Water and nutrient distribution during trickle irrigation on three soils

English: Field trails were conducted in the Free State to quantify the affect of soil hydraulic properties on the dimensions of the wetted volume below trickle irrigation emitters. Three different soil types, a non-Iuvic fine sand (6-17% silt plus clay), luvic fine sand (9.5-20.75% silt plus clay) and sandy clay loam (26.5-39.75% silt + clay)) and three emitter discharge rates 1.2, 2 and 8 I h-1 were used to determine the water distribution pattern below trickle emitters. The water content, expressed in mm water per 100 mm soil depth, was measured with a neutron probe at 10 cm depth intervals till a depth of 155 cm, for each access tube 5, 25, 45, 65, 85, 105, 125 and 145 cm away from the emitter and between two emitters. The wetting patterns were presented in diagrams, indicating the increase in water content in each cell as shades of blue. Soil texture and emitter discharge rate played an important role in the distribution of water below trickle emitters. The most acceptable way to quantify the wetted area was the ratio between the width and depth of the wetting front. The width: depth ratio of the wetted volume emphasizes the shape of the distribution pattern. It was found that this ratio increased with an increase in the average silt plus clay percentage of the wetted soil, for all the emitter discharge rates. This effect of texture was more pronounce for emitters with a discharge rate of 2 l h-1 and higher. The effect of emitter discharge rate on the width: depth ratio was different for the three soil types. For both the luvic fine sand and sandy clay loam, irrespective of initial wetness, the width: depth ratio increased sharply from the 1.2 I h-1 to the 2 I h-1 emitter rates after which it remained the same to 8 I h-1 emitters. On the non-luvic fine sand emitter discharge rate had little effect on the width : depth ratio. Practically, higher emitter rates is preferable on sandy soils and lower emitter rates for both the luvic fine sands and sandy clay loams. Because the design criteria for trickle irrigation systems in South Africa presently does not take the physical and hydraulic soil properties into account when designing trickle irrigation systems, present guidelines used in South Africa were evaluated by using important properties like soil texture, which came forward in the study. The width to depth ratio of not less than 0.8 which is currently used in South Africa was compared with actual measured data from the study. The measured width: depth ratios for all three soil types were higher than 0.8 and it increased with an increase in the silt plus clay percentage. It was difficult to set guidelines because a fixed in line emitter spacing of 60 cm was used throughout the study which caused wetting volumes to overlap, especially on soils with a high silt plus clay content. This overlapping could influence the width to depth ratio. A proposal for emitter spacing was obtained by multiplying the measured width : depth ratio by 0.8 to give the emitter spacing factor. Thus, when the required depth of wetting which is affected by rooting and soil depths, is known, the inline emitter spacing can be calculated by multiplying the wetting depth by the emitter spacing factor. The following emitter spacing factors could be used to determine the inline emitter spacing of the experimental soils: non-luvic fine sand (0.88), luvic fine sand (1.28) and sandy clay loam (2.56). A single line with a constant inline emitter spacing of 60 cm was used to determine the lateral wetting width. Pedotransfer functions, relating the wetting depth after 20 mm irrigation was applied on a wet soil to the mean silt plus clay percentage of the wetted soil volume, were derived. These functions were used to estimate the wetting depth for different silt plus clay contents, increasing with 2% intervals. Guidelines that can be used to estimate the maximum lateral spacing for a soil, from the mean silt plus clay percentage of the potential wetting volume, were derived for different emitter discharge rates. These guidelines are proposed for soils with predominantly fine sand in the sand fraction. In a second field trail the effect of timing of nitrate application through trickle irrigation on the NO3- distribution through the profile was detected. This experiment was done on a luvic fine sand with an emitter discharge rate of 2 I h-l This experiment consisted of two treatments. For the first treatment, the nitrate was applied in the beginning of a 20 mm irrigation event on a dry soil. After 4 days an additional 20 mm irrigation was applied. Nitrates moved away from the emitter and pockets of nitrate formed in the topsoil 30 cm away from the emitter. The nitrate distribution pattern stayed the same after an additional 20 mm irrigation. Nitrate spread laterally more evenly throughout the profile, with a wider and deeper distribution than when applied at the beginning of the irrigation event. The application of nitrate in the beginning of an irrigation is ideal where a single emitter supply nitrate to more than on plant for example, for crops that is spaced close together or where the emitter lines is laid between two crop rows. During a second treatment, nitrate was applied at the end of the irrigation event which was followed up by an additional 20 mm irrigation, four days later. The highest concentration nitrate stayed right below the emitter with some lateral movement. Most of the nitrates moved with the water when an additional 20 mm water was applied with small pockets of nitrate forming in the topsoil, 40 cm from the emitter. The application of nitrate at the end of an irrigation is ideal where more than one emitter is used to apply nitrate to a single plant for example, tree crops.