Managing Micronutrients in the
Greenhouse
HIL #553 Revised 8/98Douglas A. Bailey, Professor and Paul V. Nelson, Professor
Department of Horticultural Science
Proper plant nutrition is essential for successful greenhouse production of floricultural crops. As growers move towards substrates that do not contain mineral soil, micronutrient status of substrates and plants becomes more important. This bulletin outlines the major micronutrient problems that can be encountered in greenhouse production and outlines application treatments to correct micronutrient imbalances.
Micronutrient Excess
Excesses Can Cause Deficiencies. Excessive application of micronutrients probably accounts for more micronutrient disorders in the greenhouse than does insufficient application. Excessive application of micronutrients, in addition to toxicities, can lead to micronutrient deficiencies. Deficiencies in this case are due to antagonisms between micronutrients during plant uptake. When two nutrients are antagonistic, a super-optimal concentration of one in the substrate (soil) will suppress plant uptake of the other.
A high level of iron in the substrate commonly causes manganese deficiency and to a lesser extent can suppress zinc uptake (Table 1). Conversely, a high level of manganese in the substrate causes iron deficiency and also to a lesser extent, zinc deficiency. Super-optimal levels of copper cause zinc deficiency and conversely, high levels of zinc cause copper deficiency. Thus, it is possible to encounter deficiencies of iron, manganese, copper or zinc as a result of excess application of other micronutrients. These deficiencies can occur even when a normally sufficient concentration of the deficient micronutrient exists in the substrate.
Table 1. Common
micronutrient antagonisms.
|
|
|---|---|
| High soil level of: | Results in low plant level of: |
- Most substrates, whether commercially or self-prepared,
contain micronutrients.
- Most commercially formulated greenhouse fertilizers contain
micronutrients. Fertilizers prepared by the greenhouse firm as an
alternative to commercial products are often formulated with
micronutrients. Generally, plants respond well to the combination
of micronutrients in substrates and fertilizers.
- Specific fertilizers are commercially available for use in
"soilless substrates" that have higher micronutrient
concentrations than the standard greenhouse fertilizers. The
differences in micronutrient content between the standard and the
soilless substrate formulations of one given fertilizer analysis
are given in Table 2. The increase of micronutrients in the
soilless substrate formulation ranges from 100 percent for iron to
over 1000 percent for molybdenum. This third source of
micronutrients can be justified in a soilless substrate where the
pH level is 6.0 or higher because the availability of
micronutrients is strongly reduced in this situation.
- Research has indicated that, as is the case for organic field
soils, the optimum pH level for organic (soilless) greenhouse
substrates can be one pH unit lower than that desired for
mineral-soil based substrates. The optimum pH range for soil-based
substrates is 6.2 to 6.8 while for soilless substrates it is 5.6
to 6.2. When the pH of the substrate solution decreases, the
availability of all micronutrients except molybdenum increases.
Molybdenum availability decreases; however, deficiency of this
nutrient is not known to be a problem in any floral crop except
poinsettias. Thus, growing at a lower pH is equivalent to making
an addition of micronutrients to the plant. Growers who maintain a
pH level below 6.0 should consider using the standard fertilizer
formulations rather than soilless substrate formulations that
contain higher levels of micronutrients, unneeded in this
situation.
- Often the four causes of increased micronutrient availability just discussed lead to excessive availability. Excess of one or more members of the micronutrient group can block uptake of another, bringing about a deficiency of the latter. It is then easy to mistakenly diagnose the total problem as a micronutrient deficiency. Without further information, correction is usually sought by applying a complete mixture of micronutrients. This makes the situation worse because the causal nutrients which are in excess become even higher in concentration. Even though the deficient nutrient is increased in the substrate, its uptake is not effectively increased.
