Soil Analysis

Today you will test some soil for five characteristics: soil texture, acidity, nitrogen content, phosphorus content, and potassium content. These characteristics determine how well plants will grow in a particular soil.

I. Soil Texture

Soil Source:_______________________________

The texture of a soil determines the amount of air and water the soil can hold. Plant roots need liberal supplies of both.

Air is critical as a source of oxygen gas and as a repository for excess carbon dioxide gas. These gases are needed for and produced by respiration in root cells, respectively. If there is not much air space in the soil, then roots "suffocate" because they cannot carry out respiration. The roots then die, the plant wilts, and then dies. Large soil particles do not pack tightly and therefore provide air spaces in the soil. On the other hand, soil consisting of extremely fine particles packs tightly and permits little air in the soil to support root function.

Water from the roots is the supply of this important chemical for the shoot system (stem, leaves, flowers, fruits, and seeds). The water is a reactant in photosynthesis and a product of respiration. Water makes up about 90% of plant cells by weight, and is the critical solvent in the cell. The evaporation of water through the stomata in the epidermis of the shoots cools the plants and prevents overheating. A soil with very large particles drains too extensively and plants will lack sufficient water, will wilt, and perhaps even die. A soil with extremely fine particles holds tremendous amounts of water and can hold so much as to exclude air from the soil. In that case, the roots die, the plants wilt, and perhaps will die as well.

Thus, the perfect soil texture for growing plants is a compromise between fine particles (clay), medium particles (silt), and coarse particles (sand). The range of soil textures that support plant growth are called "loam."

A. ON THE WEEK BEFORE LAB! Fill a 100 ml graduate cylinder to the 70 ml mark with sifted soil. Fill the cylinder with sodium hexametaphosphate (surfactant) solution (2 teaspoons of Calgon per quart). Cover the top of the cylinder with plastic wrap. Stir/agitate the soil completely and thoroughly so that there is no unmixed soil at the bottom of the cylinder. Top-off the cylinder with surfactant. Continue inversion-stirring for at least five minutes. Label the cylinder with your group name. Set the cylinder aside until the next class meeting in a place where it will not be disturbed.

B. On the regular Lab Exercise day, and disturbing the cylinder as little as possible, measure the volume of the sand layer in the bottom of the cylinder. Since this is the coarsest particle size, it will have dark voids between particles (ask your instructor to help you distinguish the sand from the silt).

Volume of the sand layer ________ ml

C. Determine the volume of the sand + silt layers. The measuring line will appear just below the clay layer which is very smooth and usually light in color. Silt has a few voids and the particles are visible; clay has no voids and the particles are too small to be visible.

Volume of the sand + silt layers ________ ml

D. Determine the volume of the sand + silt + clay layers. The measuring line will appear between the clay layer and the dark water above it.

Volume of the sand + silt + clay layers ________ ml

E. Calculate the volume of the silt layer by subtracting the number from part B above from the number in part C above.

Volume of silt layer ________ ml

F. Calculate the volume of the clay layer by subtracting the number from part C above from the number in part D above.

Volume of clay layer ________ ml

G. Rinse out your cylinder completely and thoroughly as described by the instructor. Dry it with a paper towel, and return it to the proper location.

H. Calculate the percentage contribution for each of the three layers by dividing their individual volumes by the total soil volume (D). If your calculations are correct, the sum of these percentages should be 100! Check your work!

Sand ______% Silt ______% Clay ______% Total ______%

G. Place a small X on the diagram below where the three layer percentages would plot out together.

What is the texture classification for this soil?_______________


II. Soil Acidity (pH)

The amount of acid or alkalai in the soil determines the availability of many nutrients for plant growth and maintenance. It is important that the soil have the correct balance of acid and alkalai for the amount of nutrients to be released from the soil particles for their use. The balance is measured on a scale of acidity called pH. The scale goes from 1 (very acid) to 14 (very alkaline) with the neutral pH of 7 in the middle of the scale. Distilled water has a pH of 7, vinegar is typically pH 4, and soap has a pH of about 10. If the pH of soil is too high or too low, the nutrients are either locked onto the soil particles or are washed out of the soil by rain. Even applications of fertilizer to such a soil are useless and wasted.

Most plants grow best when the soil pH is between 5.5 and 6.5 (on the slightly acid side of neutral). If soil tests too low, the pH can be raised by applying lime (calcium and magnesium compounds). If soil tests too high, the pH can be lowered by applying sulphur or aluminum sulphate.

You will use a commercial test kit to determine the pH of your soil sample.

A. Fill a microcentrifuge tube to the 1/2 ml mark with well-sifted soil.

B. Add the "Lime" test solution until the level of the liquid is at the 1 ml mark.

C. Close the cap of the tube and, holding the cap in place with your finger, shake the tube gently for 30 seconds.

D. Place the tube in the centrifuge and spin for 1 minute.

E. Remove the microcentrifuge tube from the centrifuge and decant the supernatant liquid into a clean 10 x 75 mm test tube.

