How to Test Soil Acidity and Alkalinity? What Are the Hazards of Soil Acidification on Crops?

The soil pH most suitable for the growth and development of various vegetables is different, but slightly acidic soils with a pH of generally 5.5 to 6.5 are suitable for most vegetables. However, soil acidification has become a serious problem in many places. Soil acidification has adverse effects on soil physical properties, nutrient availability, nutrient balance and transformation, which in turn affects the growth and development of vegetables. If you want to know the acidity and alkalinity of the soil, you can use the simple method of colorimetry to measure. The tool used in this method is litmus test paper purchased from a chemical reagent store.

First, randomly take a little soil in the four corners and the middle of a piece of cultivated land, put it into a clean glass or beaker, stir evenly, add pure water according to the ratio of soil: water = 1: 2 (you can also use boiling water instead), then stir it well and let it stand. After the soil particles are precipitated, take a test strip and put one end of the test strip into a clear liquid, and remove it quickly after 1~2 s. At this time, there will be a color change at one end of the test paper, quickly compare the color with the standard color comparison board, and the color plate similar to the color of the wetting place will be found, and the value displayed is the pH value of the soil sample taken. This process should be completed within 30s. Litmus test paper should be kept in a sealed, dry, dark environment.

Most vegetables are suitable for growing in slightly acidic to neutral soils, and over-acid and over-alkali soils are not suitable for vegetable growth, except for acid-and alkali-tolerant vegetables. The harm of soil acidification to crops: the essence of soil acidification is the increasing number of hydrogen and aluminum ions. The adsorption performance of aluminum ion is strong, it can compete with the exchangeable base ion for the negative charge in the soil, so that the base ion is leached continuously, and the base saturation of the soil is gradually decreased, which leads to the enhancement of acidity. The consequence of soil acidification is the continuous leaching of base ions such as calcium and magnesium ions, resulting in poor soil nutrients.

When the soil colloid is full of hydrogen ion and aluminum ion, the soil will lose the buffering ability to acid. In case of acid rain, its pH value is easier to reduce, thus causing serious damage to crop growth. Another serious consequence of soil acidification is the increase of toxic elements such as hydrogen ion, aluminium ion and manganese ion.

Excessive hydrogen ions in soil solution will affect the permeability of plant root membrane, thus interfering with the transport process of ions on the root surface. The increase of hydrogen ions in soil has a serious antagonistic effect on the absorption of other cations by vegetables and other crops. The aluminum ions in the soil can also affect the division of the root tip cells and weaken the root respiration. Aluminum will increase the soil's fixation effect on phosphorus and reduce the effectiveness of phosphorus. Long-term and excessive absorption of aluminum by plants can cause plant poisoning and loss of production, and in severe cases can even lead to death.

Acidified soil also accelerates the loss of mineral nutrients in the soil, changes the soil structure, leads to poor soil and affects the normal development of plants. For acidified soil, due to leaching of calcium ions, its aggregate structure and agglomerates will be greatly reduced, resulting in soil compaction, water retention, fertility reduction and anti-buffering capacity reduction. In the long run, the ability of vegetable crops to withstand natural disasters such as drought and waterlogging will be weakened. Soil acidification can change the population structure of soil microorganisms, the individual growth of bacteria becomes smaller, the growth and reproduction rate decreases, resulting in a decrease in the number of major microbial groups that decompose organic matter and its proteins, such as bacillus, brucellosis and related fungi, which will affect the virtuous cycle of nutrients, resulting in a reduction in agricultural production.

Acidified soil can also reduce the number of ammonifying bacteria and nitrogen bacteria in soil, and decrease the ability of ammoniation and nitrification of soil microorganisms. At the same time, acidic soil is also ideal for fungi breeding, which increases crop rhizosphere diseases and is difficult to control. In particular, it will cause the increase of root swelling disease of cruciferae and bacterial wilt and verticillium wilt of eggplant vegetables. Acid deposition can also cause acute and chronic damage to crops.

Acute damage refers to the contact of crops with contaminants such as strong acid or high concentration of sulfur dioxide, and the leaves die in a short time. In severe cases, leaves, branches, ends and even the whole plants die. Chronic damage refers to the long-term contact of crops with weak acid or low concentration of sulfur dioxide and other pollutants, causing plant chlorosis or pigment changes, destroying the normal metabolic activities of crop cells, leading to cell death. The symptoms is premature defoliation of crops. This happens in acid rain areas or long-term excesses of sulfur dioxide.

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