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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Pasture Improvement With Special Reference to Trace Elements
F. W. CRADOCK, B.Sc. (Ag.), Chemist's Branch, Dept. of Agriculture.
The popularity of pasture improvement in this State is well emphasised by the increased areas established each year. For example, in 1948-49, 2¾ million acres in N.S.W. were estimated to be sown to pasture. This included some 1½ million acres of paspalum. In 1953, 4¾ million acres were under improved pasture; but by 1954 the area had increased to six million acres. The 4 million acres and 6 million acres include the 1½ million acres of paspalum. In the last six years there has been, therefore, an increase of over 3 million acres of sown pasture.
With this increase in pasture acreage certain problems have arisen as far as establishment is concerned. The most important individual factor in the establishment of pasture is the fertilizer superphosphate. The importance of superphosphate in this field can be gauged from the following figures:
In 1942-43, 15,000 tons were used on pastures and in 1951-52, 83,000 tons; representing a six-fold increase, whilst the amount of superphosphate used on crops has not altered materially.
RAINFALL:
The incidence and amount of rainfall are also vital to pasture establishment. In the south west, Dwalganup, a species of sub-clover, has been established now on areas receiving as little as 17 to 19" of rainfall. On the northern slopes and plains, pasture now is established successfully on areas receiving 20-24" of rainfall per annum. The establishment is influenced by the technique used and the varieties chosen; for example, in Queensland lucerne is sometimes used as a row crop to avoid competition with weeds until it is established.
SURVEY BY DEPARTMENT & C.S.I.R.O. OFFICERS :
In order to gauge the importance of various factors responsible for pasture establishment, a survey was undertaken by Messrs. R. Weir and F. Hartridze of the Department of Agriculture and Mr. R. Fawcett of C.S.I.R.O. They inspected trials which had been established in areas throughout the State, extending from the Queensland to Victorian border. In all they visited some 88 trials and made observations on growth and also took yield cuts. Of the 88 trials inspected 54 responded very markedly to superphosphate, 11 showed increased growth resulting from molybdenum dressings, 14 responded to a ton of lime and 10 to 2 cwt of lime; whilst 23 of the trials showed ineffective nodulation of the trials which did not respond to superphosphate, some trials had been established on old pasture which previously had received heavy superphosphate dressings, often up to 10 cwt. per acre, and others were severely affected by drought and poor nodulation. In the type of trials examined it was not possible in all to sort out responses to sulphur from these responses where phosphorus was primarily deficient. Apart from superphosphate, molybdenum and lime responses few of the elements showed any significant effects. There were depressions in some areas due to copper applications. Molybdenum response mainly occurred on sedimentary rocks such as sandstone and shales.
MOLYBDENUM REQUIREMENTS OF PLANTS :
Many species of plants have different requirements for molybdenum. Grasses have a very low molybdenum requirement. Subclover uses more molybdenum than grasses, whilst medics have a comparatively high molybdenum requirement. Subclover, for example, in its plant material may contain 1 to 2 parts per million, grasses may only contain 0.1 or 0.2 parts per million. This fact was nicely illustrated in a number of plots located in the Wagga area. Plots which had been sown to subterranean clover and medics for a number of years were replaced with wheat. Where wheat succeeded the medics growth was poor and the wheat plot appeared to be suffering from nitrogen deficiency Medic plants in adjoining plots were sprayed with molybdenum and they responded quite markedly. From this it is concluded that the molybdenum shortage in the medics was affecting their ability to fix nitrogen. Nitrogen starvation was then shown in the subsequent wheat plots.
C.S.I.R.O. WORK:
Usually when studying mineral requirements of plants and pasture it is impossible to separate the needs of plants for major and minor elements. K. McLoughlin of C.S.I.R.O. carried out a very extensive survey of soils in N.S.W. in the high rainfall areas. He sampled approximately 100 soils and then used these for pot work. Subsequent results revealed that phosphorus deficiency was more acute than sulphur deficiency, which in turn was more acute than molybdenum deficiency. Mr. McLoughlin obtained a phosphorus response in 68 per cent of the soils, a sulphur response in 31 per cent of the soils and no molybdenum response in the absence of other elements. Continuing on further he found that once sulphur was corrected there was then a phosphorus response in 87 per cent of the soils. When phosphorus and molybdenum deficiencies were corrected it was found that 81 per cent of the soils responded to sulphur, and when phosphorus and sulphur deficiencies were corrected 31% of the soils responded to applications of molybdenum. From this it can be seen that there is little use in looking for the response to individual elements whilst some of the more important elements may be lacking.
ANDERSON TYPE TRIALS:
This fact has been realised for many years and it is on this basis that Mr. A. J. Anderson of C.S.I.R.O. devised a simple pilot trial to test the lack of response due to the omission of any one element from a mixture of all the necessary elements for plant growth. This type of trial now has been accepted by the farmers and Departmental officers and has been established on many hundreds of sites throughout N.S.W.
METHODS OF DIAGNOSING DEFICIENCIES:
There are a number of methods of diagnosing mineral deficiencies in plants which when used in conjunction with one another are quite useful but used alone present certain difficulties.
Soil Analysis: Has received a lot of publicity in recent years but to carry out useful soil analyses very detailed work is involved. The popular conception of soil analysis is to take a sample and submit it to the Department to have all deficiencies determined. However, this picture is much more complicated and soil analysis to be of any use involves very careful sampling and detailed analysis In the case of trace elements it is usually unsatisfactory to attempt to determine deficiencies by means of soil analysis.
