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Main Index > Markets > Horticultural Applications > Potting Soils > Perlite Gradation Perlite Gradation and Peat/Perlite Mixture Tradition has dictated the use of medium or
coarse grades of perlite in propagating media and in planter mixes. Peat moss is a common
component in both types of mixes. Using several grades of perlite in varying proportions
with peat moss provides an opportunity to compare the suitability of different perlite
grades for horticultural use. Tests were conducted using three different
grades of perlite: fine, medium, and coarse. Fine grades of horticultural perlite usually
are smaller than 1 mm in size and fall largely in the USDA's sand classification range of
coarse and very coarse; coarse grades of horticultural perlite are usually larger than 1mm
(often larger than 2mm) and can fall in the USDA's sand classification range of fine
gravel.(For more information about perlite gradations, read our guide on Standard Gradations of Perlite---Fine, Medium, Coarse Defined.) Tests of peat/perlite mixtures were carried
out in 1974 and repeated in 1977. Of course, the peat moss source was different for each
series of tests. As a result of differences in peat moss quality, all data are not
completely comparable but do indicate general trends. Lowest Wet Bulk Density With Perlite Coarse Grades Both perlite and peat moss are known to be
light in weight with dry bulk density values in the range 5.1 to 8.6 lbs/cu.ft. The higher
the proportion of peat moss, the lower the dry bulk density. Since the grower or user is normally working
with moist material, wet bulk densities are of more practical interest. Lightweight
growing media in containers are much less expensive to handle and ship. The lowest wet bulk density obtained was 23.3
lb/cu.ft. for a mixture of 25% peat and 75% coarse perlite. The highest wet bulk density
was 38.1 lb/cu.ft. for a mixture of 75% peat and 25% fine perlite. The difference between the average dry bulk
density and average wet bulk density is 26.1 lbs/cu.ft. or slightly more than 3 gals. of
water per cu.ft. of mix. To fully appreciate the wet density values of
these mixtures, it should be noted that wet bulk densities of soils and sands run well
over 100 lb/cu.ft. Since finer grades of perlite have far more
surface area and will hold far more water, it is logical that the highest wet bulk
densities occurred with fine grades of perlite and the lowest wet bulk densities occurred
with coarse grades. Different proportions of perlite to peat had
the least amount of influence with the finer grades of perlite, whereas higher proportions
of coarse perlite consistently lowered the wet bulk density of the mix. It therefore
appears that the finer grades of perlite have water holding capacities similar to peat
moss, and coarser grades of perlite provide more aeration and have lower water holding
capacities. Absence of Air or Water Is Injurious In any growing medium, the space not occupied
by solids constitutes pore space which is utilized partially for holding water and
partially to provide air so plant roots can funtion normally. An absence of either is
injurious to the growing plant. The higher the total porosity, the greater the
potential to provide water and air. The smaller the pores,the greater the water retenton
and the lower the air supply. Conversely, the larger the pores, the less water retained
and the greater the amount of air space in the mixture. Peat moss is well known for high porosity and
perlite has a similar property. The effect of perlite grade is one of increasing porosity
as the gradation becomes coarser. Differences in peat/perlite mixes are not large but are
consistent in this respect. Another consistent trend is for total porosity
to increase modestly as the proportion of peat moss increases regardless of perlite grade.
Differences are not great but indicate slightly higher total porosity for the peat mosses
used compared to the perlite sources. Moisture Retention is Substantial The quantity of water held in a growing medium
is best measured on a volume percentage basis. In testing, the lowest water retenton was
found in mixtures of 75% coarse perlite and 25% peat, and the highest water retention
occurred in mixes with 25% fine perite and 75% peat. The general trend is for moisture retention to
decline as perlite particle sizes become coarser and to increase as peat moss percentages
increase. Overall, the differences are not of high magnitude and the quantity of water
held in all cases is substantial compared to sand or soil. Oxygen is Essential After saturating a growing medium with water
and after drainage has ceased, there will be a portion of the total pore space occupied by
air. This is the "free porosity" or "air space after drainage." Since
oxygen is esssential to healthy root activity, it is important that this value be of
sufficient magnitude. In field soils, values of 5% or more are usually considered
favorable. In container media, values of 15% to 25% are desirable. In the tests that were conducted, the lowest
free porosity was 21% for the mixes with 75% fine perlite and 25% peat moss, and the
highest was 46.1% for the mixes with 75% coarse perlite and 25% peat. The finer grades of perlite do not
consistently affect the free porosity when peat moss fractions are high. The coarser
grades of perlite produce extra high free porosity when the proportion of perlite is high
and this value declines as the proportion of peat moss increases. Since all values are in the 15% or higher
range, it must be concluded that no problems from inadequate aeration would be expected in
any of the blends tested. This suggests that the best mix would be that with the highest
water holding capacity. The exception would be in propagation where high mist frequency
might require a maximum free porosity and minimum moisture retention. Under these
circumstances, the best compromise might be to use 100% coarse perlite as the propagating
medium. Summary and Conclusions Perlite and peat moss have physical properties
in the same order of magnitude. Perlite however can be produced uniformly whereas peat
moss is likely to vary. Low density media can be produced with all grades of perlite and
peat moss in any desired porportions. Highest moisture retention occurs with the finest
perlite grades which is comparable to peat moss in this property. Coarser perlite grades
have proportionally lower mosture retention and higher free prorosity compared to peat
moss. Aeration characteristics are excellent for all grades of perlite/peat moss mixtures
and for all component mixes. These data confirm what most growers have known for many
years---peat moss/perlite combinations provide outstanding physical properties for
propagation and growing media. In 1974 and 1977 when these tests were conducted, what may not have been appreciated by many growers was the practicality of using finer and finer grades of perlite for many horticultural applications and mix designs---nor could anyone have anticipated the successful use of 100% fine perlite as is being done in several commercial operations in the world today.
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Information given herein is from sources considered
reliable, but no guarantee of accuracy can be made or liability assumed. Your
supplier may be able to provide you with more precise data. Certain compositions or
processes involving perlite may be the subject of patents. |
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