Use of Molybdenum in Rodney County

New Zealand Journal of Agriculture, Volume 96, Issue 5, 15 May 1958, Page 439

Use of Molybdenum in Rodney County

By

C. M. BUCK,

Fields Instructor, Department of Agriculture, Warkworth

THE first experiment with molybdenum topdressing on pasture in Rodney County ' began in 1950 in the Te Hana district at the suggestion of E. B. Davies, Senior Principal Scientific Officer, Department of Agriculture, Hamilton. Subsequently a number of trials were carried out in other parts of the county. The excellent responses obtained in these trials induced farmers to topdress considerable areas with molybdenum phosphate and. some remarkable results have been obtained.

RODNEY County is in the lower portion of the North Auckland Peninsula and has a farmable area of about 270,000 acres, of which twothirds are flat to undulating and onethird hilly. Though molybdenum responses have been obtained on both flats and hills, the hill country farmer will gain the greatest benefit.

Most of the hill country soils, on which liming is seldom economic, are derived from sandstone and these soils show a great response to molybdenum. On the other hand much of the easiercontoured soils have been well limed; up to 30,000 tons of lime have been applied annually in the county. With the consequent reduction in acidity the soil molybdenum appears to have been made available to plants, and molybdenum does not generally give outstanding responses when applied to heavily limed soils on the lower country.

Past Topdressing Practice In the past the topdressing of these hill country soils has been confined to the application of basic slag, superphosphate, serpentine superphosphate, and a mixture of basic slag and super-

phosphate. Superphosphate and serpentine superphosphate alone give only slight pasture responses, but when they are sown with heavy rates of lime good pastures are often produced. Unfortunately the application of the heavy rates of lime required is not practical on hill country. Basic slag alone gives a fair response on these soils, but this finely ground material is not suitable for aerial topdressing and when it is mixed With superphosphate for aerial application the response appears to be more or less in direct relation to the amount of basic slag applied.

Experiments with Molybdenum Experiments on these hill soils have demonstrated that molybdenum will

give outstanding responses. Sodium molybdate applied alone in solution has in some instances given better responses than 3 cwt. of basic slag per acre. Pastures not previously topdressed have been changed from paspalum, danthonia, browntop pastures into dominant white clover and subterranean clover pastures solely by the application of molybdenum. Clovers are much more susceptible to molybdenum deficiency than are grasses, and molybdenum responses are usually in the form of increased clover. After a year or more of good clover

growth it is then noticeable that the build-up in nitrogen is sufficient to stimulate the grasses, and perennial ryegrass becomes established. Another feature of molybdenum responses has always been the increased palatability. Hill country farmers may take advantage of this by making the first application of molybdenum to the shady hill faces, where fern and manuka invasion is often a problem. These weeds may then be checked by the stock which will be attracted to these areas. In this way bracken fern was almost eliminated from the molybdenum treated plots of one trial.

. One hill country experiment designed to test the effect of serpentine superphosphate, basic slag, and lime at various rates with or without molybdenum has given the following results, in which 0 means no visible difference from control; 1, slight difference from control; 2, fair difference from control; 3, good difference from control; 4, very good difference from control; . 5, excellent difference from control.

No molyb- Molybdenum denum 1J cwt. of serpentine superphos- ’ phate plus 1J cwt. of lime .. 0 2J 3 cwt. of serpentine superphosphate and 3 cwt. of lime in alternate years .. .. 0 2J 3 cwt. of serpentine superphosphate .. .. .. .. 0 2J Control . . . . . . .. 0 2 3 cwt. of serpentine superphosphate plus 3 cwt. of lime 1 3J 3 cwt. of serpentine superphosphate plus 1 ton of lime . . 1J 4 3 cwt. of basic slag . . 2 4 1J cwt. of basic slag plus 1| cwt. of serpentine superphosphate 1J 3|

