How You Can Use Compost As A Substrate Additive

How You Can Use Compost As A Substrate Additive

Longwood Gardens, an 1,100-acre display garden in Kennett Square, Pa., has a compost production facility that produces more than 2,500 cubic yards of quality compost every year. This compost is used in a variety of ways at the gardens, including greenhouse production.

Using compost as a substrate additive for greenhouse crops has several benefits. First, most compost is locally produced from waste products, making it one of the most sustainable substrate ingredients. This feature can also be used as a marketing tool to let consumers know they are buying a “green” product.

Properly produced compost also contains large amounts of beneficial microorganisms and has been shown to provide some level of disease suppression. The beneficial microorganisms out-compete the pathogenic microorganisms, making it more difficult to establish themselves in the substrate and on the roots of growing plants. Additionally, compost typically contains a nutrient charge that can offset some fertilizer costs.

Compost Issues

Although compost has several benefits, there are also several negative issues that can be associated with compost, which include phytotoxicity, lack of uniformity, particulate contaminates and availability. Potential toxins in compost include herbicides, heavy metals and salts. If you are producing your own compost, monitoring feedstocks is essential to reduce potential toxins. One example of a feedstock not suited for compost production is turf grass clippings containing herbicide residue. Unwanted particulate contaminates, such as stones or trash, can typically be removed with screening. Issues with uniformity and availability tend to be regional and also need to be considered. Someone considering purchasing compost as a substrate additive should be sure the properties of the finished product fall within the ranges presented in Table 1.

Compost is unlike many of the other substrate components used in the greenhouse industry. Although it typically resembles finely ground pine bark, chemically it is much different (Table 2). Peat and pine bark are the two most widely used substrate components in the horticulture industry. Both have a low pH and low amounts of fertility. Compost on the other hand has a high pH and high fertility of electrical conductivity (EC). These properties must be considered when using compost as a substrate component.

Table 1. Characteristics of High Quality Compost *
pH 6.5 to 7.5
Soluble salts ≤5 mS
N, P and K all should be ≥ 1% (dry weight)
Bulk density 800-1000 lb/yd3
Moisture content 40 to 50 percent
Organic matter content 50 to 60 percent
Particle size Can pass through 1/2-inch screen
Trace elements/heavy metals Meet U.S. EPA part 503 regulations
Growth screening Must pass seed germination and plant growth assays
Stability Stable to highly stable
*Taken from: Stoffella and Kahn. 2001. Compost Utilization in Horticultural Cropping Systems. Lewis Publishers. 

The high EC or salt charge that is associated with compost can be dealt with in two ways: The first is leaching the substrate prior to planting. Figure 1 shows substrate mixes with 0, 10, 20 or 30 percent compost by volume and the resulting EC after a number of irrigations applied over a three-week period. The first two irrigations were one week apart and the following were applied twice a week. The pots did not contain plants and this irrigation scheme was used to simulate what might happen in a plant production setting. The figure shows about half of the salts associated with the higher rates of compost were leached out of the pot during the first two weeks.

The second method used to deal with the high nutrient load is reducing the fertilization rate. Many commercial substrates are formulated with a startup nutrient charge. Substrates created with compost have this feature built in. The composition of the elements in the compost-based starter charge will vary based on the feedstocks used to create the compost. At Longwood Gardens, the compost feed stocks include green plant material, woodchips, horse manure and food waste. The nutrient salts are dominated by potassium and phosphorus. Additionally, with higher rates of compost, nutrients will likely be released from the compost for weeks or even months after planting. This gives the grower an opportunity to reduce the overall amount of fertilizer required to produce the crop.

Substrate pH is another consideration when creating mixes with compost. Table 2 shows compost has a much higher inherent pH compared to peat and pine bark. This can be dealt with by adjusting the limestone rate used when creating a substrate mix. Table 3 shows the resulting pH of mixes created with 0 to 40 percent compost and 0 to 6 pounds of limestone per cubic yard of substrate. This table gives an indication of substrate pH a grower can expect with using compost in mixes. However, the values will vary based on the source of compost and the inherent pH of the other substrate ingredients. The table shows that with higher rates of compost, the amount of lime required is reduced and in some cases should be completely eliminated.