Table 2. The content of
individual micronutrients in a general and a soilless
substrate commercial formulation of 20-10-20 and the
micronutrient concentration increase in the soilless
substrate formulation.
|
|||
|---|---|---|---|
| Content (%) | Increase (%) | ||
Nutrient
|
Standard | Soilless | |
| iron | |||
| manganese | |||
| zinc | |||
| copper | |||
| boron | |||
| molybdenum | |||
- The first and most obvious step is to stop application of
micronutrients. Some fertilizer companies offer fertilizers
without micronutrients. Otherwise, growers can formulate their own
fertilizer without micronutrients.
- The second step is to raise the pH of the substrate. The
availability of all micronutrients except molybdenum decreases as
the pH rises. Iron availability decreases tenfold when the pH
level is raised one unit. Extreme shifts should be avoided. It is
sufficient to move the pH level to the upper end of the acceptable
pH range for the crop.
Three methods can be used for raising substrate pH:- A shift in the fertilization program from ammoniacal
nitrogen (urea, ammonium nitrate, ammonium sulfate) to nitrate
nitrogen (potassium nitrate, calcium nitrate) sources will
bring about a gradual rise in pH.
- Limestone may be applied to the substrate surface at a rate
of approximately 1 lb/yd3 per 0.1 unit rise in pH
desired. This rate equals 1/8
tsp per standard 6 inch pot per 0.1 unit increase in pH
and 1 3/8 tsp per 6 inch pot
for a 1.0 unit increase. These rates are for soilless
substrates. Lower rates may suffice for soil-based substrates.
Limestone reacts very slowly, thus two to six weeks may be
required for a response. There are flowable suspensions of
limestone that are more effective (faster reacting) than
surface applications of ground limestone. Consult the label of
commercial products for precise rates based on existing and
desired pH.
- For a rapid rise in pH, hydrated lime has been used.
Caution should be taken to avoid contact with green tissues and
neither limestone nor hydrated lime should be applied directly
to substrates containing ammonium-containing fertilizers such
as MagAmp or Osmocote. The high pH caused by these liming
materials at the surfaces of such fertilizers can convert
ammoniacal nitrogen to ammonia gas which is highly injurious to
roots and foliage. To raise the substrate pH approximately 1
unit, one pound of hydrated lime should be mixed for five
minutes in 5 gallons of water. Allow the mixture to settle
overnight; and then apply only the liquid at the rate of
one quart/ft2 of bench area. This equates into 8 fl
oz. per 6 inch pot. The substrate should be moist prior to
applying the drench.
- A shift in the fertilization program from ammoniacal
nitrogen (urea, ammonium nitrate, ammonium sulfate) to nitrate
nitrogen (potassium nitrate, calcium nitrate) sources will
bring about a gradual rise in pH.
- The third measure which might be taken for alleviating a micronutrient toxicity involves manipulation of antagonistic pairs of nutrients. If the micronutrient present in excess is a member of an antagonistic pair (Table 1), make an application of the other member of the pair. For example, an excess of manganese, in addition to causing manganese toxicity, can result in iron deficiency. Application of iron alone will suppress manganese uptake and will increase iron uptake, thus alleviating both problems.
There are three systems for diagnosing nutrient status. The best diagnostic tool for micronutrients is foliar analysis. Visual observation of symptoms works but requires that damage be present. Most damage cannot be corrected. Commercial soil tests do not generally identify levels of all micronutrients. On the other hand, accurate tests and standards have been established for foliar analysis of all micronutrients. While the minimum and maximum critical foliar levels for micronutrients can vary for a few crops, these values do tend to be fairly standard for most crops. The general critical foliar levels for some floral crops are presented in Table 3. Plant foliar analysis is only $4.00 a sample, if submitted to the NCDA Plant, Soil and Solution laboratory, and is a wise investment for crop security. Contact your county agent for plant analysis sample sheets and sampling instructions.
