Researchers studied how adding phosphorus and manganese fertilizers to soil affects the availability of iron to wheat plants. They found that phosphorus moved iron from one form to another in the soil, while manganese had specific effects on different iron fractions. Increasing phosphorus levels in the soil decreased available iron for the wheat plants, and high phosphorus levels reduced iron uptake by the plants. The amount of phosphorus relative to manganese in the soil and in the plants was related to the iron concentration in the wheat. Iron associated with oxides and organic matter in the soil was important for the wheat's iron nutrition.