F. Compare the color of the transparent solution with the "LIME TEST (pH)" color chart. Record the pH indicated next to the color swatch matching most closely the color of the solution.

pH ________

G. Calculate the type and amount of soil additive needed to adjust a 50 ft. x 100 ft. garden of the soil you sampled to pH 6.0. Assume it takes 68 lbs. of ground limestone per 1000 sq. ft. to raise the pH 1 unit. Assume it takes 15 lbs. of aluminum sulphate per 1000 sq. ft. to lower the pH 1 unit. If your soil test indicates the pH is already 6.0, then do the calculation below assuming your results were pH 5.0 instead.

Name of Additive needed _________________________________

pH change needed ______ units

Amount of additive to use on 1000 sq. ft. to change to pH 6.0 _______ lbs.

Area (=width x length) of 50' x 100' garden __________ sq. ft.

Amount of additive to use on 50' x 100' garden _______ lbs.


III. Nitrogen Content

An appropriate supply of nitrogen gives plants healthy dark-green foliage. It promotes the growth of vegetative parts of the plant (root, stem, leaf). It should be abundant for crops like grass, cabbage, asparagus, onions, lettuce, and spinach. Too much nitrogen, however, can cause growth to be too rapid, can cause the plant to grow tall and soft, to fall over, and can reduce yield. Excess nitrogen can also delay and prevent flower and fruit formation in other crops. Therefore, you must compromise between the high nitrogen levels needed for true vegetables and the lower amounts needed for fruit crops (beans, corn, squash, peas, strawberries, etc).

Plants lacking sufficient nitrogen will be short, thin, and yellowish green, particularly in the lower leaves. The leaves will turn yellow, and will brown down to a crispy state. In monocots, such as grasses, the tip of the leaf browns first and the browning progresses toward the leaf sheath.

A. Rinse out your microcentrifuge tube completely.

B. Put well-sifted soil until it fills the tube to the 1/2 ml mark.

C. Add the "Nitrogen" test solution until the level of the liquid is at the 1 ml mark.

D. Close the cap of the tube and, holding the cap in place with your finger, shake the tube gently for 30 seconds.

E. Place the tube in the centrifuge and spin for 1 minute.

F. Remove the microcentrifuge tube from the centrifuge and decant the supernatant liquid into a clean 10 x 75 mm test tube.

G. Compare the color of the transparent solution with the "NITROGEN TEST (N)" color chart. Record the data indicated next to the color swatch matching most closely the color of the solution.

Matching color bar label ____A-E

% N in fertilizer to be used ____%

N content ______ ppm

N content ________ lbs./acre

H. Calculate the amount of fertilizer (Ammonium nitrate = 33 percent nitrogen content) needed for your 50' x 100' garden. First, determine how much is needed for 1000 sq ft by dividing the "%N in fertilizer to be used" (from above) by 33 (the percent nitrogen in the fertilizer) and multiplying the result by 50 lbs. Ammonium nitrate and urea are excellent and relatively inexpensive sources of nitrogen for lawn and garden use.

33% N Fertilizer needed per 1000 sq ft. ______ lbs.

33% N Fertilizer needed in whole garden ______ lbs.


IV. Phosphorus Content

Phosphorus is essential for flower, fruit, and seed production. It is an important part of DNA (the genetic molecule). It also facilitates seed germination. The supply of phosphorus determines, in part, the rate that the plants reach sexual maturity. It is usually difficult to have too much phosphorus because the supplies in soil are typically so limited.

Plants lacking sufficient phosphorus usually have purplish leaves, petioles, and stems. They grow slowly and mature very late in the season (if then). The yield of crops like corn, beans, peas, squash, cucumber, etc. will be very low under phosphorus deficiency.

A. Rinse out your microcentrifuge tube completely.

B. Put well-sifted soil until it fills the tube to the 1/2 ml mark.

C. Add the "Phosphorus" test solution until the level of the liquid is at the 1 ml mark.

D. Close the cap of the tube and, holding the cap in place with your finger, shake the tube gently for 30 seconds.

E. Place the tube in the centrifuge and spin for 1 minute.

F. Remove the microcentrifuge tube from the centrifuge and decant the supernatant liquid into a clean 10 x 75 mm test tube.

G. THIS STEP IS IMPORTANT!! Scrape the end of the tin wire until it is shiny and use it to stir the clear solution in the tube for 30 to 40 seconds to develop a blue color.

H. Compare the color of the transparent solution with the "PHOSPHORUS TEST (P)" color chart. Record the data indicated next to the color swatch matching most closely the color of the solution.

Matching color bar label ____A-E

% P in fertilizer to be used ____%

P content ______ ppm

P content ______ lbs./acre

I. Rinse the tin wire carefully with tap water, and then with distilled water. Return the tin rod to the envelope of the kit for use by another group.

J. Calculate the amount of fertilizer (Superphosphate = 18 percent phosphorus content) needed for your 50' x 100' garden. First, determine how much is needed for 1000 sq ft by dividing the "%P in fertilizer to be used" (from above) by 18 (the percent phosphorus in the fertilizer) and multiplying the result by 50 lbs. Treble superphosphate and Superphosphate are relatively inexpensive sources of phosphorus for lawn and garden use.

