Exercise 8: Greenhouse Soil Testing

The greenhouse represents a unique environment for plant growth. The growth medium often consists of non-soil or partial soil mixtures. Other materials such as peat, perlite, vermiculite, bark, coconut fiber (coir), compost, etc. are currently being used in many combinations. Desirable characteristics of soil amendments include a high volume of air space, moderate water-holding capacity, stability against compaction and decomposition, low soluble salt levels, uniformity between lots, and light weight. If weight is not important, sand or calcined clay (basically ground-up brick of known particle size) is used for coarse aggregate to promote drainage and aeration.

Often, little attention is given in greenhouse mixes to nutrient supplying ability because fertility management practices in the greenhouse differ considerably from field management practices. Nutrients are usually supplied as slow-release fertilizers or, more commonly, as a nutrient solution when watering. Artificial soil or soil-less mixtures often offer increased aeration and avoid water-logging problems but, as a rule, provide relatively low nutrient-holding capacities. Therefore, the quantities of nutrients released throughout the growing season are rather small, and plant growth on a day-to-day basis is very dependent upon the soluble nutrients in solution. Hence, it is important to continually monitor the level of soluble nutrients. This can easily be accomplished by taking soil samples every two weeks and sending to a soil testing laboratory. If fertilizer adjustments are required, the additional nutrients can be added in the next irrigation water. Research has shown that saturation extracts give a reliable measure of plant-available nutrients for the greenhouse.

If soil is used, a typical greenhouse soil mix consists of 1/3 loam soil, 1/3 peat, and 1/3 coarse aggregate (sand, perlite, vermiculite, etc.). Most of the pH- and cation-buffering capacity of this mix is provided by the soil. A common artificial mix of 1/2 peat and 1/2 vermiculite contains even less buffering capacity. As a result of low buffering capacities, only limited quantities of dry fertilizer can be added to these mixes without creating excessive soluble salt levels. Phosphorus is the exception because most P fertilizers have low salt indices.

Until a few years ago, most soil testing laboratories handled greenhouse soils in a similar fashion as field soils. Current work suggests that better techniques are available, such as the saturated soil extract method. The major problems in handling greenhouse growth media in the same way as field soils are related to handling and diagnostic interpretations. Because of the artificial components, drying, grinding, and sieving of greenhouse samples often result in significant alterations of the physical properties of the sample. Furthermore, interpretation of the results must take into account the bulk density of the growth medium being analyzed. Bulk densities of greenhouse media can range from 0.2 to 1.2 g/cm³. One can easily see if a certain weight of the medium is tested, the total volume will vary drastically according to the bulk density. The moisture content required for the saturated extract, however, is automatically compensated for by different bulk densities of a greenhouse mixture. Also, when a large volume of growth medium is used, the effect of a heterogeneous sample is minimized.

If one wishes also to evaluate micronutrients, a saturated paste extract containing 0.005 M DTPA has been found to be effective. Use of 0.005 M DTPA extract has little effect on other extractable nutrients or the soluble salt measurements.

The electrical conductivity of the saturated paste extract (mmhos/cm or dS/m) multiplied by 700 gives an approximation of total soluble salts in mg/L. Thus, the equations below can be used to calculate the percentage of calcium, magnesium, and potassium in the greenhouse mix.

Total soluble salt concentration (mg/L) = electrical conduction × 700 (mS/cm)

Percentage nutrient = nutrient concentration in extract (mg/L)/total soluble salt concentration (mg/L) × 100

The desirable soluble salt and nutrient levels vary with the greenhouse crop being grown and management practices. Desired nutrient balance is to have 8 to 10% of the total soluble salt as nitrate nitrogen, 11 to 13% potassium, 14 to 16% calcium, and 4 to 6% magnesium. If chloride and sodium are determined, their percentages should be less than 10%. Adjustments in available nutrient levels can be made by adding water-soluble fertilizers. Dr. Darrell D. Warncke of Michigan State University has been active in developing greenhouse soil tests. Data in Table 5 have been developed by Dr. Warncke to aid in interpreting greenhouse soil test results.

 

If using DTPA extraction, the adequate ranges for key micronutrients are boron (0.7-2.5 mg/L), iron (15-40 mg/L), manganese (5-30 mg/L), and zinc (5-30 mg/L).

Most soil testing laboratories are set up to routinely test for the nutrients listed in Table 5. Micronutrients are often tested by using an Inductively Coupled Plasma Emission Spectrograph (ICP). In this exercise, we will determine the amount of soluble calcium, magnesium, sodium, and potassium in the greenhouse sample. We also will calculate these three nutrients as a percentage of total salts. Does your sample contain adequate calcium, magnesium, and potassium for growth of a greenhouse crop?

 

Procedure

Note: The filtrate from step 4 of the exercise on soluble salts can be used for this exercise. The first four steps below duplicate those of the soluble salts exercise and can be bypassed if that filtrate is used.

1. Place approximately 125 mL of a greenhouse mix in a 250-mL beaker so water can be added.

A volume basis is used because of the large variation that may occur in bulk densities. The exact weight of soil is not critical.

2. Obtain 100 mL of deionized water in a graduated cylinder. Add the water to the soil in small increments until the mix is saturated.

Stir the mix with a spatula as the water is added. When saturated, the paste glistens, flows slightly when the beaker is tipped, and slides freely and cleanly off the spatula (for all soils except clays). After standing, free water should not collect on the soil surface. If the paste stiffens or loses its glisten, add more water and remix. If free water is on the surface after standing, add more soil and remix.

3. Allow the mixture to stand 1 hr, then recheck for saturation.

Some time must be allowed for the soluble salts to dissolve and diffuse through the added water.

4. Filter as directed by your laboratory instructor.

The equipment must be dry to avoid dilution of the extract.

5. Pipette 1.00 ± 0.01 mL of the extract from the greenhouse mixture into an Erlenmeyer flask and add 99.0 ± 0.1 mL of deionized water. Stopper the flasks and mix their contents thoroughly by inverting and shaking them several times.

Thorough mixing is essential so that a small sample of the diluted extract will be representative of the whole.

6. Determine Ca²⁺, Mg²⁺, K⁺, and Na⁺ contents using an AAS. Report in mg/L of each element in solution.
7. Calculate Ca²⁺, Mg²⁺, K⁺, and Na⁺ contents in mg/L of the extract (note that it was diluted 100 fold before being analyzed.

Remember from Exercise 8, one method for calculating concentrations in dilution problems by using the following formula: C₁V₁ = C₂V₂

where

• C₁ = initial concentration of a solution
• V₁ = initial volume of a solution
• C₂ = final concentration of a solution
• V₂ = final volume of a solution

8. Determine the percentages of Ca²⁺, Mg²⁺, Na⁺, and K⁺ of total soluble salts.

 

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