Rice Farming

Mid-South Rice Fertilizer Economics

Deriving recommendations and considering cost

By Dustin Harrell

Now that we have finally closed the books on one of the longest and most challenging rice growing seasons ever, we can begin to focus on the 2009 crop. What weighs heavily on most producers minds this year, as in previous years, is the high cost associated with growing a rice crop – namely diesel fuel and fertilizer costs.

There is a lot of uncertainty when it comes to fertilizer costs and understandably so. This uncertainty comes from the dramatic increase in fertilizer costs that we saw last year. I try to keep up with local fertilizer prices the best that I can during each growing season, and I have plotted the local retail cost of urea, triple super phosphate and potash that I was quoted last year just to illustrate the wild fertilizer rollercoaster ride that we took during 2008. By the time this article is published, I am sure that these prices will once again have changed, hopefully for the better.

What will certainly not change in 2009 is the fact that a 7,000 pound per acre rice crop takes up approximately 112 pounds N, 60 pounds P2O5 and 168 pounds K2O. If your soil cannot supply all of these needs, then fertilizer will need to be applied or a decline in rice yield is probable.

With proper scouting of fields, weed, insect and disease problems can be identified and controlled fairly effectively with chemical applications before the problem gets out of hand. Unfortunately, nutrient deficiencies are not as easily identified at the beginning stages. The term “hidden hunger” was coined to illustrate the fact that you can have a nutrient deficiency and not even know it. If the nutrient deficiency is not addressed, deficiency symptoms will eventually occur. However, by this time some of your yield potential has already been lost.

Because nutrient deficiencies are harder to identify early on and sometimes may go unseen, it is the fertility program that is the first place a producer will target when trying to reduce production costs. Consultants and researchers are commonly asked questions like: Can I cut back on fertilizer this year to save some money; and what is the most economical rate of a particular fertilizer on my farm? While these questions are hard to answer exactly, a better understanding on how fertilizer recommendations are made will help answer these questions and illustrate the economic importance of fertilizers, even when prices are not favorable.

How nitrogen fertilizer recommendations are made
Nitrogen recommendations are evaluated on a yearly basis by research scientists from each of the rice-producing states using variable rate fertilizer N trials. These trials are established on multiple locations in an effort to evaluate the variable N response on different soil types. Each variety, or potential variety release, is tested in separate trails. Rates of N generally range from 0 to 210 lb/A on silt loam soils and up to 240 lb/A on the heavy clay soils of the Mississippi Delta. A minimum of three years of yield response data from these trials are used to create the official state recommendations.

A question I am often asked is why are official LSU fertilizer N recommendations in a range of rates and not simply one rate for a particular cultivar? The answer is quite simple; it is because all soils are not created equal. Soils differ in their ability to supply N. Therefore, a one-size-fits-all recommendation is really not feasible.

Even though some states’ recommendations may provide a single recommended N fertilization rate, the rate represents an average response across multiple trials and locations. These rates are still adjusted for a high and low N responsive soil on an individual basis.

Let me give you a quick example. For example, consider the results of the fertilizer N response of the newest varietal release from the LSU Rice Research Station – Jazzman. This variety was less responsive to N on a Kaplan silt loam soil as compared to a Crowley silt loam due to differences in the two soils ability to supply native N. On the Kaplan soil, we produced approximately 5,000 lb/A rice without any N fertilization.

Comparatively, on the Crowley silt loam soil only approximately 2,000 lb/A rice was produced when no N was applied. A good N soil test for rice is currently not available and would greatly improve and fine-tune current N fertilizer recommendations. Hopefully, we will see a rice N soil test in the near future. Until then, knowing the history of N responsiveness at your farm is really the only way to fine-tune your N recommendations.

Changes in fertilizer price affect optimum N rate
Now let’s take a look at the how the price of N fertilizer affects the optimum N rate of fertilization. Economical optimum N rates can be determined by utilizing a ratio of the price of N and the price of rice in addition to the N fertilizer response curve.

Let’s use our data from the Jazzman N trial on a Crowley silt loam soil in 2008 as an example (Figure 1). We will use a $12 cwt. rice selling price ($0.12 per lb rice) and a $500 per ton urea ($0.54 per lb N) cost in our example. Plugging in this information, we can see the optimum N rate is approximately 171 lb N/A.

So how is the optimum N rate affected when urea costs increase and rice prices fall? Let’s look at a worst-case scenario, say urea increases to $850 per ton ($0.92 per lb N) like we saw in October of 2008 and the selling price of rice falls to $10 per cwt. ($0.10 lb rough rice). In this case we see that the optimum N rate declines only 10 lb/A. That is a fairly small decline in N needs relative to extreme changes in the price of fertilizer and grain. Therefore, for current cultural practices, these data suggest that it is not profitable to decrease N application rates. So can you really afford to cut back on N fertilizer use this year? ...Not really.

Economics of P and K
Fortunately, P and K needs of next year’s rice crop can be determined easily with the use of a basic soil test. A basic soil test will cost you around $7-$15, depending on what lab you choose to run the analysis and if you take the samples yourself or if you have someone else take the sample for you. Either way, the resulting analysis will give you a P and K recommendation for next year’s crop. In some cases, you may not need to apply P and K fertilizer at all and a simple $8 soil test may save you a considerable amount of money.

Because fertilizer prices have increased so much, we really need to move from the mind set of just applying P and K on every field at the same rate just because we have done it that way for years. It is much better to know how much P and K are actually needed and apply based on those needs.

Like soils, all soil test recommendations are not made the same either, and you must be aware of the differences in the recommendations you receive. For example, the LSU AgCenter’s Soil Test and Plant Analysis Laboratory provides P and K recommendations based on a crop maintenance philosophy. That is, that the fertilizer recommendations you receive are based only on what is required for next year’s crop and how much of that need that your soil can supply.

However, other labs may provide fertilizer recommendations based on a build-up philosophy. A build-up fertilizer recommendation includes the P and K needed for next year’s crop plus additional fertilizer in excess of the crop’s need to build-up soil test levels.

Depending on your situation, one philosophy may be better than another. For example, if you own the land you may want to use the build-up philosophy. On the other hand, if you are on the last year of a lease, do you really want to apply fertilizer beyond the crop’s needs? The bottom line is to know what philosophy your fertilizer recommendations are based on and, if needed, modify those recommendations based on your needs and fertility goals.

Dustin Harrell is an assistant professor with the LSU AgCenter. Contact him at (337) 788-7531 or DHarrell@agcenter.lsu.edu.