Agri-mat and grass mulch effects on selected soil physical and chemical properties, runoff, crop growth and yield in South Africa

Abstract

The current research work was conducted to understand the dynamics of nutrients from the organic-inorganic (algae, agri-mat, grass and lime ammonium nitrate) amendments into soil through mineralization, their movement thereafter from the soil to the crop. In addition, the effect of organic materials on some soil physical and chemical properties and crop productivity was investigated through a series of different but interconnected laboratory, glasshouse and field experiments. The first experiment was carried out through a 2-month incubation study to investigate nutrient release (mineralization) capacity of inorganic-organic treatment combinations and organic amendments alone on sandy loam and loam soil. The objective of the incubation study was to determine the nitrogen-mineralization rate of different organic mulching materials singularly and when combined with lime-ammonium nitrate under two soil types. The incubation study was conducted in the laboratory at Agricultural Research Council – Soil, Climate and Water in Arcadia, Pretoria. The experiment consisted of sandy loam and loam soil, each treated with the following seven treatments T1 = control, T2 = 5g of dry algae per 100g of soil, T3 = 5g of ground agri-mat per 100g of soil, T4 = 5g of ground grass per 100g of soil, T5 = 75kgN.ha-1 using Lime Ammonium Nitrate (LAN) + 2.5g of dry algae per 100g of soil (50%NF + 50%DA), T6 = 75kgN.ha-1 using LAN + 2.5g of ground agri-mat per 100g of soil (50%NF + 50%GA), T7 = 50%NF + 50%GG. Each treatment was replicated three times and the experiment was laid out in a completely randomized design. Three experimental units per treatment were drawn at 0, 3, 7, 15, 30, 45, and 60 days for mineral-N (NH4 + -N and NO3 - -N) content determination. The results from the incubation study indicates that all the treatments where organic amendments were added had higher mineralization rates compared to the treatments where organic amendments were applied singularly. The highest amounts of Nitrate-N (619 mg.kg-1 ) and Ammonium-N (507.9 mg.kg-1 ) were both recorded in the 50% dry algae (DA) and 50% compound fertilizer treatment combination at day 15 under sandy loam soils. Following the incubation study, a 2-month glasshouse experiment was conducted to study effect the following five treatments: T1 = control, T2 = 50%DA + 50%CF, T3 = 50%GA +50%CF, T4 = 50%GG + 50%CF, T5 = 150 kgN.ha-1 using LAN (100%CF) on spinach growth and yield using the same two soil types. All five treatments were replicated three times for each of the two soil types and the experiment was laid out in a completely randomized design. The treatments with sole application of organic amendments were replaced with sole application of nitrogen fertilizer (LAN) in the glasshouse experiment due to their low mineralization rates as observed in the incubation study. The sole nitrogen fertilizer treatment in the incubation study was excluded since nitrogen is already in plant available form in lime ammonium nitrate. The glasshouse experiment was conducted in a glasshouse at Agricultural Research Council – Vegetable and Ornamental Plants, in Roodeplaat, Pretoria. The objective of the glasshouse experiment was to determine the combined effects of different organic mulching materials with lime-ammonium nitrate (LAN) in comparison to sole application of LAN on spinach growth and yield. The findings from the glasshouse experiment indicates that the (50%DA + 75 kgN.ha-1 ) treatment produced higher values for plant height [34.3 cm (PTA soil) and 41.3 cm (DBN soil)], leaf length [25 cm (both DBN and PTA soils)], number of leaves [14.7 cm (PTA soil) and spinach yield [202.06 g.pot-1 (PTA soils) and 72.19 g.pot-1 compared to other treatments. The glasshouse experiment results might not give precise indication of the field conditions due to various factors (i.e, glasshouse environment with shorter growing period, restricted root growth due to pot size, etc). Therefore, two-season (winter and summer) field experiments were conducted concurrently at two different agro-ecological zones of South Africa and repeated twice. The objective of the field experiments was to assess the effect of agri-mat and grass mulch on soil water regime, temperature and crop yield. The field experiments were conducted in Agricultural Research Council – Vegetable and Ornamental Plants in Pretoria and in AgroEco Hub, Newlands West, Durban. The following five mulch treatments were established in each site: i) Full Agri-mat mulch cover (100%AG), ii) Half agri-mat mulch cover (50%AG), iii) Bare (Control), iv) 6 tons.ha-1 of grass mulch (6t.GM), and v) 3 tons.ha-1 grass mulch (3t.GM). Each treatment was replicated three times to make 15 plots per site and both sites were arranged in randomized complete block design. The objective of these experiments was to determine the effects of agri-mat and grass mulch on soil water regime, temperature and crop yield In both sites, maize was planted during the summer season and spinach was planted during winter. For each season, the experiments were running concurrently with one-week difference in planting dates – planting in Pretoria was conducted a week after planting was finished in Durban. For both seasons, in each site, the experiment was conducted under the same plots with five mulching treatments that were replicated three times. Soil moisture and temperature sensors were installed at the beginning in all 15 plots in each site. The results from the field experiments suggest that mulching with 100% agri-mat treatment can increase crop yields for better food security by improving soil water and moderate soil temperature. Alternative to maize or wheat straw as valuable