Why sample
Testing greenhouse media provides an assessment of the material to adequately provide plant nutrients and provide a good rooting environment, i.e. low salt content and adequate pH. See tables and information below for interpretations of test results (Table 1), approximate rates of materials to modify pH (Table 2), approximate amounts of sulfur or dolomitic limestone required to modify pH (Table 3), rapid corrective measures to adjust root-medium pH (pH info), modifying electrical conductivity/salt level (EC info), correcting macronutrient deficiencies (Table 4), and correcting micronutrient deficiencies (Table 5).
Interpretation and recommendation guidelines
The desirable pH, soluble salt and nutrient levels for greenhouse or soil-less media vary by the greenhouse or nursery crop and by the management practices used. The ratings and recommendations given below are general guidelines for results obtained from water or DTPA saturation extracts.
Table 1
Test result interpretations*
| Analysis | Low rating | Acceptable rating | Optimum rating | High rating | Very high rating |
| Soluble salt (mmho/cm) | 0 to 0.75 | 0.75 to 2.0 | 2.0 to 3.5 | 3.5 to 5 | > 5.0 |
| Nitrate-N (ppm) | 0 to 39 | 40 to 99 | 100 to 199 | 200 to 299 | > 300 |
| Phosphorus (ppm) | 0 to 2 | 3 to 5 | 6 to 10 | 11 to 18 | > 19 |
| Potassium (ppm) | 0 to 59 | 60 to 149 | 150 to 249 | 250 to 349 | > 350 |
| Calcium (ppm) | 0 to 79 | 80 to 199 | > 200 | ||
| Magnesium(ppm) | 0 to 29 | 30 to 69 | > 70 |
*Taken from Warneke, D. 1998. Greenhouse Root Media.
Adequate micronutrients ranges (using the DTPA Extraction Method) are: boron 0.7 to 2.5 ppm; copper 0.5 to 1.5 ppm; iron 15 to 40 ppm; manganese 5 to 30 ppm; and zinc 5 to 30 ppm.
Modifying pH
It is best to modify greenhouse media before establishing plants. Some rapid corrective measures can be used to adjust media that already contain plants. However, amendments such as iron sulfate and hydrated lime – used to change pH – will burn most plants. Apply amendments only to the root medium. Rinse plants with water if material comes in contact with plant surfaces. Some plants may be sensitive, so test a small area or a few plants before treating a large area. A pH adjustment of 0.5 to 1.0 unit is rapid, but effects are short-lived. Recheck the pH within a week and reapply if necessary.
The effectiveness of amendments and their application rates will vary with different media. The amount of change is dependent on the type, concentration and fineness of the material used and the buffering capacity of the greenhouse media. The attached tables contain recommendations that are approximations.
Table 2
Approximate rates of materials to modify pH of a root medium*
| Amendment | For bench soils lb/100 sq ft |
For potting soils lb/20 bu or 1 cu yd |
For potting soils oz/2.5 bu |
Rate of pH change | Length of change |
| To lower pH 0.5 to 1.0 unit | |||||
| Finely ground sulfur | 0.5 | 0.25 | 0.5 | slow | long |
| Aluminum sulfate† | 3 | 1.5 | 3 | rapid | short |
| Iron sulfate†‡ | 3 | 1.5 | 3 | moderate | short |
| To raise pH 0.5 to 1.0 unit | |||||
| Ground limestone | 5 | 2.5 | 5 | moderate | long |
| Dolomitic limestone | 5 | 2.5 | 5 | moderate | long |
| Hydrated lime# | 2 | 1 | 0.75 | rapid | short |
*Adapted from B.E. Whipker. Iowa State University.
†Can be applied as a soil drench in enough water to cover 100 sq ft bench area or equivalent soil area of potted crops.
‡Iron sulfate will increase a root media’s EC and may release toxic levels of micro-elements from the root media’s exchange sites.
#Hydrated lime is corrosive. Avoid contact with skin and metal. Hydrated lime may displace ammonium from the root exchange sites of the root medium into the soil solution causing root injury. Avoid using hydrated lime if high levels of ammonium fertilizer are present in the root system.
Table 3
Approximate amounts of sulfur or dolomitic limestone required to modify the pH of various types of root media to pH 5.7 from a stated beginning pH.*
| Beginning pH | 50% peat and 50% sand | 50% peat and 50% bark | 100% peat |
| Sulfur required to lower pH to 5.7 | |||
| 7.5 | 1.7 lb per cubic yard of medium | 2.0 lb per cubic yard of medium | 3.4 lb per cubic yard of medium |
| 7.0 | 1.2 lb per cubic yard of medium | 1.5 lb per cubic yard of medium | 2.5 lb per cubic yard of medium |
| 6.5 | 0.8 lb per cubic yard of medium | 1.0 lb per cubic yard of medium | 2.0 lb per cubic yard of medium |
| Dolomitic lime or equivalent amount of calcium to raise pH to 5.7 | |||
| 5.0 | 1.7 lb per cubic yard of medium | 2.5 lb per cubic yard of medium | 3.5 lb per cubic yard of medium |
| 4.5 | 3.7 lb per cubic yard of medium | 5.6 lb per cubic yard of medium | 7.4 lb per cubic yard of medium |
| 4.0 | 5.7 lb per cubic yard of medium | 7.9 lb per cubic yard of medium | 11.4 lb per cubic yard of medium† |
| 3.5 | 7.8 lb per cubic yard of medium | 10.5 lb per cubic yard of medium | 15.5 lb per cubic yard of medium † |
*Adapted from M.L. Albrecht. 1991. Plant nutrition, fertilizers, water quality and pH. Kansas Flora. 9(3):3-7.
