Crops: The Lynch File - Your fertilizer rate is more important than placement

Tuesday, February 3, 2009

Some myths and misconceptions about different ways of placing phosphorus in the soil

by PAT LYNCH

There is a notion that if you put a little bit of fertilizer on the seed, that will be better than putting more fertilizer away from the seed. Some growers believe this.
It is like saying that, if you put a little bit of feed closer to a cow or pig, that is just as good as feeding them their total requirement but putting that feed a few feet away from them. You cannot grow a crop with inadequate fertilizer rates.

A crop utilizes 10-30 per cent of the applied phosphorus. Thus, if you apply one of the common liquid starters at four to five gallons per acre, you are applying about 13 pounds per acre of plant-available phosphorus. Of this, the plant will get to use 1.3 to four pounds per acre.

A corn crop will need 35-40 pounds per acre of total phosphorus. If you are only applying 1.3 to four pounds per acre in a liquid starter, then the soil must supply the other 30-plus pounds. If you are on a high-testing soil with phosphorus levels in the 40s, this is no big deal. But if you have soil that is testing in the 40s for the amount of extractable phosphorus, there is a small probability of getting an economic response from any applied phosphorus.

And the idea that somehow a liquid phosphorus is more available than dry is not true. If you could make phosphorus more available, then the crop would have less available to it. This is because that form of phosphorus that is plant-available is also the form that is tied up in the soil. Once phosphorus gets into the form that is taken up by the plants (H2PO4), this form is also available to be tied up by the soil.

There is a myth about the extra boost from phosphorus fertilizer seed placed in cold soils, but there is very little research to back this up. Typically, the research is done with a fertilizer that contains phosphorus and nitrogen, but there is no way to separate the influence of the two. If the seed-placed fertilizer gave the benefit, which nutrient did it?  

There is also very little research done to compare the benefits of seed-placed phosphorus versus broadcast. Some of the oldest research was done by the department of Land Resource Science at the University of Guelph in the late 1960s and early '70s.

This classic piece of research compared seed-placed liquid versus dry broadcast over three soil types and seven to eight years. It showed a three to four per cent increase in yield by using a liquid seed-placed fertilizer versus nothing. But it also showed a similar yield increase with broadcast fertilizer versus nothing.

Some of the more recent starter versus broadcast research comes from Florida. This research indicates that there is no difference between broadcast P and seed-placed. There are other U.S. on-farm-trials which indicate the same. Some of these reports comment that there are hybrids which are responsive to seed-placed fertilizer and other hybrids which are not.

One of these reports suggested that there could be a bigger advantage to seed-placed fertilizer with later planting than earlier planted corn, the reason being that later planted corn grows a smaller root system relative to the top growth under warmer soil conditions. Under cooler early weather, the corn plants grow relatively more root system. This is because, typically, in the cool days of early May, the air is cool but the sun warms the soil, resulting in a higher ratio of root mass to top growth as opposed to later when the soil and air is warmer.

These comments are about seed-placed phosphorus only. If you need zinc (which more soils in Ontario are needing) or magnesium, the best way to apply these nutrients is with a starter fertilizer.

In the future we are going to see higher yielding corn hybrids. These hybrids will have a superior root system to what we are currently working with. This extended root system will be more capable of getting phosphorus from a wider soil area. BF

Pat Lynch, CCA (ON), is head agronomist for Cargill in Ontario.