Soil Test Laboratory

Preparing your soil samples

Take the soil sample from about 2-4" below the surface.

Place your soil sample into a clean container. Break the sample up with the trowel or spoon and allow it to dry out naturally. This is not essential, however it makes working with the sample easier. Remove any small stones, organic material such as grass, weeds or roots and hard particles of lime. Then crumble the sample finely and mix it thoroughly.

Set a small amount of soil aside after cleaning (about a spoonful) for the pH test. The rest will go into the Nitrogen, Phosphorus, and potassium (K) tests.

Testing Tips
N, P & K Tests Only: Extraction Process
  1. Fill the extraction tube to the 30 ml line with distilled water.
  2. Add two Floc-Ex Tablets.
  3. Cap tube.
  4. Mix until the tablets have distintegrated.
  5. Remove the cap.
  6. Add one heaping teaspoon of soil.
  7. Cap tube.
  8. Shake for one minute.
  9. Transfer the sample to a holding container.
  10. Let the sample stand until the soil settles out (30 minutes to 24 hours, dependent on soil). The clear solution above the soil will be used for the Nitrogen, Phosphorus, and Potassium tests. A fine clay soil will take much longer to settle out than a coarse sandy soil. The clarity of the solution will also vary. The clearer the better, however cloudiness will not affect the accuracy of the test.
When to test your soil

Testing your soil should be routine at the end of the growing season after harvest, again just before planting, and periodically during the growing season.

The importance of pH

Soils are known as "sweet" if they are alkaline and "sour" if they are acid. Values of pH 7 indicate a neutral soil; above pH 7 is alkaline and below pH 7 is an acid soil. Most Soils are within a range of highly acidic pH 4 to alkaline at pH 7.5 to 8.

When pH is at the proper level, nourishment available in the soil is "unlocked", ready to give the plant the necessary vitality to produce, multiply, and resist disease.  If the pH is too high or too low, this nourishment is locked out.  Fertilizer applications are wasted.  While some plants thrive on alkaline soils, others thrive on acidic soils.  Most garden and field crops prefer the pH to be between 5.8 and 6.6, slightly acidic. 

Phosphorus is a plant food lacking in soils after they have been under cultivation for a while.  When the pH is high, above 7.8, phosphorus locks up (bonds) with calcium to form an insoluble calcium phosphate compound and becomes unavailable to the plants.   When the pH drops below 7.8, phosphorus is again released in a soluble form.

Nitrogen has a special relation to pH.  Nitrogen must be broken down by soil bacteria (cyanobacteria) before it can be absorbed by plants.  These soil bacteria do their best work between a pH of 5.8 and 6.6.  Therefore in highly acidic or alkaline soils there is very little breakdown of nitrogen.  Plants can become starved for nitrogen under these conditions even with liberal quantities of fertilizer.

When the pH goes above 8.0 potash (potassium) becomes unavailable to plants.

If you wish to grow plants not suited to the pH of your soil, you can change the pH. You can make an acid soil more alkaline by adding lime. Slightly acid soils can be made more acid by adding peat, iron sulphate or flowers of sulfur. It is, however, more difficult to convert an alkaline, lime-rich soil into an acid soil.

pH affects the availability of plant foods - Nitrogen, Phosphorus and K (Potassium) - and prevents the spread of soil borne diseases. Check it regularly - at least twice a year - because nature tends towards the acid side.

Working with your soil - not against it - will help you achieve more successful results. Any changes in pH should be small scale - 1/2 to 1 pt at the maximum.

Adjusting pH

Altering pH does take time so do not expect an immediate change. After adding lime or iron sulfate to your soil, retest for pH in 40 to 60 days - If results are still significantly off, retreat your soil , not exceeding recommended application levels.

When to add Lime

You can correct pH at any time of year but it is best (in temperate Northern hemisphere) to start in Fall and check progress in Spring. Allow one month to pass between adding lime and adding fertilizers.

How to make your soil more acid

To make soil more acid, dig in plenty of peat, compost and manure, and retest in a few months. For quick action use ammonia sulfate at the rate of 1 oz./sq. yd. or flowers of sulfur. But remember ammonia sulfate also adds Nitrogen (ammonia is NH3).

