Nutritional requirements of rice crop for grain or silage expressed as a function of the yield unit (n = 240 samples of six commercial hybrids).
Crop nutritional management must be oriented so as to achieve economically convenient yields for the producer along with an efficient use of resources and concern for the environment. The use of information about soil chemical properties and experience of the behavior of each species under fertilization conditions with variations in soil chemical properties allow adjusting nutrient rates for different production situations. This chapter provides basic information about the nutritional requirements of the main cereals cultivated in the world, nutritional management strategies, and the nutritional value of using residues. This information is a guide for the producer to determine a nutritional management strategy using information provided by analyzing soil chemical properties for different productivity scenarios.
Silage corn is an important food supplement in pastoral systems, particularly in the dairy industry where the main characteristics are high dry matter (DM) yield and metabolizable energy content . Dairy cow producers in the central south and south in Chile use silage corn as feed between summer and winter, which allows extending lactation and increase production. This is done year-round in the central zone.
In the corn plant, the ratio DM grain:leaves + stem is considered as an important index of the nutritional value of the forage because of the high digestibility of the grain  as well as its starch content.
Nutrient concentration in the corn crop is highly variable just like in other crops and is associated with the genetic material used, the environment, and the agronomic management employed.  pointed out that the critical nitrogen (N) concentration in corn plants (aerial part) varied between 0.91 and 1.2% for corn with a development cycle that fluctuated between 141 and 154 d.  points out N concentrations between 0.72 and 0.80% for a semi-late corn hybrid in central Chile, which was fertilized with N rates similar to its extraction. For a similar experiment, this author indicates phosphorus (P) and potassium (K) concentrations that fluctuated from 0.10 to 0.11% and 0.60 to 0.64%, respectively, in fertilization conditions with increasing rates of these nutrients . Meanwhile,  showed nutritional fluctuations between 0.57 and 1.15% for N; 0.13 and 0.23% for P; 0.74 and 1.2% for K; 0.18 and 0.40% for calcium (Ca); and 0.10 and 0.17% for magnesium (Mg) in two mid-season silage hybrids cultivated during two consecutive seasons with different fertilization treatments in central south Chile. The same study includes the high variability found in the N, P, Ca, and Mg concentrations between both genetic materials used as has also been corroborated in other fertilization experiments conducted in the same crop.
For sweet corn, macronutrient concentrations in ears of eight hybrids in the harvest stage evaluated in the United States corresponded to 1.35-2.10% for N; 0.24-0.32% for P; 0.98-1.24% for K; 0.037-0.083% for Ca; 0.10-0.15 for Mg; and 0.09-0.15 for sulfur (S), whereas concentrations of these nutrients in the aerial residue corresponded to 1.96-2.49 for N; 0.19-0.27% for P; 2.35-3.26% for K; 0.33-0.41% for Ca; 0.21-0.28% for Mg; and 0.18-0.22% for S .
For grain corn, fertilization experiments carried out by Instituto de Investigaciones Agropecuarias (INIA) in central Chile indicate N concentrations from 0.77 to 1.18%; P concentrations from 0.13 to 0.17%; and K concentrations from 0.29 to 0.30% [5,6]. Meanwhile, experiments conducted on different commercial hybrids in central south Chile (2003 to 2010) indicate that macronutrient concentration in the grains at the harvest stage fluctuated between 1.13 and 1.77% for N; 0.31 and 0.52 for P; 0.37 and 0.58 for K; 0.010 and 0.037 for Ca; 0.14 and 0.19 for Mg; and 0.11 and 0.15 for S (n = 160 samples).
While there are many commercial hybrids with differences in earliness in their crop cycle, in general all of them exhibit a marked extraction of nutrients that is accentuated at the sixth leaf as shown in Figure 1.
It should also be noted that the high K extraction in this crop, which is expressed as K2O, is far superior to the N requirements, which also occurs in many plant species.
As for nutrient extraction carried out only for corn grain,  point out N:P:K extractions of approximately 16:3:3.3 for each ton of grain, respectively. [5,6] indicates N:P:K extractions with means of 9:1.4:2.5 for each ton of grain, respectively. The differences identified by these authors are due to the abovementioned variations in the genetic materials used.
