Peanut production in Zambia is often characterized by low yields and high aflatoxin incidence in harvested kernels. Soil amendments such as farmyard manure have shown potential to increase yields and reduce pre-harvest aflatoxin incidence. The aim of the current study was to evaluate the effects of composted cattle manure on soil properties that relate to yield and pre-harvest aflatoxin contamination of peanut kernels. Research evaluated the effects of composted cattle manure on soil respiration, plant-available water (PAW), peanut yield and pre-harvest aflatoxin contamination in a field experiment conducted in two successive rain-fed cropping seasons starting in December, 2015 and ending in April 2017, in Chongwe District, Zambia. Six (6) levels of compost were incorporated into the top 10 cm of the soil at rates of 0, 4.5, 12.0, 19.5, 27.0, and 34.5 metric tons/ha 1 wk before planting. There was a strong positive relationship between levels of compost and soil microbial respiration (R2=0.84) and PAW (R2=0.86). Secondly, compost manure was associated with increases in pod (R2=0.65) and kernel (R2=0.61) yield. The kernel yield potential of the planted cultivar was achieved at the rate of 12 metric tons per ha. Thirdly, there was a reduction in total aflatoxin levels with increasing levels of compost (R2=0.85). The improvement in peanut yield and the decrease in aflatoxin concentrations in kernels can be attributed to the improvement in soil moisture retention capacity and soil microbial activity arising from manure amendments. This study demonstrated the potential of compost manure to increase soil microbial activity, PAW, peanut yield and minimize aflatoxin contamination at field level.
Peanut (
Some of these production constraints such as poor soil health and drought stress are strongly associated with the reportedly high levels of aflatoxin contamination in harvested kernels (
Plant stress factors such as drought are linked to both productivity and aflatoxin contamination. Because drought stress is an important factor in the proliferation of
Compost is relatively cheap and is a sustainable means of restoring soil health and promoting peanut productivity. Soil health is defined as the ability of the soil to function as a living system (
The field experiment was conducted under rain-fed conditions at Kasisi Agricultural Training Centre, located at 15.2498 S, 28.4836 E in Chongwe District, Zambia. The site is located in agro-ecological region IIa, which has mean annual rainfall ranging from 800 to 1000 mm (
A composite soil sample from the study site was constituted by mixing 8 random subsamples collected from the top 20 cm soil layer using an auger. The duly constituted composite sample was air dried, passed through a 2 mm sieve and then characterized using standard laboratory soil analysis procedures. To determine soil texture, the soil was first dispersed using 5% sodium hexametaphosphate (calgon) and then determined the particle size distribution using the hydrometer method (
Treatments for the field experiment were 6 levels of composted cattle manure (compost) applied to each experimental plot. The rates of application were 0, 4.5, 12.0, 19.5, 27.0 and 34.5 metric tons/ha. The application process involved; uniformly spreading the compost on the soil surface by hand and then mixing it with the soil in the top 10 cm using a hoe in a fine tillage operation. Compost was applied after the first continuous rains of the cropping season and was allowed to settle for one week before planting. Each rate of application was replicated 6 times resulting in 36 experimental plots. Treatments were laid out in a latin square experimental design. Each experimental plot measured 25 m2 with a 1 m border between plots. Thus, the experiment covered an area of about 0.12 ha. The experiment was conducted for two successive cropping seasons; December 2015 to April 2016 and December 2016 to April 2017. These two growing periods are hereafter referred to as 2016 and 2017 seasons, respectively.
The compost for this study was prepared conventionally in compost heaps consisting of cattle manure mixed with spoiled hay arranged in windrows. These windrows were moistened when necessary and turned regularly until the materials were decomposed to stable compost. The compost was characterized for selected chemical properties using standard laboratory methods. Some of the properties of the compost included a C/N ratio of 11, pH in 0.01 M CaCl2 of 7.2 and total phosphorus, potassium, calcium and magnesium levels of 1.2, 1.3, 5.4 and 1.6 mg/kg, respectively.
Soil tillage was done by hand using a hoe. Thereafter, the ploughed area was levelled into a fine seedbed. A red-colored, virginia, bunch type peanut cultivar known as MGV 4, was then planted in rows at the recommended planting spacing of 75 cm x 10 cm, inter-row by intra-row spacing, respectively. Each plot had a total of 6 plant rows giving a net plot of 4 rows after subtracting the 2 border rows. Crop management practices were done manually and included regular weeding whenever weeds appeared and ridging at the on-set of the pegging stage. Major weeding was done four times per growing season. The crop was harvested by digging out plants with a hoe at physiological maturity, 130 d after planting. The pods were stripped from the plant by hand, packaged in polythene bags and then dried in an electric vacuum oven (D-6450 Hanau, Heraeus Instruments, Germany) set at 45 C to a gravimetric moisture content of about 10%, which took about 72 hr of continuous drying.