18% P Fertilizer needed per 1000 sq ft. ______ lbs.

18% P Fertilizer needed in whole garden ______ lbs.


V. Potassium Content

Potassium is important for carbohydrate (sugar and starch) manufacture by plants. When sufficient potassium is available, plants produce stiff, erect stems, and the plants are more disease resistant. When insufficient or excess potassium is in the soil, plants contain too much water, are susceptible to frost injury, and growth is reduced. Since roots are important storage areas for carbohydrate, root crops like carrot, turnip, and radish and tuber crops like potatoes are enhanced by supplemented potassium.

Plants with deficient potassium have mottled, spotted, or streaked leaves. The leaves curl strongly at the ends. The leaf margins die and flake out, leaving a ragged edge. Poor root development may lead to the plant toppling over as the stem grows.

A. Rinse out your microcentrifuge tube completely.

B. Put 12 drops of "Potash #6" and 12 drops of "Potash #7" into the clean microcentrifuge tube. Mix them thoroughly.

C. Add well-sifted soil until it fills the tube to the 1/2 ml mark; the liquid should be above this mark.

D. Close the cap of the tube and, holding the cap in place with your finger, shake the tube gently for 30 seconds.

E. Place the tube in the centrifuge and spin for 1 minute.

F. Remove the microcentrifuge tube from the centrifuge and decant the supernatant liquid into a clean 10 x 75 mm test tube.

G. Compare the color of the transparent solution with the "POTASH TEST (K)" color chart. Record the data indicated next to the color swatch matching most closely the color of the solution.

Matching color bar label ____A-E

% K in fertilizer to be used ____%

K content ______ ppm

K content ________ lbs./acre

H. Calculate the amount of fertilizer (Potassium sulfate = 50 percent potassium content) needed for your 50' x 100' garden. First, determine how much is needed for 1000 sq ft by dividing the "%K in fertilizer to be used" (from above) by 50 (the percent potassium in the fertilizer) and multiplying the result by 50 lbs. Potassium chloride and potassium sulfate are relatively inexpensive sources of potassium for lawn and garden use.

50% K Fertilizer needed per 1000 sq ft. ______ lbs.

50% K Fertilizer needed in whole garden ______ lbs.


VI. Using Prepared Fertilizers

The calculations you have performed for the various fertilizers needed for your 50' x 100' garden were based upon use of a single fertilizer that contained an abundance of a single nutrient (N, P, or K). These calculations allow you to optimize the amount of each of the nutrients for your particular soil. It also turns out that this is probably the most inexpensive way to fertilize, providing you have learned how to do the calculations.

Most people do not have the knowledge to do the calculations, and many do not live near agricultural supply companies (there are two in the Willimantic area). These folks rely upon prepared fertilizers that contain a mixture of all three major nutrients (N, P, and K). The amount of each nutrient in the mixture is shown on the packaging as the guaranteed analysis. This is a sequence of three numbers separated by hyphens. By convention, the sequence of numbers is always the percentage of N-P-K. So a fertilizer with an analysis of 5-10-5 contains 5% N, 10% P, and 5% K. Various combinations of nutrients are available in bags of fertilizer sold at lawn and garden shops. These are quite expensive compared to the single-nutrient fertilizers.

If you have to use these prepared fertilizers you would choose a bag of fertilizer with an analysis close to your soil test results:

A. Locate the "%N in fertilizer to be used," "%P in fertilizer to be used," and "%K in fertilizer to be used" values on pages 4, 5, and 6, respectively. These numbers would represent the ideal prepared fertilizer for your soil. What is the ideal analysis for your soil?

______ - ______ - ______

B. If this exact fertilizer were available, you would apply 50 lbs. per 1000 sq. ft. How much would you need to apply to your 50' x 100' garden?

________ lbs.

C. Unfortunately this ideal fertilizer is probably not available. Therefore, you must compromise on the analysis. To select the best compromise, round the nitrogen percentage down, the phosphorus up, and the potassium up. When you have found the fertilizer with the closest analysis to your ideal, you would apply 50 lbs. per 1000 sq. ft.

D. Assume that the local store has every possible combination of fertilizer analysis based upon multiples of fives. In other words, round your values in part A (above) off to the nearest appropriate multiples of five. What analysis would most closely match your needs?

______ - ______ - ______

If you had no soil test results to help you determine which fertilizer to use, a good "shot in the dark" for general purpose would be 20-20-20 (a balanced fertilizer). For your vegetable garden 5-10-5 would be generally useful, but for fruit crops 5-15-5 would be better. For your lawn 20-5-5 or 20-0-0 would be appropriate. Understand however, that fertilization without a soil analysis can waste time, effort, and money, and is likely to not produce ideal conditions. Your local agricultural extension service can perform soil analyses for a modest fee. For this area UCONN is the appropriate contact.


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