Interpretation of electrical conductivity measurements from ceramic suction cups, wetting front detectors and ECH2O-TE sensors

Electrical conductivity (EC) measurements are often used to identify and address soil salinity issues in irrigated cropping systems. In this study, measurements of soil solution EC (EC-sol) collected in ceramic suction cups (SCs), wetting front EC (EC-wf) collected in Fullstop wetting front detectors (WFDs) and soil bulk EC (EC-bulk) measurements made using ECH2O-TE sensors and converted to EC-sol, were compared. As a result of different methods of measurement and different components of soil waterflow being sampled, variations in EC measurement between SCs and WFDs were observed. EC-sol was usually higher than EC-wf, as expected for this system, due to incomplete mixing between the draining and resident soil water during infiltration. For periods of high solute leaching, however, the opposite can occur, indicating that WFDs are sampling when solutes are first mobilised at the beginning of the leaching event. The ECH2O-TE sensors were less effective in measuring the short-term EC dynamics but were able to detect general changes in soil salinity. This could reflect difficulties estimating soil EC-sol from measured EC-bulk, especially at low soil water contents. Each of these instruments show good potential for application to guide salinity management practices, but a more detailed study on a range of soils subjected to different watering regimes is needed to further improve interpretation of EC measurements and their application.The Water Research Commission (Project 1574), the National Research Foundation, the Cooperative Research Centre for Irrigation Futures and CSIRO.http://www.plantandsoil.co.zanf201

Modelling nitrogen leaching : are we getting the right answer for the right reason?

The complexities and challenges in quantifying N leaching have led to development of a range of measurement and modelling techniques, but none are widely applied. Observations that N moves more slowly than water through the soil profile has resulted in different approaches being used to simulate impeded N movement in crop models: (i) by accounting for nitrate NO−3 adsorption to the soil, (ii) by considering incomplete mixing between resident and draining soil water fractions or (iii) a combination of both.We compare and discuss strengths and weaknesses of these approaches. Our inability to directly measure model parameters (especially with regards to simulating N dynamics), and the risk of compensating errors during model testing and calibration, often results in low confidence in simulated N leaching. We caution that our current ability to simulate N leaching is in most cases not yet well enough developed for reliable and accurate predictions. We recommend a more strategic approach involving better linking measurement and modelling to improve understanding of the critical soil processes that control N leaching as one way of further improving our understanding and quantification of N leaching.http://www.elsevier.com/locate/agwathb201

Ecological and agro-economic study of small farms in sub-Saharan Africa

Land degradation, rising population and poverty in sub-Saharan Africa threatens the agricultural sustainability and productivity, quality of the environment and socio-economic wellbeing of rural populations. We studied farm ecological, economic and social sustainability, productivity and production risks in the Mbeere District of Eastern Kenya. We used a soil nutrient monitoring methodology to collect data from 30 households. Ecological sustainability was threatened by soil nutrient decline at rates of 1.7 kg P and 5.4 kg K ha$^{-1}$ half year$^{-1}$ while N was nearly balanced in soils. Soil phosphorus and potassium stocks, in the cultivated soils, declined at rates of 0.3% and 0.1% half year$^{-1}$, respectively. Farm economic returns were positive, albeit low, and could not sustain the livelihoods of the households. All the 30 households were living below the poverty line of 1 US dollar a day. Farm productivity was low, with livestock and yields of major staple food crops below on-farm target yields. To spread out the risks of production, farming households were cultivating an average of 4.7 crop fields, keeping more than two types of livestock and practising intercropping systems. Intercropping maize-beans reduced nutrient decline and raised household incomes compared with monocropping of either of the two crops. Despite the low rates of nutrient decline, high risks of production and the low crop yields, the livestock productivity and farm economic performance put the sustainability of these farming systems into question. The low levels of nutrient decline in small farms averaging at 1.7 kg P and 5.4 kg K ha$^{-1}$ half year$^{-1}$ contrasts with the high nutrient depletion rates on macro-scale levels, e.g. 20–40 N, 3.5–6.6 kg P and 20–40 kg K ha$^{-1}$ year$^{-1}$ for Eastern African countries and 22 kg N, 2.5 kg P and 15 kg K ha$^{-1}$ year$^{-1}$ for sub-Saharan Africa. These findings indicate that the extent of nutrient decline and conservation differs across sub-Saharan Africa. The positive contribution of intercropping to nutrient balances suggests the need to encourage farmers to adopt such systems rather than monocropping

Exploring options for integrated nutrient management in semi-arid tropics using farmer field schools: a case study in Mbeere District, eastern Kenya

