Understanding Soil Chemistry

Objectives Describe the meaning and importance of soil fertility; Explain the role of organic matter, soil depth, surface slope, soil organisms, and nutrient balance in soil productivity; Explain the role of soil reaction (pH) and liming in soil chemistry; and Explain the role of soil colloids, ions, and the cation exchange capacity (CEC) in soil chemistry.

Understanding Soil Chemistry Soil Fertility

Importance of Soil Fertility Ability of a soil to provide nutrients for plant growth Involves storage and availability of nutrients Vital to a productive soil Fertile soil is not necessarily a productive soil Why? Poor Drainage, Insects, Drought, and other factors can limit production

Importance of Soil Fertility Existing plant-soil relationships affect soil productivity External factors control plant growth Air Temperature Light Mechanical support Nutrients Water Plants rely on soil to provide all these except light

Importance of Soil Fertility Water and air fill pore spaces in soil Factors that affect water in the soil also affect air in the soil Nutrient availability is influenced by soil and water balance and temperature Root growth is influenced by soil temperature as well as soil, water, and air

Understanding Soil Chemistry Soil Productivity

Role of organic Matter What is organic matter? Plant, animal, and microbial residues Varying stages of decay Adequate levels of O.M. benefit soil by: Improving physical condition and tilth Increases water infiltration Decreases erosion Supplies plant nutrients

Role of organic Matter Organic matter contains about 5% N(nitrogen) N is not immediately available Decomposition must occur first Fertilizer (inorganic) N is needed for non- legume plants Organic matter also provides soil with a variety of other mineral elements that plants need to be healthy Some include: phosphorus, magnesium, calcium, sulfur, and others

Role of Soil Depth Soil Depth Plants need soil to be: Depth of soil material favorable for plant root penetration Plants need soil to be: Deep Well-drained Correct texture Correct structure Deeper soils provide for more nutrient storage

Role of Soil Depth Roots can extend 3-6 feet or more when soil depth permits Rooting depth is affected by Physical barriers Chemical barriers High water tables Hardpans, Shale beds, Gravely Layers and accumulations of salts are very hard to correct

Role of surface Slope Surface slope is part of what? Topography Determines the amount of runoff and erosion Also determines: Irrigation method Drainage Conservation practices The steeper the land the more management is needed More labor and equipment costs

Role of surface Slope At certain slopes, soil is no longer suitable for row crops. What are some row crops? Potential productivity is largely determined by ease of erosion along with percent of slope

Role of Soil Organisms Many different organisms live in the soil Usually in the first foot of top soil Depend on organic matter for food Factors that affect the amount soil organisms: Moisture Temperature Aeration Nutrient supply Soil pH Crop that is being grown

Role of Soil Organisms Some of the microscopic organisms cause many favorable soil reactions decay of plant and animal residues speed nutrient cycling. Other reactions can be injurious development of organisms that cause plant and animal diseases

Role of Nutrient Balance Nutrient balance is a vital concept in soil fertility and crop production Nitrogen is the first limiting nutrient in non- legumes Without adequate amounts of the other nutrients, N cannot do its best As Nitrogen fertilization raises yields, the crop demands more of the other nutrients

Understanding Soil Chemistry Soil pH

Soil pH The measure of acidity or alkalinity of the soil pH is determined by the concentration of hydrogen ions(H+) and hydroxyl ions(OH-) in the soil solution Pure water has an equal number of H+ and OH-

Soil ph Another term for alkaline is basic Acid = high in H+ Basic = high in OH- A soil with a pH of 6.0 has 10 times as much active H+ than neutral

Soil ph Plants will not thrive in soil that is too acid or too alkaline The soil pH directly affects the availability of mineral nutrients Example Low pH: phosphates may be bound chemically in the soil so plants are unable to access them Also means that nitrate production in the soil is reduced

Soil PH Several factors influence Soil pH Organic matter Parent material Rainfall leaching Forest vs. grassland Harvesting crops Legumes Soil depth Fertilizer application Flooding

Soil ph How to measure soil pH Which do you think is more accurate? Indicator dyes pH meter Which do you think is more accurate? The meter If our soil’s pH was too acidic what would we add to amend it? Lime

Lime requirement Soil pH is not the determining factor on how much lime to add What else do we need to consider? Texture of soil Amount of organic matter The amount of clay and organic matter in a soil will determine the ability of a soil to resist pH change More clay/organic matter = more pH buffering The process is very complex Example: Al and water

Lime requirement Lime requirement Lime reduces soil acidity Amount of Ag limestone needed to establish the desired pH range for the cropping system being used Lime reduces soil acidity Look at Al and water example Uses Ca ion Soil will become more and more acidic if a liming program is not followed We do not apply lime to soils here in Northern Utah very often… Why?

Lime Requirement In the case of high pH Lowering pH is not usually economical These are generally high in salts Find plants that tolerate high salt levels

Understanding Soil Chemistry Cation Exchange Capacity

Cation Exchange Capacity As soils form Some particles are broken down into extremely small particles Chemical weathering breaks them down until they cannot be seen with the naked eye The smallest are called colloids Colloids are responsible for chemical reactivity in soil Each colloid has a negative(-) charge

Cation Exchange Capacity The neg. charge will attract positively(+) charged particles What does that mean if they run into another negative? Cations: Positive Charge Anions: Negative Charge Colloids act like a magnet and holds cations Other cations can take their place They are exchangeable

Cation Exchange Capacity Can a soil hold more cations than it has colloids? no We need to know our soil’s CEC Higher CEC = more cations can be held Depends on clay and organic matter Lots of clay = higher CEC More organic matter = higher CEC

Cation Exchange Capacity Leaching is reduced by higher CEC Knowing our CEC will help determine how much fertilizer to apply and how often