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Soil & pH
Soil texture and soil pH will influence how you use fertilizer.
SOIL TEXTURE
Soil with good texture can hold adequate water, oxygen, and nutrients for plants.
Soil with poor texture won't.
You can improve the texture of any soil by mixing in organic matter such as purchased
planting mixes, soil conditioner, peat moss, composted manure, or compost. To
add organic matter to a bed or planting hole, mix it into the soil with a turning
fork or tiller as you cultivate. You will want to use a layer at least 5cm (2")
deep to mix into the soil; more if the ground is very hard clay or extremely sandy.
Most soils are either porous sand or heavy clay. In some regions, very fine silt
is also common. Clay and silt pack down hard and do not drain well. Sand particles
are much larger than particles of clay or silt leaving more space for water to
run through which is why sandy soils dry out so fast. Clay soils stay waterlogged
after a heavy rain.
The ideal garden soil is loamy. Which means that most of us end up having to improve
at least part of the soil in our yard, especially in new neighborhoods where most
of the topsoil was scraped away before building. Organic matter in the soil holds
moisture yet improves drainage (figure that one out). It also increases oxygen,
which is needed by roots and helps the soil store nutrients until plants are ready
to use them.
TIPS FOR FERTILIZING SANDY SOIL
Because of its structure, sand does not hold on to nutrients very well. Many plant
nutrients dissolve in water, so they tend to leach out of sandy soil fast. For
this reason, it is important to use a timed-release fertilizer that won't wash
through the soil before plants can make use of it. Gardeners who have sandy soils,
often have to fertilize more often than gardeners in areas that have naturally
rich, loamy soils.
Sandy soil dries out quickly, so it's important to use mulch to slow down evaporation
from the soil's surface.
TIPS FOR FERTILIZING CLAY SOIL
Clay soil has many tiny spaces that hold on to water and nutrients. Because water
that holds dissolved nutrients moves through clay soil slowly, it's important
to mix fertilizer into the soil well.
If you have a problem site that stays wet and packed down all the time, consider
making raised beds. Besides improving the soil's drainage and texture, people
are less likely to walk in clearly marked beds. Footsteps squeeze the air out
of clay soil, which makes it even more hard and dense.
SOIL pH
The subject of pH (the measure of soil acidity) often intimidates new gardeners,
but it shouldn't. Soil acidity is easy to test and problems are usually easy to
correct.
Soil pH affects the ability of the soil to release the nutrients in fertilizer.
If the pH level is too high or two low, nutrients can get "locked up"
in the soil chemistry and become unavailable to plants.
Soil pH is measured with the numbers 1 to 10. A pH below 7 is acid. A pH above
7 is alkaline. Most plants thrive in a pH of between 6 and 6.8. Check soil pH
by buying a soil test kit or pH meter. You can also find out how to send soil
samples to your soil testing lab by contacting your local soil lab service.
If needed, adjust the pH of your soil by applying lime to acid soil or by applying
sulfur or gypsum to alkaline soil. See the rates in the box to the right.
Most plants grow well in soil with a pH between 6.0 and 6.8, but some do better
if the soil pH is a little higher (cabbage family vegetables, buffalograss) or
slightly lower (azaleas, rhododendrons, evergreens).
Soil pH changes slowly, but it does change. Check your soil every year or two,
or more often if your plants do not grow well despite proper fertilization.
CHANGING SOIL pH
Pounds of Lime Needed to Raise Soil pH to 6.5 (Kg per 100 m2)
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Soil pH
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Sand
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Soil Type Loam
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Clay
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6.0
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8 Kg
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14 Kg
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20 Kg
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5.5
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18 Kg
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31 Kg
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41 Kg
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5.0
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27 Kg
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45 Kg
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61 Kg
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4.5
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33 Kg
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61 Kg
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82 Kg
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4.0
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41 Kg
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72 Kg
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94 Kg
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Note: For amounts greater than 25 Kg, split into multiple treatments of 25 Kg.
or less with a 2 week interval between applications.
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Pounds of Sulfur Needed to Lower Soil pH to 6.5 (Kg per 100
m2)
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Soil pH
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Sand
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Soil Type Loam
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Clay
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8.5
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18 Kg
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23 Kg
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27 Kg
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8.0
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10 Kg
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14 Kg
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18 Kg
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7.5
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4 Kg
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7 Kg
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10 Kg
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Apply lime or sulfur with a drop type spreader to avoid kicking up too much dust. |
DID YOU KNOW?
Much of the organic matter you add to the soil will disappear within a year or
two. It decomposes. This is why it is important to add organic matter each time
you plant. Eventually the organic matter leaves behind humus, a rich, more lasting
form of organic matter. A lazy way to keep adding organic matter is to mulch your
plants with finely ground or shredded bark. As it decomposes, it adds to the soils'
organic content.
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