Potassium and Trends of Potassium and Phosphorus Deficiency

FERTILISERS, LIME, AND TRACE ELEMENTS

by Cornelius During, B.AGR.SC., formerly Farm Advisory Service, Department of Agriculture, Wellington.

FERTILISERS, LIME, AND TRACE ELEMENTS

The main products of New Zealand agriculture are milk, meat, and wool. These are obtained from cattle and sheep which derive their feed almost entirely from pasture, that is, from grasses and clovers. Nevertheless, the largest acreage of crops is grown for supplementary livestock fodder and a much smaller acreage for human consumption. Since most of the fertilisers used in New Zealand are applied to pastures, it is this practice that is mainly discussed.

The Fertility Turnover of Pastures

Pastures take up very large amounts of plant nutrients from the soil. The herbage is then consumed by stock and the nutrients contained are in great part returned to the soil in the form of dung and urine. There is in this way a cycle of fertility from soil to pasture herbage to livestock – to urine and dung – and back again to the soil and a return to herbage. This cycle is not a perfect one. The nutrients are continually being depleted in various ways. A proportion of them is retained in animal products, such as milk. P. D. Sears (J. Brit. Grassl. Soc., Vol. 5, 1950, pp. 267–280) estimates that 600 lb gallons of milk contain the equivalents of the following fertilisers:

  • 160 lb sulphate of ammonia (nitrogen).

  • 60 lb of superphosphate (phosphorus).

  • 15 lb of potassium chloride (potassium).

  • 25 lb of agricultural limestone (calcium).

These nutrients are also taken up into the flesh and bone of the grazing animals, and this means that their sale causes further depletion. The largest losses, however, of precious minerals may occur within the farm. Transfer in dung and urine from grazed areas to sheds, races, stock camps, gateways, hedges, and trees, leaching generally and especially under urine spots and fixation by the soil, all remove from circulation a serious proportion of the plant nutrients.

Expressed as equivalents of well-known fertilisers, limestone, and trace elements, the annual turnover of nutrients of first-class pasture is approximately as follows:

Nitrogen as sulphate of ammonia 20–25 cwt/acre.
Phosphorus as superphosphate 4 cwt/acre.
Potassium as potassium chloride 5–6 cwt/acre.
Sulphur as superphosphate 300 lb/acre.
Magnesium as Epsom salts 250 lb/acre.
Calcium as superphosphate 400 lb/acre.
or Calcium as 80% pure limestone 250 lb/acre.
Copper as bluestone ½ lb/acre.
Molybdenum as sodium molybdate 1/3 oz/acre.

Nitrogen may suffer the largest losses, followed by calcium, potassium, sulphur, phosphorus, and magnesium.

Under New Zealand conditions adequate amounts of nitrogen are supplied to pastures by clovers, particularly white clovers. The root nodule bacteria of these legumes have the ability to convert atmospheric nitrogen into organic forms which become available to the associated grasses. For this reason, the main purpose of applying fertilisers, lime, and trace elements to the pastures is to stimulate clover growth so that, in turn, grasses may benefit from the nitrogen supplied by these species. Good clover growth means good grasses will grow well.

Clovers, particularly white clovers, need a continuous good supply of phosphorus, potassium, sulphur, magnesium, and several trace elements. In addition, soils must not be too acid. Where soils cannot release these needed elements right through the year at an adequate rate to maintain vigorous clover growth, or where soils are too acid, the supply of nitrogen to grasses will diminish, pasture production will fall, and low-fertility demanding species of grasses will replace high-fertility demanding grasses. This would be the situation on almost all soils in New Zealand if no fertilisers or lime were used. Fertiliser, lime, and trace elements, therefore, are needed to supplement the natural supply of these nutrients from the soil. They are also used to make good any losses which occur under pasture farming as already discussed.

The supplying power of soils for individual nutrients varies considerably. That for phosphorus is inadequate in nearly all soils of New Zealand, that for sulphur and molybdenum is inadequate in a large proportion of soils. The supplying power of potassium depends not only on soil type but to a great degree on the amount of potassium lost on individual farms and paddocks in past years. It also depends on the level of pasture production. Thus paddocks frequently hayed or cropped in the past are usually more potassium-deficient than others. On farms where pasture production is high, there is a need for a rapid release of potassium from the soil and so potassium deficiency is more likely to occur there than on farms with low-quality pastures. Moreover, losses of potassium by transfer and under urine spots are thought to be higher on highly productive farms than on low-producing farms, and higher on dairy than on sheep farms.

Phosphorus

The fertilisers most commonly used to supply phosphorus are: (1) Superphosphate containing this element in a water soluble form, (2) “Aerial superphosphate” (90 parts of superphosphate reverted with 10 parts of ground serpentine rock) in which the phosphates are partly water soluble, and (3) Serpentine superphosphate (75 per cent parts of superphosphate reverted with 25 parts of ground serpentine rock) which contains phosphorus mainly in a water insoluble but yet readily available form.

