Using Potash Fertilizers (part 2)

The mysteries of potash fertilizers

Read the previous part of the article

Potassium in soil

The total potassium content in soil is almost always higher than phosphorus and nitrogen combined. Most of all potassium is found in heavy soils, as it is a part of clay minerals. In clay and loamy soils, the total amount of K ₂ O often reaches 2%, and sometimes even up to 3%. Less potassium in sandy, sandy loam and especially peat soils. Most of the potassium in soils is in an insoluble and poorly assimilated form for plants.

The supply of plants with this element on different soils is determined not so much by its total content in the soil, but by its readily soluble compounds.

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Potassium is most accessible to plants, adsorbed on the surface of soil colloids (exchangeable potassium). Its content in soddy-podzolic sandy loamy soils is in the range of 0.09-0.2 meq., In podzolized loamy soils - 0.15-0.40 meq. per 100 g of soil. From the total potassium content in the soil, this form is only 0.8% (sandy loam), 1.5% (loam). Nevertheless, exchangeable potassium plays a very important role in the nutrition of agricultural plants. This is due to the relatively easy transition of this potassium into the soil solution (water-soluble potassium) when exchanged for other cations.

Root hairs of plants easily assimilate exchangeable potassium in exchange in an equivalent ratio for hydrogen ions that are constantly on the surface of root hairs. The condition for such an exchange is a close approach between the aqueous films of soil colloids and root hairs. Water-soluble potassium is only 1 / 5-1 / 10 and even less of the amount of exchangeable potassium. For example, the content of water-soluble potassium varies in the arable layer of sod-podzolic soils from 0.04 to 0.09 meq. per 100 g of soil.

The appearance of water-soluble potassium in the soil is a consequence of a number of processes:

a) hydrolysis of potassium minerals;

b) destruction of minerals by root exudates of plants;

c) the action on these minerals of nitric acid, accumulated by nitrifying bacteria, and other acidic products of the vital activity of microorganisms;

d) displacement of exchangeable potassium by salts entering the soil with fertilizers and by products of plant root excretions.

The main constituent of root exudates is carbon dioxide and hydrogen ions. Due to root secretions, the pH around the root drops to 4 or even 3.5. This can explain the better assimilation of nutrients by the plant (including potassium) from poorly soluble compounds in direct contact with the root system.

Studies have shown that in the soil, along with the constant transition of potassium from hardly soluble to water-soluble and exchangeable forms, potassium is fixed in a non-exchangeable state. This phenomenon is called soil fixation of potassium.

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Potassium fixation by soil

Fixation is especially noticeable with alternating moistening and drying of the soil, and a higher temperature more strongly promotes the fixation of potassium.

Fine-dispersed soil fractions, which have a greater absorption capacity, fix potassium more strongly. The fixation of potassium in a non-exchangeable form is influenced by the organic matter of the soil, as well as the soil reaction of the environment. Humous organic compounds and alkalization of the reaction caused by the addition of lime increase the conversion of potassium into the non-exchangeable form; destruction of humus and artificial acidification of the reaction to pH 4.5-5.5 reduce the fixation of potassium in the soil. Soils systematically fertilized with potassium, with its new addition, bind potassium in a non-exchangeable form weaker, since they have already bound this element earlier.

An illustrative example of the economic damage caused by the fixation of potassium in the non-exchangeable form from fertilizers by the soil is the long-term data for the alluvial soils of Holland, lying on moraine loams. Here, the fixation reached 21 and even 59% of the potassium introduced over the years.

So, the fixation of potassium in the soil in a non-exchangeable state noticeably reduces its absorption by plants from fertilizers. As measures to combat the non-exchangeable uptake of potassium by the soil, it is recommended:

a) apply potash fertilizers to a sufficient depth to exclude the influence of drying out, which is inevitable in the upper part of the arable layer;

b) to embed potash fertilizers locally, in a certain layer of soil or in foci (without mixing with a significant volume of soil);

c) apply optimal doses of potash fertilizers regularly every year.

There is some balance between exchangeable and non-exchangeable potassium in the soil, which is established very slowly. So, it was noticed that in field conditions the lowest content of exchangeable potassium is observed in autumn, this is explained by its consumption by plants developing in the spring-summer period. But by the next spring, the content of exchangeable potassium in the same soil increases, and in wet years it is much stronger than in dry years (in wet years, plants use potassium of the soil better, and in dry years, potassium of fertilizers).

The replenishment of exchangeable potassium reserves is explained by the gradual, but constant restoration of equilibrium between both forms of potassium in the soil, displaced under the influence of the plant. The roots release hydrogen ions, which displace other cations, among them potassium, from soil minerals.

Be that as it may, all these phenomena do not remove the question of introducing potash fertilizers into the soil.

Potash fertilizers

Silvinite contains on average 15% K2O, this is a low potassium content, which makes it difficult to transport. The bulk of sylvinite is supplied for processing into potassium chloride. Effective for growing beets.

Potassium chloride - the main potash fertilizer - more than 50% K2O. Potassium chloride is highly caked, especially fine-crystalline, during transportation and storage. It is used as the main fertilizer for all crops, as it contains the least amount of chlorine.

40% potassium salt is a mixture of potassium chloride with sylvinite, contains 41-44% K2O. Good fertilizer for crops sensitive to sodium (all types of beets).

Potassium electrolyte is a waste in the production of magnesium from carnallite, contains 39-42% K2O and impurities of sodium and magnesium chlorides.

Kalimag (K2S04.2MgS04 and impurities) is a fertilizer obtained after grinding langbeinite and removing NaCl from it by leaching. К2О - 16-19% plus magnesium up to 5%. Potassium-magnesia salts are very suitable for fertilizing potatoes, especially on light soils, fruit crops and clover.

Potassium sulfate. Potassium sulfate contains 45-52% K2O, non-hygroscopic; very valuable fertilizer, especially for crops sensitive to chlorine and responsive to sulfur.

Recently attention has been attracted by industrial waste - cement dust containing potash (K2CO3), bicarbonate (KHCO3) and potassium sulfate. All these forms of potassium in cement dust are of high value for all plants and especially for those suffering from chloride salts. Contains - 10-15% K2O.

Ash as a potassium-phosphate-lime local fertilizer. Potassium in ash is contained in the form of carbonate salt K ₂ CO ₃ - potash. This form of potassium is good for all crops, and for plants sensitive to chlorine (potatoes, buckwheat, tobacco, lupine, grapes, etc.), it is better than raw potassium salts, and often superior to potassium chloride. A valuable component of ash is trace elements (boron in it is about 0.06%).

The content of potassium, phosphorus, calcium in ash is subject to strong fluctuations depending on soil and climatic conditions, species characteristics of plants, their age, etc.

Ash yield when using wood fuel is on average about 2 kg per 1 m² (or 4 kg per 1 ton of dry wood).

Coal ash is of no value as a fertilizer: the content of K ₂ O and P ₂ O ₅ in it does not exceed 0.1-0.4%.