The fourth 'R' - putting nutrients in the right place

Wednesday, April 3, 2013

Some nutrients are more mobile than others, so no 'one-size-fits-all' system will work. A checklist of the best and worst options for nutrient placement on your farm

by KEITH REID

The fourth "R" in the 4Rs of Nutrient Stewardship is getting nutrients in the Right Place. After all, if a plant can't reach nutrients when it needs them, the right form, time or rate won't matter.

There are three reasons for placing fertilizer or manure in specific locations – first, to put nutrients where plants can reach them; second, to save nutrients from loss; and, third, to prevent injury to roots, particularly in small plants.

Since roots grow through the soil rather than on the surface, it makes sense that nutrients placed below the soil surface will be more readily available for uptake than nutrients on top of the ground. This does not mean that fertilizer left on the surface is unavailable to plants, otherwise surface broadcast fertilizers into winter wheat or forages would not work at all. It does mean that anything applied on the surface needs to get moved into the soil somehow. How easily this happens varies among nutrients.

Nitrogen (N) is the most mobile of the macro-nutrients, particularly in the nitrate form, so surface-applied N will move into the soil with rainfall or irrigation. Phosphorus (P) is the least mobile nutrient, so what is put on the surface will stay near the surface, while potassium (K) is intermediate in mobility.

The low mobility of P also means big advantages to banding it where the plant roots can reach it. Over the long term, P and K can accumulate near the soil surface if there isn't regular tillage and the fertilizers are surface-applied. In dry years, this can lead to nutrient deficiencies, because the soil will dry out too much for roots to absorb the nutrients.

N is the fertilizer at greatest risk of loss to the air. Urea (including the urea in UAN) breaks down when it is moist, releasing ammonia. If this happens below the soil surface, the ammonia rapidly dissolves in the soil water, and the resulting ammonium binds to soil particles, but ammonia released at the surface can drift off into the air. Thirty per cent or more of surface applied urea can be lost in this way. The same thing can happen to the ammonium in manure left on the surface of the soil, so most of the rapidly available N in manure can be lost.

P and K on the soil surface do not disappear into the air, but they can be washed away when there is runoff. This usually represents only a small percentage of the total that is applied, so it is not a major economic loss, but the P in the runoff can lead to water quality impairment downstream from the farms. Placing the P below the soil surface can eliminate, or greatly reduce, P loss.

The other side of this coin is that subsurface placement, particularly in a band close to the seed row, can lead to too much of a good thing. High concentrations of salts, or of ammonia released from urea or di-ammonium phosphate, can lead to fertilizer burn of the roots. This occurs when there is interaction among the nutrient source, rate and placement. Materials high in N or K can only be banded at low rates near the seed, while P fertilizers are generally safe.

So how are you to make placement work in your cropping system? Every farm is different in the crops and livestock they grow, the soils and landscapes they farm and the equipment available to them, so there is no "one-size-fits-all" system for applying fertilizers. This complexity is in addition to the differences in behaviour among the various nutrients. Balancing all of these factors means making compromises, so I will try to outline the best and worst options for each of the major nutrients.

Nitrogen. N fertilizers should be injected or incorporated to avoid losses through volatilization, particularly for manure or fertilizers containing urea. If injecting or banding, it should be placed between the rows rather than close to the seed to avoid fertilizer burn. If you must surface-apply N, broadcast in cool conditions when rain is forecast. Greatest volatile losses will occur if the N is applied to a warm, moist soil surface, followed by a week with no rain.

Phosphorus. Banding P close to the seed (or with the seed) will always improve P uptake, although it may not always provide a significant yield increase. This can be a challenge with large planting equipment, although high-capacity air carts that tow behind the planter have solved many of these issues. Broadcast P should be incorporated soon after application, if at all possible.  

Potassium. Placement is least critical for K, except in very infertile soils. In this case, banding a small amount of K near the seed in addition to broadcast K will give better yields than the broadcast K alone. Avoid high rates of K near the seed, however, or you will risk fertilizer burn.

The common practice for many corn farmers of broadcasting K before planting and working it in, followed by banding all of the P requirements along with a bit of N and K at planting, then injecting side-dress N between the rows comes pretty close to meeting the ideal placement for each of the nutrients. BF

Keith Reid is manager (eastern Canada), Soil Nutrient and GHG Management Agriculture and Agri-Food Canada, Guelph.