The two-page instruction sheet that accompanies the Luster Leaf Rapitest© Soil Test Kit1 could be a useful curriculum resource for chemistry teachers. This instruction sheet is available as a pdf file.2 The instruction sheet is well written and contains much information relevant to introductory general chemistry. It could serve to introduce students to agricultural/horticultural chemistry, and provide ideas for lessons, assignments, projects, and quiz or test questions. This article will briefly describe the contents of the instruction sheet and suggest possible uses in the general chemistry curriculum by posing questions for students to answer or consider. The uses are primarily in the areas of naming compounds, formulas of compounds, balancing equations, percentage composition calculations, and unit conversions.
The soil test kit itself may be purchased from garden centres or other retailers.3 The kit contains apparatus and chemicals for four types of soil chemistry tests: pH; nitrogen; phosphorus; and potassium contents. The chemical gel-packs can be replenished by ordering them from the company.4 However, use of the soil test kit as a laboratory exercise for chemistry students is not recommended. The method is very simple and would be more suitable for students in earlier years. There may also be problems with respect to the safety and disposal of the chemicals used in the testing.
Soil Test Kit Instruction Sheet – Contents
- Using the Rapitest Soil Test Kit
- Preparing Your Soil Samples
- pH Test
- Nitrogen, Phosphorus & Potash Test
- To Raise or Lower pH of your Soil
- Adjusting pH
- Soil Types
- Adjusting Soil pH – How Much to Apply (Table)
- Fertilizer Recommendations
- Feeding Prior to Planting (Table)
- Feeding Established Plants and Beds (Table)
- Special Recommendations for Lawns (Table)
- Safety and Hygiene
Preparing Soil Samples
The term ‘soil test’ is a misnomer, since it is not the soil that is tested, but rather a water extract. An exact analogy is brewing tea from tea leaves:
1. Why is the content of the water extract a measure of soil quality for growing plants?
2. Is a particular shovelful of soil exactly the same as any other shovelful that might be taken?
At this point it is possible to introduce students to the concept of the representative sample of an analyte, and have them grapple with the idea of how difficult it is to obtain a small sample that truly represents a large amount of a non-homogeneous solid material.
The pH of the aqueous extract is tested by the addition of a gel capsule. The contents of this capsule are listed in its Safety Data Sheet, available as a pdf file by request from Luster Leaf customer service. The determination of pH is by visual matching to a comparison colour chart. The observed soil pH result may be checked against the recommended pH for any particular plant on a suppled list.5 You get two choices of chemical agents for raising soil pH: dolomitic/calcic limestone or hydrated lime, and two agents for lowering soil pH: iron sulfate or aluminum sulfate. The quantities to be added are given for three soil types in units of lbs per 100 square feet.
Agricultural chemistry, like many industries, continues to use conventions and nomenclature that are traditional or arcane or both. In the United States where the kit is manufactured, the units of measurement are both traditional and arcane.
3. What are the chemical names and formulas of calcic limestone and hydrated lime?
4. By what chemical reaction is limestone an alkali material in water?
5. What are the formulas of iron sulfate and aluminum sulfate?
6. By what reaction is aluminum ion an acidic material in water?
7. Convert 8.0 lbs per 100 square feet into kg per m2.
8. Plan a soil amendment for a 10 m x 6 m plot of soil testing as pH 7.0 to grow soybean at pH 5.5 – 6.5. Visit a chemical supply outlet to determine what agents are available, how they are packaged, and how much they cost. Plan an application methodology.
N, P, and K Tests
The content level of each macronutrient in the aqueous extract is tested, in each case by the addition of a gel capsule containing reagents. As for pH testing, there are Safety Data Sheets available as pdf files listing the chemical content of each of these capsules. The macronutrient content levels are estimated by colour intensity very approximately by eye using a 5-colour-intensity-level comparison chart:
level 0 (depleted); level 1 (deficient); level 2 (adequate); level 3 (sufficient); level 4 (surplus)
9. Define a macronutrient for plants. Why are nitrogen, phosphorus, and potassium macronutrients?
10. Define a micronutrient for plants. List five necessary micronutrients for plants.
Fertilizer additions are divided into three sections: prior to planting; established plots; and lawns.
For feeding prior to planting, five agents are listed with their macronutrient content quoted (sic):
- dried blood (N = 11 %)
- nitrate of soda (N = 16 %)
- bone meal (P2O5 = 19 %)
- triple superphosphate (P2O5 = 46 %)
- muriate of potash (K2O = 60 %)
11. Calculate the percentage by mass of phosphorus in P2O5.7
12. Calculate the percentage by mass of potassium in K2O.7
A table in the instructions gives recommendations for adding agents to remedy the deficiencies of a soil sample. These additions are given in units of ounces (mass) per 100 square feet of plot. The Questions below include examples.
