Agriculture Reference
In-Depth Information
dioxide out of the soil, and it interferes with nutrient uptake. Wet soils also
warm slowly in the spring, interfering with tree growth and adversely affect-
ing soil micro- and macroorganisms.
Classifying types.
The type of soil, referred to as soil texture, has much to do
with water drainage. We classify soils by their relative percentages of sand,
silt, and clay. Sand particles are the largest and do little more than provide
water drainage, gas exchange for the roots, and anchorage for tree roots.
Sand does not hold water or nutrients well. Silt particles are the next smal-
lest and hold water and greatly help form soil aggregates. Clay particles are
the smallest soil particles. They help with aggregate formation and hold wa-
ter and nutrients.
An effective blend of the three types of soil particles provides good wa-
ter- and nutrient-holding capacity, but also good water drainage, gas ex-
change, and root penetration. Soils that are too sandy are droughty and re-
quire much irrigation. Heavy clay soils drain poorly and encourage root dis-
orders.
Figure 2.3 shows how soils are classified, based on their percentages of
sand, silt, and clay. To use the triangle, locate the percentage of sand, silt,
and clay in your sample. Starting at those points, draw lines parallel to the
adjacent counterclockwise side. Guidelines are shown in the figure. The point
where the lines intersect is your soil type or texture. Say, for example, your
soil particle size distribution test showed sand = 45 percent, silt = 35 percent,
and clay = 20 percent. The classification for this sample would be loam —
an excellent choice for orchard soil, provided that it is deep enough and well
drained.
