A study of a set of hydrosequences and chronosequences in Manawatu County

Abstract

Some hydrosequences and chronosequences developed on windblown sand in the Manawatu-Rangitikei sand country were sampled and soil morphogenesis outlined by detailed profile descriptions. The extent to which the state factors have been controlled in the sequences was discussed.

A review included elements of soil and state factor analysis, the properties of oxygen deficient soils, some misunderstood aspects of iron and phosphorus translocation in soils, the findings of hydrosequential and quasi-hydrosequential studies and the results of some previous work in the Manawatu sand district.

Soils with high water tables were found to have, with respect to those of lower normal water-table levels, the following attributes accumulations of clay and silt sized material of postulated loessial origin; greater accumulations and sharper depth gradient of organic matter, CEC capacities mirroring the organic matter accumulation, lower bulk densities; higher cation saturation values, higher HCl extractable P contents; lower Tamm's oxalate Al levels, similar or lower levels of Tamm's oxalate Fe levels; similar total P losses; similar non-occluded P levels; slightly lower occluded P levels as a consequence of reducing conditions; and markedly lower organic P levels possibly after losses due to increased hydrolysis with increased soil reaction and reduced sorption.

With soil development under freely drained conditions the initially rapid organic matter accumulation became increasingly slow. In 3 x 10³ years oxidizable carbon, total N and organic P increased at rates of 22, 2.6 and 0.18 kg.P/ha.m/yr. respectively. The Cₒₓ/Nt and Cₒₓ/Pₒ ratios widened from 5.8 to 11.1 and 28 to 102. A corresponding increase in CEC but reduction in cation saturation levels was noted. Increased accumulations, due to weathering, of clay and silt sized material, occluded P and Tamm’s oxalate Al and Fe were found.

Nett losses of 50% of "time zero" content of HCl extractable P were observed while nett total P losses of 17% of "time zero” levels occurred over a period of 3 x 10³ years. The rate of nett total P loss from a metre profile was 0.27 kg. P/ha.m./yr. Soil development under very poorly drained conditions with respect to freely drained environs results in a more rapid organic matter accumulation. In 3 x 10³ years, oxidizable carbon and total N have increased at 42 and 0.41 kg. P/ha.m./yr. respectively. Microbial decomposition is suggested to be maximal for intermediately well drained soils and restricted by the lack and excess of moisture in excessively and very poorly drained situations. Organic P enrichment occurs at a rate of only 0.06 kg/ha.m. P/yr. Losses of HCl extractable P are lower but total P nett losses are similar. A reduced rate of accretion of Tamm's oxalate Al but similar rates for Tamm’s oxalate Fe were noted in the studied gley soils.

The study delineates soil development under gleyed conditions for the soils indicating the extent to which gleying retards, and even reverses, the chronosequential transformations and modifies some of its pathways.