Soil Management – Assessment, Modification and Fertilization
Managing trees in urban areas is more than adding fertilizer and pruning. It requires an understanding of soil physical and chemical properties. This information is necessary to develop a plan for ongoing care and management. Available resources, such as certified arborist, professional agronomist, local extension specialist, soil web survey, and soil testing laboratories should be consulted with to develop soil maintenance programs.
Soil modification is the physical or chemical altering of soils to improve conditions for growing plants. Aerating, tilling, composting, mulching, and fertilizing, are a few practical ways to modify soil. Positive actions to modify soil will almost always lead to plant response. Managing trees in urban areas should be thought as a long-term commitment. This may require multiple applications of various fertilizers and organic residues, over many years.
The Natural Resource Conservation Services (NRCS), Soil Web Survey has one of the best resources on soil data, mapping, chemical and physical properties. Professionals use this resource to understand soil characteristics and limitations that may restrict growth. Large stature trees should be planted in areas that are suitable for long-term sustainability. Often tree growth is limited by soil quality and space.
Accredited soil testing laboratories can analyze soil chemical properties and provide detailed information and recommendations for plant nutrition. It is important to consult with a professional agronomist, or certified arborist who specializes in soil and plant nutrition, prior to modifying soils. A standard soil test is used to evaluate soil exchange capacity, pH and nutrient levels. Advance testing, such as saturated paste test is sometimes necessary to determine the availability of nutrients in soil solution.
It is very helpful to have a standard test and a saturated paste test for the same site, especially when dealing with poor site conditions. Landscapes that are highly disturbed, irrigated with well water, or in areas near recent road salt applications, would be candidates for both testing methods.
Interpreting soil test requires training. Some laboratories will provide fertilizer recommendations that may seem excessive. For this reason, it is best to consult with a local practitioner that can interpret the information provided by the laboratory, and make reasonable applications.
The American National Standard Institute (ANSI), A300-Part 2, Soil Management standard is a reference for professionals. An arborist with a good understanding of soil fertility is able to interpret the ANSI standards. It is important to follow the standard while making fertilizer recommendations. For instance, the ANSI standard states that fertilizers with 50% water insoluble nitrogen, should be preferred. Also, fertilizers with less than 50% salt index, should be preferred.
Prior to fertilizing trees, it is important to understand your soils and its limitations. Nutrient deficiencies should be addressed using products that are designed for the specific task. Applications using liquid humates, fluvic acids, seaweed extracts and other trending methods for fertilizing trees should only be used as an additive. In order to carry out biological processes, plants require appropriate amounts of specific nutrients. These trending methods have not been proven to correct nutrient deficiencies.
Always address site conditions that may inhibit plant growth. Tools such as a pneumatic air-spade can reduce soil compaction levels. Adding soil amendments, such as compost and peat moss will improve soil aggregation and biological activity. Mulching soils around trees will reduce further compaction and improve water retention.
It is preferable to use native compost and mulch. Native mulches have been demonstrated to increase populations of beneficial organisms (antagonistic pathogens). Many harmful plant pathogens are considered opportunistic. These beneficial microorganisms found in native mulches are effective in reducing populations of opportunistic pathogens.
Soil web survey can provide information about the soil depth and volume that is not easily seen from above ground. Soil depth may be the limiting factor for growing trees in a certain area. Large stature trees require more soil volume to reach its maximum potential. In shallow soils, where bedrock is within the top 10-15 inches, plant growth is greatly limited. Plant growth is reduced when root development is restricted (e.g., parking islands, street planters).