Calcium in Soils

A deficient level of calcium in the soil impairs plant absorption and retention of other ions such as magnesium. The mechanism responsible for plant discrimination between K+ and Na+ depends on the presence of Ca2+. A small amount of calcium in a foliar solution increases the plant’s uptake of potassium by as much as 150% by changing cell membrane permeability.

Maintenance of the plant tissue cell wall structure depends on calcium cross-linkages.

The brown discoloration that occurs in most calcium deficient tissue could be the result of increased leakage of phenolic compounds into the cytoplasm. Oxidation of some of the enzymes would then occur.

There are basically four biological roles of calcium in the growth and development of plants:

• The effects on plant membranes
• The effects on cell wall structure
• The role in enzyme activities
• The interactions with phytohormones

Phosphates in Soils

Several factors influence the availability of phosphates to the plant.

Soil pH influences the ionic character of phosphorus. At a low pH, P is principally in the H2PO4 form. At intermediate soil pH levels, HPO 2/4 is predominant, and at higher values, PO is present. Plants can absorb all of these forms, but this ionic nature of phosphorus also influences the way it binds to soil colloids. Anions are strongly attracted to and absorbed by soil particles at lower pHs. As the pH increases, HCO3 is able to replace the bound phosphate by anion exchange. In addition to exchangeable binding with soil particles, phosphates, at low soil pH can form insoluble and unavailable salts with iron and aluminum cations.

In soils with increased calcium and a high pH, CaCO3 can greatly reduce the plant’s available phosphate.

Organic matter contains large amount of the total soil phosphate, but these forms are not readily taken up by plants unless there is first an enzymatic cleavage of the phosphate bond. This cleavage is achieved by the enzyme phosphatase, which is produced by microorganisms.

Potassium in Soils

The following are general elemental relationships. The most overlooked element in agriculture, other than carbon, is potassium. Potassium levels have rarely been associated with nitrogen levels, but they should be.

Potassium is directly involved in:

• The synthesis of carbohydrates, proteins, oils and certain organic acids
• Acceleration of certain enzymatic actions
• The reductase reduction of nitrates which are fundamental for the synthesis of protein
• Increasing photosynthetic activity under low light conditions
• Facilitating the transport of carbohydrates inside the plant
• Cellular division
• Regulation of nitrogen absorption by the plant
• Increasing the resistance of plants to disease

We have found, in corn, potatoes and grasses, that when excess nitrogen fertilizer is added to the crop during the growing season, there is a direct decline in the level of potassium present in the plant. Also, as the potassium level declines, the manganese level increases to a dangerously high amount in the plant. The same elemental relationship seems to exist whether soil-applied or foliar-applied nitrogen applications are done.

Potassium strengthens a plant vascular tissue, and under adequate levels, increases the cell wall thickness up to three times. Potassium uptake is inhibited in most plants grown in soils with high magnesium and low oxygen levels. When potassium and magnesium levels are too low, use of the fertilizer form Sulfate of Potash Magnesia, which is 22% K and 11% Mg is appropriate, and closely reflects the ideal balance between these two elements.

Potassium is the plant’s cellular liquid antifreeze system because it remains present, increasing the salt level, while decreasing the liquid freezing point.

Remember, the more nitrogen that is added to a crop, the busier the potassium becomes and the greater the need.

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