• Productive soil depends on its texture, structure and biological properties.salad and vegetable plants growing in allotment image by scalesy from Fotolia.com
  
  Soil is the most important resource in agriculture. Topsoil erosion and the degradation of soil effects 65 percent of the earth, greatly reducing agricultural productivity, according to The Center for Earth Leadership, an organization that trains citizens for leadership roles in the sustainability movement. Topsoil is the uppermost 6- to 10-inch layer of soil. Human life and health depend on productive, fertile topsoil.
  
  

Texture

• Soil texture is determined by the proportional amount of sand and clay that it contains. Sand and clay are made of different mineral particle sizes ranging from very fine (clay) to large (sand and gravel). Sandy soil has large pore space between particles. Pore space is filled with water or air. Clay soil has smaller pore spaces but a larger amount, making it possible for clay soil to absorb and retain more water. Clay soil particles bond tightly together after they have been wet and dried out, making them difficult to work with. Sandy soil drains quickly and does not support plant root structures well. Ideal garden soil contains both soil textures to provide plant support, good drainage and the high mineral content of clay.
  
  

Structure

• "While soil texture is of great importance, the grouping or aggregation of soil particles has a great deal to do with its productivity," according to soil biologist John Howell of the University of Massachusetts Amherst. Soil structure is created by the arrangement of particles and aggregates. Aggregates are clumps of mineral particles held together with organic matter. Soil texture is often described as loose, hard-packed, granular or cloddy. Soil with minimal organic matter content has little structure; soil with adequate organic matter content can be formed into a loose ball when held. The addition of organic matter improves the structure of both loose and hard-packed soil. Earthworms add viscous juice to soil which help bind particles together to improve structure.
  
  

Biological Chemical Properties

• The chemical properties of soil account for much of their differences or similarities. Soil tests analyze the micro- and macro-nutrient content of soil. Nitrogen, phosphorus and potassium are macronutrients that are needed in the largest amounts by growing plants. Micronutrients are essential to the intricate cellular process of transforming soil into usable food plants, but they are needed in smaller amounts. Nitrogen is needed in the largest amount for plant growth and is the chemical most overused in non-organic fertilizers. "Nitrogen over-load" is the term used by the World Resources Institute to describe the pollution problems caused by excessive use of nitrogen-based fertilizers. Adding compost to soil provides a slow, steady release of essential nutrients.