Ag Instructor Vic Martin: What is a “Good” Soil?

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Great Bend Tribune
Published July 28, 2019

Even through a hot, dry period, the National Drought Monitor indicates no drought conditions in Kansas.  Not even any extremely dry conditions.  Soybeans, corn, and milo are progressing.  A good deal of corn has or is pollinating.  Some milo fields are heading out and beans are blooming.  A timely rain would be nice, as would temperatures staying in the upper 80s to lower 90s during the day and around 70 at night.  It will be interesting to watch the race regarding grain/bean development as we head towards fall.  Many producers are praying for a late frost.  After all the wet weather the past ten months and how it affected field work, planting and crop growth; let’s take a moment to discuss what makes a good crop soil.  At least the Readers’ Digest version.

An ideal soil should provide an appropriate environment for plant growth, The Goldilocks’ zone.  Not too wet.  Not too dry.  Not too hard.  Not too soft.  Not too acid.  Not too basic.  Not too high a nutrient concentration and not too low.  So what do we need?

  • First up is texture, the proportions of sand, silt, and clay.  The ideal soil normally exhibits fairly equally the properties of all three.  Too much clay and soils hold too much water, are prone to compaction when wet, and very hard when dry.  Too much sand and soils hold too little water and nutrients.  Clayey soils tend to have a higher bulk density while sandy soils are lighter and a poorer, less stable structure.
  • Bulk density, the weight of a unit volume of undisturbed soil, should indicate a soil that is approximately half “stuff,” soil particles and organic matter, and half “not stuff,” or porosity.  These pores should range from small to large and be as continuous as possible to facilitate root growth and water movement.  Ideally, pores should be half-filled with water and half-filled with air. 
  • Organic matter also plays a large role in proper water movement, nutrient holding capacity, bulk density, soil structure and microbial activity.  Ideally, a soil organic matter level of approximately ten percent is ideal for this purpose.
  • Soil pH, acidity, matters regarding nutrient levels, microbial activity, soil structure and crop growth.  For most common crops, the ideal pH is a range from 6.2 to a little over 7 (neutral).  Legumes such as alfalfa, soybeans, and peanuts prefer a pH closer to neutral or even a little higher.  Potatoes grow better at lower pH levels.
  • Nutrient levels determine plant growth and microbial activity.  We have discussed macro- and micronutrients before and it is a complex issue.  Too little or too much of a nutrient inhibits crop and nutrient growth.  Excessive levels of certain nutrients can be toxic to plants and other organisms or promote disease pressure.
  • Finally, microbial activity also plays a vital role and is either positive or negative.  Microbes can promote diseases or loss of nutrients such as nitrogen and sulfur under the proper conditions but their overall activity is essential for nutrient cycling, the breakdown of residues into stable organic matter, nitrogen fixation for legumes.  Many plants benefit from mycorrhiza associations that facilitate nutrient uptake.  In fact, some plants can’t survive without this fungal infection.

Next week, how can we work towards this ideal soil.

           

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