Basic Slag

New Zealand Journal of Agriculture, Volume 84, Issue 4, 15 April 1952, Page 315

Basic Slag

Pasture Topdressing in the North Island

By

C. DURING,

Assistant Investigating Officer, Department of

Agriculture, Wellington THE merits of basic slag as a phosphatic fertiliser have been ■ much discussed. Comprehensive experiments with this by-product of the steel industry were carried out overseas late in the 19th century and as a result the use of basic slag was recommended on soils rich in organic matter, soils not too dry, and on clay soils responsive to lime. Sir E. J. Russell in his bulletin "Artificial Fertilizers in Modern Agriculture" observes that clay land usually responds better to the application of slag than any other type of soil, particularly if the original vegetation contains clovers or allows for the spread of clovers. IN New Zealand in 1912, when already a considerable number of experiments had been carried out with basic slag and basic slag mixtures, B. C. Aston, then Chemist of the Department of Agriculture, wrote in the “Journal of Agriculture” as follows: “Basic slag of all substances most nearly realises the pastoralist’s dream of a magic something the touch of which should clothe the languishing earth with a perennial verdure”. Less lyrically in another article in the same year he stated the conditions under which basic slag seemed most beneficial to pastures. Aston considered that it required a climate with well-distributed rainfall; that it was better on clay soils than on sandy soils (although the coarse sandy pumice soils of the North Island were greatly benefited by slag); and that it was an alkaline fertiliser containing an excess of lime, and as most New Zealand soils are deficient in lime, he considered that they should respond to slag. Since 1912 many experiments have been conducted with basic slag. Most of them, about 400, were observational pasture topdressing trials, but some were mowing trials from which accurate records of pasture production were obtained for a number of years. Observational pasture topdressing trials are usually conducted for 3 years. During this time the vigour, palatability, and pasture species composition of the various plots are noted. Though these features give in most cases a good indication of pasture responses, it is possible that on first-class pastures capable of high production without the aid of fertilisers differences of 25 per cent, in pasture production can escape the eye of the trained observer, but on poor pastures responses of this magnitude are less likely to be missed. Northland A comprehensive review by E. H. Arnold of fertiliser requirements and responses of Northland pastures based on observational topdressing trials was published in the Journal of Agriculture” for February 1951. The information can be summarised as follows: 1. On pastures which have not received adequate lime: On such pastures basic slag has given responses Prior to those from superphosphate or serpentine superphosphate on the moderately weathered and quite fertile consolidated

sands of the west coast, and probably also on the limestone hills, although experimental evidence from these is less complete. On the other hand basic slag is usually superior to superphosphate and other phosphatic fertilisers on the more fertile volcanic soils derived from basalt, on the more fertile brown and grey clays derived from sedimentary rock, on the old terraces which once were alluvial flats, and, to a minor extent, on the more recently formed alluvial clays and meadow soils. On some soils basic slag is very much better than other phosphatic fertilisers. These include the more leached and infertile basalt soils, all soils derived from dolerite and andesite rock, and the poor gumland clays. Much of the superiority of basic slag lies probably in a lime effect, for all soils of Northland except the consolidated sands need lime to carry good pasture. 2. On pastures which have been adequately limed: When adequate amounts of lime have been applied basic slag has given equivalent results to superphosphate on old alluvial terraces, on the clay and meadow soils from more recent alluvium, and on the better gumlands and clays. In some soils, however, basic slag maintains its superiority over superphosphate even in the presence of applied lime. On volcanic soils, that is, soils derived from basalt, andesite, and dolerite, basic slag seems a little better than superphosphate even on well-limed pastures. The same seems true of the very poor sandy and pipe-clay gumlands. South Auckland District The most important soils of South Auckland, Waikato, and Rotorua-Taupo districts are derived from volcanic ash. The central plateau is covered with Taupo pumice and related ash showers. Pumice has also been deposited by rivers in the Waikato basin and in the Matamata district and is in places mixed with alluvium derived from the sedimentary rocks such' as sandstones, limestones, and mudstones. To the west of the pumice country, from Ohura to Huntly, andesitic ash (the Mairoa showers) — somewhat similar to the ash of Taranaki most undulating country and hills. North of the Mairoa showers the older andesitic Hamilton showers have formed an important series of soils. > Hamilton and Mairoa ash showers: Soils derived from the Hamilton showers cover most undulating country from Ohaupo northward to Tuakau, Pukekohe, and Patumahoe. They appear as brown clay loams on red-brown or red clayey subsoil with a tendency to dry out badly in summer. The Mairoa showers influence soil types from Ohaupo southward to Taumarunui County. Along the west

coast from the Mokau River to Raglan Mairoa ash has also covered all undulating and slightly hilly country and has been washed off only from the steeper hills and the more recent alluvial plains. The texture of the soil is friable and the colour ranges from light brown to dark chocolate. The darker and more andesitic soils derived from these showers occur west of the south Waikato basin in the coastal hill region.

