Kwantlen Farm School

1. Kwantlen Farm School Soil Texture and Structure Chris Thoreau February 24, 2012

2. Soil Texture Soil texture refers to the relative amount of sand, silt, and clay found in a soil The mixture of these components affects the feel of the soil as well as water, nutrient, and pore space interactions

3. Soil Texture • Mineral Components • Sand • Silt • Clay

4. Soil Texture • Sand • Largest soil mineral particles (.02 – 2 mm) • Formed greatly from physical processes • Spherical/erratic in shape • Sand = little rocks • Larger pore spaces • Good drainage • Does not hold a charge • Difficult to compact

5. Soil Texture • Silt • Size between sand and clay (.002 - .02 mm) • Usually physically formed out of sand • Hold and releases water well • Flat or round in shape • Holds very little charge • Feels soapy • Carried in moving water

6. Soil Texture • Clay • Smallest soil mineral particle (< .002 mm) • Holds water very well • Holds strong negative charge for mineral adsorption • Susceptible to compaction • Platy-/flat-shaped particles • Various lattice structures

7. Soil Texture Mineral ratios determine soil texture

8. Soil Texture • Clay • Understanding structure of clay is important for: • Compaction • Water holding • Cation adsorption • Soil cultivation • Clays are categorized by their layer structure • Relationship of Si-tetrahedral and Al-octahedral sheets • 2:1; 1:1; 4:1; 5:2

9. Soil Texture 2:1 Clay • Shrink and swell 1:1 Clay No change

10. Soil Texture • Shrink and Swell of Clay • Interlayer space expandswith increasing watercontent in soil • Space contracts as wateris removed • Clay can crack when it shrinks

11. Soil Texture • Why is Texture Important? • Water Infiltration • Water Storage • Fertility • Aeration • Trafficability • Soil texture knowledge is the key to developing an overall soil maintenance and improvement plan

12. Soil Texture • Notes: • We do not change the texture of soils • We can change the characteristics of certain textured soils • We change soil characteristics through: • Additions of organic matter • In soil and on top of soil • Cultivation practices • Raised beds

13. Soil Texture Attributes of Different Soil Textures

14. Soil Texture • We can determine the texture of the soil by feeling it • Ribbon test • Ball test • Jar test • Laboratory tests give more accurate results

15. Soil Texture • Texture Questions?

16. Soil Structure • Soil Structure: How the soil fits together • Primary particles are arranged into secondary particles called aggregates (or peds)

17. Soil Structure • Why is Structure Important? • Pore space • Air and water movement • Rooting space • Nutrient storage and release • Contributes to soil resilience • Cultivation • Erosion resistance

18. Soil Structure • How does aggregate formation occur? Flocculation + Cementation = Aggregation • Flocculation: • Primary pulled close together (into flocs) by attractive forces (electrostatic forces, H bonding) • Cementation • Primary particles held together by cementing agents • Carbonates; clays; OM; Oxides

19. Soil Structure

20. Soil Structure • Soil Aggregates are classified by their shape

21. Soil Structure • Ideal structure: Spheroidal • Typical in A Horizon • Rounded; loose • Granular (porous) or Crumb (very porous) • Greatly affected by soil management OR mismanagement • Improved with OM additions and microbial activity

22. Soil Structure • Soil structure is particularly important in providing adequate pore space for: • Root growth • Water movement • Gas exchange • Microbial activity • Macrobial activity

23. Soil Structure Structure can be easily observed in the soiland structural stability, or aggregate stability, can be measured in the lab Structure can be improved, to a point, by soil cultivation. Soil cultivation is also a great way to destroy structure

24. Soil Consistency • Related to texture • Very important when considering soil cultivation • Dependent on: • Texture/clay content • Clay type • Soil water content

25. Soil Consistency

26. Soil Consistency • Cultivating soil when too dry • Breaks aggregates into small pieces • De-aggregates • Can result in dust • Very damaging to soil structure • The drier the soil – the more it acts like powder

27. Soil Consistency • Cultivating soil when too wet • Where to start?! • Compaction • Risk and depth of compaction increases in wet soil

28. Soil Consistency Cultivating soil when too wet The wetter the soil - the more it acts like water

29. Soil Consistency

30. Soil Consistency • Soil consistency, determined greatly by water content and percentage of clay, plays a major role in when soil can be cultivated! • Not as crucial when hand digging…

31. Good Soil Structure • We promote good structure in soil by: • Minimizing cultivation (especially in sandy soils) and using appropriate cultivation methods • Avoiding compaction (especially in clay soils) • No tractor in wet soil! • Especially careful with clays • Cultivating at proper soil consistency • Adding various types of organic matter regularly • Maintaining a proper pH • Promoting microbial life – especially fungi • Always keeping the soil covered • Preferably by crops

32. Soil Structure and Texture • Soil texture influences soil’s ability to aggregate • Clay soils aggregate more readily • Sandy soils have les stable aggregates • Organic matter, plant growth, and microbial activity all contribute to aggregate stability