†Additions of more than 10 pounds of dolomitic limestone per cubic yard often causes micronutrient deficiencies.
Rapid adjustment of pH
These are rapid corrective measures to adjust root-medium pH in pots or benches that already contain plants.
To lower pH
- Dissolve 1 to 2.5 pounds of iron sulfate in 100 gallons of water. Apply to the root medium.
To raise pH
- Option 1
Mix 1 pound of hydrated lime in 100 gallons of water. Allow the mixture to settle. The clear solution can be applied to the root medium. - Option 2
In a plastic bucket, mix 1 pound of hydrated lime with 3 to 5 gallons of warm water. Allow the mixture to settle and pour off the clear solution into another plastic bucket. Apply the clear solution with a fertilizer injector set at 1:100 or 1:128.
Modifying electrical conductivity (salt level)
If the electrical conductivity (EC) is too high, leaching the medium with water will reduce the salt level. A corrective procedure includes a normal irrigation followed immediately by another irrigation. After this, allow the medium to dry. If further leaching is required, repeat the double irrigation. Recheck the EC level to determine whether the leaching has been successful in lowering the EC to an acceptable range. A low EC is usually indicative of inadequate fertilization. Increase the rate or frequency of fertilization.
Correcting nutrient deficiencies
Table 4
Correcting macronutrient deficiencies*
| Deficient nutrient | Corrective procedure and recommended formulations | oz/100 gal † | gm/liter † |
| Nitrogen | Use a high-N fertilizer Balance the N to K2O 24-7-15 calcium nitrate (15-0-0) ammonium nitrate (34-0-0) |
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| Phosphorus | Use a fertilizer formulation with P equal to half or more of the N level 20-10-20 or use one application of diammonium phosphate 18-46-0 or monoammonium phosphate 11-56-0 |
32 | 2.4 |
| Potassium | Use a high K2O fertilizer formulation 20-5-30, 17-5-24, 15-11-29, 15-10-30 or use potassium nitrate to balance N to K2O 13-0-44 |
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| Calcium | Use the N-source calcium nitrate 15-0-0 or use a high-calcium fertilizer 20-0-20, 21-0-20, 15-0-15, 15-5-15, 13-2-13 |
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| Magnesium | Apply magnesium sulfate (Epsom salt) or use a complete magnesium-containing fertilizer 13-2-13, 14-0-14, 15-15-15 17-0-17 | 32 | 2.4 |
| Sulfur | Apply magnesium sulfate (Epsom salt) or use a sulfur-containing fertilizer 20-18-18, 20-18-20, 20-9-20 | 32 | 2.4 |
*From Nelson (1996)
†Rates apply to general greenhouse crops. Lower rates may be required for lightly fertilized crops. Rates should be halved for plug seedling crops. These corrective procedures are to be applied once. Subsequent applications should be made only after a soil or foliar analysis test indicates a need. All fertilizers are to be applied as a substrate drench.
Adjustments in available nutrient levels can be made by the following additions: 2 oz of calcium nitrate (15-0-0) per cubic yard (75 g per cubic meter) to increase the nitrogen test level 10 ppm; 1 lb of concentrated superphosphate (0-46-0) per cubic yard (600 g per cubic meter) to increase the phosphorus test level 5 ppm; and 1.5 lb potassium nitrate (13-0-44) per cubic yard (55 g per cubic meter) to increase the potassium level 100 ppm.
Table 5
Correcting micronutrient deficiencies*
| Micronutrient and source | Bench drench | Container drench | |||
| Analysis | oz/100 sq ft | g/100 sq ft | oz/100 gal | g/100 gal | |
| Iron ferrous sulfate |
20 percent |
16 |
454 |
50 |
1418 |
| Boron sodium tetraborate (borox) boric acid |
11 percent 17.5 percent |
0.5 0.6 |
14.2 17.0 |
1 0.65 |
28.4 18.4 |
| Manganese manganese sulfate |
33 percent |
0.25 to 0.5 |
7.1 to 14.2 |
0.1 |
2.8 |
| Zinc zinc sulfate |
23 percent |
0.25 to 0.5 |
7.1 to 14.2 |
0.1 |
2.8 |
| Copper copper sulfate |
25 percent |
0.25 to 0.5 |
7.1 to 14.2 |
0.1 |
2.8 |
| Molybdenum ammonium molybdate† sodium molybdate† |
54 percent 46.6 percent |
0.025 0.029 |
0.7 0.8 |
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