Soil, Organic Matter, Microorganisms and Crop Production

Soil organic matter is essential to productive soils. Organic matter addition addresses some of the soil physical properties discussed in Lecture 4 as well as providing nutrients to the crop plants that grow in these soils.  

II. Learning Objectives
 

	To learn the relationship between organic matter and soil fertility.
	To understand the relationship between organic matter and climate.
	To be able to suggest ways to maintain organic matter in soils.
	To understand the role of microorganisms in this dynamic organic matter/soil continuum.

III. Overview

Soil organic matter (O.M.) is to soils what a blood pressure is to humans. O. M. level indicates the health of the system. With tillage and crop production, organic matter declines. This is influenced directly by temperature, so soils in warm climates loose OM even faster with tillage.

Soil organic matter addresses soil texture by making a sandy soil hold more water if OM is high, and loosens a tight (clay) soil allowing more air and water movement. Soil OM provides nutrients slowly by microbial release, prevents the loss of nutrients (especially nitrogen) in a leaching environment. So, soil OM addresses both the physical and chemical needs of crop production.

The problem is, with continuous tillage, especially in warm climates, keeping OM levels up is nearly impossible. Unfortunately, many of the world's poorest nations are in the tropics and subtropics, and are faced with increasing population and decreasing food production. The best source of fertility for crops in this zone is not expensive chemical fertilizers (that are not available or beyond reach of a farmer's pocketbook), but the use of OM to recycle nutrients, fix atmospheric nitrogen and deliver nutrients to plants throughout the growing season. This lecture deals with one of the most important components of sustainable cropping systems worldwide. OM offers one of the best fixes for world food supply and hunger.
 

IV. Introduction
 

  Soil organic matter is essential to productive soils.

  It promotes desirable physical and chemical properties.

  It is a life base of a diverse population of microorganisms.

  It is a storehouse of many plant nutrients.

  Organic matter includes

• Products of intermediate decomposition
• Finely divided residual compounds resistant to decay

Two primary  Improvement of physical properties of soil Enhancement of microbial activities

CONCEPT  Soil organic matter is one of the best indications os good soil tilth (nutrition and structure).

• Surface mulches control soil erosion
• Reduce evaporation loss
• Stubble mulching

• Almost all N in non-fertilized soils tied up in O. M.
• Phosphorus (P) commonly present (20-50%) but can be as high as 70%
• Plant nutrient elements contained in O. M.
• Potassium (K), Calcium (Ca), Magnesium (Mg), Copper (Cu), Zinc (Zn), Manganese (Mn), and Iron (Fe) associated with O. M. complex
• Decomposition makes mineral elements available to plants
• Farm operators must manage this
• Maintain good supply of soil moisture
• Provide drainage
• Replenish O. M. supply of soil moisture.
• Soil humus has a high capacity to absorb and exchange cat ions
• Increases with pH
• In oxidized tropical soils, having primarily kaolinite clay with low CEC, most exchangeable cat ions on O. M. fraction
• Products of O. M. decomposition hold soil particles together in aggregates
• Maintaining soils in good physical condition is enhanced by periodic additions of O. M.
• Addition of decomposable O. M. to soils in poor physical condition results in slight improvements
• Most desirable granular structure are usually found in soils just plowed out of grass covers
• Other aggregating forces include
• Moldboard plow
• Wetting and drying
• Freezing and thawing
• Growth promoting substances have been identified
• Auxin
• Gibberellins
• IAA
• Under some conditions, these may stimulate root development
• Phytotoxins - certain crop residues injure germination and growth of crops
• Wheat straw retards corn

	When virgin soil placed under cultivation - rapid decline in O. M.
	Virgin soil of arid regions typically low in O. M. and nitrogen.
	The O. M. content of soils is the balance between rates of addition and rates of decomposition.
	O. M. retention in soils is influenced by both the quantity and type of clay present.  Soil humus adsorbed most by montmorillonite, less by illite, and least by kaolinite types.
	Even productive soils with good tilth benefit from frequent additions of O. M.