Table 2. Inherent pH and electrical conductivity (EC) of peat, pine bark and compost
  pH EC (mS)
Peat 3.0 to 4.0 0.1 to 0.5
Pine bark 4.0 to 5.0 0.1 to 0.5
Compost 6.5 to 8.0 2.0 to 6.0

Longwood Gardens has an extensive compost production and use program. Compost feedstocks are constantly monitored for quality while finished compost is monitored as well. Currently, compost is used at rates of up to 30 percent for greenhouse crops, and this rate varies based on crop requirements. If growers decide to use more than 20 percent compost, substrate should be leached or fertilization should be avoided or reduced for the first one to two weeks and then applied at slightly lower concentrations. Although this should prevent over-fertilization, EC along with pH should be closely monitored, especially when trialing a new substrate mix. Growers should also adjust lime rates accordingly in order to prevent nutritional disorders due to high substrate pH.

Table 3. Resulting substrate pH of substrate made with 0 to 40 percent compost and 0 to 6 pounds of limestone per cubic yard of substrate. Remaining substrate ingredients were pine bark (25 percent), perlite (15 percent), vermiculite (15 percent), calcine clay (5 percent) and peat moss (0 to 40 percent depending on compost rate)
Lime 0% 10% 20% 30% 40%
0 lb • yd-3 4.9 5.2 5.7 6.2 6.6
2 lb • yd-3 5.6 6.0 6.4 6.6 6.8
4 lb • yd-3 6.2 6.4 6.6 6.8 6.9
6 lb • yd-3 6.4 6.6 6.7 6.8 6.9

Leave a Reply

More From Media...
Coleus 'Colorblaze Velveteen' (2015 University of Tennessee Field Trials)

November 28, 2015

2015 University of Tennessee Gardens (Knoxville and Jackson, Tenn.) Field Trials Results

See the 2015 field trials results (includes photo gallery) for University of Tennessee Gardens in Knoxville and Jackson, Tenn.

Read More
Feature Image Cob 700 (NewLux)

November 28, 2015

16 LED Lighting Solutions For Your Greenhouse

Narrowing in on the right LED lighting product often comes down to considering your specific crop needs and growing requirements to see what works best for your application. Here are 15 LED products to take into account when choosing the right fit for your greenhouse.

Read More
Begonia 'BabyWing Red' (2015 Louisiana State University Field Trials)

November 27, 2015

2015 Louisiana State University (Hammond, La.) Field Trials Results

See the 2015 field trials results (includes photo gallery) for Louisiana State University in Hammond, La.

Read More
Latest Stories
Stockosorb Crystals_with water Agriculture leaf (Evonick)

November 21, 2015

9 Sustainable Growing Media Products For Superior Green…

Manufacturers are delivering new growing media products to help growers attempt to minimize their footprint without sacrificing quality. Here are nine new products to consider for your greenhouse operation.

Read More
PittMoss is an engineered fiber, comprised of recycled newspaper and other natural materials, that has been processed to better hold air and water while delivering nutrients to the plant

November 12, 2015

The Latest Advancements In Growing Media And Soils

From increased automation to new soil amendments and water-saving technologies, media manufacturers offer sustainability-focused solutions to meet growers’ unique needs.

Read More

September 25, 2015

Canadian Sphagnum Peat Moss Association Announces Early…

According to an August 31 survey of members of the Canadian Sphagnum Peat Moss Association (CSPMA), whose members represent approximately 95 percent of all North American peat production, the peat harvest season has been adequate, but not strong, and could cause shortages and potentially higher transportation costs. Down To The Dirty Details The survey inquired about the status of CSPMA members’ 2015 Actual Harvest (including an estimate of what can be expected to be harvested for the remainder of the season) as a percentage of their 2015 Expected Harvest. The lack of a strong harvest overall may challenge peat availability. The Prairie Provinces (Manitoba 98 percent, Saskatchewan 88 percent and Alberta 94 percent), experienced early favorable weather conditions and a strong start to the year. This helped to minimize periodic, negative, weather-related conditions during the balance of the harvest season, and the harvest numbers are close to achieving the expected amounts. […]

Read More
Bob’s Market and Greenhouses’ Ron Morris pours Stockosorb into the hopper for distribution on the conveyor line