Experimental records generated by fertilization studies of the corn crop for grain and silage allow estimating the nutritional requirements shown in Table 1. Requirements in this table are shown as a function of the yield unit. Nutritional requirements in Table 1 are highly variable and this variability depends on many factors among which are genetic material, soil physicochemical properties (nutrient availability in the environment), and also climatic conditions as pointed out by [4-6,9].
|N||14 – 26||6.9 – 14.5|
|P2O5||6 – 13||3.1 – 6.9|
|K2O||16 – 38||8.2 – 23.2|
|CaO||3.0 – 7.5||1.5 – 4.5|
|MgO||3.2 – 7.4||1.7 – 4.0|
|S||1.4 – 2.6||0.8 – 1.3|
|Fe||0.24 – 0.41||0.109 – 0.185|
|Mn||0.04 – 0.06||0.019 – 0.029|
|Zn||0.03 – 0.05||0.013 – 0.021|
|Cu||0.002 – 0.009||0.001 – 0.004|
|B||0.016 – 0.018||0.007 – 0.008|
Nutrient distribution in the corn plant is shown in Figure 2. This figure shows that a large part of N and P extracted by the corn plant is concentrated in the grain, whereas the aerial residue concentrates an important part of K and Mg absorbed by the plant, this was also pointed out by [5,6]. In turn, Ca is mainly concentrated in the aerial residue. Therefore, when incorporating corn residues, a large part of K, Ca, and Mg are recycled, which contributes to reducing the requirements of these nutrients in the next crop to the extent that the incorporated residue achieves its biological decomposition in the soil.
Regarding the effect of applying different nutrients to the crop, various authors have demonstrated positive effects of applying N, P, K, Ca, and Zn [4,5,10-16]. The favorable effect of organic amendments as a fertilizer source has been demonstrated on grain and dry matter yield for silage using fresh amendments (for example, poultry and swine litter) and composted manures (pig compost, dairy compost, and others), leaving a positive residual effect on nutrient availability for the next crop [2,10,17-24].
Another management factor to consider in corn crop fertilization is residue management. Some theoretical models indicate that the N contained in the residues is gradually mineralized by the soil microbial biomass once it has been incorporated into the soil and is available for the next crop. However, this availability has not been evidenced under experimental field conditions since residue decomposition is also due to initial fractionation thereof (trituration) and soil temperature. Furthermore, soil biomass tends to generate humic compounds , which exhibit a C/N ratio that fluctuated between 18 and 22 ; therefore, an important fraction of N contained in the residue will be part of the humic compounds that will be synthesized by the soil’s biological activity, situation that allows increasing soil organic matter content and improve its physical, chemical, and biological properties.
 indicate that superficially applying three residue levels (including a control without residues) in a corn grain monocrop fertilized with increasing N rates (from 0 to 225 kg ha-1) through various fertilization sources, reduced soil inorganic N availability with a slightly negative effect on yield as compared with the treatment with no residues. In addition, soil temperatures were negatively affected by the increasing residue rate. For evaluated N sources, the best effect was achieved by ammonium nitrate over the use of urea.
 indicate that using three residue levels (including a control without residues) in a corn grain crop managed with two tillage systems (chisel and no-till) and two increasing N rates (between 67 and 268 kg ha-1) exhibited a variable effect on yield according to the rainfall level and tillage practice used. Thus, for low rainfall and tillage conditions, total residue removal (control) produced a higher grain yield than using residues at the highest rate. Residues applied at high rates with no-till for this low rainfall level had a positive effect on yield. In summary, for normal rainfall conditions, the level of residues used in tillage conditions had no effect on yield, while for no-till management, the total (control) or partial removal of residues produced a higher grain yield than using residues at the highest level.
This crop is one of the three most important cereals for human consumption along with corn and rice. History indicates that wheat (
Wheat plants absorb the N nutrient as nitrate ion or ammonium ion. The way in which N translocation occurs depends on the absorbed N source and root metabolism . Absorbed N is transported by the xylem to the leaves as nitrate ion or could be reduced in the roots and transported in inorganic form as amino acids or amides. A large part of the absorbed ammonium has to be incorporated in organic compounds in the roots . In terms of phloem, N is a mobile nutrient, thus under deficiency conditions this element can be retranslocated from the older to the younger leaves and then translocated from there to the developing grains. The principal forms of organic N in the phloem sap are amides, amino acids, and ureides . The nitrate and ammonium ions are not present in this sap, but mainly in the xylem. Nitrogen in the wheat crop is the major component of proteins, amino acids, enzymes, and nucleic acid . It is also part of the mature grain, mainly concentrated as proteins in the endosperm which is the part that makes up the flour .
