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

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