Template:Chapter 4.16.2

4.16.2 - Balanced fertilization

Dylan P. Harding, University of Guelph, Canada

Introduction

There are approximately 14 different nutrients that plants need to grow (Principals of Plant Nutrition, 2001). These nutrients are classified broadly by the relative quantities in which they are needed for the growth of plants. The “macronutrients”, which are needed in relatively high amounts, include nitrogen (N), phosphorus (P), and potassium (K). Carbon and oxygen are also included in this group however as they are provided by the air they are not considered soil nutrients. The “secondary nutrients” are sulphur (S), calcium (Ca), and magnesium (Mg), which are needed in moderate amounts for plant growth. “Micronutrients” include zinc (Zn), molybdenum (Mo), manganese (Mn), iron (Fe), copper (Cu), and boron (B), all of which plants need in relatively small amounts. The specific quantity of each nutrient needed by a crop will vary according to species and the growth environment. Deficiencies are most common for macronutrients because much greater quantities of these nutrients will be removed by crops each year, however secondary or micro-nutrient deficiency can potentially be equally limiting for crop growth. Secondary and micro-nutrient deficiency is especially prevalent in developing countries where national fertilizer policies that almost exclusively promote nitrogen and phosphorus fertilizer use (e.g. India) are in place (B. K. R. Rao et al., 2012).

Figure 1. Visualizing the importance of balanced fertilization. Image from: http://www.yara.com/doc/37694_2012%20Fertilizer%20Industry%20Handbook%20wFP.pdf

Each board of the barrel represents the available quantity of each nutrient relative to the amount it is needed in by the plant. The level of the water represents the growth potential of the plant. In the above image, nitrogen is the limiting nutrient. Although all the other necessary elements for the growth of the plant are available, the plant does not have enough nitrogen to continue growing. Thus, it cannot make use of the other available nutrients and no further growth occurs. In this situation, only making more nitrogen available to the plant would enable further growth whereas adding any of the other nutrients would not provide any benefit. Deficiency of any nutrient will affect a crop in the same way.

Each element required by plants has a unique, natural biological function. Nitrogen is a building block for amino acids, which make up proteins, while phosphorus is a component of DNA, and potassium is required to maintain hydraulic pressure within plant cells and transmit biological signals (Mendel and Kirkby, 2001). Similarly, each of the secondary and micro- nutrients has a specific function within plants. For example, zinc is required for ~100 “master genes” in plants to function properly (Li et al., 2013), and molybdenum is required for biological nitrogen fixation by bacteria that are symbiotic with legumes (Bambara & Ndakidemi, 2010) (see Chapter 5: Maximizing Legume Productivity through Molybdenum Addition). With few exceptions, one mineral cannot serve the purpose of another and thus an abundance of one nutrient will not make up for a shortage of another.

For example, if a fertilizer such as di-ammonium phosphate (DAP, 18-60-0) is the only one that is applied to the soil for several seasons in a row, crops will rely exclusively on potassium already in the soil and quickly deplete the supply. When this occurs, additional application of DAP will have no effect. Similarly, if only NPK (macronutrient) fertilizers are applied to a soil for an extended period of time, the supply of at least one micronutrient within the soil will eventually become exhausted, making further application of NPK fertilizers ineffective until the supply of the missing micronutrient(s) is replenished. Put simply, every nutrient required by plants must be regularly provided to the soil in a balance that is similar to the needs of the crops under cultivation. This will rarely require the application of every nutrient every year, and careful observation of plant symptoms as well as regular soil testing are important strategies to recognize and avoid nutrient deficiencies (See Chapter 2: Soil Testing). Where soil testing is impractical, the most effective method of determining whether or not balanced fertilization will have a beneficial effect on crop growth is through doing a small split-plot trial. To do this, a small test plot should be established in which half receives balanced fertilization, and the other half does not.

Although regularly returning organic matter to the soil will generally reduce the risk of nutrient deficiencies developing, absolute deficiency of a mineral within an area will not be alleviated through this practice as returning on-farm organic matter to the soil will only recycle the minerals that were initially there (Bonilla & Bolanos, 2009). Manure from animals raised on crops or pasture grown on mineral deficient soil will often not contain appreciate quantities of the deficient nutrients. Deficiency is exacerbated through the removal of a certain quantity of each mineral from the soil each year when crops are harvested. Because of this consistent removal of minerals from the soil, replenishing the soil with an outside source of minerals may eventually become necessary, and is a current necessity for many already depleted soils. When this is the case, the introduction of synthetic fertilizers can be extremely beneficial.

