriculture to improve soil fertility management and on-farm research {{!}} Integrated Crop Management|website=crops.extension.iastate.edu|access-date=2019-05-22}}</ref>

For example, in the United States corn and soybean growing regions a grid distribution with a resolution of 2.5 acres per grid (one sample for each 2.5 acre grid) is offered by many precision agriculture soil test service providers. This is generally referred to as grid soil testing.

=== Storage, handling, and moving ===
Soil chemistry changes over time, as biological and which chemical processes break down or combine compounds over time. These processes change once the soil is removed from its natural ecosystem (flora and fauna that penetrate the sampled area) and environment (temperature, moisture, and solar light/radiation cycles). As a result, the chemical composition analysis accuracy can be improved if the soil is analyzed soon after its extraction — usually within a relative time period of 24 hours. The chemical changes in the soil can be slowed during storage and transportation by freezing it. Air drying can also preserve the soil sample for many months.

===Lab testing===
[[File:Soil test.JPG|thumb|Soil testing in progress]]
Soil testing is often performed by commercial labs that offer a variety of tests, targeting groups of compounds and minerals. Laboratory tests often check for plant nutrients in three categories:
* Major nutrients: [[nitrogen]], [[phosphorus]], and [[potassium]]
* Secondary nutrients: [[sulfur]], [[calcium]], [[magnesium]]
* Minor nutrients: [[iron]], [[manganese]], [[copper]], [[zinc]], [[boron]], [[molybdenum]], [[chlorine]]

The amount of plant-available [[phosphorus]] is most often measured with a chemical extraction method, and different countries have different standard methods. Just in Europe, more than 10 different soil phosphorus tests are currently in use and the results from these different tests are not directly comparable.<ref>{{Cite journal|last1=Jordan-Meille|first1=L.|last2=Rubæk|first2=G. H.|last3=Ehlert|first3=P. a. I.|last4=Genot|first4=V.|last5=Hofman|first5=G.|last6=Goulding|first6=K.|last7=Recknagel|first7=J.|last8=Provolo|first8=G.|last9=Barraclough|first9=P.|date=2012-12-01|title=An overview of fertilizer-P recommendations in Europe: soil testing, calibration and fertilizer recommendations|journal=Soil Use and Management|language=en|volume=28|issue=4|pages=419–435|doi=10.1111/j.1475-2743.2012.00453.x|s2cid=98596449 |issn=1475-2743}}</ref>

[[Do-it-yourself]] kits usually only test for the three "major nutrients", and for soil [[acidity]] or [[pH level]]. Do-it-yourself kits are often sold at farming cooperatives, university labs, private labs, and some hardware and gardening stores. [[Electrical meter]]s that measure pH, water content, and sometimes nutrient content of the soil are also available at many hardware stores. Laboratory tests are more accurate than tests with do-it-yourself kits and electrical meters. An [http://us.wlabs.com/wp-content/uploads/2013/02/samplesoilreport.pdf example soil sample report] is provided for reference by Wallace Laboratories LLC.

In order to avoid complex and expensive analytical techniques, prediction based on regression equations relating to more easily measurable parameters can be provided by [[Pedotransfer function|pedotransfer functions]]. For instance, soil bulk density can be predicted using easily measured soil properties such as soil texture, pH and organic matter.<ref>{{Cite journal |last=Qiao |first=Jiangbo |last2=Zhu |first2=Yuanjun |last3=Jia |first3=Xiaoxu |last4=Huang |first4=Laiming |last5=Shao |first5=Ming’an |date=2019-01-01 |title=Development of pedotransfer functions for predicting the bulk density in the critical zone on the Loess Plateau, China |url=https://doi.org/10.1007/s11368-018-2040-1 |journal=Journal of Soils and Sediments |language=en |volume=19 |issue=1 |pages=366–372 |doi=10.1007/s11368-018-2040-1 |issn=1614-7480|url-access=subscription }}</ref> 

Soil testing is used to facilitate fertilizer composition and dosage selection for land employed in both agricultural and horticultural industries.

Prepaid mail-in kits for soil and ground water testing are available to facilitate the packaging and delivery of samples to a laboratory. Similarly, in 2004, laboratories began providing fertilizer recommendations along with the soil composition report.

Lab tests are more accurate and often utilize very precise [[Flow injection analysis|flow injection]] technology (or Near InfraRed (NIR) scanning<ref>[http://www.kake.com/story/40168844/on-the-spot-real-time-and-affordable-soil-testing-technology-is-enhancing-the-agricultural-value-chain-in-uganda On-the-spot, real-time and affordable soil testing technology is enhancing the agricultural value chain in Uganda.]</ref><ref>[http://ictupdate.cta.int/2019/01/14/on-the-spot-easy-and-affordable-soil-testing-for-kenyan-smallholder-farmers/ On-the-spot, easy and affordable soil testing for Kenyan smallholder farmers]</ref>). In addition, lab tests frequently include professional interpretation of results and recommendations. Provisory statements included in a lab report may outline any anomalies, exceptions, and shortcomings in the sampling, analytical process or results.

Some laboratories analyze for all 13 mineral nutrients and a dozen non-essential, potentially toxic minerals utilizing the "universal soil extractant" ([[ammonium bicarbonate]] [[DTPA]]).<ref>{{cite web|url=http://us.wlabs.com/suitable-import-borrow-topsoil-or-reclaimed-soil/ |title=wlabs.com |publisher=Wallace Laboratories LLC |access-date=2012-11-08}}</ref>