Cooperative Extension Fact Sheet FS848 | December 2014
It is necessary to maintain optimum nutrient levels in the soil or growing media to maximize growth in ornamental plant production. Since plant size along with plant quality dictates sale price, nursery growers certainly recognize this relationship. There is also the need to recognize that adequate moisture levels must be maintained as medium nutrient levels increase to avoid soluble salt injury to the roots. Excessively high soluble salt levels can damage roots, severely restrict plant growth, causing undesirable foliage damage (salt-burn) and possible death of the plants. A systematic salinitymonitoring program is thus required to successfully produce high-quality plants.
Several methods can be used to determine the salt concentration or electrical conductivity (EC) of the soil solution from soil and soilless growing media. Two of these methods are best suited and recommended for EC monitoring in ornamental plant production: the saturated media or paste extract (SME) and the pourthrough (PT) method.
The SME is widely used in university and commercial soil/media testing laboratories. Numerous research studies and fertility trials have aided in developing general interpretation guidelines for growing media and mixes used in ornamental plant production. The general guidelines given in Table 1 can be used in making preliminary judgments of the results obtained with the SME method. Keep in mind, however, that the desirable pH, soluble salt and nutrient levels will vary with plant species and their specific management and cultural practices. For example, it is well known that woody ornamentals are generally more sensitive to salinity than herbaceous plants like perennials. As a rule of thumb, the soil solution EC should not exceed 2 to 3 mmhos/cm (same as mS/cm or dS/m) for most woody ornamentals. However, this level may be too high for some salt-sensitive plants like azaleas.
|Soluble salts (dS/m)*||0–1.0||1.0–2.0||3.0+|
|EC (in dS/m) =||Total soluble salts reading
Although SME is the method most widely used by commercial soil testing laboratories, it poses some difficulties to growers. Achieving the saturation point may be ambiguous and extracting of the soil solution may be cumbersome. Furthermore, the EC reading obtained by the SME method is lower than the environment EC roots experience after irrigation and drainage (called field capacity in regular field soils and container capacity in container media), and even lower than that occurring before irrigation.
This method was developed in the Virginia Polytechnic Institute and State University nursery crops research program by Dr. Robert Wright in the 1980’s. While the SME is applicable to a wide range of plant growing conditions, PT is most applicable to container crops.
The practical advantages of the PT procedure over SME are: (a) no medium is actually handled; (b) there is no danger of rupturing controlled-release fertilizer particles, which may cause high salt readings; (c) no specialized equipment is required for extracting the soil solution; (d) the time required for each extraction is reduced; and (e) plant roots are not disturbed. The results obtained by this method are believed to more accurately reflect the root-zone salt and nutrient conditions at container capacity. However, and contrary to the SME procedure, the current interpretation guidelines for PT (Table 2) have been developed from a limited number of species (like Ilex crenata) and growing media (mainly media based in pine-bark). Caution is recommended when interpreting your results. Refine the guidelines according to your crops and cultural conditions and practices.
|Analysis||Soluble salts (dS/m)||N||P||K (ppm)||Ca||Mg|
|Level in leachate||0.6–2.0||75–100||10–15||30–50||10–15||10–15|
The quality of irrigation water plays a significant role in the production of ornamental crops, determining which crops, if any, can be grown. It also determines how irrigation and other cultural practices must be managed. Thus, in most instances, testing and evaluating of irrigation water should precede any other considerations when planning ornamental crop production or purchasing agricultural land for this purpose. Of all the parameters listed in an analysis of water for irrigation purposes, salinity is the main criterion to be considered. An ideal irrigation water for ornamental crops should be less than 0.5 mmhos/cm. Waters with EC readings higher than 0.75 mmhos/cm have been shown to present increasing salinity problems. Only close monitoring and careful management will prevent salt accumulation when high-EC waters are used for irrigation.
The highest concentration of salts, expressed in electrical conductivity (EC) units, is normally present before a plant is irrigated. The resident soil solution is displaced down the container or soil profile by incoming irrigation water or liquid feed. Reduction of the soil solution salinity is, however, dependent on the irrigation water EC and the volume applied. To prevent and control salinity buildups when using typical irrigation waters or nutrient solutions, it is recommended to apply enough water to produce a leaching fraction of 10-30%. The leaching fraction (LF) is the percentage of applied water that must leave the root zone or container to accomplish the degree of leaching desired.
|Leaching Fraction =||Volume Leached||X 100|
When the level of soluble salts becomes excessive, higher leaching fractions must be produced. For container growing media, leaching with one container volume of good quality water (low EC) will remove most of the soluble salts in the resident soil solution. At higher soluble salt levels, an additional leaching cycle should take place 24 hours later to remove additional soluble salts that have entered the solution.
Other measures to help control and/or maintain soluble salt levels are: (a) keep adequate moisture in the growing medium, (b) avoid applications of dry fertilizer or highly concentrated nutrient solutions to a dry growing medium, (c) avoid fertilizers that give a high salt stress for a given amount of nutrient (high salt index), and (d) be alert to changes in environmental conditions (like temperature and humidity) that affect plant transpiration, soil water evaporation, and nutrient release from slow or controlled-release fertilizers.
The following articles go into additional detail on attributes and procedures for use of the PT testing system.
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