Recent investigation of soils in the semi-arid tropical regions of India has indicated widespread deficiency of the secondary and micro- nutrients sulphur, boron and zinc (B. K. R. Rao et al., 2012; B. K. R. Rao, Srinivasarao, Sahrawat, & Wani, 2010; Rego, Sahrawat, Wani, & Pardhasaradhi, 2007; Sahrawat, Wani, Pardhasaradhi, & Murthy, 2010; Srinivasarao, Wani, Sahrawat, Rego, & Pardhasaradhi, 2008). Rainfall has traditionally been considered to be the limiting factor on plant growth in these areas, which has discouraged farmers from thoroughly considering soil fertility (Rego et al., 2007; Sahrawat et al., 2010). However, field trials performed in these areas have observed significant yield increases (20-80%) for a wide variety of crops through the addition of sulphur, boron, and zinc compared to the farmers’ usual fertilization practice. Crops tested in these trials include maize, castor, groundnut (peanut), mung bean, finger miller, sunflower, and soybean (B. K. R. Rao et al., 2012; Rego et al., 2007; Sahrawat et al., 2010; Srinivasarao et al., 2008). Micronutrient addition increased crop yields when added alone, but had increased effect when added alongside nitrogen and phosphorus (B. K. R. Rao et al., 2012). In multi-year trials, micro-nutrient application was shown to improve yield in comparison to controls each year for the duration of the test period (B. K. R. Rao et al., 2012; Rego et al., 2007; Sahrawat et al., 2010). Areas where only macronutrients have been applied for long periods of time are at especially great risk of developing micronutrient deficiency, and the reintroduction of these minerals into the soil can often significantly improve yields (Mahajan, 2009).

In situations where farmers rent the land that they cultivate there will often be less interest in using organic matter to build long term soil fertility. Although long term management plans should ideally be developed for all soils, this is unfortunately not always a realistic expectation. One option for degraded soils is to add synthetic fertilizers in one or more seasons to rebuild nutrient levels, and then to integrate longer-term organic strategies to recycle the minerals that have been introduced. In situations where immediate benefit is the primary concern, microdose fertilization can often bring a good return on investment with low initial costs. Please see Chapter 4: Fertilizer Microdosing for further detail.

As nutrients will be removed from the soil every year for human consumption, it is ultimately necessary to recover nutrients from human waste in order to have a fully sustainable agricultural system. However, there are significant challenges to re-introducing nutrients from human waste to agroecosystems in a fashion that is safe for human health. Further research in this field is necessary to responsibly take advantage of this potential resource.

References

1.Bambara, Sylvie, & Ndakidemi, Patrick A. (2010). Phaseolus vulgaris response to Rhizobium inoculation, lime and molybdenum in selected low pH soil in Western Cape, South Africa. African Journal of Agricultural Research, 5(14), 1804-1811.

2.Bonilla, Ildefonso, & Bolanos, Luis. (2009). Mineral Nutrition for Legume-Rhizobia Symbiosis: B, Ca, N, P, S, K, Fe, Mo, Co, and Ni: A Review (Vol. 1). Li, S. Y., Zhao, B. R., Yuan, D. Y., Duan, M. J., Qian, Q., Tang, L., . . . Li, C. Y. (2013). Rice zinc finger protein DST enhances grain production through controlling Gn1a/OsCKX2 expression. Proceedings of the National Academy of Sciences of the United States of America, 110(8), 3167-3172. doi: 10.1073/pnas.1300359110.

3.Mahajan, A, Gupta, R. D. . (2009). Components of INM System: Springer, Po Box 17, 3300 Aa Dordrecht, Netherlands.

4.Rao, B. K. Rajashekhara, Krishnappa, K., Srinivasarao, C., Wani, S. P., Sahrawat, K. L., & Pardhasaradhi, G. (2012). Alleviation of Multinutrient Deficiency for Productivity Enhancement of Rain-Fed Soybean and Finger Millet in the Semi-arid Region of India. Communications in Soil Science and Plant Analysis, 43(10), 1427-1435. doi: 10.1080/00103624.2012.670344.

5.Rao, B. K. Rajashekhara, Srinivasarao, C., Sahrawat, K. L., & Wani, S. P. (2010). Evaluation of Stratification Criteria for Regional Assessment of Soil Chemical Fertility Parameters in Semi-arid Tropical India. Communications in Soil Science and Plant Analysis, 41(17), 2100-2108. doi: 10.1080/00103624.2010.498539.

6.Rego, Thomas J., Sahrawat, Kanwar L., Wani, Suhas P., & Pardhasaradhi, Gazula. (2007). Widespread deficiencies of sulfur, boron, and zinc in Indian semi-arid tropical soils: On-farm crop responses. Journal of Plant Nutrition, 30(10-12), 1569-1583. doi: 10.1080/01904160701615475.

7.Sahrawat, K. L., Wani, S. P., Pardhasaradhi, G., & Murthy, K. V. S. (2010). Diagnosis of Secondary and Micronutrient Deficiencies and Their Management in Rainfed Agroecosystems: Case Study from Indian Semi-arid Tropics. Communications in Soil Science and Plant Analysis, 41(3), 346-360. doi: 10.1080/00103620903462340.

8.Srinivasarao, Ch, Wani, S. P., Sahrawat, K. L., Rego, T. J., & Pardhasaradhi, G. (2008). Zinc, boron and sulphur deficiencies are holding back the potential of rainfed crops in semi-arid India: Experiences from participatory watershed management. International Journal of Plant Production, 2(1), 89-99.