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Composts as a Soil Amendment

N. V. Hue and H. Ikawa
Department of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources
University of Hawaii at Manoa


According to the Environmental Protection Agency (EPA), the United States in 1993 generated 207 million tons of municipal solid wastes (MSW), or 4.4 pounds per person per day. The EPA further projected that MSW will increase to 218 million tons in the year 2000, yet the per capita generation rate will decrease for the first time to 4.3 lbs/person/day. Thanks to recycling and composting.

Figure 1. Making compost on Oahu by the windrow, turned pile method.

Composting is a controlled biological decomposition process which converts organic constituents of wastesinto humus-like materials that may be used as soil amendments or organic fertilizers. Thus composting seems to be a logical and environ- mentally sensible way to reduce and reuse our wastes. Yet, one may wonder why composting has not been more widespread as a mode of waste anagement. The answer lies partly in the following facts. (1) Being biological, composting is a rather slow process and is affected by many environmental factors, such as temp- erature, moisture, and aeration. (2) There is a lack of legal and commercial standards for compost. Many products of differing quality are available in the market, and all are called "compost," but they have little in common in terms of plant nutrient values or soil amending characteristics. (3) There is a lack of knowledge on proper uses of composts: using a wrong compost might stunt or even kill plants.

Compost Quality

In 1994 we did a survey of many composts that were commercially available in Honolulu in an attempt to provide the public with the information necessary for selecting proper composts. As shown in Table 1, composts varied widely in their nitrogen (N) contents and carbon-to-nitrogen (C/N) ratios. Apparently, those with C/N ratios > 25 were made primarily for such uses as mulches (to control weeds and regulate soil temperatures) and soil organic matter enhancers (to increase the water holding capacity of soils). A few composts with C/N ratios < 25 and N > 1.0% could be used as a source of slow-release organic N as well.

Table 1. Nitrogen levels and C/N ratios of commercial composts available in Honolulu in 1994.

Compost source/Brand and origin N % C/N ratio
Niu organic compost, Aloha Agric Consultants, HI 0.28 139
Garden compost: wood products, Redi-Gro, CA 0.32 122
Redwood compost, Cascade Forest Products, CA 0.36 151
Redwood compost, Redi-Gro, CA 0.52 96
Local compost: forest products, HI 0.72 41
Sphagnum peat moss, Lakeland peatmoss, Canada 0.88 47
Peat moss, Canada 0.88 48
Local compost, HI 0.96 22
Organic compost: aged mushroom products, CA 1.00 42
Organic compost: rice hull, wood chips, sludge, CA 1.08 19
Nitrohumus, Kellogg, CA 1.52 13
Niu compost: chicken manure mix, Aloha Agric. Consultants, HI  2.04 8

Composts as a Source of Nitrogen

--A double edged sword-- Proper uses of composts must be observed; the wrong compost may hurt plants. To illustrate this point, we mixed a wood-based compost (C/N = 80) and a chicken manure-based compost (C/N = 8) at 25 and 50% by volume with a relatively fertile soil from Waimanalo, Oahu. Tomato seedlings were grown as a test crop. Visual inspections revealed that tomato plants grown in the woody compost mixes were stunted and yellow, due to N deficiency. This compost probably tied up (immobilized) N from the soil and starved the plants of its N nutrition. By contrast, plants grown in the manure-based compost were green, healthy and much taller than plants of the control.

Table 2 shows dry matter weights and plant N contents, which quantify the N problem.

Table 2. Dry matter weights and N concentrations of 5- week-old tomato plants as affected by composts.

Treatment Source Rate Dry Weight (g/pot) Nitrogen (%)
Soil (control)   0.84 1.10
Woody 25% 0.20 0.80
  compost 50% 0.22 0.74
Manure 25% 18.26 1.55
  compost 50% 24.20 2.04

Compost Reduces Soil Acidity

Roots try to get away from the hostile acid soil of the control

Many soils in Hawaii are acid, high in aluminum (Al) and low in calcium (Ca), and so are harmful to roots and unsuitable for plant growth. We hypothesized that organic matter in a well-aged compost can detoxify Al and increase crop yields. To test this hypothesis, we selected an acid soil (Manana series) from Waiawa, Oahu, which had an initial pH of 4.5 and high Al. We established four treatments: (1) control (no lime, no compost), (2) 2 ton/ac of lime, (3) 10% compost, and (4) 25% compost by volume ( 75 ton/ac). All treatments received equal amounts of N, P, K, Ca, S and micronutrients for adequate growth. Corn was planted as a test crop. Corn dry weights, harvested one month after planting, clearly showed benefits of the compost in correcting soil acidity. The 25% compost treatment had the highest yield (8.9 g/pot) as compared with 6.2 g/pot in the control and 7.7 g/pot in the lime treatment. The 10% compost treatment produced as much dry matter as the lime treatment.

Compost Increases Phosphorus Availability

Along with the acidity problem, many Hawaii soils are severely deficient in phosphorus (P) because P reacts strongly with Fe and Al minerals in soils. Based on this knowledge, we hypothesized that organic matter in composts can compete with P fertilizers for reactions with soil minerals, thereby leaving more P in soil solution for use by plants. To test this hypothesis, we selected a P-deficient soil (Leilehua series) from Waiawa, Oahu, which barely supports plant growth without P fertilization. We then set up five treatments: (1) control (100% soil), (2) 75% soil + 25% compost by volume, (3) 50% soil + 50% compost, (4) 25% soil + 75% compost, and (5) 100% compost. (The compost used was made of tree trimming from UH campus by the Division of Landscaping and Grounds, UH.) Plant nutrients other than P were added equally to all treatments. Corn was grown as a test crop.

The accompanying figure shows that corn seedlings grew best in the presence of 75% or 100% compost (the last two pots from the right). By contrast, corn plants grown in soil alone were P deficient, stunted and grew poorly (the left most pot).


Composting is an environmentally sound means of reducing our society's wastes. The compost products can be used as soil amendments and plant growth media. However, they must be used properly. Not all composts can serve as a source of slow-release, organic N. Those with C/N ratios > 25 may tie up N from the soil and deprive plants, particularly fast growing crops, of their N nutrition. On the other hand, organic matter in composts can have many positive effects on soils from reducing soil acidity to increasing P availability.

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