P. W. (Paddy) Shannon
Agricultural Consulting Services

We apply fertiliser for two reasons. First, to build up the fertility of the soils on our farms, orchards or gardens to get the production we need to make them economic. Second, to keep the soils in that condition.

If we do not keep applying fertiliser, the fertility we built up will be lost. Every time we sell produce from our land, be it plants, grain, animals, meat, wool or milk, some of the fertility from our soils is sold with it. Where animals are run, some of the fertility is lost in dung and urine spread on stock camps, races and yards. Soil erosion, nutrient fixation and leaching can also reduce soil fertility.

So, unless we continue to apply fertiliser, these nutrient losses will reduce soil fertility. With this reduction, the production we get from our soils will fall and make farming or growing uneconomic.

What does a fertiliser do? Basically, a fertiliser releases plant nutrients into the soil. Plant roots can only absorb nutrients once they are in the soil.

If we need the nutrients to build up soil fertility we want them released very quickly, so they can begin to do their job. This stage of farm or orchard development is often very costly, so we need to see a quick improvement to let us get a return from our investment. For this to happen, the fertilisers we use for development should be fast-acting, completely releasing the nutrients they contain in a matter of days or weeks, not months or years.

Where we apply fertilisers to maintain production after we've finished our development, the needs are different. As long as the fertiliser releases the nutrients it contains into the soil faster than the plants remove them, the form of the fertiliser matters very little. It can be rock, reverted, soluble, granulated or powder - or any combination of these. All it has to do is be capable of being spread evenly and then releasing the nutrients it contains into the soil.

Why is this? It is because plant roots take up nutrients from the soil, so it is the soil, and not the form of fertiliser, that controls the release of nutrients to the plant. All we have to do is make sure that the nutrients we apply in our fertiliser are able to be released into the soil. Provided the form of the fertiliser is such that it will release its nutrients, we can make our choice based on the cost of the nutrients supplied. You can discount most of the claims for superiority of different forms - there are no miracle processes involved, just basic soil chemistry - and base your choice purely on the amount of nutrient supplied and the price.

This works because in the initial development process, what we were actually doing was establishing a "capital base" of fertiliser in the soil. The amount of nutrients that can be used in a year depends on the size of this "capital base". Unless we replace what we use there will be less capital to release plant-available nutrients for next year's growth.

Maintenance fertiliser is applied to re-build our "capital base", not to feed the plant directly.

Lets put some numbers on this to illustrate the point, using phosphorus (P) as an example of a plant nutrient. On an intensive beef farm, carrying 20su/ha, we will need to grow nearly 13,000 kg of dry matter per hectare to feed the animals. This growth will extract some 45kg of P per hectare from the soil.

Of the P in the grass actually eaten by the animals, some 10 kg of P per hectare will leave the farm as produce (beef animals sent to the works) and 2 kg of P/ha will be lost in fertility transfer to stock camps and races. 33kg of P/ha will be returned to the soil from dung, urine and decomposing plants. On many soils in New Zealand, 13 of these 33 kg of P/ha will be lost through fixation, leaching and erosion, so our total loss comes to 25kg P/ha each year. Most soils will release some phosphorus from the weathering of minerals within the soil. If we allow 3 kg P/ha/year for this, it gives a balance of 22 kg P/ha lost each year.

This leaves 23 of our original kilos of P available for re-use in the next year. If we do not replace the other 22, our "capital base" will shrink and it will not be able to continue to supply us with enough P to grow grass to feed 20 stock-units per hectare. So, we either put in 22 kg of P/ha to let our soil continue to supply P to the plants at a rate of 45kg per hectare per year, or face a loss of production.

The biggest flow of nutrients each year is from the soil to the plants, not the fertiliser to the soil. The soil had to supply, in this case, over twice the amount of nutrients that were needed as fertiliser.

Soil conditions are therefore more important than the form of fertiliser once we have our "capital base" of soil fertility as big as it needs to be.

How much of a reduction will we see if the 22 kg of P from our example is not put back? We have to have 310 to 320 kg of P/ha in the soil's "plant available bank" to allow us to withdraw 45 kg P/ha each year to drive plant growth. If we do not replace the 22 kg P/ha that is lost, we will only have 290kg left. Initially, this will only cause a slight reduction in production, causing a drop to 19.5 su/ha. If we continue to with-hold fertiliser, after five years we will see a 20% drop in production and if we still continue without fertiliser, we will end up being able to carry only 7 to 8 su/ha, producing less than half of what we used to do.

The small reduction over the first two or three years explains why some fertiliser programs based on using very small amounts of nutrients, either finely-ground or in liquid form, appear to work. The plain fact is that they do not; what they "trade" on is the fact that the "capital base" already in the soil will initially sustain production. Indeed, some of these merchandisers know this, and will suggest to their clients that they have a "holiday" from their program after two or three years and apply some "straight" fertiliser. What this will do is provide the "capital base" with a partial top-up to keep the bogus program appearing to work for another few years before the inevitable collapse occurs.

To sum up, fertilisers are needed to build up soil fertility and to maintain it at a level that will keep your property economic. The job of the fertiliser is to supply nutrients in a form that can be stored in the soil and then released to the plant roots.

While fast-release fertilisers are needed during the development phase, you can use any form of fertiliser to maintain production if it releases its nutrients fast enough to replace plant withdrawals from the soil. Choose fertilisers based on the cost per kilogram of nutrient applied - do not be swayed by claims of superiority based on some chemical property of the fertiliser concerned.

Plants take up more nutrients per year from the soil than you apply in fertiliser. Not all of these nutrients are lost because some are recycled from dying plants, dung and urine. You need to put on fertiliser to replace the plant nutrients which are lost through product sale and fertility transfer.

To the plant, the soil is the most important source of nutrients, not the fertiliser.