Recommendations for lawns

For established lawns, Nitrogen is the most essential nutrient to promote lush growth and deep, green color. Phosphorus and Potash, in lesser quantities, are all important for strong root formation and growth. Compound fertilizers will supply all 3 nutrients, or you select an individual fertilizer with a Nitrogen, such as our rapid feed Nitrogen with 44% Nitrogen.

Feeding your plants a "healthy" diet

There are at least six major plant foods which are essential to plant growth - carbon, hydrogen, oxygen, nitrogen, phosphorus and potash (potassium), the later three being expressed as N, P, and K.  Carbon is obtained by plants from carbon dioxide in the air, hydrogen from water, and oxygen is produced by plants themselves from water.  It is the later three that are not generally available and often have to be supplied by fertilizers.

NITROGEN (N) is essential to the proper functioning of plant metabolism. It increases the protein content of food crops and is needed by most leafy vegetables, foliage plants and grass. Nitrogen gives plants their dark green color (chlorophyll) and helps the growth of leaves and stems. Lawns, foliage plants and dahlias require a regular supply of nitrogen.  Too much nitrogen can cause too rapid growth, leading to softened plant tissue and general weakness of the plant.  At very high levels nitrogen can cause the leaves to "burn" and the plant to die.  At flowering time too much nitrogen can cause the plant to return to leaf production prematurely.

PHOSPHORUS (P) is the most important nutrient in root formation, creating good fibrous root systems. It enables plants to get off to an early start and hastens maturity. Phosphorus encourages blooming and seed formation, helps plants' resistance to wintry weather and disease and increases the vitamin content of plants. Lettuce, potatoes, carrots, for example, require good reserves of phosphorus.

POTASH (K) is probably one of the better known foods because it stimulates flowering and makes fruit tastier through converting sunshine into starches and sugars. Tomatoes, strawberries, beans, peas and flowering plants require especially high levels of potash.   Potash is also important in the formation and transportation of starch, sugar, and other carbohydrates in the plant.  Plants with a proper supply of potash form stiff stalks and healthy, disease resistant tissue.  Potash also promotes root growth.   Too much potash can cause plants to take up too much water and become weak.

Plants also need sulfur and calcium, many general fertilizers supply these compounds.   In addition plants often need micronutrients: trace amounts of metals such as magnesium, iron, copper, zinc, manganese, molybdenum, and the element boron.  The environment often has sufficient quantities of these trace elements.  Some plant foods also provide these elements.  Some plants will flower only if fed the appropriate amount of a particular micronutrient.  See page 145 of Botany for Gardeners for more information on micronutrients.

Deficiency Symptoms (p146-147 of Botany for Gardeners)
  1. Nitrogen: Leaves turn yellowish-green, slow and dwarfed growth, drying up or "firing" of leaves which starts at the bottom and proceeds upward.
  2. Phosphorus: Purplish leaves, stems, and branches; slow growth and maturity.
  3. Potash: Mottling, spotting, streaking, or curling of leaves, leaves scorched or burned on margins, plants weak and subject to collapse.
Nutrient Replacement

Plant food deficiencies can be corrected by adding nutrients in the form of fertilizers. Nutrient replacement can be approached in different ways:

  1. Utilize compost to enrich soil.  Black and brown decayed plant and animal matter is called humus.  Humus is a key to gardening success.  It is created by microorganisms (bacteria and fungi) and macroorganisms (earthworms, mites, grubs, and nematodes) digesting organic matter.  It should be remembered that organisms that break down organic matter require nitrogen. 

    Humus forms slowly and is often only a very thin layer in the tropics.  In the tropics most if not all of the available nutrients are taken up and held in living organic matter.  Tropical top soils, the layer with humus, are typically very thin and easily eroded. 
  2. Apply a general, slow-release fertilizer (usually organic) at the beginning of the growing season prior to planting. This creates a food bank for the plants to draw on and balances any deficiencies.
  3. Feed established plants with the food they prefer, particularly in the case of vegetables, fruit and lawns. In all instances, feeding can be supplemented with a liquid feed (usually chemical) throughout the growing season.
Compound Fertilizers

A compound fertilizer is a mixture of all the essential foods a plant needs. Compounds are available in slow release (granular) and quick-acting (powder and liquid forms) and can be for general feeding or for specific plant needs.