The farmer field school (FFS) approach was used in semi-arid eastern Kenya in the period 2002–2003 to explore technology options for addressing declining soil fertility and to institute learning processes on integrated nutrient management (INM).The farmer field school (FFS) approach was used in semi-arid eastern Kenya in the period 2002-2003 to explore technology options for addressing declining soil fertility and to institute learning processes on integrated nutrient management (INM). Participatory diagnosis of soil fertility constraints and experimental design workshops led to the formulation of the INM-FFS learning curriculum and choice of INM technologies for testing in the central learning plot. INM technologies jointly chosen for testing included farmyard manure (FYM = T-1), diammonium phosphate (DAP = T-2), combined application of FYM and DAP (T-3); and T-3 combined with Tithonia diversifolia applied as green manure (T-4). Maize was used as a test crop. The treatments were replicated twice using a pair-wise design and data collected, bi-weekly, using the agroecosystem analysis (AESA) framework to aid learning and data analysis with farmers. Farmers' evaluation of the trials was conducted at the end of the study period using matrix scoring and ranking. Treatments with combined application of organic and inorganic materials had better agro-economic performance than sole application of either FYM or DAP (T-4 > T-3 > T-2 and T-1) and they had a high value cost ratio (VCR > 2). The technologies of T, to T-3 did not have a positive impact on either nitrogen or phosphorus balances. However, T-4 resulted in a positive partial N balance. Farmers' evaluation corresponded well with the majority of the quantitative agro-economic analysis. The study showed that there is a potential to use FFS for INM technology development and testing by stimulating interactions, farmer learning and closer working relationships between farmers, research scientists, extension and the institutions that they represent

Obtaining the parameters required to model labile phosphorus for South African soils

Modelling phosphorus (P) in the environment can increase our understanding of potential transfer pathways into receiving water bodies as well as the plant availability of this nutrient in soil. Many current models make use of algorithms originally developed for the EPIC model over two decades ago. These algorithms were developed primarily using continental USA soils. Obtaining the required input parameters can therefore be challenging when applying this approach to soils not classified according to the USA system, and for soils for which similar parameters are not available. In this paper, new equations for the estimation of labile P from Ambic P, Bray 2 P and the modified ISFEI method are proposed. Guidelines for the classification of South African soils as calcareous, slightly weathered and highly weathered are further suggested, and we propose that only topsoil properties be used for this purpose. Depending on the amount of soil information available, this classification can be achieved using the clay fraction SiO2:Al2O3 molecular ratio, the sum of exchangeable Ca, Mg, K and Na, or a newly proposed categorization system for South African soil forms. It is clear that the above approaches should be thoroughly tested and relevant local research carried out to improve our ability to model P in South African soils

The benefits of conservation agriculture on soil organic carbon and yield in southern Africa are site-specific

Conservation agriculture (CA), with reduced tillage, permanent soil cover and diversified cropping systems, is advocated in southern Africa to improve soil quality, reduce input costs and mitigate climate-induced risks. However, improvements in terms of yield and soil organic carbon (SOC) under CA are slow and variable and many small-scale farmers are unable to buffer themselves against potential short-term financial losses. In this study we examined the effects of CA-related management practices on SOC sequestration and productivity at two medium-term sites on a sandy soil (eight year trial) and clay soil (six years) in maize producing areas of South Africa. Using field data, current input costs and market prices for crops, we calculated the gross margin for each system. Treatments compared conventional ploughing under maize monoculture with reduced tillage, intercropping and crop rotation. On the clay soil, SOC was increased under reduced tillage (57.6 t C ha−1) compared to conventional tillage (54.9 t C ha−1) while there was no difference for the sandy soil (19.7 t C ha−1 average across treatments). Profitability was most strongly influenced by seasonal rainfall, but was higher on the sandy soil than the clay soil, with an average gross margin of R11,344 ha−1 and R5,686 ha−1, respectively. This study has demonstrated that while certain CA practices can create site-specific benefits for farmers, it is highly dependent on local weather and soil conditions. For the clay soil an additional payment scheme would be required to reward farmers in southern Africa for C-sequestration to make CA profitable and achieve increased C-mitigation through soil sequestration.CMS, LHS, MPH and RPR were supported by the German Federal Ministry of Education and Research via the ‘Limpopo Living Landscapes’ project within the SPACES programme (Grant Number 01LL1304A ). The field trials were supported by ARC-Grain Crops Institute (GCI) (Buffelsvlei) and ARC-ISCW (Zeekoegat), and funded by the Maize Trust of South Africa.http://www.elsevier.com/locate/still2019-11-01hj2018Plant Production and Soil Scienc