by This is the next in my series of 4 articles on feeding roses.
Opinions vary, as will your rose blooms...
(c) 1998 by Scott G.***inson, All rights reserved.
Permission given to the Honolulu Rose Society to reprint by
the author, 3/6/98.
Rose Fertilizers Defined, part II
So, now that we know what the big three fertilizer numbers are and what they
mean, I will delve into what plants do with the big three and also discuss
soil pH as it is key to how fertilizers are made available to plants.
Nitrogen:
Nitrogen is a key element in plant growth. It is used to make proteins
and chlorophyll. Without nitrogen your roses will not produce new growth,
and that will result in far fewer flowers. This is because most types of
roses (ie., hybrid teas and floribundas) bloom on new wood (recent green
growth). Nitrogen is used the most when roses are growing fastest in the
spring. Nitrogen is highly mobile in plants and in soil. It is also water
soluble, and thus may be leeched from the soil with heavy watering. Its
high mobility in plants is an indicator for detecting that they are lacking
nitrogen; if low on nitrogen the old leaves in plants will turn yellow. This
is because plants will sacrafice the older leaves and move the available
nitrogen to the newly growing ones. If you feed them they will usually
restore the levels of nitorgen in the older leaves, and they will turn green.
This type of leaf yellowing is called "nitrogen-deficiency chlorosis."
Soils in semi-arid areas like the coast of central and southern California
tend to be nitrogen poor. Some high level nitrogen sources include urea
(33-0-0 or 46-0-0) which is produced in protein breakdown in animals.
Urea can be used directly by plants. Ammonia and nitrites are very high in
nitrogen, but they cannot be used directly by plants. Microbes break these
down into nitrates quite rapidly though, and nitrates can then be used by
plants. Many combinations of ammonia and nitrites appear in fertilizers.
Sulphate of Ammonia (21-0-0) is a typical example commonly used as lawn
fertilizer. Note that this and urea can burn rose leaves though, so I do
not recommend using them on roses (if you do use them, use them in low
doses and dissolved in water).
Other good sources of nitrogen for roses are *** meal (13-0-0) and
various rose formulas like Miracle-Gro for Roses[TM] (18-24-16) and Shultz
formula for roses (19-24-24). Other types of water-soluble chemical
fertilizers are usually high in nitrogen, such as Miracid[TM] (30-10-10)
and 20-20-20 fertilizers available in many brands. I like 20-20-20; it
tends to be cheap and certainly has all that roses and orchids need for
food. Kirkland Plant Food is a Price Club/Costco wholesale brand that is
a cheap water-soluble fertilizer (20-20-20). I prefer water soluble
fertilizers here in California where any amount of compost or mulch will
degrade in a matter of months. I also like water soluble fertilizer for it
ability to carry elements that tend to bind in soil (see below). Steer and
chicken manure are also a good source of nitrogen, and they are really
cheap. Steer manure is a good soil conditioner as well, and will feed the
worms which help loosen clay and hard soils.
Phoshorous (from phosphate):
Phosphorus is used by plants to capture light which is used to produce sugars
in the photosynthesis process. It also promotes good root growth and flower
production. Without phosphorus your roses will grow slower and the stems may
turn reddish/purple in color, be weak, or fall off. The blooms may also be slow
to open, and the roots will not grow well. Phosphorous does not move easilly in
plants. Also most forms of it do not move well in soil. This is because it
attaches chemically to compounds in the soil, and latches onto it. In dry form
it must be dug well into your soil where the roots can get at it. This makes
it hard to distribute phosphorous in established gardens. Liquid phosphorous
will be mobile for the time it remains soluble. This is another reason I like
and use liquid fertilizers.
Bone meal (3-15-0) is an excellent and traditional source of phosphorous.
Superphosphate (0-45-0) and Superbloom (12-55-6) are other excellent
sources, and they are more soluble. These come in dry form though and thus
should be dug well into the soil where the roots can get to it to be effective.
A good time to dig these in is after hard pruning roses in winter, or when
planting bare root roses, or when transplanting. Roses do not have that many
surface roots to get destroyed when digging these in and around them. Other
good sources of phosphorous are seaweed, fish meal, rock phosphate, and guano.
Several so-called "Super Bloom" and "Super Fruit" fertilizers have 0-10-10 or
2-10-10 formulas and they are relatively high in phosphorous. They are often
sold in liquid form as well, and thus have good solubility. A cheap source
of phosphorous is plain old fireplace ashes (ashes are alkaline though so be
carefull when using in alkaline soil).
