Leaching and surface runoff losses of sulphur and potassium from a Tokomaru soil : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Soil Science at Massey University
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Date
1979
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Massey University
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Abstract
Sulphur and potassium surface and subsurface drainage water losses from grazed pastures on a yellow-grey earth soil, the Tokomaru silt loam, were investigated in field experiments. Runoff losses from undrained and drained pastures fertilised in spring or autumn were measured over a six week winter interval in 1976. Losses from undrained pastures were measured throughout the runoff season in 1977. In 1977, S and K leaching losses from pastures fertilised in spring or autumn, were determined by measuring tile drainage water losses and monitoring changes in soil S and K levels. An attempt was also made to relate soil S and K levels to tile drainage water losses. This field study illustrates that SO4
-S is readily leached in the Tokomaru silt loam. Losses in tile drainage waters occurred from all depths above the mole drains (i.e. 45 cm depth) during individual flow events. On average 7.5 kg dissolved S04
-S ha-1
was lost from the two non-irrigated pastures fertilised in spring. An additional 6.7 kg SO4
-S ha-1
was discharged in tile drainage waters from two irrigated pastures fertilised in spring (i.e. total 14.2 kg SO4
-S ha-1
). Evidence indicated that SO4
-S may have bypassed the drains in water seeping beyond the fragipan. An autumn application of fertiliser S (45 kg S ha-1
) significantly enhanced the extent of leaching. The equivalent of 10% of the applied S (4.47 ± 1.5 kg SO4
-S ha-1
) was leached over a period of 17 weeks from July 1 to September 21. Losses occurred throughout this period. On average, 15.2 kg SO4
-S ha>-1
was discharged from the two non-irrigated pastures fertilised in autumn. An additional 3.4 kg SO4
S ha-1
was lost from the two irrigated pastures. An appreciable quantity (13.8 kg SO4
-S ha-1
) of the fertiliser S applied in autumn but not leached in tile drainage waters, was recovered as water soluble SO4
-S, leached below the 20 cm depth (i.e. below the zone from which pasture species are likely to obtain most of their S. Over a period of six weeks in 1976, 0.9 kg SO4
-S ha-1
was lost in surface runoff from an undrained pasture fertilised (19 kg S ha-1
in superphosphate) in spring. Less SO4
-S was lost from the associated drained plot (0.2 kg SO4
-S ha-1
). Undrained and drained plots fertilised in autumn (57 kg S ha-1
in superphosphate) lost 8% and 1.8% of the S applied (i.e. 5.5 and 0.9 kg SO4
-S ha-1
) respectively. In 1977, on average only 0.8 kg SO4
-S ha-1
was transported in surface runoff off two undrained plots fertilised (36 kg S ha-1
in superphosphate) in spring. An average of 8.0 kg SO4
-S ha-1
was lost from two plots fertilised (55 kg solution S ha-1
) in autumn. Hence surface runoff is an important S loss mechanism if undrained plots are fertilised in autum. Sulphur received in the rainfall over a five month interval in 1977 amounted to 3.1 kg ha-1
. From these results it was concluded that total drainage water losses from non-irrigated, drained pastures were likely to be largely offset by S received in the rain in 1977. A significant net S loss (in relation to annual pasture S requirements) will have occurred from pastures irrigated the preceding summer and/or fertilised in autumn. Sulphur fertilisation in autumn and winter is not recommended. Under the conditions likely to prevail at this time an appreciable fraction of the applied S may be lost in drainage waters. Results of this study indicate that leaching is not an important K loss process in the Tokomaru silt loam. Dissolved K leaching losses from pastures fertilised in spring or autumn averaged 4.66 and 4.05 kg K ha-1
respectively. Potassium surface runoff losses are generally of no consequence. In 1976 only 1.1 kg K ha-1
was lost from an undrained pasture fertilised (50 kg K ha-1
) in spring, whilst 0.3 kg K ha-1
was discharged from the associated drained plot. A minimal fraction (3%) of the K applied in autumn (50 kg K ha-1
) to an undrained plot was lost in surface runoff. Less than 1% of that applied was discharged from the associated drained plot. Throughout 1977, on average, 1.35 kg K ha-1
was discharged from undrained plots fertilised (57 kg K ha-1
) in spring. An additional 3.75 kg K ha-1
was lost from pastures fertilised (55 kg K ha-1
) in autumn. Rainfall K additions measured ever a five month interval in 1977 were low (total 1.4 kg K ha-1
). However, because of the trend for K concentrations to vary on a seasonal basis it was concluded that K received in rainfall throughout 1977 was likely to largely offset total drainage water losses from undrained and drained pastures. The results indicate that K deficiencies in pasture on K retentive yellow-grey earth soils are not attributable to drainage water losses. Regression analyses showed that SO4
-S concentrations in leachate, but not SO4
-S loadings, were significantly related to water soluble soil SO4
-S levels (0-40 cm), determined at frequent intervals during the drainage season, if the quantity of water percolating through the soil is measured. No relationship was found between measured water soluble or ammonium acetate extractable soil K levels and leachate K concentrations or loadings.
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Keywords
New Zealand Tokomaru, Soil chemistry, Runoff