for Improving Soil Fertility & Crop

Dr. Allen L. Stout

What makes a soil fertile and productive? According to noted soil scientists, "the fertility of the soil is in direct proportion to the number and activity of soil microorganisms" (Dr. Stanley Wedberg). And, "the humus content, plus active microorganisms is equivalent to a high degree of fertility" (Dr. S.A. Waksman).

Soils contain anywhere from practically none up to 0.5% by weight of living organisms in the top 6 inches or foot. This includes hundreds of types and varieties of organisms such as algae, fungi, bacteria, actinomycetes and larger organisms such as earthworms, etc. What can this small percentage of the soils composition do for soil fertility and crop production?

Beneficial Functions of Soil Organisms

Scientists have discovered that soil organisms perform a number of important functions essential for good crop production including:

Proper soil management is important for maintaining beneficial biological activity and functions in the soil. As a result of the wide-spread use of chemical fertilizers and pesticides for many years, the importance of humus and the biological and biochemical functions in the soil have been virtually ignored. Over the years, beneficial biological factors have declined in most soils to the point that the condition of soils has deteriorated considerably and the effectiveness and efficiency of fertilizers and other chemicals have also declined. In many cases residues won't decompose and have become a trash problem harboring insects and disease organiams and act as a barrier to water perculation.

Management Practices to Enhance Beneficial Soil Biological Activity

Management practices to restore and maintain beneficial biological activity and functions include:

  1. Apply manures, Composts, crop residues and crop rotations if necessary to feed the biological system.

  2. Correct soil mineral deficiencies and imbalances. Calcium is especially important in correcting pH and cation balance. A proper chemical balance will help release many nutrients and prevent toxic conditions.

  3. Proper tillage is important to aerate the soil and allow proper water movement. Surface crusts should be broken up, subsoiling may be necessary to break hard pans, residues should be incorporated in the surface to increase water intake and stop erosion.

  4. Biological seed inoculates, such as varieties of Rhizobia for legumes and Mycorrhiza, and other soil inoculates such as Azotobacter may be beneficial to restore potent beneficial types of organisms that may have degenerated in poorly managed soils. Inoculations may need to be repeated periodically until the soil is brought up to a fertility level that will support the more potent beneficial types of organisms.

  5. Many soil organisms go dormant when soil conditions are unsuitable or toxic. Some microbial bio-catalyst products(bio-activtors or bio-stimulants containing vitamins, hormones, enzymes, co-enzymes and other biological factors) have been proven to be higthly effective in promote beneficial biological multiplication and activity to help restore residue decomposition and improve soil tilth and fertility, especially when used as part of a balanced total soil management program.

Under ideal conditions microorganisms multiply at a rapid rate. A few ounces of an inoculant may multiply to several tons of active living organisms in a short time.

Microorganisms have first choice for available nutrients in the soil. In some soils, microbes may be limited by nutrient deficiencies or they may temporarily tie up deficient nutrients resulting in crop deficiencies. Soil and plant testing and a good fertilizer program are important to correct deficiencies to gain the maximum benefits from the biological factors.

The crop and its record of productivity on a specific field is the best test of soil fertility. The ability of a soil to grow a good crop cannot be tested accurately in the lab. Every crop has specific requirements and a built-in genetic potential for production. This potential is seldom reached because of limiting factors. To achieve maximum production, crops must have the right balance of readily available minerals, water, carbon dioxide, nitrogen, oxygen, various organic substances, light, proper temperature and suitable environmental conditions, and all these at the right times. Plant requirements change through­out its growth cycle from germination to maturity.

More than 16 e1ements are presently known to be essential for plant growth, and the importance of others is beginning to be recognized. Not all elements are required by all plants, but all are necessary to some and many are essential for the biol­ogical functions in the soil. Carbon hydrogen and oxygen are derived from air and water; nitrogen from air and soil organic matter; and phosphorus, potassium, sulfur, calcium, magnesium, sodium, iron, manganese, boron, copper, cobalt, zinc, molybdenum, vanadium, chlorine and silicon from soil min­erals and organic matter. Plants also contain iodine and selenium, elements essential to animals, and many other elements whose functions are not totally known. Approximately 95% of these plant nutrients come directly from air and water. Another 1 to 2% (nitrogen) comes from air and soil organic matter or fertilizers. The remaining 3 to 4% (minerals) come from the available mineral and organic matter reserves of the soil and fertilizers.