Table 3. Interpretative
ranges for micronutrient values (reported in ppm) obtained
from foliar analysis of selected floricultural
crops.
|
||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Carnations | Greenhouse Azaleas | |||||||||||
| Nutrient | Nutrient | Excess | ||||||||||
| boron (B) | boron (B) | >200 | ||||||||||
| copper (Cu) | copper (Cu) | >20 | ||||||||||
| iron (Fe) | iron (Fe) | >175 | ||||||||||
| manganese (Mn) | manganese (Mn) | >400 | ||||||||||
| molybdenum (Mo) | molybdenum (Mo) | --- | ||||||||||
| zinc (Zn) | zinc (Zn) | >69 | ||||||||||
| Chrysanthemums | Poinsettias | |||||||||||
| Nutrient | Nutrient | |||||||||||
| boron (B) | <21 | 21 to 49 | 50 to 100 | 101 to 124 | >124 |
boron (B)
|
<21 | 21 to 29 | 30 to 100 | 101 to 200 | >200 | |
| copper (Cu) | <6 | 6 to 24 | 25 to 75 | 76 to 80 | >80 |
copper (Cu)
|
<3 | 3 to 5 | 6 to 10 | 11 to 15 | >15 | |
| iron (Fe) | <51 | 51 to 59 | 60 to 500 | 501 to 525 | >525 |
iron (Fe)
|
<51 | 51 to 99 | 100 to 300 | 301 to 500 | >500 | |
| manganese (Mn) | <21 | 21 to 29 | 30 to 350 | 351 to 800 | >800 |
manganese (Mn)
|
<41 | 41 to 79 | 80 to 300 | 301 to 650 | >650 | |
| molybdenum (Mo) | --- | --- | --- | --- | --- |
molybdenum (Mo)
|
<0.51 | 0.51 to 1.00 | 1.01 to 5.00 | 5.01 to 806 | >806 | |
| zinc (Zn) | <16 | 16 to 20 | 21 to 50 | 51 to 55 | >55 |
zinc (Zn)
|
<16 | 16 to 24 | 25 to 60 | 61 to 70 | >70 | |
| Foliage Plants (General) | Roses | |||||||||||
| Nutrient | Nutrient | |||||||||||
| boron (B) | <25 | 25 | 26 to 100 | 101 to 124 | >124 |
boron (B)
|
<31 | 31 to 39 | 40 to 60 | 61 to 400 | >400 | |
| copper (Cu) | <6 | 6 to 24 | 25 to 75 | 76 to 80 | >80 |
copper (Cu)
|
<6 | 6 to 7 | 7 to 15 | 15 to 17 | >17 | |
| iron (Fe) | <51 | 51 to 59 | 60 to 500 | 501 to 525 | >525 |
iron (Fe)
|
<51 | 51 to 79 | 80 to 120 | 121 to 150 | >150 | |
| manganese (Mn) | <21 | 21 to 29 | 30 to 350 | 351 to 800 | >800 |
manganese (Mn)
|
<31 | 31 to 69 | 70 to 120 | 121 to 250 | >250 | |
| molybdenum (Mo) | --- | --- | --- | --- | --- |
molybdenum (Mo)
|
--- | --- | --- | --- | --- | |
| zinc (Zn) | <16 | 16 to 20 | 21 to 50 | 51 to 55 | >55 |
zinc (Zn)
|
<16 | 16 to 19 | 20 to 40 | 41 to 50 | >50 | |
| Geraniums | Other Crops (General) | |||||||||||
| Nutrient | Deficient | Low | Sufficient | High | Excess | Nutrient | Deficient | Low | Sufficient | High | Excess | |
| boron (B) | boron (B) | |||||||||||
| copper (Cu) | copper (Cu) | |||||||||||
| iron (Fe) | iron (Fe) | |||||||||||
| manganese (Mn) | manganese (Mn) | |||||||||||
| molybdenum (Mo) | molybdenum (Mo) | |||||||||||
| zinc (Zn) | zinc (Zn) | |||||||||||
Prior to adding micronutrients to correct a deficiency, check the substrate pH and make sure it is in the recommended range. If not, take steps to correct the pH before adding extra micronutrients. If there is still a problem then consider additions of the deficient micronutrient. There are three alternative methods of application for micronutrients:
- Dilute concentrations may be applied in combination with
macronutrients during each fertilizer application throughout the
crop cycle. Sources, rates, and the final elemental concentration
of each micronutrient are given in Table 4. This table will be
helpful for those growers who formulate their own fertilizer and
want to apply one or more but not all of the micronutrients. When
all of the micronutrients are desired most commercially prepared
fertilizers can be used since they contain all micronutrients.