Again, if your soil is acidic, phosphorus in the soil will be less available
to plant roots. Acid soil will latch onto phosphorous and phosphates, and thus
more phosphorous compounds must be used to make it available to plant roots.
Use water soluble types of phosphorous fertilizer for better effectiveness
in acid soils.
Potassium (from potash):
Potassuium allows sugars to move freely though plants, and thus contributes to
the vigor of growth. It is hard to detect if your roses are lacking in potassium,
but sometimes the leaf margins will yellow and you may see underdeveloped flower
buds. Potassium is not very mobile in soils, and only about 1% that is in the
soil is useable by plants (the so called "exchangable posassium"). A very cheap
form or potassium fertilizer is plain old fireplace ashes. Again, they tend to
be alkaline though, so do not use in high amounts if you soil is already alkaline.
If you have acidic soil you can use them more libearally, but they must be dug
into the soil well to be of any use.
Some of the fertilizers listed above have higher amounts of potassium in them,
but it tends to be the lowest percentage element in most fertilizers. Some rose
formulas have a high percentage of potassium, like Shultz's for roses (19-24-24)
and some are quite low, like Ortho Rose Pride (8-12-4). Several forms of soluble
potassium exist, and again it is a good reason to use soluble fertilizers becasue
they are better at delivering the goods to the roots. Potassium Sulfate (0-0-50)
is the strongest form available and is usually cheap. Also potassium nitrate
(13-0-44) is very high in this element and combines it with nitrogen, but this
is usually on the more expensive side. Seaweed and guano are good sources of
potassium as well.
A word about soil pH:
To those without a chemisty degree (I did poorly in college chemistry) here is a
simple explanation about pH. pH is an accronym for "Potential (or P-factor) of
Hydrogen." pH is a negative base 10 logarithmic scale (a numerical measurement)
centered around a value of 7 (seven). 7.0 is pH neutral; neither acidic or
alkaline, a balance between and the pH of pure water. Any values above 7 are
considered alkaline (also called base), and anything below 7 are acidic. Pure water
is neutral, but most water is usually alkaline or acidic, depending on the solids
dissolved in it. The cause of acidity/alkalinity is due to levels and combinations
of hydrogen and hydroxide ions, but I will not get into that. It is sufficient to
know if the soil is either acidic or alkaline. Soils that are alkaline are salled
"sweet" soils. Soils that are acidic are called "sour" soils. Roses grow in just
about anything... they are pigs, remember? But they are more apt to grow better
in soils that are between a pH of 6 and 7.2 (though I have grown then in 7.4 soil
with little trouble).
Do note however that these log scale values are not linear. They are exponents
of 10. A pH of 9 is not ~slighly~ more alkaline than a pH of 8, it is *TEN TIMES*
more alkaline than a pH of 8. Thus it is not simple to change soil pH, and I
do not try (with exception of mounding acidic compost/peat moss to my azalias
which have surface roots that thrive on these acidic goodies). You can buy a soil
tester at the nursery and test your soil for pH. However, these have the drawback
that they only test the surface of the soil unless you dig well below it. Soil near
the surface can tend to be more acidic due to added compost, rotting leaves, and the
addition of topsoils or peat moss. It is better to know what the pH is at the root
level of plants. *Insider tip* A cheap method of testing soil pH is by planting
hydrangias. They are like litmus paper, as they will bloom pink in alkaline soil,
and blue in acid soil. This will also be a good indicator of what the average soil
pH is in your yard. If you are really cheap, look at your neighbor's hydrangias
instead... ;). Soil pH is usually the same in any given region too, so you can
also ask around to find out.
Some compounds can be used to radically change the soil pH. For example R.A.C.
is a soil acidifier, and it will turn your soil acidic. The R stands for
Rhododenderon, the A for Azalia, and the C is for Camelia. These are acid loving
plants, and RAC is capable of changing the soil around a small area from alkaline
to acidic. Also another trick is to add one cup of sulfer per camelia bush to
acidify the soil around it for about 3 years. Some other examples: pine needles,
compost, MirAcid[TM] plant food, gypsum, sulfate of ammonia, and Epsom salts are
all acidic. Fireplace ashes are alkaline.
So to review: Nitrogen is used to make proteins and chlorophyll. It is highly
mobile in plants and water soluble. Phosphorus is used to capture sunlight to
produce sugars (the real plant food). It is not highly mobile in plants or soil.
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