Utilizing Compost and Plant-Based Mulch to Enhance Soil and Plant Health
Posted on Dec. 12, 2023 / Fertilizer , Soil / Subscribe 0
By: Jackie Jamison
What is soil? According to the Soil Science Society of America, soil is “the unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants.” It is made up of solid matter, including mineral material and organic material, as well as pore space, which holds both water and air. Soil organic matter, although a relatively small portion of soil itself, has a disproportionately large effect on soil health and function, and ultimately on plant health. It is largely responsible for important soil functions such as water storage and filtration, nutrient cycling, providing habitat for soil organisms, and acting as a medium for plant growth.
Figure 1. Pie chart showing soil components. Organic matter, although a smaller percentage of most soils, has a disproportionately large effect on soil health and function.
The addition of organic matter to soil has many positive effects that ultimately contribute to healthy plants and landscapes. This can be in the form of fresh plant residue, cover crops, compost, manures, or plant-based mulch. The key point is that by adding organic matter, we are feeding the soil, and the soil (and the microbes within it) then feeds the plants in our landscapes.
Figure 2. This image shows a Kula soil with organic matter in various stages of decomposition.
Figure 2: Adding organic matter to soil results in positive changes to soil structure and function. Adapted by Magdoff and Van Es (2020) and modified from Oshins and Drinkwater (1999).
Figure 3. Another way of adding organic matter to the soil is through the living roots of plants, including cover crops such as the buckwheat (Fagopyrum esculentum), cow pea (Vigna unguiculata), black oats (Avena strigosa), daikon radish (Raphanus sativus) and brown mustard (Brassica juncea) pictured here.
Organic matter plays an important role in nutrient availability. It feeds plants directly as it’s decomposed, and 1% organic matter on 1 acre can release 20-30 lbs. of nitrogen (N), 5 lbs. of phosphorus (P), and 2 lbs. of sulfur (S). It also both stores (through cation exchange capacity) and physically protects (through chelation) nutrients. This ultimately leads to improved and sustained nutrient availability to plants, less nutrient loss, and reduced pollution in our waterways.
Figure 4. A negative charge on organic matter increases cation exchange capacity and chelation of positively charged micronutrients. Adapted from Magdoff and Van Es (2020).
One simple way of adding organic matter into landscape soils is by adding compost. Adding compost provides numerous benefits for soil health and plant growth. It helps to loosen clayey soils and aggregate sandy soils, thereby aiding in water infiltration and storage. By increasing the cation exchange capacity of soils, compost also helps soils hold onto more plant nutrients. Compost also helps to balance the pH of soil, bringing it closer to a neutral range, where the majority of plant nutrients are available. Compost also provides nutrients directly to plants. Those nutrients are mostly in organic forms, meaning that they are slowly released into the soil for plant uptake, and are less prone to leaching and loss than other fertilizers.
Figure 5: Soil after the addition of well-decomposed compost, showing dark color of organic matter and visible soil aggregates.
Another effective way of adding organic matter into the soil is by using plant-based mulches such as wood chips. Utilizing wood chips as mulch can help utilize a waste stream from tree trimming jobs while simultaneously providing a benefit to plants and soils. Plant-based mulches decompose over time, providing valuable organic matter to the soil as they do so, with all the benefits discussed above. In addition, by covering the soil, they help reduce evaporation and decrease the water requirements of landscapes, particularly important considering increasing drought conditions across the islands. They also reduce competition from weeds, reduce erosion, and help maintain lower soil temperatures. Lastly, they can provide an aesthetic benefit to the landscape.
Figure 6. Pohinahina (Vitex rotundifolia) surrounded by wood chip mulch.
Figure 7. Partially decomposed wood chip mulch in the landscape, showing fungal hyphae and earthworms. Photo credit: Hannah Lutgen
References:
Magdoff, F. and Van Es, H. (2021) Building Soils for Better Crops. Sustainable Soil Management, IV Ed, Sustainable Agriculture Research and Education (SARE) Program, National Institute of Food and Agriculture, (USDA).
Oshins, C. and L. Drinkwater. 1999. An Introduction to Soil Health. A slide set previously available from Northeast SARE.
Jackie Jamison is an Extension Faculty Member in Urban Horticulture and the Master Gardener Program Coordinator at the University of Hawai‘i at Mānoa, College of Tropical Agriculture and Human Resources.
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