When looking at nutrients for crops, two key figures to keep in mind are crop uptake versus crop removal. Crops require a lot of nutrients to grow but some portion of those nutrients are left behind when the crop is harvested.
Crop uptake of a nutrient is the amount of a nutrient used by a crop when it’s growing. For example, the Manitoba Soil Fertility Guide lists nitrogen uptake for a 400 cwt/ac potato crop at 205-251 lbs./ac. This means that if you’re planning to grow a 400 cwt/ac potato crop, you should have that much nitrogen available for the potatoes or you will be limited by the available nitrogen. Under ideal conditions, a potato crop with only 150 lbs of available nitrogen might only produce 300 cwt/ac of potatoes.
Crop removal is a measure of the amount of nutrients actually removed from a field by the harvested crop. Using the potato example again, nitrogen removal is listed at 115-141 lbs./ac. This means that roughly half of the nitrogen taken up by the potato plant remains in the field after harvest. The nitrogen from the potato plants apart from the tubers goes back into the soil. Preventing the escape of these nutrients is one of the reasons why erosion control is emphasized for potato production.
The difference between uptake and removal plays a big role in manuring fields. One of the problems that we’ve encountered is a rise in phosphorus levels in manured fields. Pasture fields have very little removal of phosphorus from the soil because the cattle take up very little phosphorus from the grass. A typical hay field where the grass is baled up and hauled away has removal rates of around 30 lbs. P2O5 per acre. A typical pasture with only grazing and no hay removal has removal rates of around 10 lbs. P2O5 per acre.
If you want to maximize nutrient recycling within a field, remove as little of the crop as you can. One example of this would be to work crop residues back into the field after harvest. If you want to maximize nutrient removal, remove as much of the crop as you can. For example, bale up the straw and ship it elsewhere. The choice of crop and of practices can have dramatic effects on the levels of crop update versus removal.
Post a Comment