Characteristics of Soil

Let Nature Do It

by Randy Scott, Consulting Rosarian

Woodbridge, VA



Soil is made up of varying ratios of minerals, air, water and organic material.  Soil is healthy if it consists of roughly 40% mineral, 23% water, 23% air, 6% organic material and 8% living organisms.


Soil texture is concerned with the relative proportions of mineral particles of various sizes in a given soil.  These particles are grouped into 3 basic categories:  sand, silt and clay.  Sand particles are the largest ones in soil other than gravel or other rocks.  Intermediate sized particles are called silt.  The very smallest particles in soil are clay.


Heavy and light are commonly used to describe soils.  They refer to the easy of tillage, and not to soil weight.  Heavy soils are commonly finer soils, which require more horsepower because the higher clay content makes them more sticky.  Light soils have a higher percentage of sand (thus coarse textures), stick together less, and require less “muscle” to till.


The rate of water percolation is another way to describe the texture of soils.  Soils percolate water at different rates.  Soil should be watered only as much and as fast as the soil can absorb without runoff.  Sandy soil absorbs more than two inches of water per hour.  It as very porous.  Loam soils absorbs from 0.25 to 2 inches per hour.  The soil is loose and porous but holds water quite well.  Clay soil absorbs less than 0.25 inches of water per hour.  Clay soil is dense with few air spaces between particles and holds water so tightly that little water is available to the plants.  This very dense soil also prevents oxygen from reaching plant roots.


As if it’s not enough to worry about the structure of a soil and its fertility, there other factors that have an influence on both of these qualities and upon the type of organisms that develop in the soil.  The two other factors are acidity and alkalinity, measured by pH readings, which are also necessary to consider.  The breakdown, intake and utilization of minerals in plants are essentially chemical and biological reactions.  Most reactions in living systems take place at a specific pH.  Roses and many other plants do well when the pH is between 6.4 and 7, just a shade on the acid side.  Most eastern soils are usually acidic.  In many Virginia locations the pH is as low as 5.0.  Although in some mountain regions there are pockets of soil that have a limestone base and can have a pH that is slightly basic.  So to achieve an optimum range regular adjustment is required.  A soil analysis is good to have every several years to help guide your fertilization schedule.  This will also include a pH test and an organic content measurement.  Lime is often added to clay soil to counter its natural acidity.  As a general rule apply 5 pounds of limestone per 100 square feet.  Dolomitic lime is also a favorite because it also contains magnesium.  To determine the effect of the application it is wise to have a pH meter of your own to monitor the pH of your garden.  Regions with high pH will need to lower it.  Sulfur compounds are generally used to achieve this at a rate of 1.6 pounds per 100 square feet.


What is the best way to improve the soil in your garden?  The most important thing we can do to improve the soil is to begin restoring the organic content.  Most of us do not have the benefit of living on land that has remained at its natural level.  In developed areas all the topsoil has been removed during the home building process leaving little more than a compact, sterile subsoil with no organic content in which to grow our roses.  An organic content of 5% is considered a minimum acceptable organic content in soils.  It is the organic material in the soil that provides a repository for nutrients as well as a home to many beneficial organisms that help to decompose dead organic matter, make nutrients available to plants, provide aeration, regulate moisture retention, soil temperature, and provide a system of checks and balances between the harmful and beneficial components of the soil.  Most soils contain less than the minimum 5% organic matter.  Since it is a proven fact that insects, weeds and diseases are much less of a problem in healthy soil, it should be obvious that amending the soil with good, organic material is a necessity.


Since soil conditioning materials and organic fertilizers are slow working in general, they should be mixed into the soil at least a month ahead of planting or transplanting.  To help existing plants, organic materials may be worked into the soil between plants at any time or added as a side dressing.  As the amendments decompose their benefits will be realized.  You can repeat this as decomposition occurs.


What are some of the best organic soil conditioning materials?  Start by establishing a compost pile.  Most anything organic can be added:  grass cuttings, leaves, straw, pine needles, shrub clippings and coffee grounds.  Save your kitchen disposal by saving your kitchen scraps for the compost pile, but avoid any animal scraps or grease and oils that can attract pests.  Almost any plant material is suitable.  This provides a simple system for you to have a great garden.  One of the top rose exhibitors in the country went to this system years ago.  His garden consistently produces prize-winning roses using only compost and basic 5-10-5 or 10-10-10 fertilizers.  Where animal manures are available, they are probably the best source of fertilizer and organic matter for the rose garden.  Use manures that have been composted so any weed material will not germinate.  Manures vary greatly in their content of nutrients.  Their composition varies according to type, age, and condition of the animal, kind of feed used, degree of decomposition, moisture content and the amount of bedding litter.  They may be as high as 4.35% N, 2% P and 2% K in some cases.  Some of the best features of animal manures are that they provide most of the micronutrients needed and help to establish biological activity from the microorganisms in the manure.  Processed poultry manure is not likely to provide these microorganisms, but any composted manures such as horse, cow, sheep and poultry will.


Some non-composted amendments may not immediately affect any biological processes but may be a great amendment to change the physical character of your soil.  Agricultural “Perlite”, peat moss, and super fine hardwood mulch work wonders to break up clay soil.  Adding sand to clay soil is generally thought to be a questionable practice.  Sand combined with many of the minerals in the clay is likely to make it more like cement.  Improve your roses by starting at the soil level.  Give nature a little help through the use of manures and composting amendments.