Superphosphate fertilizers are suitable for all types of soil (acidic, neutral, alkaline, saline), but are most effective on non-acidic or low-acid soils.
Read more: >> Phosphorus disorder
>> Phosphate fertilizer, things to know
1. Types of soils and phosphorus absorbing environments of plants
Vietnam’s soil is classified into 11 main groups, the area ratio of the soil groups is as follows: sandy soil 1.70%, alluvial soil 10.81%, saline soil 4.05%, alkaline soil 5.92%, gray soil 7.47%, valley soil 1.20%, peat soil 0.80%, feralit soil on basic and neutral magmatic rock 8.53%, other feralit soil 47.09%, ferral humus on mountain 11, 14%, inert erosive soil with gravel 1.29%.
pHKCl values of some main soil groups: Sandy soil 5.5 – 6.5; saline soil 5.5 – 6.5; alkaline soil 2.5 – 4.5; alluvial soil 4.5 – 6.0; glay soil 4.0 – 5.0; infertile gray soil 4.0 – 5.0; feralit soil 4.0 – 4.5. Most plants only grow well within a certain pHKCl limit; pHKCl < 3 and > 8 will be very restrictive for many crops; pHKCl = 3 – 4 is moderately restrictive and pHKCl = 4 – 5.5 is moderately restrictive.
The group of plants that are very sensitive to high pH includes cabbage, cotton, sugar beet, sugarcane… grow well on soil with pH = 7 – 8. The group of plants sensitive to high pH includes wheat, corn, soybean, peanut. , sunflower, cucumber, onion… grow best on soil with pH = 6 – 7. The group of plants less sensitive to high pH are tomato, carrot, rice, sorghum, cassava, potato, sweet potato, Flax, coffee, pepper, orange, tangerine, longan, watermelon… can grow in the pH range = 4.5 – 7.5 and most suitable pH = 5.5 – 6.0. The group of plants sensitive to low pH are tea, pineapple, and many green manure plants… grow best on acidic soils with pH = 4.5 – 5.5 and are adversely affected on neutral and alkaline soils. Obviously, the soil is like a tree, followed by liming, fertilizing to match the tree, the soil and the weather.
Except for the feralit soil group (on limestone or basalt), alluvial soil in the Red River and Mekong River has relatively good total phosphorus content, while most of the remaining soil groups in Vietnam are poor in phosphorus. The pH range of the soil is most favorable for phosphorus to be dissolved and easily absorbed by plants at 5.2 – 6.5. When the soil pHKCl is below 4.5, iron phosphate is formed in the soil, aluminum phosphate is difficult to dissolve, so it affects the efficiency of phosphate fertilizer use, so it is necessary to apply lime.
The amount of lime applied to improve the soil depends on the type of crop and the soil texture, light or heavy: very acidic soil pHKCl < 3.5 apply 1.0 - 5.0 tons CaO/ha, acidic soil pHKCl = 3 ,5 - 4.5 apply 0.7 - 2.0 tons, acidic soil pHKCl = 4.5 - 5.5 apply 0.5 - 1.0 tons. The amount of lime in the composition of superphosphate or fused phosphate fertilizer only provides calcium nutrition for plants, but has little meaning in improving acidic soil.
Research results show that phosphorus in Vietnamese soil is mainly in the form of iron phosphate 3; This form of wet rice can be used (due to iron reduction, phosphorus release), but it will be difficult for dry crops. Plants absorb phosphorus most readily in the water-soluble form; Plants absorb phosphorus in the form of H2PO4 – at low pHKCl and HPO42 – at high pHKCl. Phosphate-absorbing plants are very weak at the seedling stage, so they need to be lined with soluble phosphate fertilizers such as superphosphate, DAP, etc. to provide the plants in a timely manner.