August 13, 2015

How Bob’s Market And Greenhouses Improved Growing…

My father started our company 45 years ago growing bedding plants, mainly early season production and finished plants for our West Virginia market. It was in the early 1980s that we started growing earlier spring production and shipping materials to southern markets, and by the late 1980s, we also produced pansies for fall. We started using hydrogels when they first came on the market in the early 1990s and found that they really helped with our production by keeping plants healthier for these new markets. Over the years, we’ve grown to be a large young plant producer and have a sizable business growing finished plants in cell packs, 4 1/2-inch pots, 6-inch pots, gallon containers, hanging baskets, multiple sizes of large containers and large baskets for municipal use. Creating The Ideal Soil Mix With our old system, it took several workers to mix pre-made soil with slow-release fertilizers in cement […]

Read More

December 2, 2014

Grow-Tech Announces BioStrate, Its Newest Hydroponic Gr…

Grow-Tech LLC recently announced the release of BioStrate Felt, a biobased textile specifically engineered for the growing of hydroponic microgreens and baby salad greens.

Read More

November 18, 2014

7 New Media And Light Products For Greenhouse Productio…

New media and light products cover a broad sweep of growing conditions.

Read More
Oakland Nursery plantings in Columbus_featured

November 17, 2014

Oakland Nursery Simplifies Streetscape Plantings And Ma…

The outdoor decorative containers that Oakland Nursery plants and maintains in downtown Columbus, Ohio, enhance the look of the city’s buildings and streets and hinder vandalism.

Read More

October 27, 2014

Peat Moss May Be In Short Supply This Year

Adverse weather conditions in Canada have played havoc with the peat moss harvest.

Read More

September 24, 2014

Canadian Harvest Of Peat Moss Is Below Average For 2014

The harvest season has been challenging, according to the Canadian Sphagnum Peat Moss Association (CSPMA), with lower-than-expected harvest of peat moss across most production regions in Canada, due primarily to adverse weather conditions across the country.

Read More

July 23, 2014

Liming Requirements And pH Modification For Pine Wood C…

In the last of a four-article series highlighting the production and use of pine wood chips as aggregates in greenhouse substrates, the researchers found growers do not need to adjust their production practices when 20 percent pine wood chips are used as a perlite replacement.

Read More

May 14, 2014

How Pine Wood Chips In Substrates Affect Plant Growth R…

This is the second article of the four article series highlighting the production and use of pine wood chips as aggregates in greenhouse substrates.

Read More

April 3, 2014

The Processing And Properties Of Pine Wood Chips

In the first of a four article series highlighting the use of pine wood chips as alternative aggregates to perlite in greenhouse substrates, researchers from North Carolina State University discuss the processing and physical properties of pine wood chips.

Read More

April 3, 2014

Water And Media Are The Foundations Of Your Business: T…

Electrical conductivity (EC) and pH, as well as water alkalinity, have the biggest effects on nutrient availability. Learn how to keep track of them through three common methods for better monitoring in the greenhouse.

Read More
PlugEase from Acme Group

March 3, 2014

Acme Group Introduces PlugEase, A Line Of Recyclable Ag…

The Acme Group recently announced its new line of recyclable Agrifabrics and plant plug substrate, PlugEase. The products make recycling affordable for greenhouse growers, farmers and horticulturalists.

Read More
Emerald Coast Growers

February 26, 2014

Emerald Coast Growers Constructs New Soil Facility

Emerald Coast Growers has constructed a new, consolidated soil mixing facility to increase efficiency and allow for easier custom blending by crop.

Read More
Combination pH and EC meter. Photo courtesy of Hanna Instruments

February 5, 2014

Test Media pH And EC With The 2:1 Technique, Pour-Throu…

Avoid a buildup of soluble salts and create an environment most conducive to nutrient uptake with these three common media testing methods.

Read More
Fertiss Growing Medium from Oasis Grower Solutions

November 18, 2013

Three New Options For Growing Media

From special blends to mycorrhizae, here are some new options for growth media.

Read More

November 14, 2013

Growing Media: To Mix Or Not To Mix

Thinking about making your own growing mixes to lower costs? There are many things to consider before taking the plunge.

Read More
[gravityform id="35" title="false" description="false"]