A deficiency of N in plants greatly reduces growth rate. In the case of cereals , tillering is poor and the leaf area is small; both the number of spikes per unit of area and the number of grains per spike are reduced. Since this nutrient is a component of chlorophyll, its deficiency is seen as a generalized yellowing or chlorosis of the leaves, appearing first on the lower leaves while the higher leaves remain green. In cases of severe deficiency, a generalized chlorosis is noticeable in the whole plant. Finally, it decreases crop yield and the grain protein content . Excesses of N are less evident than its deficiency. They include prolonged plant growth, dark green coloration of foliage, increased plant susceptibility to the attack of phytopathogens, and a delay in crop maturity.
The concentration of N in the wheat plant decreases over the phenological periods reaching values of 3.5-4.2% at the full tillering stage to 0.9-1.2% at the harvest maturity stage . The adequate concentration in the higher leaves before the spike formation stage fluctuates between 1.75 and 3.0% . In the case of wheat grain, N concentration fluctuates between 1.6 and 2.4% at harvest and surpasses other nutrients. Phosphorus deficiency restricts plant development, delays growth, tillering, root development, and maturity. Deficiency symptoms normally start in the oldest leaves and are characterized by a blue-greenish to reddish color, which can lead to a reddish color and bronze tints that normally start from the edges. Leaves often have a darker green color than normal plants. This is because the expansion of cells and leaves is delayed more than chlorophyll formation, so that the chlorophyll content per leaf area unit is higher . A symptom of P deficiency is the decrease of the stem/root ratio and less growth of all the growth points. Extremely high P levels can result in toxicity symptoms, which generally occur as aqueous points in the leaf tissue, eventually becoming necrotic . In very severe cases, P toxicity can provoke plant death.
Wheat plant P concentration decreases with the maturity process and can vary between 0.23 and 0.30% at the full tillering stage and decrease to values of 0.12-0.18% at harvest maturity stage .
Potassium deficiency is produced as chlorosis along the edge of the leaf followed by burning and bronzing of the old leaf tips. The affected area is curled when the deficiency of this element increases. The symptoms of K deficiency appear in the old leaves due to nutrient mobility. The affected plants are generally stunted and have shortened internodes. These plants exhibit slow and stunted growth, weak culms susceptible to lodging, higher incidence of pests and diseases, lower yields, curled grains, and low grain quality . Plants with K deficiency can lose the control of the respiration rate and exhibit internal water deficit. High K concentrations contribute to increasing plant tolerance and resistance to diseases and pests.
Wheat plant K concentration decreases with crop maturity, fluctuates from 3.8-4.5% at the full tillering stage, and decreases to 0.9-1.2% at the harvest maturity stage .
Calcium deficiency produces small, twisted, dark green leaves . Although all the growth points are sensitive to Ca deficiency, the root meristems are the most affected. Calcium is a non-toxic mineral nutrient even in high concentrations and is very effective to detoxify high concentrations of other mineral elements in plants . Moreover, high Ca contents within the plant raise tolerance and resistance against diseases and pests.
Wheat plant Ca concentration decreases with maturity and reaches values of 0.28-0.30% at the full tillering stage to levels of 0.08-0.10% during harvest maturity.
Magnesium is an element that easily translocates from the older parts to the younger parts; therefore, its deficiency symptoms first appear in the oldest parts of the plant. A typical symptom of Mg deficiency is interveinal chlorosis of the old leaves in which the veins remain green but the area between them turns yellow. When the deficiency becomes more severe, leaf tissue uniformly turns chlorotic, then brown and necrotic. Leaves are small and break easily .
Wheat plant Mg concentration tends to decrease with maturity  and can fluctuate from 0.14-0.16% at the full tillering stage to values of 0.05-0.07% at harvest maturity.
Common Zn deficiency symptoms in wheat are arrested plant growth, poor tillering, light green coloring, yellowing, whitened spots, chlorotic stripes on both sides of the central vein, and small leaves. The internodes are short and the flowering, fructification, and maturity processes can be delayed. A high soil P concentration can induce Zn deficiency . The toxicity of Zn can be translated in a reduction of root growth and leaf expansion followed by chlorosis. This is associated with concentrations higher than 200 mg kg-1 Zn in the tissue. The excess of Zn can induce Fe deficiency, which is recognized by interveinal chlorosis in the new leaves of the plant.
Wheat plant Zn concentration decreases with maturity and reaches levels of 12 to 20 mg kg-1 at the full tillering stage and 10 to 12 mg kg-1 at cereal harvest maturity .
