Soils: formation and fertility Chapter 5 etc. Soils: formation and fertility

Outline 1. Nutrient Cycles What are nutrient cycles? major cycles (Ch 5-1) Water Cycle (Ch 5-2) Carbon Cycle (Ch 5-3) Nitrogen Cycle (Ch 5-4) Phosphorus Cycle (Ch 5-5) Sulfur Cycle (Ch 5-6) 2. Nutrient Cycling & Sustainability (Ch 5-9) 3. Soil (Ch 5-8) layers/profiles, texture & porosity, acidity Soil Erosion and Degradation (Ch 14-6) Soil Conservation (Ch 14-7)

Soil Profile A soil profile is a description of the soil horizons (layers) from the surface where the plants are growing to the geologic material (rock or sediment) at the base of the soil. This is usually considered the “parent material” from which the soil formed.

Soil Types Soils can be categorized into two major types: Organic soil - a soil in which more than half of the soil profile thickness is composed of organic matter (>30 cm thick, >20% organic matter). Mineral soil - a soil whose properties are determined by mineral (rock) matter, rather than organic matter.

Organic Soil example: Surface organic layer (O-horizon) is > 30 cm thick.

Humus-partially decomposed organic matter. Organic matter (OM) importance: Organic acids and colloids in OM “bind” contaminants and help make nutrients available to plants. OM holds onto water and releases it slowly. OM in soil is a “carbon reservoir”. Dark brown or black color in soil usually indicates high nitrogen content (and lots of humus).

Components of Soil Organic Matter (OM)

Soil Organisms “By-products from growing roots and plant residue feed soil organisms. In turn, soil organisms support plant health as they decompose organic matter, cycle nutrients, enhance soil structure, and control the populations of soil organisms including crop pests.” - USDA/NRCS

3. Soil Characteristics

3. Soil Characteristics

Mineral Soil Horizons A horizon - Surface horizon of a mineral soil. It has most of the organic matter. Often dark brown or dark gray. E horizon - Present in some acid (low pH), forest soils. A zone of “Eluviation” (washing out) of clays, and iron and aluminum oxides. Often light gray or white. B horizon - A zone of accumulation of clays, and iron and aluminum oxides. May be reddish in color. May have blocky or prismatic texture. C horizon - A zone that is relatively unaffected by biological activity. It may be the “parent” material of the soil.

How do horizons form? soil horizon animation

Particle Size: Clay- Very fine (microscopic grains) Silt- Fine Sand- Medium Gravel- Large

What type of soil is 20% Clay, 70% silt, And 10% Sand? Silty Loam

Loams: Roughly equal mixture of clay, silt, sand and humus.

Water-Holding Capacity Soil Texture Nutrient-Holding Capacity Water- Infiltrating Capacity Water-Holding Capacity Permeability Clay Good Poor Silt Medium Sand Loam

Soil Porosity is the amount of ‘pore space’ a soil has. Porosity is affected by the sizes, shapes and arrangements of particles. It is also influenced by plants and soil organisms that create channels and “pores” in soil. Compaction reduces pore space. Soil holds less water or air.

Soil Permeability A measure of how connected the pore spaces are to each other. Determines the water and air capacity of the soil. Clay will “hold onto” water because the clay mineral grains are negatively charged at their edges. Some clays also swell when water is added. START

Soil Fertility Soil fertility is influenced by Porosity/compaction Drainage Nutrient content Nutrient availability Salinity and other toxins

What are the major soil nutrients? macronutrients: Sulfur, Phosphorous, Magnesium, Calcium, Potassium, Nitrogen, Oxygen, Carbon and Hydrogen. micronutrients: molybdenum, copper, zinc, manganese, iron, boron, chlorine. (less important) Most soil nutrients are positively charged ions (cations).

Nutrient availability The three biggest factors for nutrient availability for a moist soil are clay content, organic matter (OM) content, and pH. The Cation Exchange Capacity (CEC) is a measure of how available nutrients are to plants in the soil. It is a lab measurement. CEC can be estimated by determining the amount of Clay Organic matter IF the pH is about 7.0 (neutral)

Why does clay matter? Nutrients are mostly cations (positively charged ions). Clays are negatively charged at their edges. Clays hang on to cations loosely enough, so they are available to plants.

Why does OM matter? Its surface is negatively charged, so it has lots of places for positive ions to loosely attach and become available for plants to use. It tends to contain a lot of carbon and other nutrients in the big organic molecules that make up the OM itself. It’s food for the decomposers, etc. The chemistry gets messy, but that’s the gist of it…

Why does pH matter? H+ competes with other nutrients for sites on OM and clays. During acid leaching…nutrients are “washed away”.

Tropical soils: why do clay, OM and pH matter? Organic matter is broken down rapidly in the tropics (it’s hot!) and is removed. High rainfall leads to lots of leaching Leaching of soils in the tropics leads loss of clays in upper layers and “laterization.” Tropical soils are generally low in fertility as a result.

Sodium: poison in soil Sodium modifies the structure of some clays to “seal” the soil. Water does not penetrate. Too much sodium (salt) is toxic to most plants. Excess sodium can be tolerated by some plants that have evolved to deal with it. If salt builds up, it must be leached away by flooding, or other chemicals (calcium sulfate) must be applied to the soil to restore fertility.

What is NOT generally considered a benefit of organic matter It contributes to overall soil fertility by helping make nutrients available to plants. It binds contaminants. It stains your clothes when you dig in the dirt. It retains and slowly releases water. It is a carbon “sink,” meaning that it sequesters carbon and keeps it out of the atmosphere.

The B horizon is A soil layer dominated by organic matter. A mineral soil layer that has some deposits of iron oxides, aluminum oxides, and clays that turn it red. Or it may have a prismatic or blocky structure. Parent material that is virtually unaltered. A layer or zone of “eluviation” where iron oxides and aluminum oxides have been leached out by water.

What is the difference between porosity and permeability? Only porosity can be improved by soil organisms, not permeability. Porosity can be reduced by compaction (squishing the soil), but permeability cannot. Porosity is a measure of total pore space, while permeability measures how connected the pores (holes) are. You measure porosity by pouring water through the soil. You can’t measure permeability this way.

How does clay improve soil fertility? It doesn’t. Clay decreases soil fertility. Clay increases the availability of nutrients to plants. Clay increases the permeability of the soil. Clay is a nutrient.

What is the optimal pH for soil fertility for most food crops? a. Less than 7.0 (acid) b. 7.0 (neutral) c. Greater than 7.0 (alkaline)