Some lime-reverted superphosphate is also used, particularly in the South Island, where it is desirable to drill in small crop seeds with a phosphatic fertiliser. Superphosphate in contact with turnip, swede, chou moellier, rape, clover, and other small seeds may cause serious loss of germination. Fully reverted and therefore water-insoluble forms do not cause germination injury.

The use of serpentine-reverted superphosphate is peculiar to New Zealand. Serpentine superphosphate has three advantages. It is superior in physical qualities to ordinary superphosphate in that it flows more freely, does not set hard on storage under damp conditions, and does not rot bags. It contains about 5 per cent of magnesium, a considerable proportion of which has been shown to be available to plants. In contrast to superphosphate it stores reasonably well when mixed with soluble potassic fertilisers. Its main disadvantage is a lower phosphorus and sulphur content per unit than that in superphosphate. Both fertilisers are the same price. Aerial superphosphate combines good physical qualities with a higher phosphorus and sulphur content than that in serpentine superphosphate, but it is not satisfactory when mixed with potassium chloride, and it is expected to cause germination damage if sown in contact with small seeds.

The greater part of unimproved land needs phosphatic fertilisers. Exceptions are some of the fertile river flats of both Islands, and some of the soils in the dry parts of Central Otago, and of inland South Canterbury and, possibly, of Marlborough. Therefore it is seldom a question of whether to use or not to use phosphatic fertilisers. The question is rather how much to use initially during the period of pasture improvement, and how much to use to maintain pasture production at a desirable level.

Initial Rates of Application

Recommended rates of application of superphosphate needed to obtain vigorous initial growth of pasture sown on cultivated but previously unimproved land are approximately as follows:

  1. Rainfall 40 in. – 200 in.

    1. poor initial phosphorus fertility, 6–9 cwt/acre.

    2. medium initial phosphorus fertility, 3–5 cwt/acre.

  2. Rainfall 30 in. – 40 in.

    1. poor initial phosphorus fertility, 3–5 cwt/acre.

    2. medium initial phosphorus fertility, 2–3 cwt/acre.

  3. Rainfall 15 in. – 30 in.

    1. poor initial phosphorus fertility, 3–5 cwt/acre.

    2. medium initial phosphorus fertility, 1–2 cwt/acre.

Where unimproved phosphate-deficient pasture containing little clover is oversown with clover seeds, initial rates of application of phosphatic fertilisers seldom need to exceed 3–4 cwt/acre because heavier rates cannot be utilised fully straight away. It is usually important, however, to consolidate pasture improvement by regular annual dressings for several years.

Maintenance Rates of Application

Rates for maintenance requirements depend on the original fertility of the soil and its ability to retain applied phosphate in a form which is not readily available to plants. The correct rates depend on the amount of growth of the pasture which, together with management factors and type of farming practised, determine the losses of phosphorus through transfer of fertility and through stock and stock produce sold off the farm. Very little experimental work has been done on maintenance requirements, but it appears that these may vary from 1 cwt superphosphate per acre every few years on the more fertile soils of the South Island to 3–4 cwt per acre each year on some of the “high phosphate fixing” volcanic soils of the North Island.

Crops

The following common crops appear to have a greater need for readily available phosphorus than grass-clover pasture – potatoes, barley, and all brassica species including turnips, swedes, chou moellier, rape.

Autumn-sown wheat does not need a very generous supply of phosphorus, but spring-sown wheat may approach barley in its sensitivity to low levels of available phosphorus. The position of oats in relation to other crops and to pastures is not well known.

Potassium and Trends of Potassium and Phosphorus Deficiency

Potassium chloride (often called muriate of potash) is the cheapest fertiliser per unit of potassium and by far the most widely used. Potassium sulphate contains 20 per cent less potassium than the chloride and is used for special purposes only, mainly for small fruit and tobacco leaf production.

The areas recognised as needing potassic fertilisers to maintain reasonably high-producing pastures are growing larger year by year. This spread of potassium-deficient areas may be caused in part by the losses of potassium under pastoral farming and in cash cropping; recent research into soil fertility, however, is leading to better recognition of long-standing potassium deficiencies. The soils most liable to become potassium deficient are those highly leached or in areas with a high annual rainfall. These are the soils with a severe initial phosphorus deficiency as well. Hence, where potassic fertilisers must be used, phosphatic fertilisers are usually also needed. In practice, therefore, potassic fertilisers are mainly applied as potash-phosphate mixtures either as potassic serpentine superphosphate or, less commonly, as potassic superphosphate stabilised by 3–6 per cent of ground limestone. There are, however, differences in the long-term fertility trends of phosphorus and potassium. While regular applications of phosphatic fertilisers result in a gradual rise in the level of available phosphorus, regular applications of potassic fertiliser do not have the same beneficial, long-term effect. On the contrary, the light rates of application of potassic fertilisers commonly used may not completely arrest a downward trend in potassium fertility. For this reason intensively top-dressed areas may gradually become less deficient in phosphorus and more deficient in potassium. The ratio of use of phosphatic fertilisers to potassic fertilisers is therefore expected to become narrower.