When fertilizers are self-formulated, commercial products
containing all micronutrients can be added into the fertilizer.
Some of these products include Peters STEM, Peters Compound 111,
and Miller's Mitrel M.
- The second method of application calls for higher
concentrations to be applied one time as a normal watering. See
Table 5 for sources, rates, and final elemental concentrations to
be applied in a single application.
- The third method involves a single foliar application of micronutrients. Sources, rates, and concentrations for foliar sprays are given in Table 6. Foliar sprays are very useful where root injury due to such factors as disease or a poorly drained substrate would reduce root uptake of nutrients. However, the greatest risk of plant injury exists with foliar application. Spraying should be avoided during the midday heat. Early morning, after sunrise, is an effective time for application. Plant uptake is enhanced by the increased drying time which occurs during the moist conditions in the morning. Nutrient uptake through the leaves is also greater in the light period than at night, thus making morning applications more desirable than evening sprays. Incorporate a recommended spreader/sticker into micronutrient sprays for more effective coverage. Use rates similar to those employed when applying pesticides.
Table 4. Sources, rates,
and final concentrations of one or more micronutrients with
every liquid fertilization.
|
|||
|---|---|---|---|
Micronutrient
Source
|
Weight of source/100 gal | Final conc. (ppm) | |
| oz | grams | ||
| iron sulfate -- 20% iron OR | |||
| iron chelate (EDTA) -- 12% iron | |||
| manganese sulfate -- 28% manganese | |||
| zinc sulfate -- 36% zinc | |||
| copper sulfate -- 25% copper | |||
| borax -- 11% boron OR | |||
| solubor -- 20% boron | |||
| sodium molybdate -- 38% molybdenum OR | |||
| ammonium molybdate -- 54% molybdenum | |||
Table 5. Sources, rates,
and final concentrations of micronutrients for a single
corrective application of one or more micronutrients applied
to the soil.*
|
|||
|---|---|---|---|
Micronutrient
Source
|
Weight of source/100 gal | Final conc. (ppm) | |
| oz | grams | ||
| iron sulfate -- 20% iron OR | |||
| iron chelate (EDTA) -- 12% iron | |||
| manganese sulfate -- 28% manganese | |||
| zinc sulfate -- 36% zinc | |||
| copper sulfate -- 25% copper | |||
| borax -- 11% boron OR | |||
| solubor -- 20% boron | |||
| For soil-based substrates (>20% soil in substrate) | |||
| sodium molybdate -- 38% molybdenum OR | |||
| ammonium molybdate -- 54% molybdenum | |||
| For soilless substrates | |||
| sodium molybdate -- 38% molybdenum OR | |||
| ammonium molybdate -- 54% molybdenum | |||
| *Do not apply combinations without first testing on a small number of plants. Wash solution off foliage after application. | |||
Table 6. Sources, rates,
and final concentration of the micronutrient for single
foliar sprays for correcting micronutrient
deficiencies.*
|
|||
|---|---|---|---|
Micronutrient
Source
|
Weight of source/100 gal | Final conc. (ppm) | |
| oz | grams | ||
| iron sulfate | 4 | 113.4 | 62 Fe |
| manganese sulfate | 2 | 56.7 | 40 Mn |
| zinc sulfate | 2 | 56.7 | 56 Zn |
| tri basic copper sulfate (53% copper) | 4 | 113.4 | 159 Cu |
| sodium molybdate OR | 2 | 56.7 | 57 Mo |
| ammonium molybdate | 2 | 56.7 | 81 Mo |
| *Do not apply combinations without first testing on a small number of plants. Use the same spreader- sticker product and rate with the above foliar sprays as used with insecticide and fungicide sprays. | |||
.
No comments:
Post a Comment