Plant nutrient sources and method of application by element

Nitrogen (N)

Broadcast applications of organic compost or manure may preclude the need for additional preplant N, but usually not for postplant N. Where preplant responses are certain, N can be banded in the soil as ammonium sulphate, ammonium phosphate, potassium nitrate, urea or urea-ammonium nitrate at 25-100 kg ha-1 (25-100 lb acre-1) of N as the element. Side-dressings of ammonium sulphate and potassium nitrate are effective, but prohibitively expensive in large operations. Repeated foliar applications of solutions of urea or urea-ammonium nitrate, at 2-, 3- or 4-week intervals, are the mainstay of the nitrogen programme for most large operations. Cumulative quantities of 200-600 kg ha-1 (200-600 lb acre-1) of N are typical, depending on the local requirements.

Extensive research has been conducted on the effects of various nitrogen sources and methods of application on the distribution of nitrogen in the plant and the subsequent effect on growth and fruit production (Sideris et al., 1934, 1936, 1938, 1947; Sideris and Krauss, 1937; Sideris and Young, 1946, 1947; Su, 1957; Py, 1962; Subramanian et al., 1974, 1977, 1978; Chadha et al, 1975, 1976; Py et al., 1987). For a more detailed discussion of nitrogen and other nutrients, see Malézieux and Bartholomew, Chapter 7, this volume.

An important aspect of nitrogen management is the potential conversion of soil-applied nitrogen to nitrates and the accumulation of these nitrates in the fruit during the later stages of development through early fruit ripening. Application of ammoniacal nitrogen or urea to the leaves almost completely avoids this problem. High nitrates in the fruit are particularly important in fruit canning, where the nitrates rapidly react with the tin coating of the cans, leading to high levels of dissolved tin and development of hydrogen gas, which produces swollen cans (see Hepton and Hodgson, Chapter 11, this volume).

Potassium (K)

Broadcast applications of manure, sulpomag (the natural mineral langbenite containing 22% sulphur, 22% potassium and 11% magnesium), potassium chloride (muriate of potash, KCl) or potassium sulphate (K2SO4) are effective at quantities ranging from 200 to 1000 kg ha-1 (200 to 1000 lb acre-1) of K where soil K is found to be limiting. Potassium can also be banded as sulpomag or as potassium chloride, nitrate (KNO3) or sulphate at 100-500 kg ha-1 (100-500 lb acre-1) of K. Side-dressings and foliar applications are also utilized. The requirement for potassium is high, as the plant and fruit remove significant quantities of this element. Plant tissue analysis should show at least 0.20% K on a fresh-weight basis at forcing and responses have been obtained at even higher levels. In sandy soils, potassium should be applied throughout the growing cycle to ensure adequate levels at the time of forcing.

Significant controversy surrounds the form in which potassium should be applied to pineapples, mostly the result of research in Hawaii, where potassium sulphate was shown to be superior to potassium chloride. Organic preplant sources work very well under almost all conditions, but frequently organic sources are not available in sufficient supply. Potassium chloride is almost always the least expensive potassium fertilizer, while potassium nitrate and potassium sulphate are more expensive but provide other nutrients.

Some research has shown fruit quality improvements with the use of potassium sulphate, but this response is by no means universal, and considerable areas of pineapple are fertilized with foliar applications of KCl with no apparent adverse effects on either yield or fruit quality. To resolve this issue in any particular growing area, comparative trials should be established at various times of the year. Data should be gathered on plant growth, K levels in tissue, and fruit quality attributes to determine if the additional costs associated with K2SO4 are justified.

Phosphorus (P)

Phosphorus is most notable for its early enhancement of rooting and yet small effect on final fruit yield. Preplant applications of 25-150 kg ha-1 (25-150 lb acre-1) of P are often made because of the importance of this element in the development of a strong root system. Phosphorus may be adequate in some soils, but soil analysis may not always reveal the true availability to the plant. Where soil P levels are very low, mycorrhiza may also play an important role in the transfer of this nutrient from the soil to the pineapple plant (Aziz et al., 1990). However, soil applications always require an actively growing root system in good health, with adequate moisture supply, for nutrient supply and uptake. In acidic soils, in which pineapple are often grown, finely ground rock-phosphate may be an effective method of supplying phosphorus.

Responses have been obtained to foliar applications of ammonium phosphate, monoammonium phosphate or ammonium polyphosphate. These responses indicate an inadequate uptake of phosphorus by the root system even in areas where standard methods of soil analysis indicate that levels of phosphorus are adequate.

Calcium (Ca)

Lime, gypsum and manure are excellent sources of Ca and typically 200-2000 kg ha-1 (200-2000 lb acre-1) Ca are broadcast over the field. However, amounts should be determined based on the desired pH and Ca status of the soil. Recommendations for broadcast calcium are complex for pineapple and should be made in conjunction with a reliable soil-testing laboratory and a qualified agronomist. Foliar applications of calcium nitrate or calcium chloride are also possible, but are rarely made and usually pertain to applications after differentiation to enhance fruit quality.

Calcium has been called white gold in reference to the favourable results associated with its use in pineapple nutrition. Dramatic improvements in both fruit yield and quality have been obtained as a result of application following the steady, long-term reduction of soil calcium from the continuous cropping of pineapples. The method of calcium application and incorporation must be well understood if favourable results are to be consistently obtained and potential disasters associated with high soil pH and increased incidence of Phyphthora root and heart rots are to be avoided. The judicious use of calcium to increase its content in soil and raise soil pH results in enhanced uptake of other nutrients. At the same time, the additional calcium produces stronger cell walls, which result in better plant growth and development of fruit that are more resistant to bacterial diseases (see Rohrbach and Johnson, Chapter 9, this volume).

Repeated foliar applications of ferrous sulphate (1% solution) in the same foliar solution with N are very effective. Leaf deficiency symptoms for iron are distinctive for pineapple and are an excellent guide to the need for iron. Where iron deficiency is certain, foliar-applied Fe will increase the yield of pineapple in proportion to the amount required to remove an observed Fe deficiency. Application rates can be determined for the current crop and empirically for subsequent crops based on visual monitoring of leaves. Cumulative amounts from repeated foliar sprays may vary from 2-10 kg ha-1 (2-10 lb acre-1) of Fe in the plant crop and 1 to 5 kg ha-1 (1-5 lb acre-1) of Fe in the ratoon.

To avoid precipitation, ferrous sulphate should not be used in a foliar solution with phosphate or boron. Care must also be taken to avoid foliar burning due to the high osmotic potential of ferrous sulphate. In the case of low-volume applications, there should be no runoff into the leaf axils where the sensitive basal white tissues are located. The quality of the ferrous sulphate, which is green in colour, needs to be confirmed, since the iron is readily converted in moist air to the ineffective ferric form. Chelated sources of iron are effective, but can be prohibitively expensive and may deteriorate in storage. Soil or drip application of iron, even in chelated form, is costly or not consistently effective, or both.

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