Phosphorous Fertility

Phosphorous (P)

There are two distinct approaches to managing phosphorous (P); one is referred to as the “Sufficiency Approach” and the other is the “Build (or Target) and Maintain Approach”.  OMAFA P guidelines for corn as outlined in this section adhere to the Sufficiency Approach. For a more detailed explanation of the two approaches and how they influence P decisions, see Chapter 9, Soil fertility and nutrient use[BR1] .

Adequate phosphorus is necessary for optimum corn growth and yield, although the response to these nutrients is not as evident as with nitrogen. Phosphorus deficiency does not show any unique symptoms; phosphorus-deficient plants will be stunted and may have a darker green or purplish colour. Purple leaves may also be an indication of cool weather stress or root injury (see Figure 1[cmb2] ). Phosphorous will exhibit “hidden hunger,” where yields are reduced by a deficiency even where no deficiency symptoms are visible.

Phosphate guidelines for corn are presented in Table 1.

Table 1. Phosphate (P2O5) Guidelines for Corn

For information on the how to use these tables or if an OMAFA-accredited soil test is not available, see fertilizer guidelines.

Where soil tests indicate that large amounts of phosphorus may be required, the major portion may be broadcast and incorporated in the fall or spring. Where soil tests show a moderate or small requirement for these nutrients, apply a fertilizer containing nitrogen (preferably in the ammonium form) and phosphorus, or nitrogen, phosphorus and potassium as a starter at planting. All of the phosphorus and some of the potassium may be applied in a band 5 cm (2 in.) to the side and 5 cm (2 in.) below the seed (refer to Table 9–24. Maximum safe rates of nutrients, in Chapter 9, Soil fertility and nutrient use).

Seed-Placed Fertilizer

Field trials over several years have shown that an application of 10–15 kg/ha (9–13 lb/acre) P₂O₅ directly with the seed will give greater yield increases than 20 kg/ha (18 lb/acre) P₂O₅ in a side band application. At phosphorus soil tests of 13–45 ppm, this “with-seed” application is more likely to give a profitable response than a side-band application. At soil tests below 13 ppm, application of 10–15 kg P₂O₅/ha (9–13 lb P₂O₅/acre) with the seed may also be profitable, but cannot replace the requirement for additional phosphorus in the side band or broadcast application.

Fertilizers applied with the seed that contain nitrogen in the ammonium form must be low in salt and must not contain either urea or diammonium phosphate. They must also be distributed uniformly to avoid toxicity to the germinating seed. Application of more than 15 kg/ha (13 lb/acre) P₂O₅ with the seed in 75-cm (30 in.) wide rows is not advised.

Maximum Safe Rates of Fertilizer

Applying too much fertilizer to corn may result in crop injury, either from excessive salts or ammonia (Figure 2). The more concentrated the fertilizer and the closer it is to the seed, the greater the risk of crop injury and the lower the safe rate. Maximum safe rates are given in Table 9–24. Note that slight reductions in crop growth and yield are possible with these application rates under adverse weather conditions.

Phosphorous (P): Band vs. Broadcast

Band applying phosphorus (P) is more likely to result in profitable corn yield increases when compared to the same amount applied broadcast.  A review of Ontario research trials indicated that applying 50–70 kg-P₂O₅/ha (45-62 lb-P₂O₅/acre) in a 5 cm x 5 cm (2 in. x 2 in.) band, had average yield increases that were three times higher when compared to broadcast applied P.  Only banded P when applied at rates between 50 –70 kg-P₂O₅/ha resulted in yield increases that on average were profitable.

Table 2. Average grain corn yield and profit response from broadcast and banded phosphate, shows the average grain corn yield and profit response to broadcast and 2 x 2 band applied P.

Table 2. Average Grain Corn Yield and Profit Response to Broadcast and Banded Phosphate1