Soil and Plant Nutrition

The relationship between soil quality and plant quality
The importance of soil, as a medium for plant growth, is sometimes overlooked. Most evidently seen to serve the purpose of physical support; soil also provides plants with essential nutrients, protection from various toxins, and can function as a moderator of temperature fluctuations. In addition, soil aides in water availability due to its water holding capacity, and plant respiration due to its ability to provide ventilation.

Soil Basics
Physical properties: Soil functionality and manageability is dictated by its physical properties. Primary characteristics include texture, structure and color. Texture (or soil particle size distribution) usually is considered to encompass soil fragments ranging 2mm (sand) to 0.002mm (clay) in size. The middle ground is termed silt and is typically the ideal texture for agricultural conditions. Structure refers to how the soil particles are grouped or aggregated (glossary link) together, defining pore space between particles. Whereas texture remains fairly constant over time, structure is susceptible to alteration depending on any number of activities that may affect the soil at any given time such as drainage, grazing, or tillage. Color can be a good indicator of the amount of soil organic matter (SOM), a crucial component in a given soil; light color indicates low SOM levels and darker color indicates higher SOM levels.

Chemical properties: Soil particles are highly reactive with electrically charged surfaces which serve many purposes such as aggregate stability and more tightly bound binding nutrients among others. Cation exchange capacity (CEC) (glossary link), base saturation (glossary link), and pH (glossary link) buffering are some of the major illustrations of these important chemical properties of soil particles. The exchangeability of positive charges (cations) is an important property that is helpful in classifying soils in more defined terms for the assessment of their fertility and environmental behavior. Increasing base saturation increases the availability of essential nutrients in the soil. Base saturation is determined by the ratio of CEC present in the form of basic cations such as calcium, magnesium potassium and sodium; this is inversely proportional to the acidity of the soil. The pH buffering mechanism of soils is an attempt to correct for limit extremes of over acidity or alkalinity (glossary link) that may be occurring so neither extreme prevails.

SOM: Composed of a variety of carbon containing organic substances, soil organic matter (glossary link) plays a critical role in the CEC, soil water-holding capacity, and global carbon cycle. Although SOM is only a small percent of soil mass, it is largely responsible for aggregate formation and stability as well as a nutrient supply for soil microorganisms. The quality of SOM is crucial to the growth and enhancement of plants and thus illustrates the quality of the soil.

Photo: Topsoil produced from composting.

Nutrients: Essential Elements
Macronutrients: There are 17 macronutrients needed for normal plant growth. Those available from the air and water include carbon, hydrogen and oxygen. Minerals obtained from the soil that are used in large amounts (hence MACROnutrients) include calcium, magnesium, nitrogen, phosphorus, potassium and sulfur.

Read more about Nutrient Topics & Deficiencies from Iowa State University Extension.

Micro or trace nutrients: Other soil minerals that are used in smaller amounts (hence trace or MICROnutrients) include boron, cobalt, copper, chlorine, iron, manganese, molybdenum, and zinc.

Processes or methods of fixation, release, and removal are major influences on the availability of nutrients to plants. Some of these nutrients can be taken in through the leaves, but most are taken up by the roots. The pH of the soil may affect the absorption of nutrients or the level of nutrient toxicity present. Properly balanced nutrients are more likely to produce successful agricultural crops, maintain fertile soil conditions, and promote healthy biological activity.

Soil Testing
What is it? Generally used for agricultural purposes, the testing of soils is meant to evaluate the chemical status of soils and measure the degree of nutrients accessible for plant uptake. Soil organic matter is often what is being tested for, and since the levels present are susceptible to change, regular testing is recommended.

Why is it important? Soil tests usually assess SOM, able to test for base saturation, CEC, and soil pH. With such information, recommendations for soil enhancement can be made more available.

Where do I go for a soil test? Visit A&L Labs in Modesto, CA and Portland, OR or the North Carolina Department of Agriculture & Consumer Services Agronomic Division.

How do I interpret my soil test?

