Managing Extreme Soil Environments Acid Soils

Soils are acid (pH below 7.0) due to chemical properties of the parent rock from which they were formed and/or because of the rainfall-

induced leaching of soil cations, particularly Ca and Mg. The process is exacerbated by heavy N fertilization with acidifying fertilizers, e.g., ammonium sulfate, urea, or ammonium nitrate.

Deleterious effects of soil acidity include (1) aluminum (Al) and Mn toxicity to plants; (2) limited soil availability of P, Ca, and Mg; (3) reduced plant uptake of N and K; and (4) poor soil structure. Acid soils require liming to raise their pH to 7.0. The amount of lime can be determined by soil chemical analysis. When soil acidity is associated with low Mg availability, dolomitic lime is used to simultaneously enrich the soil in this element.

Alkaline Soils

Alkaline soils (pH above 7) are formed when evapotranspiration exceeds precipitation, as occurs under semiarid and arid climates, leading to the accumulation of cations in the soil, particularly Ca, Mg, and/or sodium (Na). Alkaline soils may also form in moderate climates when derived from rocks rich in calcium carbonate. Alkaline soils often contain free calcium carbonate, in which case they are called calcareous. Growing fruit trees on alkaline/calcareous soils poses serious challenges, such as deficiencies of Fe, Zn, Cu, and Mn, and low availability of P. Remedies include (1) foliar applications of Mn, Zn, and Cu; (2) soil applications of chelated forms of Fe; (3) sodding to increase soil Fe availability; and (4) judicious water management to avoid waterlogging, which is conducive to the development of Fe deficiency.

Salinity

Soil salinity develops when soils accumulate an excess of inorganic salts. This happens when evapotranspiration raises ground-water or perched water tables containing salts to the soil surface. Salinity may also develop when water containing high levels of soluble salts is used for irrigating crops. Remedial actions include (1) improving soil internal drainage and leaching the accumulated salts with an excess of irrigation water and/or (2) irrigating with water of acceptable quality. When the soil contains an excess of Na (sodic soil), the remedial actions include application of gypsum (CaSO4) or, when the soil is calcareous, sulfur (S). This allows the excess ofNa in the exchange complex to be replaced with Ca. Heavy irrigations to leach the excess Na must follow. Good internal soil drainage is a prerequisite for this measure to be successful.

As growers increasingly adopt environmentally friendly technologies to gain universal acceptance for the fruit they produce, mineral nutrient management will become more precise. The past practice of applying fertilizers as "an insurance policy" is no longer acceptable and has been replaced by practices based in sound science. With further advancements in estimating plant mineral nutrient requirements, acquisition, and use, new fertilization practices will supply nutrients only when needed to organs requiring them the most, and at optimal times from the standpoint of plant needs. Not only will this approach assure high productivity and quality of fruit, but it also will effectively protect the environment and conserve the soil.

Related Topics: ORCHARD FLOOR MANAGEMENT; ORCHARD PLANNING AND SITE PREPARATION; PLANT NUTRITION; SUSTAINABLE ORCHARDING

SELECTED BIBLIOGRAPHY

Miller, Raymond W. and Duane T. Gardiner (1998). Soils in our environment. Upper Saddle River, NJ: Prentice-Hall.

Organizing Committee of the International Symposium on Mineral Nutrition of Deciduous Fruit Crops (2002). World overview of important nutrition problems and how they are being addressed. Proc. Fourth Internat. Symposium. Hort-Technol. 12:17-50.

Peterson, A. Brook and Robert G. Stevens (1994). Tree fruit nutrition. Yakima, WA: Good Fruit Grower.

Swezey, Sean L., Paul Vossen, Janet Caprile, and Wail Bentley (2000). Organic apple production manual. Oakland, CA: Univ. of California, Div. of Agric. and Nat. Resources.

Swietlik, Dariusz and Miklos Faust (1984). Foliar nutrition of fruit crops. Hort. Rev. 6:287-356.

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