Agricultural Limestone and Molybdenum

These two materials must be considered together. This is because liming increases the availability of molybdenum to plants. And so this trace element can replace lime to a varying degree in many soils, deficient in available molybdenum. The importance of molybdenum as a substitute for lime depends on many factors such as the following:

Where the main beneficial effect of limestone is one of releasing available molybdenum to the plant, the use of molybdenum can replace liming. Where lime is needed to eliminate soil conditions which are detrimental to pasture or crop growth as well as to counteract molybdenum deficiency, lime alone or lime and molybdenum may be used in varying combinations.

In practice, the choice of using molybdenum alone, lime alone, or molybdenum and lime is often determined by economics. On molybdenum-deficient hill country, a moderately vigorous pasture can be obtained by applying molybdenum alone. If thought payable, this pasture may be further improved by using small quantities of lime. On flat, ploughable land of a closely related soil type, however, first-class pasture may be desirable. This may be possible only with the assistance of heavy rates of lime, the use of which would eliminate molybdenum deficiency. Thus on one and the same soil molybdenum, molybdenum and small quantities of lime, or heavy rates of lime without added molybdenum may be used depending on the desirable degree of utilisation of individual paddocks and on the cost of applying lime. New Zealand probably has a greater proportion of soils low in available molybdenum than any other country in the world.

Very small amounts of molybdenum are needed to ensure an adequate supply to pastures and crops. Usually molybdenum deficiency is eliminated by 2½ oz of sodium molybdate applied every three to six years, depending on the soil type.

In New Zealand, lime is applied to reduce the acidity of the soil but very rarely if ever to supply calcium. On strongly acid cultivated land, up to 3 tons of limestone per acre may be needed to obtain the desired reduction in soil acidity. More commonly 1–2 tons of limestone per acre are adequate. On uncultivated land it has been found that as little as 2–6 cwt of lime per acre may produce a worth-while improvement in clover establishment and vigour.

When excessive acidity has been corrected, the amount of lime needed to keep the soil reaction at a satisfactory level has been determined experimentally only at Marton. The results obtained at Marton are being adjusted to other districts, making allowance for factors such as rainfall, soil type, and the size of the initial dressing of lime which has been applied. In districts with 25–30 in. rainfall, it is considered that 1 ton of lime every six to 10 years is adequate maintenance. In districts with a rainfall of 35–45 in., 1 ton per acre every four to five years appears adequate, and in higher rainfall areas 1 ton of lime every three years may be needed to prevent excessive soil acidity.

Sulphur

Sulphur deficiency is very common in New Zealand. Since superphosphate contains about 10 per cent of phosphorus and 11 per cent of sulphur, this fertiliser serves as the main source of sulphur. In addition, small amounts of sulphur, commonly between ½–5 lb/acre and rarely exceeding 7 lb per acre, are added to the soil in rainfall. Many soils in the drier districts of Otago, Canterbury, and Marlborough are only slightly phosphorus deficient but strongly deficient in available sulphur. There is, moreover, the possibility that with regular phosphatic top-dressing the level of available phosphorus increases more rapidly than that of available sulphur because applied phosphorus is mainly retained in the topsoil while applied sulphur may leach quite rapidly out of the topsoil. Following superphosphate topdressing, therefore, the supply of available sulphur may not increase as much as the supply of available phosphorus. To cater for the soils on which sulphur deficiency is more severe than phosphorus deficiency but in which both deficiencies are present, a superphosphate-sulphur mixture is used in increasing quantities. The sulphur used is screened elemental sulphur. It contains a considerable proportion of fines which ignite easily on friction or if near an open flame or spark. The danger of combustion is greatly reduced by dilution with superphosphate or other materials. Hence the proportion of sulphur in a mixture designed for distribution by air is strictly controlled. The safe upper limit is set at 500 lb of screened sulphur per ton of fertiliser material or diluent.

Magnesium

Magnesium deficiency in pasture is almost unknown. On the other hand recent experimental work has shown that losses of magnesium from the topsoil may occur under urine spots and following topdressing with potassic fertilisers. With the widespread use of serpentine superphosphate and aerial superphosphate, appreciable quantities of magnesium are added to soils. It is possible therefore that these fertilisers are preventing the occurrence of magnesium deficiencies. Apple trees and possibly a few other crops of lesser importance are more sensitive to a low supply of available magnesium than pastures.

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FERTILISERS, LIME, AND TRACE ELEMENTS 23-Apr-09 Cornelius During, B.AGR.SC., formerly Farm Advisory Service, Department of Agriculture, Wellington.