Why does soil have a negative charge

This is one reason why it is important to maintain pH levels at 5. Dispersive soils have poor water entry and drainage and set hard on drying.

You can improve CEC by applying lime and increasing the pH. However, increasing organic matter is the most efficient way of improving cation-exchange capacity.

This is a unique website which will require a more modern browser to work! Please upgrade today! Cation-Exchange Capacity of Different Soil Types A cation is a positively charged ion, with fewer electrons than protons, while an anion is negatively charged, with more electrons than protons. However, little decomposition will occur if the soil is dry or very cold.

The fertilizer doesn't have to be mixed into the entire soil area, either. In fact, less N is needed if it's placed near the crop row where the plants have good access to it. This is done in parts of S. Asia with rice straw residue but is laborious and requires the addition of high-N materials such as fresh manure to encourage the breakdown of the lowN straw. For more information on composting, refer to Chapter 8. Phosphorus plays many roles in plant growth and exerts a beneficial effect on:.

Only about percent of the P you apply as chemical fertilizer to an annual crop like maize or vegetables will actually be available to it. In acid soils, much of the P gets "fixed" tied up by reacting with iron, aluminum, and manganese to form insoluble compounds. In basic soils, the added P has a similar reaction with calcium and magnesium. The amount of P immediately available from an application of chemical fertilizer depends on the amount applied but even more so on the application method used.

Some of the percent of the P that becomes "fixed" will eventually become available again to crops over the years. There's a saying that applying fertilizer P is like putting money in the bank and living off the interest.

The amount of future interest you get depends a lot on the type of soil. Some soils, especially very acid, red soils high in "tropical" clays, can have an extraordinary P fixation ability and may tie up percent applied fertilizer P in a virtually irreversible, unavailable form.

The P in organic fertilizers like compost and manure is much less subject to fixation. Temporary P tie-up by decomposing crop residues: As with N, some soil P can become temporarily tied up when low-nitrogen crop residues i. The bacteria that break down the residues need P as well as N for their growth and multiplication and end up borrowing both from the soil as explained in the previous section on nitrogen. Such tie-up can last for several weeks or more, but can be compensated for by applying P fertilizer near the row.

Legume residues break down quickly enough so that tie-up isn't a problem. Application method is vitally important: In most cases, chemical fertilizer P should not be broadcast spread but applied in a band, hole or half-circle to concentrate it near the plant row.

Refer to Chapter 9. Maintain a good level of soil organic matter: Decaying organic matter produces humus and organic acids that form complexes with iron and aluminum; this can considerably reduce their ability to tie up P. Likewise, pH's above 7. P is most available within a pH range of 6. Unlike nitrate N, P is pretty immobile in the soil, and leaching losses are virtually nil, even on sandy soils. This means there's no need to "spoonfeed" fertilizer P by splitting the dosage into two or more applications; all can be applied at transplanting or planting.

Crops such as bananas, sugarcane, and starchy root crops like potatoes, cassava, and taro have especially high needs.

If high rates of potassium are applied, plants have a tendency to take up more than they need. Some soil specialists feel that "luxury consumption" is aggravated by shortages of other nutrients.

Others feel that this problem is over-exaggerated. At any rate, limited resource farmers are unlikely to apply high enough rates of X to promote luxury consumption. Only about percent of a soil's total K is in the available form, but even this is often enough to supply the needs of some crops. Tie-up of added K is usually not a problem. Some soils high in the temperate clays such as montmorillonite can temporarily tie up some added K.

Clay types are covered in Chapter 2. However, leaching losses can be substantial on sandy soils or others that have a low C. In this case, it's best to "spoonfeed" K by making applications if chemical fertilizer is used. Acidic soils lose more K by leaching than limed ones. Returning crop residues to the soil is a good way to recycle K. For example, overuse of K in grass pastures has caused Mg deficiencies in both the grass and the livestock.

Peanuts have unusually high Ca needs and often require gypsum applications. This is more often a problem on sandy soils or other low C. When liming, it's a good idea to use dolomitic limestone a mix of Ca and Mg. It also forms part of several vitamins and is used in oil fat formation. The available form of sulfur is the sulfate ion SO 4 - which is readily leached, especially in sandy soils under high rainfall.

A good part of the soil's sulfur is in the unavailable organic form which bacteria convert to available sulfur. Organic sulfur is an important reservoir of this nutrient, since it doesn't leach in this form. As with N and P, sulfur can become temporarily tied up when large amounts of low-nitrogen crop residues i.

Sulfur retention: Appreciable amounts of available sulfur can be retained against leaching in subsoils high in tropical-type clays; plant roots can utilize this source. As little as 75 grams of Mo per hectare may cure a deficiency for several years, but kg might severely injure plants.

Boron is another touchy one. Although less common than macronutrient deficiencies, macronutrient deficiencies can be just as serious when they occur and are favored by:. Copper deficiencies are especially common on these soils. Over application of a - charged element followed by excessive water will quickly move that element through the system.

The odd anion is phosphorous. Even though it has a - charge, it is not mobile in soil because phosphorous forms are not very soluble.

It can, however, still move — not as the anion, but bound to soil particles as the particles move. Therefore, minimizing runoff is helpful in reducing phosphorus pollution.

Understanding nutrient movement in soils helps producers apply nutrients and water to maximize economic effectiveness while minimizing environmental impact. For more information on commercial vegetable production, contact Ron Goldy at ext. This article was published by Michigan State University Extension. Anions and cations in plants, oh my!

Overall charge is usually negative. Sorption Properties of Soil Organic Material Heavy metals An element in the soil may exist in three forms : Fixed, adsorbed, and dissolved. Cation Sorption: Like layer silicates, humic substances also have a surface negative charge, because of dissocation of and OH groups Sorbed ions can be exchanged, depending on relative concentrations Total amount of sorbed cations is defined as the cation exchange capacity CEC -- CEC is mostly defined by the clay and SOM content of soil.

Ion exchange is stiochiometric and reversible. Soil Type. Very strongly acidic. Strongly Acidic.