Soils & Soil Physical Properties
Part 2 – 16 | Unit 2.1
Lecture 2: Soil Properties
Bright colors (reds and yellows), indicate well-drained soils. However, color shouldn’t
be your sole indicator for determining the soil’s suitability for crops. It is possible for
a soil with bright colors to still have excess free water at points in the growing season
if the groundwater is moving quickly and has sufficient oxygen or if it is too cold for
biological activity.
ii. Organic matter
Dark colors in the soil usually indicate organic matter. However, they may also
indicate wetness (remember, wetter soils can accumulate more organic matter).
Sometimes dark colors may be derived from the parent material. This is often the
case in soils that formed from dark-colored igneous rock.
- Soil depth
Soil depth determines how far the roots can grow and how much water the soil can hold.
Depth is measured to the shallowest root-limiting layer.
a) Factors that can limit soil depth:
i. Bedrock
ii. Hardpans
- Densely compacted material (tillage pan or plow pan): Can form when farm
implements repeatedly pass through the soil at the same depth. This causes soil
particles to be pressed closer together, reducing the amount of pore space and
the size of the pores. Consequently, these pans have permeability rates lower than
those of the soil above and below them. - Natural hardpans: Can form when certain minerals, such as iron, lime (calcium
carbonate), and silica, bind to soil particles and create a cemented layer in the soil - On a field scale, growers may choose to plow or rip a soil to break up natural or
tillage pans and to increase the pore space in the soil. Another option is to use
deep-rooted cover crops (see Unit 1.6, Selecting and Using Cover Crops). In a
gardening context, growers can use double digging (see Unit 1. 2).
However, the benefits of using tillage to break up soil compaction are temporary,
especially in coarser soils. In a coarse-textured soil, such as a sandy loam, most
of the pore space added by plowing or ripping will be lost by the end of one
cropping season. While it is more difficult to break up compaction in a finer
textured soil, the benefits will last longer than they will in a coarse textured soil.
iii. Strongly contrasting textures
If the area of cultivation is very different from the surrounding soil, water or roots can
be trapped in the cultivated area. This makes it similar to having a flower pot holding
the water in or inhibiting root growth (sometimes called the “pot effect”), and can be
potentially damaging to the crop. On a small scale, this can happen if a hole dug for
planting is filled with soil amendments and the lighter soil, but not mixed well into
the surrounding soil. On a larger scale, this can happen with sandy floodplain soils
adjacent to denser soils.
iv. Water tables
- Soil temperature
Soil temperature is important to growers, especially for spring planting. Many seeds need a
certain minimum temperature for germination (see Unit 1.3, Propagating Crops from Seed,
and Greenhouse Management).
a) Factors influencing soil temperature
i. Local climate: Soil temperature is highly correlated to air temperature