crop residues, smallholder farmers can adopt agri-mat mulch as a cheap and effective soil cover practice to conserve moisture in their evaporation prone soil in order to increase crop yield and improve food security. At the end of the field experiments, soil samples were collected from the plots where spinach and maize were planted with the objective to assess the effect of agri mats and grass mulch on aggregate size distribution and stability of two different soil types. Air dried soil aggregates of approximately 10 mm per treatment in triplicates were selected from both experimental sites (in each plot, which were laid in a Completely Randomized Block Design) for image scanning and analysis using a Nikon XTH 225L micro-focus CT X-ray unit. The experiment also adopted the fast wetting method to determine the aggregate stability of a loam and sandy loam soil, which were under agri mat and grass mulch for two consecutive years.The X-ray CT analysis results show that, due to the higher frequency of storage pores (0.5-50 µm) and feeding root pores (100 - 200 µm) in the loam soil compared to the sandy loam soil, the loam soil has better capacity to accommodate more roots and had higher volumes of water reservoir for plants and microbes. In addition, the 100% agri-mat mulch treatment improved the water holding capacity of the loam soil by decreasing total macro porosity, making it less porous compared to other treatments. The aggregate stability test results indicate that the 100% agri-mat mulching cover has a greater stabilizing abilitycompared to all other mulching materials (6t.GM, 3t.GM, 50%AG and Control) in both soil types. The last experiment was conducted in a laboratory to assess the effectiveness of agri-mats made with different organic materials in controlling soil water erosion using Mini Portable Pressure Head Rainfall Simulator. The objective of this laboratory experiment was to investigate the effectiveness of utilizing different mulching materials on water infiltration and runoff. Agri-mats water infiltration and runoff test was conducted in a laboratory at Tokyo University of Agriculture and Technology, in Fuchu city, Tokyo – Japan. Results shows that when algae were used as an additional organic material during the fabrication process, agri-mat delayed runoff by absorbing more moisture than it losses. The agri-mats that were fabricated only with 100% Bagasse had a lower water infiltration rate compared with agri-mats made with 90% Bagasse and 10% Algae. The 100% dry sugarcane bagasse agri-mat treatment had higher ru-mats noff rate of 16.8 mm. hr-1 compared to mere 1.7 mm. hr-1 recorded under the agri-mat 90% Bagasse and 10% algae during the first 10 minutes of rainfall simulation test. In addition, the agri-mat made with bagasse only released magnesium at a rate of 105.7 mg. hr-1 compared to 6.6 mg. hr-1 lost from the agri-mat board made with both bagasse and algae. Agri-mats are a novel and innovative pro-smallholder farming effective mulch technology that promises to be a cheaper alternative approach to prevent sediment loss and conserve soil moisture. Agri-mats are fabricated using forestry waste, bagasse, algae, grasses, etc., thus allowing smallholder farmers to sell crop residues after harvest for profit generation whilst the soils remain covered with agri-mats. Agri-mats can last on the field for up to two years or more, depending on the climatic factors (rainfall and temperatures), soil type and quality of the organic material used during the fabrication process, before they completely decompose. The findings from this study show that in all treatment combinations (organic amendments + inorganic fertilizer), the mineralization rate was significantly higher compared to treatments where only organic amendments were applied in both soil types. In addition, the inorganic amendments improve the efficiency of organic fertilizers and vice versa, through a positive interaction on physical, chemical and biological characteristics of the soil. The findings from the glasshouse experiment indicates that the inorganic-organic treatment combination (50%DA+50%CF) can replace full inorganic nitrogen fertilizers without compromising spinach growth, yield and quality. The results from the aggregate stability test show that, in reference to the control, all mulching treatments increased the stability of soil aggregates from unstable to medium stability after a period of two years for both loam and sandy loam soil. Moreover, this research study indicates that agri-mats can play a crucial role in building soil structure that is resistant to soil erosion by improving the stability of soil aggregate and physical architecture of the soil. The results from agri-mats infiltration and runoff test experiment indicates that agri-mats allow more water to infiltrate through agri-mats than being lost as runoff. In addition, the current study shows that adding algae as one of the organic materials during the fabrication process improve the agri-mat water absorption and holding capacity and thus reduce nutrient loss through runoff. Field experiments indicates that the 100% agri-mat mulch treatment can improve crop yields and leaf nutrient quality for better food security by improving soil water regime and moderating soil temperatures, irrespective of soil type, agro-ecological zone and type of crop. However, further research is needed to assess economic viability of agri-mats relative to crop residue costs to ensure there is science-based evidence for investment/input decision making especially for poor resource smallholder farmers who wants to maximize profits. Moreover, the recommendations drawn from all future agri-mats research findings should be based on prevailing site conditions, such as climate and soil type where the research studies were conducted.