The results from this experiment indicate that there was little benefit from the application of serpentine superphosphate alone or in combination 1 with light rates of lime. Basic slag gave a better response than superphosphate plus 1 ton of lime and slag plus molybdenum gave better results than superphosphate plus molybdenum. This seems to indicate that either the molybdenum rate was insufficient to correct . the deficiency fully dr that some other trace element in the slag is producing a response. It would appear to be the latter, because in another trial nearby molybdenum at 1 oz. per acre gave outstanding results. Another interesting feature of the. trial is that’ molybdenum alone gave a better response than either superphosphate plus 1 ton of lime or 11 cwt.

of superphosphate plus 1| cwt. of basic slag and was equal to 3 cwt. of basic slag per acre. Therefore it seems that the best results can be obtained by applying molybdenum serpentine superphosphate and then a mixture of basic slag and serpentine superphosphate or serpentine superphosphate alone. All of these can be applied by aeroplane. Frequency of Molybdenum Application Because of the possibility of excess molybdenum causing an imbalance with copper the frequency of application is important. It is difficult to obtain definite evidence on just how frequently molybdenum should be applied. However, trial responses have been visible for as long as 3 years after application, though after this period there were indications that the 'response was declining. It is therefore considered inadvisable and unnecessary to apply molybdenum more frequently than once in every 4 years.. .'. Rate of Application The rate' of application is also important if any possibility of a copper deficiency is to be avoided. In the North Island molybdenum superphosphate is mixed at the rate of 1 lb. of sodium molybdate per ton of fertiliser.

This mixture, if applied at 3 cwt. per acre, will provide just under 2| oz. of molybdenum per acre, a rate which has given outstanding results; in some trials 1 oz. per acre appeared to be sufficient. . Some farmers with limited finance who realise that the molybdenum is the important ingredient of molybdenum superphosphate, as the above trial results showed, have had the mixture made at double strength (2 lb. per ton) and sown at 1% cwt. per acre, with excellent results. Soil Responses On the easier soils where liming has been carried out molybdenum is usually much less effective, but responses are still obtained. In one trial a slight molybdenum response was obtained on a soil which had been topdressed with lime at 6 tons per acre over 5 years. In these sandstone soils it appears that the responses obtained from heavy liming have partly been due to the molybdenum already present in the soil becoming available as the acidity is reduced. This is borne out by the fact that on a soil which would normally be considered to require 2 to 3 tons of lime per acre an excellent pasture was obtained by the application of molybdenum superphosphate plus 10 cwt. of lime.

The soils in Rodney County which respond to molybdenum are all those derived from sandstone and some which are derived from claystone, mainly gumland soils. However, the availability of molybdenum seems to vary within a soil type, as a molybdenum response has been obtained on a soil type in one area while pastures on the same soil type in an adjacent area did not respond. The first molybdenum trial in the county was carried out in a field which appeared to be in poor heart except for patches of dark green paspalum, resembling nitrogen patches, scattered throughout the field. These patches, varying in size from a square yard to j acre, may have been due to the higher availability of molybdenum. In some instances heavy rates of basic slag have appeared to correct molybdenum deficiency, but this has not occurred on every occasion and probably it depends on the molybdenum content of the basic slag, on the rate of application, and also on the extent of the deficiency. Molybdenum and Copper The molybdenum : copper ratio of soils is said to be important if copper deficiency in stock is to be avoided. The copper status of most soils in

Rodney County, though not high, appears to be satisfactory, but there have been instances of mild copper deficiency in cattle on soils derived from sandstone. Though considerable quantities of molybdenum have been used in the county, there has so far been no stock trouble which can be definitely attributed to molybdenum. However, farmers using . molybdenum on these soils and gumland and other soils where the kauri tree has been common should watch stock for copper deficiency symptoms and apply copper if necessary. The great improvement that usually follows molybdenum topdressing will warrant the extra expense of copper .topdressing if this element becomes deficient. However, it is advisable for farmers to proceed cautiously and apply molybdenum to only a portion of the farm each year. This will ensure that any copper upset will be minimised and also the stimulated clover which is likely to increase the bloat potential of the pasture will be present on only a portion of the farm. Molybdenum is not dangerous if it is used intelligently and the subsequent reactions of livestock are observed, and farmers should take advantage of the outstanding pasture improvement that usually follows the correction of molybdenum deficiency.