Phosphorus in Plants: What It Does and When Your Crop Needs It

So phosphorus decides how fast a young crop gets going. A seedling with enough of it roots deep and fast, tillers early and flowers on time. Later, phosphorus moves into the seed and the grain, which is why a crop short of it fills poorly and matures late.

A building block and an energy carrier

Phosphorus is a building block. It becomes part of the plant body, in its genes, its cell walls and its seeds, and the harvest takes it away. A ton of palay grain carries off roughly 2 to 3 kilograms of phosphorus, which is much less than nitrogen, but it has to be replaced all the same.

Unlike nitrogen, phosphorus does not wash away easily. The trouble is the reverse: it binds so tightly to the soil that the crop reaches only a small part of what is there. That is why where and when you place it matters as much as how much.

Where phosphorus shows first

Phosphorus moves inside the plant, so a shortage also starts on the older leaves. But it seldom yellows them the way nitrogen does. Instead the plant stays small and stiff, the leaves turn dark green, and in corn, tomato and many young plants the older leaves turn purple or reddish.

In palay the sign is fewer tillers and late, uneven maturity rather than a color. See phosphorus deficiency for the signs crop by crop.

When phosphorus in plants is most critical

Phosphorus matters most at the start. Young roots need it the day they form, and a crop that runs short in its first weeks rarely catches up, even if phosphorus comes later. So it belongs in the soil at planting or transplanting, worked in near the roots.

DAT means days after transplanting, DAS days after sowing in the field, and DAP days after planting. The days are the usual ones for a typical variety, so a long or short variety moves them a little. Timing phosphorus in plants to these windows gets more harvest from the same bags. Our guide to fertilizer timing by growth stage puts them together for a whole season.

Here is when each crop needs it most, from the anee.io stage book:

Rice, corn and other grains

Vegetables and legumes

Root crops, sugarcane and banana

When phosphorus comes too late

• Palay, transplanted: Phosphorus after panicle initiation comes too late to help the grain.
• Yellow corn: Phosphorus after silking comes too late to help the ear.

What keeps phosphorus from the crop

Most soils hold far more phosphorus than any crop needs, but little of it is in a form the roots can take. On acid soil it locks up with iron and aluminum. On alkaline and limy soil it locks up with calcium. It is most available when the soil pH sits near 6 to 7.

Cold, waterlogged or compacted soil also slows phosphorus uptake, because roots grow little there. Organic matter helps, and so does placing the fertilizer close to the roots instead of spreading it thinly over the surface. In a flooded paddy, phosphorus becomes easier to take up as the soil softens, which is one reason rice suffers less from it than upland crops.

Where phosphorus comes from

Phosphorus comes in the fertilizers you use at planting. Ammonium phosphate is the usual choice for palay and mais, with complete fertilizer when the soil also needs potassium. See our guides to 16 20 0 fertilizer and complete fertilizer.

• Ammonium phosphate (16 20 0): the common basal phosphorus for palay.
• Complete fertilizer (14 14 14): phosphorus with nitrogen and potassium.
• Diammonium phosphate (18 46 0): the most phosphorus per bag.
• Chicken manure and compost: slow phosphorus that also builds the soil.

Phosphorus questions