A good report includes samples from various sites on a given plot of land as it is common for soil compositions to change within short distances. In addition, textural classification of the soil being sampled should be known.

Practices
Tillage: Used as a means of manipulating the soil in order to modify conditions, tillage is often used in crop production. Tillage practices can have beneficial effects such as assimilation of organic matter into the soil. Over long periods of time, adverse effects of excessive tillage may occur. Eventually aggregates break down too small, which leads to decreasing pore size and hindrance of water infiltration. An alternative approach is a variety of conservation tillage practices by means of minimum tillage, mulch tillage, no-tillage, ridge till, or strip till practices which may be employed to reduce unfavorable consequences.

For reduced tillage practices, visit: www.reducedtillage.ca

Photo examples of conservation tillage:

A new cotton plant grows from the stubble of a previous crop.
Contrasting low-till and conventional tillage areas

Fertilizers: Application of fertilizers is often used to supply soils with salts that it is otherwise lacking. The most common nutrients used in fertilizers are nitrogen, phosphorus, and potassium. These are the main macronutrients required for plant growth that would normally exist in fertile soils under optimum conditions. Fertilizers must be carefully selected based on individual needs and instructions of applications must be carefully read (some compounds are not compatible with others and should not be mixed).

For information on efficient fertilizer use, visit: www.back-to-basics.net/efu/efu.html

Potential Problems
Leaching (glossary link) of nutrients into the environment can occur from weathering of minerals and natural organic compounds or from synthetic chemicals that have been applied to the soil. Leaching results in loss of nutrients necessary for plant growth. Also leaching can cause the potential consequence of eutrophication in natural lakes, streams and estuaries, in which the accumulation of chemicals in natural bodies of water nutrients leads to anaerobic (glossary link) conditions in which life is unable to exist.

Erosion is the term used to imply the loss of topsoil and degradation of soil quality making the soil less productive. It is of major concern due to the fact that it is a continually increasing process and it is an extremely hard condition to counteract. Human activity has led to land degradation which allows erosion to occur via water flow and wind intensity. The problem will persist unless precautionary steps are taken to prevent future erosion and repair that which has already occurred. For more on soil erosion, visit: www.soilerosion.net

Photo: Severe soil erosion in a wheat field near Washington State University.

Environmental Consequences
With increasing population size there is an increasing need for land use. Activities such as grazing, timber harvesting, and cultivation lead to rapid soil degradation and erosion. Over time, the consequence is less productive soils that are unable to provide necessary benefits for human survival. The greater degree of degradation, the harder it is to perform soil restoration and the greater the spread of desert like conditions of unworkable soil. For these reasons, it is important to practice good soil management and conservation.

Glossary:
Aggregate - numerous soil particles held in a single mass or cluster.

Alkalinity - having the nature of a class of substrates that liberate hydroxide ions in water, pH > 7.

Anaerobic - without air, oxygen-free environment.

Base saturation - total CEC saturated with exchangeable nonacid cations.

Cation exchange capacity - total number of cations a soil can absorb, expressed in centimeters of charge per kilogram (cmolc/kg) of soil.

Leaching - to be removed from the soil by means of liquid percolation.

pH - a logarithm of the reciprocal hydrogen-ion concentration in moles per liter of a solution, a measurement of the degree of acidity/alkalinity.

SOM - (soil organic matter) portion of the soil which contains plant and animal residues at various stages of decomposition, established by the amount of organic material within a soil sample passed through a 2mm sieve.

Sources:
Dixon, J.B., and D.J. Schulze. 2002 Soil Mineralogy with Environmental Applications. (Madison, Wis.: Soil Sci. Soc. Amer.)
Hudson, N. 1995. Soil Conservation, 3rd ed. (Ames, Iowa: Iowa State University Press).
Hughes, H.A. 1980. Conservation Farming. (Moline, III.: John Deere and Company).
McCarthy, D.F. 1993. Essentials of Soil Mechanics and Foundations, 4th ed. (Englewood Cliffs, NJ: Prentice Hall).