Fertilization

Plant nutrition is necessary for production of quality plants. Many components must be considered when developing nursery fertilization plans. If not properly considered, environmental quality, plant quality, and production efficiency may be sacrificed. The important components surrounding plant nutrition are as follows:

  1. Fertilizer application
  2. Nutrient monitoring
  3. Record keeping
  4. Irrigation
  5. Container substrates

Fertilizer Application

Nutrients may be applied to plants in a container nursery using several different strategies. The diversity of application methods allows growers to tailor application to the needs of their particular growing plan.

Fertilizers can be incorporated into growing media prior to planting, or they can be applied post-planting.

Pre-plant application

controlled release osmocote

Fertilizers may be applied as amendments to the container substrate prior to planting. This can be done to correct pH, to supply micronutrients, or to supply macronutrients.

To correct pH

Where substrate pH may be excessively low for the crop in question, dolomite limestone can be mixed with the growing substrate. This will raise the pH while providing calcium and magnesium to the growing plants. Limestone application should be done on a crop specific basis depending upon the pH preferences of the individual crop. A mixture of 4-6 pounds of Dolomite limestone per cubic yard of substrate is typically sufficient for plants that require substrate liming.

Where substrate pH may be excessively high, iron sulfate can be applied. This application serves to provide iron that is often unavailable at elevated pH while also lowering the pH. If iron is not an issue, then granulated sulfur can be used to lower substrate pH.

To supply micronutrients

Micronutrients are important for plant growth, but as their name suggests, they are required in exceedingly small amounts. A single application of commercial micronutrient fertilizers is typically sufficient for two seasons of growth. Micronutrients can also be applied as components of other controlled-release fertilizers. Composted yard waste, hardwood bark, or composted biosolids can provide micronutrients where they make up more than 10% of the substrate volume.

To supply macronutrients

Nitrogen, potassium, and phosphorous can be supplied before planting as a controlled-release media amendment. Additionally, Calcium, Magnesium, and Sulfur can be applied pre-planting. Care must be taken as most forms of these nutrients can alter substrate pH.

Post plant application

Typical production cycles involve one or several applications of fertilizer during the growing season.

Controlled-release fertilizers

These fertilizers are capable of providing nutrients over periods of several months. Controlled-release products are available many different forms with respect to types of nutrients, forms of nutrients, rate of release, etc. As a result, care must be taken to apply the proper product. Release rates are often temperature dependent making them suitable for specific regions. Irrigation rates can also influence release rates.

Fertigationsoluble fertilizer

Fertilizer application through irrigation can be an effective method. Water soluble fertilizers typically appear blue (like the picture at right) or white. In the event that this system type is utilized, steps must be taken to prevent nutrient rich runoff from flowing into nearby rivers and streams. Runoff waters must be processed to remove nutrients before they can be allowed to leave the property. Constructed wetlands, like those detailed in the constructed wetlands portion of this website, can be used to treat these runoff waters.

Nutrient Monitoring

A program for monitoring container nutrient status is a must for efficient fertilizer usage. Monitoring prevents both under-application and over-application of fertilizer. Both extremes can result in plant and/or environmental damage. Also, fertility issues can indicate excessive irrigation or inadequate substrate nutrient holding capacity.

EC (Electrical Conductivity) of substrate leachate is generally used as an indicator of container nutrient status. High EC values indicate a danger of salt stress while low EC values indicate exhaustion of applied fertilizer. The pH of leachate should be regularly monitored so that extremes in acidity and alkalinity can be avoided. Monthly or biweekly EC and pH monitoring is usually sufficient to track nutrient and pH levels. Optimal EC and pH values vary with crop. Typically, however, EC values should range  1.0-1.5 mmhos/cm while pH values should range 4.5-6.5. Always be sure to test a sample of irrigation water to check for water quality issues.

When monitoring, samples should be collected from 3-4 pots in a section of the production area. Solution can be extracted from the container by either collecting container leachate using the pour-through method or using a suction lysometer. In the pour through method, pots are irrigated, and the water that flows from the pot bottom is collected. Suction lysometers are cylindrical probes that are imbedded in the substrate. When a vacuum is applied, solution is withdrawn. EC tests are conducted on samples collected. Elemental analyses for actual nitrogen, phosphorous, potassium, calcium, magnesium, etc. concentrations) can be conducted by a laboratory like the Agricultural Service Laboratory (email:agsrvlb@clemson.edu) at Clemson or similar labs operated by other universities and extension agencies.

Record Keeping

Organized records of fertilizer applications, substrate pH, substrate EC, irrigation water pH, irrigation water EC, irrigation events, and substrate types can help growers enhance plant quality and environmental stewardship while minimizing costs and maximizing profits. Water quality issues can be recognized and diagnosed quickly, and fertilizer rates, times and formulations can be optimized for each individual crop. Substrate types can be adjusted to minimize loss of fertilizer. Irrigation events can be adjusted to prevent drought and salt stress while minimizing nutrient loss. As a result of the many dividends of record keeping, all growers should keep thorough, accurate records of all of these operations.

Interactions between Irrigation, Container Substrates, and Fertilizers

Irrigation

The volume and frequency of irrigation can significantly impact plant nutrient status. Excessive irrigation can wash nutrients from plant containers. This deprives plants of needed fertilizers while potentially leading to environmental issues. Insufficient irrigation can result in drought stress or salt stress. Drought stress, if caught quickly, can be overcome by a return to adequate irrigation. Salt stress (the buildup of salts in the container substrate) can be overcome by rinsing the salts from the container by a single large irrigation event that leaches salts from the substrate. Salt stress may result from over-fertilization, insufficient irrigation, or high salt in irrigation waters. Substrate drainage and water holding capacity can impact occurrence and severity of salt stress as well. More information on irrigation can be found in the irrigation section of this website.

Container Substrates

As mentioned in the container substrates section of this website, each substrate has different characteristics. There is a constant interaction between substrates, irrigation waters, and plant nutrients. Substrates must have sufficient CEC (cation exchange capacity) to hold nutrients while having proper pore space to hold both water and air in the pot. Substrates with high CECs retain large amounts of nutrients attached to the substrate particles while low CEC substrates allow rapid nutrient leaching. Proper media selection is critical to quality plant production, and must be carefully considered as part of all plant nutrition plans.