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Banking approach to rice fertility

As you prepare for 2004, let me encourage you to think about an important factor in making a rice crop — soil fertility.

When the term fertility is mentioned for rice production, many of you think of nitrogen and sulfur. For most rice farms in the Mid-South Delta, those two nutrients still are the only nutrients, which when applied correctly, optimize rice yields. However, many farmers need to broaden their thinking about soil fertility to include phosphorus and potassium.

I often equate phosphorus and potassium fertilization to the lesson we should have been taught at an early age about checking accounts. As long as there is money in the bank, checks can be a convenient way to pay bills; however, if there is no money to cover the check, overdraft charges and a bad name around town can be costly. The soil nutrient “bank,” if not managed properly, can drop to costly levels because of decreased yields and increased disease pressure.

Just as financial records are used to monitor credits and debits in personal finances, soil testing, crop removal charts and a good record of inputs will help monitor the health of our soils.

I often see fields where rice is short and spindly and has a dirty, dark green appearance, which is indicative of a phosphorus deficiency. These areas normally become visible five to 10 days after the permanent flood is established.

My research, which is sponsored by the Mississippi Rice Promotion Board, indicates sometimes you can apply phosphorus fertilizer into the water before midseason and obtain a yield response; however, in most situations, if a soil sample is collected prior to planting, a phosphorus application is recommended. If the $12- to $15-per-acre phosphorus application is applied between planting but prior to flooding, the investment could pay a seven- to 10-fold dividend.

If these problem areas are isolated spots in the field, the soil sampling scheme used by most growers may miss the bad area. However, in today's technological age, we can look at a picture of the whole field each year after harvest in the form of a yield map.

If you pay for yield monitoring on your farm, look for areas within your fields that appear to be problematic year after year. Go to those areas and soil sample. If the areas are not very large, you may consider applying nutrients in only the bad areas. If you have not adopted the yield-monitoring practice, your eyes and knowledge of individual fields are just as good if not better. Use that historical knowledge of your farm to make soil sampling decisions.

Many of you may have samples analyzed and find that only nitrogen and sulfur are recommended. However, by soil-sampling on two- to three-year intervals, you can better monitor fertility levels and be prepared to begin a fertility maintenance program. A maintenance application means to apply nutrients based on crop removal to fields where the present nutrient levels are high enough that a yield response would not be obtained with a fertilizer application.

Even though no immediate economical benefits are reaped from maintenance applications of fertilizer, a maintenance program allows your soils to remain at highly-productive levels, which will pay off long-term.

If soil test levels of phosphorus and/or potassium are remaining in the high range, I do not recommend a maintenance application. If soil test levels fall into the upper-medium to lower-high range, I recommend you consider starting a maintenance application program.

To make this decision, however, you need to know how much phosphorus and potassium you are removing with the grain. In a one-to-one rice/soybean rotation, about 85 pounds phosphate per acre and 95 pounds potash per acre are removed during those two years when average rice yields are 150 bushels an acre and average soybean yields are 45 bushels acre.

These numbers are larger for those fields that averaged 180 to 200 bushels an acre of rice followed by soybeans that averaged 60 to 80 bushels per acre.

For rice and soybeans, our yield averages continue to rise. We cannot continue to increase yields on fields with marginal fertility levels without replacing those nutrients. On soils that have exceptional phosphorus and potassium fertility levels, we are increasing the rate of nutrient mining as we increase rice yields through new technologies.

It is critical to sample during the correct time period and most of all, to be consistent from one sampling year to the next. Not only are soil test levels affected by crop removal, but environmental conditions can affect the results as well.

In rice-to-soybean rotation, I recommend delaying soil sampling to the late winter or early spring after the soybean crop is harvested. The soil conditions present immediately following harvest tend to cause soil test levels to be much lower than what will be present at planting time the following spring. The reason I recommend sampling behind the soybean crop is because the soil chemistry is greatly influenced by the flooded conditions of the preceding rice crop.

A lot of money is spent each year in rice production for nitrogen fertilizer, herbicides, insecticides and fungicides, but if your soil test levels of phosphorus and potassium are below critical levels, your crop does not reach its full yield potential.

In addition, some of the yield losses caused by diseases can be minimized or avoided with proper nutrient levels in the soil. As you plan for the 2004 growing season, make sure your nutrient bank can cover the expense of making a profitable crop.

Timothy W. Walker's work at the Delta Research and Extension Center in Stoneville, Miss., includes research on rice fertility and production practices. Contact Walker at 662-686-3278 (office) or 662-822-2291 (cellular) or e-mail at

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