SPECIAL SOIL PROBLEMS

Low pH (Acid Soils)

Strongly acidic soils pose a problem for many crops. There are three major facets of the problem which should be considered.

1. Some crops, regardless of the supply of available cations, just do not do well when the soil reaction is acidic. There are plant physiological problems involved in which the plant metabolism or chemistry is unable to function properly. For those types of crops which must have a more nearly neutral pH the only answer is to increase the pH or grow a more adaptable crop.

2. In many soils with very tow pH there is, simply, a drastic shortage of the needed cations, resulting in deficient crops and poor growth. Where this is the case, banding of cations to achieve partial saturation of the exchange complex may furnish adequate nutrients. It is a case of supplying the crop with the needed amounts of plant food.

3. The solubility of some elements is increased in low pH soils, while others decrease. This may pose problems of toxicity and of deficiency. Manganese toxicity may be a problem at low pH, while the problem with molybdenum may be insolubility and deficiency. The most favorable range in terms of availability of plant food elements is the pH range of 6.2to 6.8. However, a pH range of 6.0 to 7.8 is satisfactory for most field crops. At this range the problems of toxicity, solubility, etc., are most nearly balanced off, for the majority of soils. Molybdenum is a very slight problem occasionally, but seed treatment, spray or soil treatment will correct the problem. The problem is most pronounced on seedling legumes. Sometimes as little as 2 oz. per acre seed treatment is sufficient. One pound ammonia molybdate per acre as spray or soil treatment is usually adequate. Be careful to guard against toxicity to livestock from ingestion of treated forage, and especially so if using sprays.

If acid sensitive crops are to be grown on soils with a pH below 6.0, a test should be run on the soil to determine the amount of lime required to bring the pH up to a satisfactory level. The lime requirement may be up to 6 tons per acre depending upon how acid the soil is, soil texture, cation exchange capacity, and present base saturation desired.

High Salt Concentrations

Excessive concentrations of various salts occur naturally in many soils as a result of the weathering of parent material high in the contributing cations or anions. Other soils are developing or have developed high salt concentrations from irrigation or ocean spray drift and continual accumulation.

One effect of high soil salt concentration is to produce water stress in the growing crop. The crop may wilt or even die from drought while the soil is apparently moist or even wet The osmotic or absorbing power of the plant for water is too slight to remove the water from the salty solution. Accumulation of salt in the plant adds to the physiological problem. The measure of the salt problem is the soluble salt reading in millimhos per centimeter (abbreviated mmhos/cm}. The following illustrates some common problem ranges and crop salt tolerances.

Different crops vary in their tolerance to salt concentrations in the soil. Also, most crops are more sensitive to high salt concentrations in the germination and seedling stage of growth, and more tolerant to high salt concentrations as the crop matures. Therefore, if the excess salt can be leached by irrigation 2 to 6 inches deep prior to seeding, some crops may establish themselves and tolerate a higher salt concentration in the subsoil. Following is a list of crops with low, medium, or high tolerance to salt concentrations in the root zone.

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