Plant Analysis: By analysing the actual plant material itself it is often possible to obtain a direct measure of the nutrients which have been taken up by the plant. The Department is now doing routine analysis of plant material.
Visual Symptoms: The most popular means of diagnosing nutrient deficiencies in plants is perhaps by means of visual symptoms. In fruit trees and vegetables people skilled in symptoms can recognise readily plants which are being starved of one or more nutrients. Unfortunately it is much more difficult to recognise deficiency symptoms in grasses and legumes than it is in the higher forms of plant life.
Pot Culture: It is possible by means of growing plants in special pots to determine deficiencies in one growing season. Under these conditions plants receive ample quantities of water and if deficiencies do exist it is usually possible to obtain results within a year.
Field Trials: Results of work carried out by soil analysis, plant analysis and pot trials must be proved finally in the field. Before recommending large scale planting it is usual to carry out field trials to verify the responses.
RESULTS OF PLANT ANALYSIS:
In the M.I.A, there are numerous outcrops of calcareous soils. It therefore would be expected that given adequate moisture, pasture will suffer eventually, as in the case of fruit trees, from manganese, zinc and a possible boron deficiency. Some red clover plants were collected beneath a citrus tree showing clearcut symptoms of manganese deficiency. On analysis it was revealed that the red clover plants contained 8 parts per million of manganese in contrast to red clover plants collected from the Northern Tablelands; grown normally and containing from 86 to 500 parts per million of manganese. This obviously is a clearcut case of manganese deficiency as shown by plant analysis.
CONDITIONS WHERE MOLYBDENUM DEFICIENCY MAY BE EXPECTED :
Although McLouglin of C.S.I.R.O. did not find any strong correlation between molybdenum deficiency and such factors as soil type and acidity,etc., molybdenum deficiency is expected normally under acid conditions, i.e., with pHs below 4.5 and 5. Molybdenim in some soils may be absolutely deficient or it may be rendered unavailable due to acid conditions. In the first case it is only possible to correct molybdenum deficiency by the use of molybdenum salts as a soil dressing or a spray. In the second instance it is conceivable to correct the deficiency in two ways: (1) by the use of lime to lower the acidity and release the molybdenum, and (2) by further applications of molybdenum.
REASONS WHY MORE TRACE ELEMENT DEFICIENCIES HAVE NOT BEEN FOUND IN THIS STATE:
The only worthwhile trace element deficiency in pasture in N.S.W. is molybdenum. States such as Victoria, South Australia and Western Australia have fairly widespread tracts of country in which copper, zinc and manganese deficiencies exist. Zinc and manganese deficiencies usually occur on alkaline or very sandy soils. Copper deficiency may occur in alkaline soils or sandy soils or peat soils. These sands would have been originally poor in copper and zinc. The higher pHs in addition to this would tend to make these elements unavailable. In N.S.W. the soil types are not similar to those described in the other States. The Department now has a powerful Nutrition Team and trials for a number of years have been located in practically all parts of the State The results to date have shown that molybdenum deficiency is the only important trace element deficiency in pasture in this State. In very high rainfall years a few other trace element deficiencies have been reported; for example, on the red loams in Lismore, copper, zinc and molybdenum deficiency showed up. Copper and zinc showed up one year at Gosford. Slight responses to boron and molybdenum have been recorded in the Goulburn and Cooma districts. Copper deficiency has been noted in a few areas within the State, including Grafton and Tooraweenah.
TESTS FOR MOLYBDENUM:
On plants other than legumes it is possible to use an indirect test for molybdenum. Molybdenum functions in the plant by breaking down nitrates to nitrites which then go on to form amino acids and proteins. In the absence of molybdenum, nitrates accumulate in the plant material. By testing a small amount of plant material with diphenylamine in strong sulphuric acid for nitrate accumulation it is possible to determine a molybdenum deficiency. When nitrates are shown to accumulate by this test it is assumed that molybdenum is deficient. Unfortunately, this test cannot be used on legumes, but has been used with considerable success in N.S.W. on vegetables such as cauliflowers, lettuce, pumpkins,etc.
SULPHUR:
In recent years sulphur has come into the limelight. Sulphur for years has been supplied normally in the form of gypsum in superphosphate and its importance was overlooked. Sulphur occurs in 3 essential amino acids: cystine, cystein, methionine. The extent of sulphur deficiency in this State is now being appreciated. Many of our soils are deficient in both phosphorus and sulphur and therefore have been corrected in the past by superphosphate. Some of our soils, such as basalts on the southern and northern tablelands, are deficient in sulphur alone. The use of superphosphate in these areas therefore would be wasteful. Where top-dressing with superphosphate has been carried out over a number of years and adequate levels of phosphorus supplied, then further use of superphosphate could be wasteful. Phosphorus has a higher residual effect than sulphur. Sulphur losses may occur in two ways from the soil: (1) by leaching and (2) by being tied up in the organic fraction of the soil. Where phosphorus deficiency has been corrected by means of superphosphate it should be economical to supply sulphur in another form for the next few years.
As production of various soil types is raised it is possible that further mineral deficiencies may show up in years of high rainfall. In New Zealand, for example, once pastures have been brought to maximum productivity by means of superphosphate, potash deficiency has occurred. This is also the case in various European countries. It is whilst a programme of pasture improvement or soil fertility build-up is being carried out that it is well to bear in mind that the plant requires some 10 minerals for satisfactory growth, and if any one of these is limiting, then production may be affected adversely.