Crop Nutrition
University of California research as well as field experience clearly demonstrate the economic value of soil analysis before planting, and leaf tissue analysis during the growing season, to assess the fertilization requirement of rice. Soil tests for phosphorus (P), potassium (K), and zinc (Zn) (table 3), and leaf tissue analysis for nitrogen (N), P, K, and Zn are valuable aids in developing efficient plant nutrition programs (table 4).
| Table 3. Minimum soil levels of key nutrients necessary for satisfactory rice yields. |
 |
| Element |
Soil test method |
Critical value |
| Phosphorus |
NaHCO3 |
6 ppm PO4-P
(In cold years, may be as high as 9 ppm) |
| Potassium |
NH4Ac |
60 ppm K |
| Zinc |
DTPA |
0.5 ppm Zn |
Field research has established the critical concentrations of P, K, and Zn in the soil by correlating soil test values for these mineral nutrients and rice plant growth. The critical value (table 4) is that concentration of a given nutrient associated with a reduction of 10 percent or more in growth and grain yield. When soil test values are near or below the critical level, application of the deficient plant nutrient will usually prevent that nutrient from limiting growth and yield. Proper soil sampling, chemical analysis, and interpretation help avoid plant nutrient deficiencies and are valuable guides in soil fertility management.
| Table 4. Adequate and critical concentrations of nutrients in rice plants of publicly developed short statured varieties. |
|
| Plant growth stage |
Nitrogen
(% total N) |
Phosphorus (ppm extractable PO4-P) |
Potassium (% extractable K) |
Zinc (% in whole seedling plant) |
|
critical |
adequate |
critical |
adequate |
critical |
adequate |
critical |
adequate |
| Mid-tillering |
4.6 |
4.6-5.2 |
1,000 |
1,000-1,800 |
1.4 |
1.4-2.8 |
20 |
22-80 |
| Maximum tillering |
4.0 |
4.0-4.6 |
1,000 |
1,000-1,800 |
1.2 |
1.2-2.4 |
- |
- |
| Panicle initiation |
3.3 |
3.3-3.8 |
800 |
800-1,800 |
1.0 |
1.2-2.4 |
- |
- |
| Flag leaf |
2.6 |
2.6-3.2 |
800 |
800-1,800 |
1.0 |
1.2-2.2 |
- |
- |
 Analysis on dry weight basis of most recently matured leaves for Kjeldahl N, 2% HAc extractable PO4 and K.
Plants with a critical level of a nutrient produce approximately 90% of maximum yield. |
Whereas soil analysis provides insight to preplant fertilizer needs, plant tissue analysis is valuable as a way of diagnosing the nutritional status of the growing crop. Tissue analysis serves as a guide for mid-season N application and for anticipating the need for N, P, and K applications for subsequent rice crops. The value of tissue analysis depends on representative sampling, selection of the proper plant part (the most recently matured whole leaf blade also known as the 'Y-leaf', fig. 13), drying and handling of samples, analytical procedure used, and the correct interpretation of the relationship between the nutrient levels and crop growth and yield.
Figure 13. The "Y-leaf," the most recently fully expanded leaf of the rice plant, is the correct leaf to sample for tissue analysis. |
|
Nitrogen
Nitrogen (N) fertilizer is essential for nearly all commercial rice production in California. The general N rate applied is 100 to 160 pounds of N per acre. Specific N requirements vary with soil type, variety, cropping history, planting date, herbicide used, and the kind and amount of crop residue incorporated during seedbed preparation. Less N is used on land newly planted to rice; weaker-strawed varieties; where a leguminous covercrop has been incorporated; and for late plantings. Soils in continuous rice production and early plantings usually require a higher N level for optimum plant growth and sustained high yield.
In flooded rice, ammonium-based forms of N should be placed in and not on the soil to prevent losses through denitrification and volatilization. Suitable sources of N include aqueous ammonia applied pre-flood and pre-seeding (fig. 14), and ammonium sulfate or urea, which can also be used for topdressing. To achieve the highest N use efficiency in flooded soils, ammonium-N (NH4+) is applied and soil incorporated or injected to a depth of 2 to 4 inches before flooding. Although most N should be applied preplant, N may be topdressed as late as panicle differentiation to correct deficiencies and maintain plant growth and yield. Figure 15 shows a typical N response of semidwarf California varieties.
 |
 |
| Figure 14a. Aqueous ammonia is applied as a liquid that is injected into the soil at a depth of 2 to 4 inches. |
Figure 14b. To prevent volatilization, the field should be rolled and flooded as quickly as possible after an application of aqueous ammonia. |
 |
| Figure 15. A typical nitrogen response of California semidwarf varieties. |
Phosphorus
Application of 18 to 26 pounds per acre of phosphorus (P) (40 to 60 pounds P2O5 basis) incorporated into the seedbed before flooding has improved rice yields where soils are below critical levels. However, many rice fields are above the critical levels as a result of the repeated use of phosphorous fertilizers. Phosphate fertilizer may also be topdressed with good results when a deficiency occurs in the crop. Topdressing should be made early, preferably in the seedling, or not later than the midtillering stage.
Potassium
Potassium (K) fertilization is generally unnecessary in California, although yield increases and deficiency symptoms have been corrected in some soils with a low cation-exchange capacity on the east side of the Central Valley. Potassium chloride (KCl, 62 percent K2O) is best incorporated preplant at rates of 60 to 120 pounds K2O per acre. Some N/P/K mixes are also used.
Zinc
Zinc (Zn) deficiency, originally called "alkali disease," is common in high pH, sodic soils, and in areas where the topsoil has been removed by land leveling or where irrigation water is high in bicarbonate (>4 milli-equivalents [meq]). In zinc-deficient soils, rice seedling growth may be reduced and, in severe cases, stand loss may occur. Preflood surface applications of 2 to 16 pounds per acre of actual Zn, depending on the source, have effectively corrected this deficiency. Zinc deficiency occurs more frequently in cool weather during stand establishment. Zinc fertilizer in the form of zinc sulfate, zinc oxide, or zinc chelate is broadcast or sprayed on the soil surface after the last seedbed tillage for maximum effectiveness. Zinc should not be incorporated into the soil.
Iron
Although rare in California, iron chlorosis can be induced on soils that contain free calcium carbonate and where irrigation water contains more than 4 meq of bicarbonate per liter. Iron compounds have been inconsistent in correcting this nutrient deficiency, but lowering the soil pH can solve it.
ˆ top of page ˆ
|