Both Mairoa soils and Hamilton soils react similarly to basic slag. On ground requiring liming basic slag appears at least equal to and sometimes slightly better than superphosphate or serpentine superphosphate. When adequate liming is carried out, however, superphosphate and serpentine superphosphate seem to be slightly the better fertilisers. What applies to Mairoa and Hamilton soils applies equally to their alluvial derivatives, which are found south of Manukau Harbour and in the Henderson-Hobsonville district. An attempt was made to correlate progressive stages of leaching with basic slag response, because it was thought that basic slag might be better on the higher-rainfall areas than on the lower-rainfall areas, but no •evidence was found to support this. In the Hamilton soils, however, one trial found basic slag superior to superphosphate on limed and unlimed pastures. This trial was situated on Churchill clay loam, a very leached and poor soil derived from Hamilton ash and pumice alluvium. Pumice soils: A large area in the Putaruru, Rotorua, and Taupo districts is covered with soils derived from Taupo ash. These soils respond to lime only rarely. Perhaps for this reason basic slag appears more frequently to be inferior than superior to superphosphate and serpentine superphosphate, but the difference is only slight. In general, therefore, basic slag can be regarded as being as good a fertiliser as the more soluble types on the pastures of these districts. Volcanic ash soil of the Bay of Plenty: On these soils derived from the Kaharoa, Waihi, and Whakatane showers, basic slag does not seem to be 1 suitable. Even at its best in the slightly lime-responsive soils of the Waihi district basic slag seldom equals superphosphate in efficiency. Hill soils of Auckland Province: The hill soils derived from underlying

sedimentary rock tend to vary greatly within any one district. A very large number of trials are therefore necessary to cover all variations in hill soils and this is impracticable. General tendencies in pasture response to basic slag can, however, be observed, but they are as yet of limited practical value. Basic slag without lime seems superior to superphosphate without lime on the well-weathered but only moderately leached and often quite fertile hill soils of Raglan, Manukau, and Franklin Counties, but little difference is seen when pastures are adequately limed. Only on one observational trial, near Whangamarino, on very poor and leached hill clay resembling Northland gum soils was basic slag found to be definitely better than superphosphate on limed and unlimed pastures. Strangely enough, however, on the poor leached hills of Coromandel no difference could be seen between superphosphate and basic slag. Both gave good pasture responses. Much further south near Ohura trials have been conducted on quite fertile hill soils which have undergone less weathering than related soils in the north. Here basic slag was equal to superphosphate. Recent alluvial soils and meadow soils: Though the Hauraki Plains represents the largest area of these soils, pockets of' alluvial and meadow soils are found in all districts. In general no differences in pasture response have been observed between basic slag and superphosphate. Remainder of North Island The remainder of the North Island, for the purpose of observational pasture trials, extends southward from the East Cape, the Raetihi-Ohakune district, and the Mokau River. In the northern part of this region soils derived from various showers of volcanic ash are

important. Taranaki soils are developed from various showers of andesitic ash. The Ohakune-Raetihi district was covered with ash showers from Mt. Tongariro and the Taupo eruption. On the east coast from Tokomaru Bay to Eskdale various Taupo pumice showers have influenced soils on the easier hills and rolling country. 1. Volcanic soils of the Ohakune district: These soils are developed under an average yearly rainfall of 70in. Pastures respond well to liming and good results from basic slag on unlimed ground could therefore be expected. In fact basic slag appears to be very slightly better than superphosphate under such conditions, and is equal to superphosphate on pastures which have been adequately limed. 2. Volcanic soils of Taranaki: Basic slag is popular in Taranaki, and a great number of observational trials comparing. superphosphate with basic slag have therefore been carried out. In south Taranaki and the coastal districts of Waitotara County there is evidence that basic slag alone is slightly better . than superphosphate alone; where lime dressings were applied, however, no differences between the two fertilisers were detected. In north and central Taranaki, in the higher-rainfall belt, observational trials have failed to establish any superiority of basic slag. 3. Recent alluvial flats: This group covers a wide variety of soils. The soils vary from the heavy, often badly drained soils of high fertility found on the east coast and Hawkes Bay to the barren stretches of shingle and stones along some of the Hawkes Bay rivers. On the whole, where a pasture response to phosphates can be detected the quick-acting fertilisers seem slightly better than basic slag. 4. Older alluvial soils: These are mainly “light plains” soils such as in the Takapau-Ruataniwha flats in south