At harvest, the number and weight of mature grain-filled fresh pods per individual plant were determined from 6 representative plants randomly selected from the middle rows of each plot. After weighing, the pods were mixed with the other pods from plants in the middle rows for drying as described above and then shelling upon drying to approximately 10% w/w moisture content. Dry pods were shelled by hand and then temporarily stored in air-tight plastic jars at room temperature. Shelled kernels were sub divided into four equal subsample lots of about 500 g. Laboratory samples were collected by scooping 50 g samples per scoop for 4 times (200 g per 500 g subsample) from each quarter of the lots. A 120 mL plastic cup (1/2 standard cup) was used as a scooper. The samples were shaken before each scoop of sub-sample was taken. The scooped samples were added together and homogenized by shaking. The mixed sample constituted the laboratory sample, which was then ground into fine flour using an ordinary kitchen grinder (LM2211BM, Moulinex, China).
The total concentration of aflatoxins in dried kernels was determined using Neogen Afla Reveal® Q+ aflatoxin kit (Neogen Corporation, USA) within 1 wk after shelling. Ground samples were homogenized by thorough shaking. For each treatment 18 samples (6 replicates by 3 subsamples) each weighing 10 g were assayed for total aflatoxin using 30 ml of 65% ethanol (diluted from 95% ethanol, UN1170, Xilong Scientific Co., Shantou City, China) by shaking on a rotary shaker (ISO-9001-2000, Navyug, India) at a speed of 120 rpm for 3 min. The extract was then filtered through Whitman 42 filter paper. Five hundred
Soil microbial respiration and PAW were determined at 90 d (during pod-development stage) after planting as once-off soil health indicators in each of the two seasons. To determine soil respiration, composite soil samples each weighing 2 kg per plot were collected from 3 to 4 random sampling points in the top 10 cm of soil using a bucket soil auger. The samples were transported in air-tight plastic jars stored in a cooler box filled with ice blocks between jars containing soil samples. To determine carbon evolution due to microbial respiration, the evolved carbon dioxide was trapped in 1 M KOH (
To determine plant-available-water, 3 undisturbed soil samples per plot were collected from the top 10 cm of soil using standard core rings. The samples were then placed in the pressure plate apparatus to determine water content at field capacity (FC) and permanent wilting point (PWP). The samples were subjected to -10 kPa and -1500 kPa pressure for FC and PWP, respectively.
All the data collected in the experiment were managed in Microsoft Excel and SPSS version 20 statistical program. The effects of compost on PAW, soil microbial respiration, peanut pod and kernel yield and aflatoxin content in harvested kernels were determined year by year for the two cropping seasons. Each data set was checked for extreme outliers defined according to SPSS as data points with a magnitude of 3 times the inter-quartile range by first plotting box plots and removing flagged data points. There were no outliers in all the data sets. Scatter plots were used to establish whether or not there were linear relationships between the independent and the outcome variables. The central limit theorem was applied to assume normal distribution since all data sets had more than 30 observations. Simple linear regression analysis was then performed to estimate the response to each level of treatment.
The soil at the research site was a strongly acidic (pH = 4.22) sandy loam (19 % clay, 11.4 % silt and 69.6 % sand), with very low available phosphorus (0.56 mg/kg), low soil organic matter content (0.7 %) and low exchangeable calcium (0.06 cmol/kg). No measures were taken to correct neither the acidity nor nutrient deficiencies for the sole purpose of evaluating groundnut performance on marginal soils (control) common to local farmers.
The plant-available-water (PAW) in soil treated with compost manure increased with increasing levels of compost manure applied (R2=0.86) (
Effect of compost on plant-available-water. Error bars represent standard error of the mean. Plotted values represent means of 6 replicates. The R-square values for the fitted regression lines are as indicated on each line.
Effect of compost on field capacity. Error bars represent standard error of the mean. Plotted values represent means of 6 replicates. The R-square values for the fitted regression lines are as indicated on each line.
