Lime Requirement of Minimally and Moderately Weathered Soils

The lime requirement is the amount of lime required to adjust the pH to a more favorable value for the crop being grown; for soybeans it would be about 6.5. The same is true for many other legumes, including alfalfa and clover. Farmers who continuously grow corn can maintain a lower soil pH than those producing soybeans and alfalfa, because corn is less responsive to pH adjustment than are soybeans and alfalfa. The amount of lime required to adjust the pH of soils depends on the amount of pH adjustment needed and on the amount of buffer acidity. The amount of buffer acidity is related to the CEC.

It is a common practice in soil testing laboratories to make a direct measurement of the lime requirement by using a buffer solution. A buffer solution with a pH of 7.5, for example, is mixed with a known quantity of soil. After a standard period of stirring, the pH of the buffer-soil suspension is made. The amount of depression in pH from that of the buffer solution is directly related to the amount of lime needed to adjust soil pH to a particular value.

Soil Acidulation

The addition of acid sphagnum peat to soil may have some acidifying effect. However, significant and dependable increases in soil acidity are probably best achieved through the use of sulfur. Sulfur is slowly converted to sulfuric acid by soil microbes, and the soil slowly becomes more acid over a period of several months or a year. As with lime, the amount of sulfur required varies with the pH change desired and the soil CEC. Sulfur is used to change soil pH in large agricultural fields in arid regions, but it is less common a practice than the use of lime in humid regions. Sulfur is commonly used in nurseries and gardens.

(H.D. Foth, Fundamentals of Soil Science)

Find in the text above English equivalents for the following words and expressions and memorize them.

1) гидролиз

2) известковая почва

3) натриевая (солонцовая) почва

4) кислотные дожди

5) грунтовые воды

6) кислование (ацидификация)

7) буфферность почв

8) токсичность

9) подход (как метод)

10) избыточное известкование

11) пахотный слой

12) потребность в известковом удобрении

13) регулирование рН

14) подкисление почвы

15) сфагновый торф

16) подкисляющее действие

17) серная кислота

18) засушливые районы

19) рассадник, садовый питомник

TEXT 5

Micronutrients and Toxic Elements

IRON AND MANGANESE

Iron and manganese are weathered from minerals and appear as divalent cations in solution; as such, they are available to plants. Generally, in acid soils, sufficient Fe2+ and Mn2+ exist in the soil solution to meet plant needs. In some very acid soils, manganese, and to a lesser extent iron, are toxic because of the high amounts in solution. Deficiencies are common in alkaline soils where oxidized forms of iron and manganese exist as insoluble oxides and hydroxides. As a consequence, deficiencies are common in arid regions where many soils are calcareous and alkaline. In humid regions, many calcareous soils exist on recent lake plains and on exposed subsoil or parent materials. At building sites, such as home sites, acid soil may be contaminated with enough mortar from brick wall construction, or the surface soils may be mixed with highly calcareous parent material during excavation and become alkaline. Shrubs and flowers grown on these locations commonly have deficiency symptoms for iron and/or manganese. Cereals and grasses, including sugarcane, tend to have a manganese deficiency when grown on alkaline soils. Plants particularly susceptible to iron deficiency include: roses, pin oaks, azaleas, rhododendrons, and many fruits and ornamentals.

Deficiency symptoms of iron and manganese are striking and, for some plants, are very similar. In some cases, it is impossible to know whether the symptom is due to an iron deficiency or a manganese deficiency unless trials are conducted to determine which element alleviates the symptoms. In other cases, it is known from experience which element is deficient when the deficiency symptoms occur. Typical iron-deficiency symptoms or iron-chlorosis appears on leaves as yellow interveinal tissue and dark-green-colored veins, as shown for pin oak and roses in Color Plate 2. The younger leaves tend to be most affected because iron is immobile in plants. The absence of sufficient manganese stunts tomatoes, beans, oats, tobacco, and various other plants. Associated with this stunting is a chlorosis of the upper leaves. The interveinal tissue turns yellow, whereas the veins remain dark-greencolored, as shown for kidney beans in Color Plate 3. The gray-speck disease of oats is attributed to manganese deficiency and appears as graycolored areas on stems. For many nutrients, varietal or cultivar differences result in differing plant responses to the same level of available nutrients. This is illustrated by varietal response to manganese by soybeans as shown in Color Plate 4. Many chlorotic plants growing on alkaline soils have an iron deficiency and many have a manganese deficiency. Zinc may also be a contributing factor. Healthy green pin oak trees were found to need 55 centimeters of acid soil when growing in an area where many soils were alkaline and the trees were chlorotic.