By the numbers, legacy phosphorus looks like a slam dunk. But can plants actually live on it? Studies suggest that, in places with long histories of phosphorus overuse, like the U. Conventional measures of soil chemistry suggest they should apply more fertilizer, says Barbara Cade-Menun, who oversees the experiments at the Swift Current Research and Development Center in Canada.
Scientists think that as plants use up the readily available phosphorus in the fields, soil minerals and organic matter release more of the nutrient. And that means just cutting back on fertilizer could go a long way to meeting phosphorus demand and reducing runoff without jeopardizing harvests.
At some point, however, soil phosphorus drops low enough that crops become stressed. The scarcity of phosphorus in nature forced wild plants to develop strategies for securing an adequate supply. Many evolved extensive root systems that search out phosphorus. Some can also excrete chemicals to liberate the nutrient from the soil. Researchers now want to create smarter crops. Researchers then bred the trait into modern rice plants, and in farmers in Madagascar—which has naturally nutrient-poor soils—started testing some of the most promising varieties.
Sigrid Heuer, a researcher at Rothamsted who helped with the rice study, is searching for a similar gene in wheat as part of the International Wheat Yield Partnership. Besides breeding, no-till farming could help by preventing soil compaction and encouraging good root development to help plants access more legacy phosphorus.
Withers and Sylvester-Bradley have been running down the phosphorus levels in their test fields for the exact purpose of exploring these kinds of approaches. The researchers had to abandon the barley field in Cambridge because of changes in farm ownership. But at the remaining sites, phosporus levels have finally dipped low enough for them to start conducting experiments on how to help plants access as much legacy phosphorus as possible.
The first will compare the performance of existing commercial wheat varieties. The researchers had to wait longer than expected—nearly a decade—for phosphorus levels to drop back to natural levels. But that fact alone should reassure growers that they can safely cut back on the nutrient, Sylvester-Bradley says. This story was supported by a science journalism fellowship from the European Geosciences Union.
All rights reserved. Phosphorus in the soil Chemically, phosphorus is a very stable element. Uptake problems There are several possible reasons if your pastures or crops seem to get little benefit from phosphorus. Your soil is so low in phosphorus that even with some phosphorus applications, there is little available in the soil solution for the plant to take up. This is a common problem on the North Coast where superphosphate application is too low.
You have cultivated the phosphorus into the soil where it is not readily available to most crops because its effect is diluted and there is greater likelihood of the phosphorus binding with other minerals. Your soil may have reached its optimum phosphorus level, and adding extra will only maintain the level, not boost it.
This is the most usual reason for non-response in well-farmed land. Other factors may be contributing to poor crop growth. Improving uptake Do a soil test to check your phosphorus levels and see whether your soil already has enough phosphorus for plants, in which case they won't respond to extra phosphorus.
When sowing seed, place phosphorus fertiliser close to seed. This is very effective in low-phosphorus soils, for you need only half as much phosphorus as you do when broadcasting the fertiliser. Soil phosphorus cycle. Once phosphorus enters the soil through chemical fertilizers inorganic source , manure, biosolids, or dead plant or animal debris organic sources , it cycles between several soil pools via processes such as mineralization, immobilization, adsorption, precipitation, desorption, weathering, and dissolution.
Following are explanations of these processes:. Mineralization is a process through which organic phosphorus in soil is converted into inorganic phosphorus with the help of soil microbes.
Immobilization, on the other hand, is the reverse of mineralization. During immobilization, inorganic phosphorus forms are converted back to organic forms and are absorbed into the living cells of soil microbes.
Immobilization typically occurs when crop residues are incorporated in the soil. As crop residues decompose, more phosphorus becomes available in the soil solution through mineralization.
Because mineralization and immobilization processes are biological processes, they are highly influenced by soil moisture, temperature, pH, organic carbon to organic phosphorus ratio of crop residues, microbial population, etc. The phosphorus binding takes place on clay surfaces or the iron Fe and aluminum Al oxides and hydroxides present in soil.
Adsorption is a fast process and reversible in nature, meaning that adsorbed phosphorus can be released into soil solution via a process known as desorption and will be available for plant uptake.
Soils containing greater concentrations of iron and aluminum oxides have greater potential to adsorb phosphorus than soils with relatively low iron and aluminum oxides. Another soil property that favors phosphorus adsorption is the clay content. Soils with greater clay content have higher adsorption capacity than coarse textured sandy soils. Soil contains minerals that are rich in phosphorus. These minerals are classified into primary and secondary minerals. Minerals break down over time a process referred to as weathering and release phosphorus in the soil solution for plant uptake.
Primary minerals such as apatite are very stable and resistant to weathering. The proportion in which these two forms are absorbed is determined by the soil pH, when at higher soil pH more HPO is taken up. The mobility of phosphorus in soil is very limited and therefore, plant roots can take up phosphorus only from their immediate surroundings. Since concentration of phosphorus in the soil solution is low, plants use mostly active uptake against the concentration gradient i.
Active uptake is an energy consuming process, so conditions that inhibit root activity, such as low temperatures, excess of water etc. Symptoms of phosphorus deficiency include stunted growth and dark purple color of older leaves, inhibition of flowering and root system development. In most plants these symptoms will appear when phosphorus concentration in the leaves is below 0. Excess of phosphorus mostly interferes with uptake of other elements, such as iron, manganese and zinc.
Over-fertilization with phosphorus is common and many growers apply unnecessarily high amounts of phosphorus fertilizers, especially when compound NPK fertilizers are used or when irrigation water is acidified using phosphoric acid. The acceptable concentration of phosphorus in nutrient solutions is ppm, even though it was found that it can be reduced to ppm. In nutrient solutions that flow continuously the concentration can be as low as ppm. In soilless media, much like in soil, phosphorus accumulates with each phosphorus addition, and minerals of phosphorus and calcium or magnesium start to precipitate.
The types of minerals that are formed depend on the pH of the media. Phosphorus soil test level gives a measure of the capacity of the soil to supply phosphorus to the soil solution. The soil test does not measure the total amount of phosphorus in the soil, because the available amount of phosphorus is much less than the total amount. It also does not measure the phosphorus in the soil solution, because the amount of phosphorus in the soil solution is usually very low and does not represent appropriately the amount of phosphorus that plants can potentially absorb during the growing season.
Phosphorus soil test is actually an index that helps predict the fertilizer requirement of the crop. The recommendations for fertilizer application are determined based on many field tests in many soils and crops.
Different testing methods result in different values, which have to be interpreted accordingly.
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