Plant Nutrition of Greenhouse Crops
Monitoring and Managing Soluble Salts
The most common soluble salts in soils are the cations calcium (Ca+2), magnesium (Mg+2), and sodium (Na+) and the anions chloride (Cl-), sulfate (SO4-2), and bicarbonate (HCO3-). Smaller quantities of potassium (K+), ammonium (NH4+), nitrate (NO3-), and carbonate (CO3-2) are also found in most soils. Sources of soluble salts in growing media include salts from pre-plant amendments, water-soluble fertilizers, controlled-release fertilizers, irrigation water, residual fertilizer, media components, and compounds resulting from microbial decomposition of organic matter. Some fungicide products applied as a media drench also contribute to soluble salts. The usual method to quantify the soluble salts concentration in growing media is to measure the electrical conductivity (EC) of either the media solution or a media-water extract. Electrical conductivity refers to the ability of a material or solution to conduct an electrical current. As soluble salts increase in the soil, the soil solution becomes a better conductor of electricity and EC increases. For growers, it is a fairly reliable measure of the media’s fertility status as long as the major source of salts is from the fertility program and not other sources, such as the water supply. EC measurement does not differentiate between individual nutrients (nitrogen, phosphorous, potassium and so on), but simply provides the sum total of all salt content. Also, EC measurements cannot determine whether one macro or micronutrient is being absorbed at a higher rate than another. Measuring the EC of the growing media allows the grower to gauge the nutrient needs of the crop. For instance, if the EC value is high in the substrate, there is no need for further fertilization, but if it is too high, then appropriate management measures need to be taken to reduce the soluble salts. Likewise, if the reading is low, this is an indicator that the crop needs some supplementation of nutrients to avoid nutrient deficiencies as previously discussed.
Units of Measure
EC measurement does not differentiate between individual nutrients (nitrogen, phosphorous, potassium and so on), but simply provides the sum total of all salt content. A laboratory soil analysis is required to quantify nutrients.
Action of Salt Injury
Salt injury usually occurs as osmosis. The strength of the salt solution in the soil is greater than that of the solution within the plant root cells. As a result, the water in the root cells is pulled out of the cells by a force exerted by the salt solution. This causes the cell contents to pull away from the wall and collapse. This condition is called plasmolysis. When plasmolysis takes place in a large number of cells, physiological drought sets in and the root suffers from excess water loss. A plant’s sensitivity to salt is highly variable—some are very sensitive, while others are very tolerant of salt.
Many growers only check soluble salts when a problem is perceived. However, scheduled sampling and measurement of soluble salts allow the grower to track trends and make corrections before problems arise. Target ranges for EC for individual crops can be found in trade literature or on the World Wide Web.
Managing Soluble Salts
It is necessary to maintain optimum nutrient levels in the growing media to maximize growth in greenhouse crops. Excessively high soluble salt levels can damage roots, severely restrict plant growth, causing undesirable foliage damage (salt-burn) and possible death of the plants. Managing soluble salts involves an integrated approach to production. The leaching fraction is the most effective method in controlling salt buildup but other measures can be used to control and/or maintain soluble salt levels too, which include: 1) growing media used, 2) soil amendments, 3) maintaining adequate moisture in the growing media, 4) water quality, 5) fertility regime, and 5) plant tolerance.
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 to 30 percent. 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.
Good soil drainage helps to control soluble salt concentrations. Growing media should contain a substantial quantity of large pores to facilitate good drainage.
In some greenhouse soils, some improvement in drainage may be obtained by adding gypsum to the soil.
Maintaining Adequate Media Moisture
Since the concentration of soluble salts in plant tissues increases as moisture levels decrease, it is important to monitor the water content of the growing media.
The quality of irrigation water plays a significant role in the production of greenhouse crops. It also determines how irrigation and other cultural practices must be managed.
Avoid applications of dry fertilizer or highly concentrated nutrient solutions to a dry growing medium and avoid fertilizers that give a high salt stress for a given amount of nutrient (high salt index). Water-soluble fertilizers vary in how much they affect soluble salts levels in the media.
Perhaps the most effective means of managing soluble salts is to avoid producing salt sensitive plants.
Click on the following topics for more information on plant nutrition of greenhouse crops.