Land Connections  

This mailer has been provided as an avenue to disperse information pertinent to public agencies and the landscape architecture profession in hopes of fostering greater understanding and collaboration. Topics address issues that affect the built environment within which we live.


Landscape Soil Suitability

"The nation that destroys its soil destroys itself", Franklin D. Roosevelt

A college professor of mine frequently made the comment that "dirt" is found on your shoes and on the kitchen floor, soil is a living complex organism. His comment has some merit as soil can serve many functions, five main functions of which include: a medium for plant growth, a system for water supply and purification, a recycling system for nutrients and organic waste, habitat for soil organisms and a medium for engineering and construction. Each of these functions is vital to sustaining our built environment; however, the focus of this issue is soil's role in plant growth and support.

It is common knowledge that “good” or “fertile” soils are best for growing ornamental and food crops, but what exactly constitutes a “good” or “fertile” soil? The following summary will address basic information related to important soil properties including nutrients, texture, temperature and pH.

Macro and Micronutrients
Many mineral elements are absorbed and metabolized by plants, however, only 18 are essential for successful growth. These 18 essential elements are broken into two categories, (macronutrients and micronutrients), based on the relative amounts needed by plants. Macronutrients
are needed in large amounts (>0.1% of dry plant tissue) and consist of nine elements: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium and Sulfur.  (Commercial fertilizers typically include nitrogen, phosphorus and potassium). Micronutrients are needed in very small amounts (<0.1% of dry plant tissue) and consist of Iron, Manganese, Boron, Zinc, Copper, Chlorine, Cobalt, Molybdenum and Nickel.

Though seemingly unimportant, each of these nutrients plays a vital role in the growth and metabolic processes of a plant. Even a slight deficiency in a micro-nutrient can cause discoloration of leaves, spotted leaves, stunted growth and can even lead to serious plant health issues. 

Soil Texture

Soil is comprised of mineral particles varying greatly in size and material. The proportions of different-sized particles in the soil is referred to as soil texture. Soil texture can be just as important to plant growth as nutrient availability is. There are three main categories of particle sizes in soil: sand, silt and clay. 

Sand particles are between 2.0mm and 0.05 mm in diameter and feel gritty when rubbed between your fingers. These particles have relatively low surface area resulting in less holding capacity for nutrients and water.

Silt particles are between 0.05 mm and 0.002mm in diameter, and feel smooth or silky when rubbed between your fingers. These particles are much smaller than sand and therefore have far more surface area to hold nutrients and water.

Clay particles are smaller than .002 mm in diameter and when rubbed between fingers is plastic and moldable. These particles have very large relative surface areas. In fact one spoonful of clay may have a surface area equivalent to that of a football field11

A fertile soil will have a balanced mix of soil particles consisting of large sand particles, that assist in movement of nutrients, air and water through the soil, silt particles, which store and release nutrients and water, and clay particles, (needed in small amounts) which retain nutrients and water, and buffer soils from changes in soil temperature, pH and from leaching. A well balanced soil is referred to as a loam soil. See this soil chart by the USDA for more information on soil textures.

Soil Temperatures
Soil temperatures play a significant role in the growth of plants. Plants are actually impacted more by soil temperatures than above ground temperatures1. Extreme soil temperatures can limit seed germination, root functions and microbial activity as well as causing availability deficiencies of water and nutrients. Optimal soil temperatures for plant growth range between 50 and 86 degrees Fahrenheit. A well balanced soil structure can help buffer changes in soil temperatures.

Soil pH Balance

pH is the measurement of soil acidity or alkalinity. Plant species tolerance for pH levels can vary considerably, however the effect of pH on overall nutrient availability is perhaps the most important piece of information to know. The figure to the right depicts the effect on nutrient availability at various pH levels. As is illustrated, extremes at either end of the pH spectrum greatly limit nutrients. Most plants are well suited to pH levels ranging between 5.5 and 7.0.

As soils vary greatly from project to project, it is advised that for each project at least one soil sample is obtained and tested for landscape suitability. Each of the items discussed here will be addressed on the lab results with references to existing soil conditions as well as suggestions for soil amendments to improve soil conditions for optimal plant growth.

1. Brady, Nyle C. and Ray R. Weil. Elements of the Nature and Properties of Soils. Prentice Hall Inc. Upper Saddle River, New Jersey. 2000.

O'Dell Engineering

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Author: Chad Kennedy, Landscape Architect

This informational article provided by O'Dell Engineering - 1165 Scenic Drive, Suite A, Modesto CA 95350