2. Advantages and limitations of each type of phosphate fertilizer
-Superphosphate provides phosphorus, calcium and sulfur for plants. Chemical formula: Ca(H2PO4)2.H2O + 2 CaSO4 + 2H2O + 2% free phosphoric acid. Phosphate content 15.0 – 16.5% P2O5, 11 – 12% sulfur (S) and 22 – 23% CaO. Superphosphate fertilizers are suitable for all soil types (acidic, neutral, alkaline, saline), but are most effective on non-acidic or slightly acidic soils (pH = 5.6 – 6.5) with all type tree; can be used for priming, top dressing; Can be applied to the soil or mixed with water for irrigation. Due to its water-soluble nature and quick absorption by plants, Superphosphate is very suitable for short-term crops such as beans, vegetables, medicinal plants, corn, tobacco, etc., and plants that require a lot of sulfur such as cabbage oil, kohlrabi, cabbage, cauliflower, beans…
-Fused phosphorus is slightly soluble in water, soluble in weak acids (2% citric acid). Theoretical formula: 4(Ca,Mg)O.P2O5+5(Ca, Mg)O.P2O5.SiO2. Fused phosphate fertilizer has 15 – 16% P2O5, 15 – 18% MgO, 28 – 34% CaO and 24 – 30% SiO2. Fused phosphate fertilizers are suitable for acidic, washed out, hilly, infertile, old alluvial soils; with fruit trees, perennial industrial plants, legumes, green manure crops…
Do not apply fused phosphate fertilizers on slightly alkaline, alkaline saline or newly neutral alluvial soils. Fused phosphate fertilizers can be applied on acidic or alkaline soils, but on this type of soil natural phosphate fertilizers are more cost-effective and alkaline soils are not lacking in magnesium. The phosphorus in the fused phosphate fertilizer can only be converted to a soluble form in the conditions of acidic soil and rich organic matter.
In many cases, superphosphate is superior to other phosphate fertilizers due to the sulfur element (S) present in the manure. Even in acid acid soils, this saline soil shows its superiority over other phosphate fertilizers thanks to its gypsum (CaSO4) composition. The advantages of fused phosphate fertilizers can be masked by sulfur (S) deficiency even when applied on silicon (Si) and magnesium (Mg) degraded soils.
During the production process, the cropping industry has formed the main commodity crops and is difficult to replace such as rice, maize, peanut, tea, coffee, pepper, fruit trees, vegetables… because of their adaptability. and comparative advantage in economic efficiency. Therefore, the specific fertilizers are suitable for the soil, and the plants are difficult to replace. Along with a number of other physiological sour fertilizers such as potassium chloride, potassium sulphate, ammonium sulphate, superphosphate fertilizers have been used for decades and still have good effects.
The long-term use of superphosphate will acidify the soil or fused phosphorus will increase the soil pH, neither of which have been demonstrated in long-term experiments on soil in the humid tropics in Vietnam. In the world, there have been lessons learned when long-term use of non-sulfur phosphate fertilizers has led to a serious lack of sulfur for plants, which then add sulfur to the fertilizer; This is a very unreasonable and expensive job.
3. The method of using phosphate fertilizers with high efficiency
Phosphate fertilizers of any kind are precious and need to be used to achieve high efficiency. The problem is fertilizing the right soil, the right tree and the period of growth and development of the tree, the right dose, the right weather. When fertilizing plants with phosphorus, it is necessary to consider the status of phosphorus in the soil, the type of crop, the type of fertilizer; concentrated fertilizer, compost with organic fertilizer for fertilizer; balanced fertilizer for N and K2O. Based on the season to fertilize, for spring rice, the dosage is higher than that of the crop; based on the crop structure to fertilize, if the previous crop was legumes, sweet potatoes, etc. fertilized with a lot of phosphorus, in the spring rice crop, the amount of fertilizer needed will be reduced a little. To promote the advantages of two types of phosphorus, we should mix these two fertilizers together in an appropriate ratio or SX NPK containing both types of phosphorus.
Above are some advices for farmers to use phosphate fertilizers with the highest efficiency based on scientific results achieved in our country in the fields of soil and plant nutrition, crop science…
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