Principles of Agricultural Science – Plant 1

2 Structure and Depth Perception Unit 2 – Mineral Soils Lesson 2.1 Understanding Soil Properties Principles of Agricultural Science – Plant

3 Soil Structure When soil particles cling together in an arrangement known as a ped, it is called structure. Two considerations must be made for soil structure: Structure type Structure grade

4 Structure Type The shape of the ped formed is called structure type. (Huddleston & Kling, 1996) Granular – roughly spherical, like grape nuts. Usually 1-10 mm in diameter. Most common in A horizons, where plant roots, microorganisms, and sticky products of organic matter decomposition bind soil grains into granular aggregates. Platy – flat peds that lie horizontally in the soil.

5 More Defined Structure Types (Huddleston & Kling, 1996) Blocky – roughly cube-shaped, with more or less flat surfaces. If edges remain sharp, it is called angular blocky. If the edges are rounded, the peds are called subangular blocky. Blocky structures are typical of B horizons. Prismatic – larger, vertically elongated blocks, often with five sides.

6 Structureless Types The above structure types are the two extremes in terms of effects on permeability. Neither present great conditions for crop use. (Huddleston & Kling, 1996) Massive – compact, coherent soil not separated into peds of any kind. Massive structures in clayey soils usually have very small pores, slow permeability, and poor aeration. Single grain – in very sandy soils, every grain acts independently, and there is no binding agent to hold the grains together into peds. Permeability is rapid.

7 Structure Grade How strongly the ped is held together is structure grade. Four grades define soils: Strong Moderate Weak Structureless

8 Effects of Structure Good structure, or strong structure, means good aeration because of ample pore space. Organic matter is important for forming soil structure.

9 Effective Depth Relative to structure is soil evaluation criteria called effective depth. Effective depth is the zone, which plant roots can easily grow.

10 Limitations of Effective Depth Barriers for root growth are caused by: Massive soils Dense soil structure Poor internal drainage Abrupt texture changes between horizons Gravelly and large sands

11 Signs of Promise for Roots Good conditions for effective depth include: Moderate and strong structure grades Brown or red soils, which indicate good aeration

12 References Huddleston, J. H., & Kling, G. F. (1996). Manual for judging Oregon soils. Corvallis, OR: Oregon State University. Parker, R. (2010). Plant and soil science: Fundamentals and applications. Clifton Park, NY: Delmar. Plaster, E. J. (2003). Soil science & management (4th ed.). Clifton Park, NY: Delmar.