Phosphate fertilizers react with soil iron, aluminium and calcium, and a large part of the applied phosphorus can become less available in the first season. The reaction is faster in some soils than others, and it depends on pH, clay and oxide content, and the form of the phosphate. Coating MAP aims to slow that fixation, so that more phosphorus remains available to the crop in the period when it is needed.
The question for a grower is whether the slower reaction is worth the extra cost. The answer depends on how strongly the soil fixes phosphorus, how much phosphorus the crop needs, and how the product is placed. A coated product is not a cure for a poorly managed phosphorus programme, but on high-fixing soils it can reduce the amount applied for the same crop response.
Why phosphorus fixation matters
Fixation is the process by which phosphate becomes bound to soil particles in forms that roots cannot easily take up. In acid soils the binding is mainly to iron and aluminium oxides, and in alkaline soils it is mainly to calcium. Both reactions are strongest in the first weeks after application, and the plant takes up a smaller share of the phosphorus than the amount applied.
Fixation is one reason why phosphorus responses are often disappointing on high-fixing soils. A soil test may show a moderate level of available phosphorus, yet the crop still responds to fertilizer, because much of the test value is held in forms the crop cannot reach quickly. Reducing the rate of fixation can therefore change the response without changing the soil test.
How a coating changes the reaction
A coated granule releases its phosphate more slowly than an uncoated one. The coating limits the contact between the soluble phosphate and the soil particles, so less of the phosphate reacts with iron, aluminium or calcium in the first days. Over the following weeks the phosphate moves out of the granule into the surrounding soil, and more of it stays available for root uptake.
The effect is therefore a shift in timing. The coating does not change the total amount of phosphorus that the crop can take up, and it does not stop fixation altogether. It spreads the supply so that the early roots and the rapid growth phase meet more available phosphorus than they would from an uncoated product.
| Feature | Uncoated MAP | Coated MAP |
|---|---|---|
| Dissolution | Fast, most phosphate in solution early | Slower, released through the coating |
| Early fixation | Higher on fixing soils | Reduced by the slower release |
| Root uptake window | Concentrated in the first weeks | Spread over a longer period |
| Typical use | Standard basal and banding | Fixing soils and placements where early loss is high |
Placement and rate
Placement matters as much as the product. A coated granule banded close to the seed or the root zone meets the roots early, while a granule broadcast on a high-fixing soil may be fixed before the roots reach it. Banding reduces the contact with the soil surface and concentrates the phosphate where roots can find it.
The rate should be set from the soil test and the crop's removal, not from the product's nutrient content alone. On a soil with low fixation, the coated product may give little extra benefit at the same rate. On a high-fixing soil, the same crop response may be reached with a lower rate, which can offset part of the extra cost.
What to measure in the field
Compare coated and uncoated MAP in the same field, at the same rate and timing, using replicated plots. Include an untreated control so that the response to phosphorus is visible. Measure yield and, where possible, the phosphorus content of the crop at a key growth stage. Tissue phosphorus shows whether the crop took up more of the nutrient, which is the mechanism the product is meant to improve.
- Compare coated and uncoated MAP on the same field, at the same rate and placement.
- Use crop yield and tissue phosphorus, not only the soil test, to judge the result.
- Record the soil pH, the clay and the organic matter, because they explain how strongly the soil fixes phosphorus.
- Repeat the comparison over more than one season, because rainfall and temperature change the fixation and the uptake.
Cost and value
Compare the cost of the coated product with the cost of the rate it replaces. If the coated product gives the same yield at a lower rate, the cost per tonne of crop may be lower even when the price per tonne of product is higher. If it gives the same yield at the same rate, the extra cost has to be justified by a quality or a timing benefit, such as an earlier root response on a cold soil.
Include the value of the phosphorus left in the soil. Phosphorus that is not taken up by the crop in one season remains in the soil in forms that may be available later. A fair comparison therefore looks at the crop response over two or more seasons, not only the first harvest.
Soil conditions where coating helps most
Coating is most likely to help on soils that fix phosphorus strongly, such as acid soils with high iron and aluminium oxides, and calcareous soils with high free lime. It is less likely to help on soils that are already rich in available phosphorus, or on soils where the main limit is water or root health rather than phosphorus supply.
Crops with a long early root phase, and crops planted into cold soils, also benefit more from a coated product, because their roots take longer to reach the fertilizer band. A grower with these conditions should consider the coated product as part of the planting plan.
Seasonal timing and soil moisture
The benefit of a coated product depends on the season. In a wet spring the uncoated phosphate may be fixed quickly while the soil is still cool, and the coated product can hold more of the phosphorus for the roots. In a dry spring the phosphate moves little either way, and the difference between the two products may be small. Record the rainfall and the soil temperature at application, so that the result can be interpreted in the right context.
Timing should also follow the crop. Applying the coated product at planting gives the early roots the most benefit, while a later application may suit a crop that takes most of its phosphorus later in the season. The planting calendar and the crop's uptake curve should guide the choice.
Common questions
Will coated MAP always give a higher yield? Not always. The benefit depends on how strongly the soil fixes phosphorus, the crop's demand and the placement. On low-fixing soils the difference may be small.
Can I use it on every crop? It is suited to crops where phosphorus supply limits yield. Check the rate and placement for the crop and soil, and follow the product label.
How do I measure the result? Compare yield and tissue phosphorus with an uncoated control on replicated plots. Repeat over more than one season before making a farm-wide decision.
- International Plant Nutrition Institute (IPNI), phosphorus fixation and management resources.
- Kingenta internal trial data for KingCote coated MAP (to be confirmed before publication).
- University extension publications on phosphate fertilizer efficiency and placement.
- Sharpley, A.N. and Smith, S.J., published research on phosphorus availability and soil fixation (academic literature).
Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.