RE SEARCH Potatoes and soil moisture: when should I start irrigating? P otato roots extract moisture from the soil that is needed for the juice flow. The moisture cools the plant and transports dissolved nutrients. On a warm day of around 25 degrees, the crop can evaporate as much as 5 to 6 millimetres. So that can soon reach 30 to 40 milimetres a week. Most of the moisture is drawn from the layer at a depth of 25 to 50 centimetres. At the top of the ridge, the soil itself also allows evaporation, which makes the moisture content extra low. Cracking in clay soils encourages dehydration. Soil moisture has a typical stratification, in which there is lower moisture content high up in the ridge, more moisture at the bottom of the ridge, and the moisture in the subsoil is even higher (see Figure 1). Soil moisture sensors in the field must therefore be placed at the correct depth. measurements 11 juli 2018 Cracking in clay soils encourages dehydration. ometers and gypsum blocks is often even greater. The initial conclusion is that a soil moisture measurement without calibration should be seen more as a relative measure. average average - SD average + SD ridge queue bottom Figure 1: Variation of soil moisture with depth on a light sandy-loam soil. The data are averaged over sixty measuring points. Most moisture sensors measure electrical soil properties. The soil moisture is therefore measured indirectly via amperage. A laboratory calibration is necessary to convert the measurements to an absolute moisture content, because the measurements are influenced by soil composition (air, water, organic matter, clay minerals, etc.). In practice, a calibration is often skipped, so not too much value should be attached to the absolute value of a sensor in the field. The uncertainty with tensiSpatial distribution in soil moisture Another observation is that there is an enormous spatial spread in soil moisture on a micro scale. This is caused by occasional clods, crusts, cracks, worms, soil tillage, tractor tracks and other influences. The root system will adapt to these conditions by deeper or wider rooting. Table 1 shows the spatial variation of a plot in July. There are big differences in a 1 metre row. Sometimes there is only 0.01 cm3/cm3 difference (homogeneous), while in other places this difference increases over a distance of 1 metre to 0.21 cm3/cm3 (heterogeneous). The average standard deviation for a row is 0.041 cm3/cm3 (average value is 0.229 cm3/cm3). For a grid of 1 metre x 1 metre (six plants) there is slightly less spreading (average standard deviation is 0.033 cm3/cm3), from which it can be concluded that most of the differences are in the rows themselves, rather than between the rows. At plot level, the distribution of soil moisture is obviously greater (average standard deviation of 0.068 cm3/cm3), but this is mainly due to variations in soil texture and soil development. The sphere of influence of a sensor with needles of about 5 centimetres is around a few decimetres. The second conclusion is that a carefully installed soil Table 1: Spatial distribution of soil moisture for a plot of potatoes of 8.3 ha with light clay on 19 July. There were 650 measurements carried out. The standard deviation (SD) is shown. Area: Minimal soil moisture (cm3/cm3) 1 m - row 1 m2 - 6 plants 83,000 m2 – total field 0.111 0.155 0.026 Maximal soil moisture (cm3/cm3) 0.387 0.378 0.430 Average SD (cm3/cm3) 0.041 0.033 0.068 Lowest SD (cm3/cm3) 0.006 0.020 - - Highest SD (cm3/cm3) 0.084 0.094 24 Potato World 2018 • number 4 volemetric soil moisture (cm3/cm3) Pagina 23

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