Ironically, more fertilizers are being applied to the land than ever before, but with less results and at greater cost and increasing problems. An important reason is because of improper and imbalanced fertilizer treatments without regard for the biological functions in the soil.

The biological balance in the soil may be altered considerably by the fertilizer and management program. An imbalanced fertilizer program may alter the types of microbes that predominate and counteract much of what the grower is trying to accomplish. Many beneficial organisms disappear while others that flourish attack the plants or eliminate nitrogen (denitrifying bacteria) wasting costly nitrogen fertilizers and using up soil humus in the process and causing soil structure to deteriorate.

A total fertilizer management program should include proper tillage to aid the Physical condition of the soil, a balanced fertilizer program to maintain Chemical balance and a good Biological program to maintain the biological functions in the soil.

Analysis & Recommendations for Soil Physical, Chemical & Biological Conditions

Physical Condition:

Key Soil Components

The Texture, Tilth and Moisture conmdition of the soil is observed in the field while taking soil samples to determine physical problems such as hard pan, poor tilth, poor drainage, etc.

The Cation Exchange Capacity (CEC) and soil pH are analyzed in the lab. CEC is determined by the amount and type of clay and humus in the soil and is measured as milliequivalents/100 grams of soil. CEC is the capacity of soil particles (colloids) to loosely hold the cation minerals and exchange them with the plant roots for hydrogen. The pH of the soil is the acid or alkaline reaction of the soil. The Lime pH is a test used to more accurately determine the soils lime requirements.

Proper tillage practices, use of Soil Conditioners including organic matter or compost, and crop rotations are recommended practices to improve the physical condition of the soil.

Organic & Biological Condition:

Organic Matter (OM) or humus is the soil reservoir for most of the soil nitrogen, sulfur and much of the phosphorus and trace minerals. One percent OM in the top 6 inches of an acre of soil equals about 20,000 lbs of humus. A level of 2% or higher is desirable to maintain adequate soil life, good soil structure and optimum availability of minerals. The ENR is the estimated nitrogen release expected from the breakdown of soil OM and available to the crop during the crop season. Crop residues are undecomposed remains of plants that do not show up on the OM test.

The Biological Activity of the soil is observed in the field by the odor, appearance of earthwroms and beneficial microflora or by root diseases indicating parasitic and pathogenic organisms. An unhealthy or dead soil may become odorless or foul smelling, have poor tilth, poor decomposition of crop residues and loss or tieup of nitrogen and other nutrients in the soil.

Correction of low OM will require rotations with high residue crops, growing of green manure crops or additions of animal manures or compost. Improvement of beneficial soil biological activity will include the above and possibly soil biologicals and may need correction of the soil mineral balance and possibly some tillage to improve soil aeration. Improving beneficial soil biology will improve decomposition of crop residues, improve seed germination and survival, crop growth and vigor, fixation of atmospheric nitrogen, release of nitrogen from soil OM and increas availability of soil minerals.


A chemical soil analysis may give soil anions as available Nitrogen (N) from soil nitrates, ammonia and/or ENR from OM; Sulfur as available sulfates or estimated sulfur released from OM; and available Phosphorus as Phosphate (P2O5) in parts per million (ppm) or lbs per acre. Soil exchangable Cations may be given as Calcium, (Ca), Magnesium (Mg), Potassium (K) or as K2O (Potash), Sodium (Na) in ppm, lbs/acre and as Percent of Base Saturation of the CEC to indicate the relative balance of the Cations. An optimum range of Base Saturation for Ca is 60-75%, Mg 10-15%, K 2-5%, and Na <2%.

As part of a good total soil management program, the biological factors can greatly reduce fertilizer needs and costs, reduce tillage problems and competition from weeds, insects and diseases and increase production and the quality of crops. The end results are better soils, better crops, better nutrition of animals and humans and more net profit.

Copyright © 1979-2010 Allen Stout, Serf Publishing, Inc. ; Updated 11/5/2010