13. What is ‘bone meal’? What are some sources of dried blood and bone meal?
14. What is ‘soda’? What is the formula of ‘nitrate of soda’?
15. Calculate the percentage by mass of nitrogen in nitrate of soda.7
16. What is ‘muriatic acid’? What is ‘potash’? What is the formula of ‘muriate of potash’?
17. Calculate the percentage by mass of potassium in muriate of potash.7
18. A typical addition of nitrogen calls for 19 ounces of nitrate of soda to be added per 100 square feet. Covert this into g per m2.
Further convert this value to an amount of the element N in g/m2.
19. A typical addition of phosphorus calls for 10.25 ounces of triple superphosphate to be added per 100 square feet. Covert this into g per m2.
20. (a) Further convert the previous value to an amount of P2O5 in g/m2. (b) Further convert this into an amount of the element P in g/m2.
21. A typical addition of potassium calls for 2.5 ounces of muriate of potash to be added per 100 square feet. Covert this into g per m2.
22. (a) Further convert the previous value to an amount of K2O in g/m2. (b) Further convert this into an amount of the element K in g/m2.
23. The above equations are unbalanced. Balance these equations.
24. What is the classification of each reaction?
25. Write the formula and charge of the dihydrogen phosphate ion. Why is calcium dihydrogen phosphate much more soluble in water than either calcium phosphate or fluorapatite?
26. Prove by calculation that 100 g of triple superphosphate contains three times the phosphorus content by mass as does 100 g of superphosphate.
27. Per kilogram of phosphorus, triple superphosphate is more expensive then superphosphate to produce. Where then does the advantage in its use lie?
Special Recommendations for Lawns
For lawns, general purpose fertilizers are used that contain all three macronutrients. They contain more nitrogen than phosphorus or potassium, and application is based on the nitrogen test result. Typical fertilizers will be designated as 24-4-4, 24-3-4, or 30-4-4. The numbers give the percentage by mass of nitrogen, phosphorus (as P2O5) and potassium (as K2O).
28. Why is nitrogen more critical for grass lawns than phosphorus and potassium?
29. The instructions suggest adding 4.0 lbs per 1000 square feet of 24-4-4 fertilizer to a depleted lawn. Convert this to an addition value for nitrogen in g/m2 units.
30. Calculate the addition rates for the previous question for phosphorus and potassium in g/m2 units.
Using the Soil Test Kit with Students?
The instruction sheet for the soil test kit connects with multiple aspects of introductory general chemistry and can be used to generate discussion about these aspects with students. However, having chemistry students use the kit itself to perform a laboratory exercise is counter-indicated for the following reasons:
- Simplicity. The testing is too simple for chemistry students.
- Safety. The hazards of using the kit chemicals has to be considered.
- Disposal. The disposal of the test chemicals is problematic.
- Cost. The kit retails at about $25 before tax in Canada, so assume it would retail at $20 in the U.S. Each kit comes with chemicals for 10 tests of each kind. The kit chemicals can be replenished at a cost of $12 US plus shipping by ordering from the company.4 International shipping may not be available. The kit can also be found in the US at Home Depot for about $15.
- Precision. The manufacturer assures us that the test methods are “accurate”, which means that the average result of a large number of tests will be the true analysis value. But the methodology, using the human eye to judge colour density, is of very low precision. Also, the sampling method is unlikely to give a truly representative sample of a large area of soil. This lack of precision is a significant flaw for introductory chemistry students. Having spent much effort exhorting students to read and record every digit on a balance or buret, a teacher may very well not want to have them perform an experiment where only low precision is possible.
- Luster Leaf Soil Test Kit: http://www.lusterleaf.com/nav/soil_test.html
- Luster Leaf Soil Test Kit Instructions: http://www.lusterleaf.com/img/instruction/1601-soiltestkit_instructions.pdf
- Lee Valley Tools: http://www.leevalley.com/en/garden/page.aspx?cat=2,42578&p=65366&WT.mc_id=2932_21&WT.tsrc=Email
- Luster Leaf Soil Test Kit Replenishment Order Form: http://www.lusterleaf.com/img/instruction/soil_capsule_orderform.pdf
- Luster Leaf List of Plant pH Preferences: http://www.lusterleaf.com/img/instruction/ph_pref_list.pdf
- See, for example: https://www.quora.com/Why-is-fertilizer-measured-in-N-P2O5-and-K2O-instead-of-N-P-and-K
- CRC Handbook of Chemistry and Physics, CRC Press, 54th Edition 1973, Table of Gravimetric Factors, pages B-157 and following.
- Greenwood, N. N. and A. Earnshaw, Chemistry of the Elements, 2nd Edition, Butterworth- Heineman. 1997.