Wairarapa and in the Horowhenua areas. A comparison of basic slag and superphosphate on this soil series is difficult, because a large number of differing climatic conditions are involved. Large differences in response between basic slag and superphosphate have never been noted, however, and it can be said that the two fertilisers have equal merit on limed and unlimed pastures on these soils. 5. Yellow-grey earths: This class of soils has developed under the lowestrainfall conditions in the North Island and is distinguished from related types by the limited amount of leaching of soil nutrients which has taken place and by higher natural fertility. These soils are divided into the more friable and least leached soils of the undulating hills of central Hawkes Bay and the more compact and relatively more leached soils which are typified by the rolling country near Marton. The latter soils often suffer from poor winter drainage, but tend to dry out fairly quickly in the summer. Information is available only on this more compact class of soils. On the Marton Experimental Area accurate trials measuring pasture production under various phosphatic fertilisers have been studied. Here basic slag without lime is found to be as good as or slightly better than superphosphate or serpentine superphosphate without lime. On limed pastures there is little difference in the three fertilisers. In one trial the residual value of basic slag alone was found to be a little, but not significantly, better than that obtained on superphosphate alone. Observational trials on the whole confirm the Marton results. 6. Hill soils: Hill soils derived from underlying sedimentary rock tend to very greatly from paddock to paddock and from district to district. This makes an evaluation of fertiliser responses difficult. In general it can be said that on no hill soils has basic slag yet been found to give the outstanding results which are obtained on some of the hills in Auckland Province, particularly where kauri was a mature constituent of the original flora. In Wellington Province most hill country trials have shown a response to lime, but there are quite a few exceptions. Superphosphate and lime plots have been the best, followed by basic slag and lime. On unlimed ground sometimes superphosphate and sometimes basic slag has appeared to give the greater pasture improvement. There are, however, too few trials of this type to enable definite conclusions to be formed. It is of course desirable that more information on the best usage of available phosphatic fertilisers in hill country should be sought. This is being done at present and observational topdressing trials on hill country are being laid down in increasing numbers. It will be several years, however, before enough data will be accumulated to enable the most efficient phosphatic fertiliser for any particular type of hill soil to be named. Basic Slag-superphosphate Mixtures (Equal Parts) In 1912 a mixture of superphosphate and basic slag was officially recommended, probably for the first time, by

B. C. Aston in the “Journal of Agriculture”. Heating of the mixture and subsequent setting into a hard, con-crete-like mass were avoided by mixing the two fertilisers in the paddock just before spreading. This, of course, entailed a great deal of inconvenience and it may be mainly for this reason that the basic slag-superphosphate mixture has gone out of favour. Records of early trials with this substance are scanty,' but results from 12 trials carried out about 1930 are preserved. These show that in the Reporoa district the basic slag-super-phosphate mixture was slightly but consistently better than either basic slag or superphosphate alone. One very comprehensive and carefully conducted observational trial on the former experimental farm at Puwera, Northland, situated on poor gumland soil, revealed a rather remarkable superiority of this mixture over most other treatments. The basic slag-superphosphate mixture (equal parts) was compared with either fertilisers alone, with superphosphate plus annual dressings of 3cwt. of lime, and superphosphate plus 1 ton initial dressing of lime. All phosphatic fertilisers were applied annually at 3cwt. per acre. The mixture compared well with the plots receiving superphosphate plus 1 ton of lime and appeared much better than the rest of the treatments. Conclusion The results of observational pasture topdressing trials indicate that basic slag compares well with other phosphatic fertilisers on a large number of agriculturally important soils in the North Island. In the main these are soils on which pastures respond well to the application of lime. The performance of basic slag is therefore attributed in part to its high lime content. This opinion is borne out by the fact that where adequate liming is carried out serpentine superphosphate and superphosphate frequently appear slightly superior to basic slag. In many North Auckland soils, however, and possibly in some of the poorer hill country of the South Auckland and Raglan districts, basic slag is outstanding when applied on unlimed land and is still slightly better than other -tested phosphatic fertilisers on limed land. The reason for this is not known. Possibly the answer may lie with the trace elements present in basic slag. Strongly weathered soils, that is, soils of high clay content, seem to respond better to slag than weakly weathered soils, and this is particularly so where strong weathering is combined with high phosphate fixation. In the North Island rainfall seems to have little influence on the utilisation of basic slag by pastures. This is probably so because almost everywhere rainfall is sufficient and beyond a certain point additional moisture makes little difference to the solubility of slag in the soil. References "Basic Slag”, Department of Agriculture Bulletin No. 26, B. C. Aston (1913). Legend to series of soil maps of the North Island, Soil Bureau, Department of Scientific and Industrial Research (unpublished). "Volcanic Ash Showers”, Journal of Science and Technology, Vol. XII, No. 4, L. I. Grange (1931).