If you prefer to use a compound fertilizer, which should normally be applied during early Spring (slow release) and during the growing season (quick-acting), use your soil test results to apply the fertilizer more efficiency You can thus avoid the possibility of over-feeding, which can be as detrimental as a lack of food.

If, for example, you find the levels for phosphorus and potash are both drastically low, you should increase the application rate of the compound by at least 100% and vice versa. Apply half of a soluble feed in dry form directly onto the soil and the other half as a liquid feed.

If, on the other hand, the levels are widely out of balance e.g. high in phosphorus and low in Potash you should not be using a compound fertilizer. This would not only be wasteful, but would increase the already high level to a potential excess.

Where both the pH needs correcting and fertilizer is required, divide the applications over several weeks. Do not, for example, add lime and fertilizer together. Add the lime first, then allow least a month to pass before applying fertilizer.

pH Test Only
  1. Fill the test tube to the shoulder with distilled water.
  2. Add 1 soil pH tablet
  3. Use the green test tube cap to measure and add one capful of prepared soil.
  4. Cap tube.
  5. Mix by inverting ten times.
  6. Let soil settle for one minute or until a clear colored solution is visible above the soild layer.
  7. Compare colored solution to pH color chart on box by holding the tube against the white area on the box. Determine the pH of the soil.

In the following tests be aware that the answer might also be "none present" or 0 parts per million.

  1. Use pipette to transfer the clear extraction solution above the soil to a test tube until it is filled to the shoulder.
  2. Add one nitrate #1 tablet.
  3. Cap with red cap.
  4. Mix until the tablet distintegrates.
  5. Add one nitrate #2 tablet.
  6. Cap with red cap.
  7. Mix until the tablet distintegrates.
  8. Wait five minutes for the color to develop.
  9. Compare pink color of solution to Nitrogen color chart on box.
  10. Low = 20 parts per million Medium = 80 ppm High = 160 ppm
  1. Use pipette to transfer 25 drop of the clear extract solution above the soil to a test tube.
  2. Fill the tube to the shoulder with distilled water.
  3. Add one phosphorus tablet.
  4. Cap with the blue cap.
  5. Mix until the tablet disintegrates.
  6. Wait five minutes for the color to develop.
  7. Compare blue color of solution to phosphorus chart on the box.
  8. Low = 4 ppm Medium = 10 ppm High = 32 ppm
  1. Use a pipette to transfer the clear extraction solution above the soil to a test tube until it is filled to the shoulder.
  2. Add one potassium tablet.
  3. Cap with white cap.
  4. Mix until tablet distintegrates.
  5. Compare the cloudiness of the solution in the test tube to the potassium color chart on box.  Hold tube over black boxes in left column and compare it shaded boxes in right column.
  6. Low = 20 ppm Medium = 40 ppm High = 80 ppm
Prepare a report

Fill in the following table

pH Nitrogen /ppm Phosphorus /ppm Potassium /ppm
Red clay near administration 7.0 Below detectable limits Below detectable limits Medium: 40 ppm
Soil from around hibiscus near classroom 7.0 Below detectable limits Below detectable limits Medium: 40 ppm
Muck from wet area across the road 5.5 Not tested Below detectable limits Medium: 40 ppm
Black soil from forest behind agriculture area 4.5 Below detectable limits Below detectable limits Not tested
Coralline sand > 9.0 Below detectable limits Low: 4 ppm Low: 20 ppm
Safety & Hygiene

Dispose of test solutions by rinsing down the sink. Wash the comparators and caps in soapy water immediately after each use. Make sure any sediment or color staining is removed. Rinse well and dry.

The tablets are safe in normal domestic terms, but like all chemicals and pharmaceuticals, they should be put away and kept out of reach of children. Try to avoid touching the tablets. Always wash your hands thoroughly after making your tests. Do not eat, drink or smoke while using the soil test kit. Keep tablets away from food, drink and animal feed. If taken internally, drink copious amounts of water and seek medical advice.

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