Soil mapping for precise land management David Cutress: IBERS, Aberystwyth University

Farming Monthly National|June 2020

Soil mapping for precise land management David Cutress: IBERS, Aberystwyth University
Soil is one of the most essential aspects of any agricultural production system, with different soil components able to have huge effects on crop or pasture yield and quality, as well as increasingly important roles in environmental sustainability.

Farmers have for centuries been adapting their management strategies based on observed physical and visual properties of soils, with more recent technological advances assisting in this practice. Components of the soil of major interest in agricultural include; nutrient levels (carbon - C, sulphur - S, phosphorus - P, potassium K, nitrogen – N, calcium - Ca and magnesium – Mg) fluctuation in which can significantly affect plant growth, physicochemical properties such as pH, texture (clay, silt and sand) and water contents. Traditional soil sampling and mapping were performed using a “W” shape field sampling pattern before grid sampling became common, this was improved through the innovation of global positioning system (GPS) technologies to allow accurate sampling localisations. The difficulty with soil sampling and mapping, however, is the huge complexity and variability present across both large and small geographic distances. To obtain information at a significantly high resolution (to be effective across individual fields) intensive soil sampling is required which is time-consuming, costly and requires repeating regularly. One technological innovation of note which can facilitate more targeted soil sampling, to provide information for more cost-efficient mapping, is electrical conductivity (EC). When utilised alongside precision application systems such as variable-rate technologies (VRTs) EC can assist in improving crop yields, pasture management, reducing input costs and assist in mitigating potential environmental impacts associated with chemical applications.

Electrical conductivity (EC)

EC measures the voltage of electricity conducted through soils and is associated with several different factors of interest within soils. EC analysis was previously performed on extracted soil samples placed in solution, this required physical sampling. To improve time and efficiency EC has been adapted to ECa (apparent soil electrical conductivity) which can be performed on bulk soil samples in a mobile fashion. Different soil profiles can affect soil conductivity including the makeup of sand (lower conductivity), silt (medium conductivity) and clay (high conductivity) as well as soil salinity levels. Whilst these simplistic determinants can often be made, the true nature of ECa is far more complicated. ECa, in reality, is detecting the conductivity through the pore systems of soils via water. Therefore, the size, shape, connectivity and water content of pores are the biggest factors on conductivity results. Other broad factors that have the potential to be correlated with ECa results include; mineral levels, soil moisture levels, depth to the water table and soil texture. ECa is one of the most common research assessment tools in soil systems having been used in the sugar cane industry (Florida) and the potato industry (Canada), with results suggesting, significant enough, correlations between ECa results and variables such as pH, magnesium and calcium nutrients levels. Results can play a role in improving soil mapping and highlighting management zones (MZs) of interest (Figure 1). MZs show a sub-division of an area which offers consistency within a defined measurement (eg. voltage or water content etc), therefore, allowing these areas to be treated with equal input quantities.

Figure 1 Two varied management zone maps based on ECa results a) loose mapping into two management zones b) more stringent mapping into three management zones – example figure

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June 2020