Horticulture Principles and Practices

Soil Erosion
The wearing away of the
land surface by geological
agents such as water, wind,
and ice.
considered organic matter until it is incorporated into the soil. Soil erosion depletes soil
organic matter. Organic matter is important to soil productivity since it is a source of
nutrients when it decomposes. It improves soil structure by binding together mineral particles
into aggregates for better aeration and drainage. It helps to buffer soils against rapid
changes in pH. Organic matter increases the water-holding capacity of soils and gives
them their characteristic dark brown or black color. Microorganisms (e.g., bacteria, fungi,
and actinomycetes) are responsible for decomposing plant parts for easier incorporation
into the soil. Sugars, starches, proteins, cellulose, and hemicellulose decompose rapidly,
whereas lignin, fats, and waxes are slow to decompose. Organic matter acts as a slowrelease
fertilizer, since its nutrients are released gradually over a particular period.
Humus is a very stable part of the soil organic matter. Much of humus is formed
from two general biochemical processes. The chemicals in the plant residue undergo
decomposition by microbial action to produce simpler products. These breakdown products
undergo synthesis, by which the simpler products are enzymatically joined to make
more complex products such as polyphenols and polyquinones. These synthetic products
interact with nitrogen-containing amino compounds to produce a great portion of resistant
humus. Further, the synthetic process is aided by the presence of colloidal clays.
Humic particles (or humic micelles) carry a large amount of adsorbed cations (e.g., Ca 2+ ,
Mg 2+ ,H + , and Na + ) as clay micelles.
114 Chapter 4 Plant Growth Environment
4.3.3 SOIL REACTION AND NUTRIENT AVAILABILITY
A soil test showing that adequate amounts of a nutrient are present does not indicate its
availability to the plant. In addition to adequate amounts, the presence of moisture is critical,
because water is the medium in which solutes are transported through the plant.
Other factors that interfere with nutrient availability are soil temperature and soil reaction,
or pH. Plant processes are generally slowed down by low temperatures.
Soil reaction, or pH, is a measure of the hydrogen ion concentration as an indication
of the soil’s degree of acidity or alkalinity. A pH of 7 is neutral. Values above 7
are considered alkaline, and values below 7 are acidic. The pH scale is logarithmic
(Figure 4–11), meaning that a soil pH of 5 is 10 times more acidic than a soil pH of 6
and a pH of 4 is 100 times more acidic than a pH of 6. Most horticultural crops tolerate
a soil pH within the range of 4 to 8. Soil pH regulates nutrient availability. Figure 4–12
shows the relationship between pH and nutrient availability to plants. A pH of7±1appears
to be a safe range for most nutrient elements in the soil. Only iron is available at
a strongly acidic pH. Conversely, iron is deficient in the soil under alkaline conditions.
Sensitive plants (such as bluegrass [Poa pratensis]) develop iron-deficiency symptoms
called iron chlorosis, a condition in which young leaves lose their green color and
become yellowish. The difference between this kind of chlorosis and that associated
with nitrogen deficiency is that iron chlorosis occurs between the veins of the leaves (interveinal
chlorosis) and nitrogen causes a more uniform yellowing of leaves. Soil pH
affects the biotic population of soil. Fungi tend to prefer highly acidic conditions
(pH of 4 to 5), and nitrogen-fixating bacteria (Rhizobia) prefer a pH range of between 6
and 8. Table 4–8 shows the pH requirements of various horticultural plants.
Factors That Affect pH
Soil pH may rise in a soil that experiences low rainfall or is poorly drained. Salts tend to
accumulate under these conditions. Soils formed on calcareous parent material have high
alkalinity. Acidic soils (low pH) occur when soils are exposed to heavy rainfall and good
drainage such that the bases are leached into lower depths or washed away in the runoff.
Correcting pH
Low soil pH may be corrected in practice by adding limestone (CaCO 3 ) or gypsum (CaSO 4 )
to the soil to raise the pH. The choice depends on the soil pH and other characteristics.
To lower soil pH, sulfur compounds are added to the soil. Nitrogen fertilizers also tend to