The role of organic matter on soil moisture retention capacity relates to its role on aggregate stability and soil structure. Aggregate stability relates to the capacity of the soil aggregate to maintain its physical structure/shape when subjected to a given pressure while soil structure relates to the distribution of the solid phase and the pore space (liquid and gaseous phases) in a given mass of soil. Stable aggregates tend to have more pore space and are able to hold more water than weak aggregates. According to
There was an increase in microbial respiration (R2=0.84) with increasing levels of compost (
Effect of compost on soil microbial respiration. Error bars represent standard error of the mean. Plotted values represent means of 6 replicates. The R-square values for the fitted regression lines are as indicated on each line.
Results similar to our study have been reported by several authors. For instance, Chaoui
There were significant increases in pod (R2=0.65) and kernel yield (R2=0.61) with an increase in compost in each cropping season (
Effect of compost on number of pods per plant. Error bars represent standard error of the mean. Plotted values are means of 6 representative plants per replicate. The R-square values for the fitted regression lines are as indicated on each line.
Effect of compost on weight of pods per plant. Error bars represent standard error of the mean. Plotted values are means of 6 representative plants per replicate. The R-square values for the fitted regression lines are as indicated on each line.
Effect of compost on kernel yield. Error bars represent standard error of the mean. Plotted values are means of 6 replicates. The R-square values for the fitted regression lines are as indicated on each line.
Amending soils low in organic matter with compost is one of the sustainable means of improving soil fertility and crop productivity. Compost manure can enhance the growth of crops either by supplying plant nutrients or enhancing the supply and recycling of plant nutrients (
Total aflatoxin concentrations in kernels decreased with increasing levels of compost (
Effect of compost on total aflatoxin contamination in kernels. Error bars represent standard error of the mean. Plotted values are means of 6 replicates. The R-square values for the fitted regression lines are as indicated on each line.
The soil moisture content of the soil during pod-development in peanuts is not only crucial for minimizing pod colonization by
Adequate soil moisture is important to minimise soil temperature, an equally important factor influencing pre-harvest aflatoxin contamination in groundnuts (
An increase in soil microbial respiration is indicative of improved microbial activity in the soil. Compost manure inoculates the soil with microorganisms and adds nutrients to the soil (
It is noteworthy that although there were significant differences in aflatoxin levels, the observed concentrations were markedly low for the warm climatic region in which the experiment was conducted. Soil temperature data from a weather station situated within 8 km southeast of the study site indicated average soil temperatures of 23.2 C in the last 6 wk to harvesting of the peanuts coupled with a fairly distributed average annual rainfall of 905 mm during the two growing seasons (SASSCAL Weather data, Kenneth Kaunda International Airport). The weather conditions were more favourable for plant growth and development than for pre-harvest aflatoxin development and hence the low levels of aflatoxin observed across treatments. According to
Results from this study demonstrate the potential of compost manure to increase soil respiration, PAW, pod and kernel yield, and minimize aflatoxin development in peanut kernels at field level. There were significant increases in soil microbial respiration (R2 = 0.84) and PAW (R2 = 0.86) with increasing levels of compost. An improvement in soil microbial respiration is an important indicator for improved soil health. In terms of crop performance, compost had a strong positive correlation with kernel yield (R2 = 0.61) and a strong negative correlation with aflatoxin content in kernels (R2 = 0.85). With the potential yield achieved at the rate of 12 metric ton/ha, the study showed that compost can be used by local farmers for better crop performance. Additionally, the decline in aflatoxin levels with increasing levels of compost is an improvement in the quality of kernels. We therefore recommend the use of compost for better yield and lower pre-harvest aflatoxin content.
This study was funded by the U.S. Agency for International Development, under the terms of Award No. AID-ECG-A-00-07-0001 to The University of Georgia as management entity for the U.S. Feed the Future Innovation Lab on Peanut Productivity and Mycotoxin Control. We thank Idah Ngoma (University of Zambia) and Emmanuel Mulenga (Kasisi Agricultural Training Centre) for the technical support. We also acknowledge the Administration at Kasisi Agricultural Training Centre for availing the study site.
University of Zambia, Department of Soil Science, P. O. Box 32379, Lusaka, Zambia
International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Chitedze Agricultural Research Station, P. O. Box 1096, Lilongwe, Malawi
North Carolina State University, Department of Entomology and Plant Pathology, Box 7613, Raleigh, North Carolina, USA
North Carolina State University, Department of Crop and Soil Sciences, Box 7620, Raleigh, North Carolina, USA.