Compost effects on soil properties and plant growth.

Abstract

Compost production is considered an economic and environmentally friendly means to reduce the waste going into landfill. Compost application can improve soil quality and productivity as well as sustainability of agricultural production by replenishing soil organic matter and supplying nutrients. Organic matter is a vital component of a healthy soil as it plays an important role in soil physical, chemical and biological fertility. In this project, four experiments were conducted to assess the effect of compost mulch on soil properties and plant growth and nutrient uptake as influenced by compost composition and particle size, soil type and time after application. The first experiment was carried out using four composts from various feedstocks applied as mulch to a sandy loam. Their effect on soil properties, plant growth and nutrient uptake was assessed after 2,4,6,9, 13 and 18 months. The composts were Cl and C2 (from organic fraction of municipal solid waste); C3 (from manure); C4 (from straw and manure). A soil without compost amendment was used as control. At each sampling date, the soil was sampled after removing the compost mulch and the remaining soil was planted with wheat; plant growth and nutrient uptake were determined after 4 weeks. The coarse-textured compost (C2) had the least effect on nutrient availability and plant growth despite moderate concentrations of available N and P in the compost. However, its total N and P concentrations were low which, together with the coarse texture resulted in low mobilisation rates. The other three composts increased soil aggregate stability and water holding capacity although there was no measurable increase in soil organic C, suggesting that the improved soil structure was due to microbial activity (bacterial slimes, fungal hyphae). These three composts also increased soil N and P availability, plant growth and nutrient uptake with N and P availability increasing over time whereas plant growth was maximal after 4 months. The increased soil nutrient availability was due to two mechanisms: either release of P and N already present in available form in the composts or mobilisation of the nutrients during the experiment which varied with compost type. Despite the fact that the beneficial effects of compost on soil properties are generally well-known, there are few systematic studies comparing the effects of composts on soils of different texture. The second experiment was conducted to assess the effects of two types of composts derived from different feedstocks: Cl (from garden waste) and C2 (from agricultural residues and manures) applied as mulch on three soils with different clay content (46yo, 22%o and l3Yo, hereafter referred to as 546, 522 and Sl3) on soil properties as well as on plant growth and nutrient uptake. Wheat plants were grown for 35 days and to grain filling (70 days). The soil was sampled after removal of the mulch layer. The composts reduced the soil pH by 0.3-0.7 units and slightly increased total organic C compared to unamended soil. Soil respiration was highest in S13, and at grain filling it was greater in soil amended with C2 than that amended with Cl and the non-amended soil. The addition of compost significantly increased soil cation exchange capacity (CEC) n 522 and 546, but not in S13 which had the lowest CEC among the soils. C2 increased the available P concentration and macroaggregate stability in all soils compared to Cl and the unamended soil. Compost addition increased available N in 546 and 522 compared to the unamended soil with a stronger effect by Cl. Both composts increased wheat growth and shoot P concentrations with the effect of C2 being greater than that of Cl. Generally, the effect of soil type was greater than that of compost type. It is concluded that the effect of composts on soil properties and plant growth is strongly affected by soil properties. There are few studies in which the effect of several composts applied as on soil properties and plant growth has been studied under controlled conditions. To address this knowledge gap, a third experiment was carried out with six types of composts applied as mulch on two soils (13% and 46% clay, referred to as S13 and S46). Wheat was grown and harvested at 42 days and at 77 days (grain filling). Composts differed in total and available N and P and particle size with Cl, C3, C4 and C5 being fine-textured, whereas C2 and C6 were coarse-textured. Compost addition increased soil TOC and EC, but had no effect on pH. In all treatments, cumulative soil respiration was higher in S13 than in 546 and was increased by compost addition with the greatest increase with C2 and C6. Compared to the unamended soil, most composts (except C2) increased macroaggregate stability. Compost mulches significantly increased available P and N in both soils, except for C2. Compost mulches increased available N concentrations up to 6-fold in both soils with the strongest increase by C5. Most composts also increased wheat growth and shoot P and N concentrations, except C2 which decreased shoot N and P concentrations compared to the unamended soil. Most composts (except C2) increased mycorrhizal colonization by up to 50%m compared to the unamended soil. It can be concluded that fine-textured compost mulches generally had a greater effect on soil properties and plant growth than coarse-textured composts. In this experiment, a given compost had similar effects in both soil types. In most studies to date, compost effects on soils have been studied in the bulk soil without considering the distance from the compost. This distance could be important because the effects are likely to be greater in the immediate vicinity compared to further away. Thus by taking a bulk soil measurement, one may underestimate the effect in immediate vicinity of the compost whereas the effect in greater distance is overestimated. A fourth experiment was carried out to assess the effect of compost on soil properties in their immediate vicinity. This experiment was also used to assess the changes in nutrient concentrations withing the composts. Three composts from different feedstocks: Cl (from animal manures) and C2 and C3 (from the organic fraction and municipal solid waste) were applied as a layer which was separated from the soil by a mesh. Microcosms without compost served as controls. Microbial and chemical properties of the soil were determined at 0-5 and 5-10 mm distance from the mesh after 30 and 63 days. During the 63 day incubation, the total C, N and P and available N concentrations in the composts decreased whereas the available P concentration increased. The composts induced higher microbial biomass and activity, total organic C and available N and P concentrations up to 10 mm into the surrounding soil with greater effects after 30 than after 63 days. The nutrient concentrations were generally greater in soil adjacent to the two finer-textured composts with the higher nutrient concentration (Cl and C3) than in the coarser-textured compost (C2) which had lower nutrient concentrations. The 0-5 and 5-10 mm layers did not differ in most of the measured properties except for greater soil respiration and N and P availability in the 0-5 mm layer. It can be concluded that composts affect soil properties up to 10 mm distance but that the greatest effect on microbial activity is limited to the first 5 mm. The experiments showed that compost properties, particularly particle size and nutrient concentration determine the effect of compost on soils. Finer composts have greater effects than coarse-textured composts. Nutrients released from composts may derive from the nutrients already available in the composts or may become mobilised after compost application. Compost effects were modulated by soil type and changed over time.