• Crop residues
• Green manures
• Farmyard manure
• Composts
• Urban and industrial wastes

• Most important source of O. M.
• Root systems of many grasses and some legume crops used for forage or grazing are substantial

• Green manure crops are grown primarily when they can be fitted into a cropping pattern without interfering with production of regularly harvested crops
• Best results are usually obtained from the use of leguminous crops

• Where animals grazing, manure directly voided on soil, nutrients well conserved, but poor distribution - inefficient
• Where the animal litter allowed to accumulate, partially decompose and dry. Leaching/large losses of nutrients.
• Value of farmyard manure (cattle)
• 5 kg N, 2.5 kg P, 5 kg K/ton
• 40 cents/kg/ 35 cents /kg, 18 cents/kg/$3.00/ton
• Poultry manure 2x
• In some tropical areas, primary benefits are from P
• Many operators of commercial hog and beef farms consider manure too low in nutrients to justify hauling and spreading
• Cultivated soils usually best place for manure disposal
• Increased yields will at least repay costs of handling

• U. S. produces 6 million tons of sewage sludge/yr on a dry weight basis
• About 20% goes on land
• Nitrogen (N) and Potassium (P) are principle nutrients, present in substantial amounts
• Heavy metals, Zinc (Zn), Lead (Pb), Cadmium (Cd), Copper (Cu), and Nickel (Ni) present at hazardous levels
• Unless injected in soil or worked under soon after application, much ammonia N lost to volatilization
• Under good management, about 12% is lost
• N commonly limits annual application rate of sludge, metal content will determine the length of time a given area can receive sludge
• Sewage sludge have proved effective in reclaiming such land types as
• Mine spoil banks
• Dune and dredged sand
• Abandoned garbage dumps/landfills

	Diverse population of microorganisms in most productive soils totals several hundred million/gram.
	Microbial processes include  O. M. decomposition  Conversion of elements to available form Nitrogen fixation	CONCEPT  To maintain O.M., water and air are critical factors that can limit functions of microorganisms. This is the relationship between soil texture and structure and organic matter.

  Organic matter decomposition

• A modest rate of decomposition is desirable
• These conditions for modest rate average
• Available moisture
• Well aerated soil
• Ample supply of available plant nutrients
• A warm temperature (20-35 degree C)
• Soils should be well drained
• Maintaining a favorable C:N ratio
• If C:N ratio is greater than 20:1, microorganisms unable to obtain sufficient N and must draw on supplies of N in environment
• Crop plants fare badly in such competition
• Straw from cereal crops, stalks of corn and millet have C:N ratio ranging from 40:1-80:1.
• Additional N should be added to these applications of O. M. before crop is planted.

• Changes in form of soil N and the activities of microorganisms are important in farm management

Ammonification: Ammonia formed as a by-product of O. M. decomposition as C:N ratio approaches 10:1 ammonia near surface may be volatilized

Nitrification: Primary agents, nitrosomonas bacteria that oxidize ammonia to nitrite and nitrobacter group that oxidizes nitrite to nitrate - collectively called nitrifying bacteria.

Denitrification: Biological reduction of nitrate of nitrite to gaseous N. Most Important cause of loss of available N in soils. O₂ in nitrate and nitrite used by by bacteria in anaerobic conditions. Nitrogen released as N gas or nitrous oxide.

Water may leach nitrite or nitrate ions from upper well aerated parts of soil or from a pile of manure to lower moist, anaerobic area. Then denitrification occurs.

  Nitrogen fixation.

• Microorganisms can fix atmospheric nitrogen
• Inoculation and growth of leguminous plants has been major practical use of this
• Recently the discovery that nitrogen may be fixed by certain bacteria living in close association with the roots of a number of tropical grasses has attracted attention
• Nitrogen fixing microorganisms
• Azotobacter
• Beijerinkia
• Clostridium
• Blue-green algae
• Rhizobium (in nodules on roots of leguminous plants)
 

CONCEPT  Nitrogen is the most critical element limiting high production in crops.  Nitrogen fixation might be the only way to add Nitrogen to soils in developing countries.

     

Organic matter makes up the nutrient source for all the living organisms in soil. The complex of microorganisms that are responsible for the decomposition, nitrification, and nitrogen fixation in the soil surface are essential components of whole cropping systems. This organic matter is also the food for earthworms that are responsible for aeration and with their waste, contribute to the overall chemical and physical properties of the soil complex. Decomposition of organic matter can make up a significant portion of the nutrients available in many infertile soils. Organic matter also provides beneficial physical influences, reducing erosion, evaporation, and improving water-holding capacity. Organic matter management is an important component of a agroecosystem. Crop residues, green manures, animal waste, compost and urban industrial wastes can all be organic matter sources.

  

VI. Self Assessment
 

	What are the physical benefits of OM?
	What are the chemical benefits of OM?
	What materials can be used to maintain OM in soils?
	Explain the role of microorganisms in OM decomposition.
	What is the relation between OM and temperature?
	How does OM relate to the problems and potentials of food production in the developing world?