Regarding the nutritional management of the wheat crop, it must be considered that the higher the yield level, greater the requirement of nutrients for the crop will be. However, in the case of the wheat crop, it is fundamental to also consider the cultivar (commercial variety) and the growth habit. For the cultivar, differences are observed in the accumulation of proteins in the grain, thus indicating variations in plant N requirement. Each cultivar exhibits a unique genetic base that gives it unique attributes for potential yield and grain quality ; . For growth habit, winter habit cultivars generally produce higher total nutrient extractions than those of spring habit, mainly because it is more permanent in the soil. Other factors affecting variability in the nutritional requirements of the wheat crop are soil physicochemical properties, climatic conditions, and agronomic management. In this way, the same wheat cultivar sown at different sites will also exhibit different nutritional requirements for the same yield levels. Therefore, to define nutrient quantities to apply to this crop and in other crops, factors that influence these nutritional requirements must be considered.
Wheat crop nutritional requirements can be observed in Figure 3 with nutrient extraction in wheat cv. Dollinco-INIA (cultivar with alternative habit) showing gradual extraction that reaches its maximum near the end of the cultivation period, and where N is the most extracted nutrient. Nutrient extraction of this cultivar in its maximum accumulation period is 30.6; 5.7; 24.7; 7.9; and 4.1 kg Mg-1 N, P2O5, K2O, CaO, and MgO, respectively, for a grain yield of 8 Mg ha-1. For the highest extracted nutrient, other cultivars exhibit a higher K extraction, for example, the cvs. Tukán-INIA and Domo-INIA with nutrient extraction of their maximum accumulation period is 26.4-31.1; 6.0-7.4; 30.7-32.5; 4.0-8.6; and 2.1-4.2 kg Mg-1 N, P2O5, K2O, CaO, and MgO, respectively, for a grain yield of 7 and 9 Mg ha-1 for each cultivar, respectively .
The nutrient extraction curves allow adjusting and harmonizing better the fertilization criteria with the growth rate of the wheat crop. For example, in the initial phenological stages of cv. Kumpa-INIA (Table 2) a large part of the extraction of evaluated nutrients is produced. Thus, nearing the end of the vegetative stage (flag leaf visible) there has been an extraction of 80.3% N; 76.6% P; 98.4% K; 62.6% Ca; and 67.1% Mg, while for cv. Dollinco-INIA (Table 3) nearing the end of the vegetative stage (flag leaf visible) there has been an extraction of 56.9% N; 33.9% P; 62.2% K; 53.8% Ca; and 38.4% Mg. However, the accumulated absorption values are less than those of cv. Kumpa-INIA .
|End of tillering||111||27.3||16.1||36.2||18.3||15.4|
|Flag leaf visible||153||80.3||76.6||88.4||62.6||67.1|
|End of tillering||118||27.1||11.4||29.6||13.9||10.6|
|Flag leaf visible||160||56.9||33.9||62.2||53.8||38.4|
As for the effect of applying different nutrients to the crop, various authors have demonstrated positive effects of applying N, P, Zn, and liming [28,31,37-41]. For wheat crop nutrient partialization many experimental studies have demonstrated that there is a response to partially applying N and that yield is generally maximized with three applications of this nutrient, which correspond to sowing (15-20% total N), start of tillering (40-50% total N), and start of nodes (30-40% total N) [28,31,41]. Experimental studies indicate that there are differences among cultivars and soil and climatic conditions for the rate of this nutrient, but that yield is generally maximized when the N rate is determined based on the absorption requirement of the crop (replacement rate) with a variation that can fluctuate between 90 and 120% of crop consumption [31,34]. The rates of the other nutrients must be adjusted to the nutritional requirements and soil chemical properties.
The incorporation of residues of the wheat crop is a practice that benefits physical, chemical, and biological soil properties. Figure 4 shows that an important fraction of N, P, and Mg along with the greater part of K and Ca extracted with this crop will be returned to the soil along with the incorporation of residues, which will have a significant effect on the reduction rate of some of these nutrients in the next crop. However, the required application of N should be considered to achieve an adequate decomposition of the incorporated residue so as not to affect N availability for the next crop.
Among the cereals produced on a world level, rice occupies the greatest proportion of soils. Of the 147.5 million ha of soil dedicated to rice production in the world in 1989, developing countries contributed with 141.4 million ha, that is, 96% . In the 1976-1980 period, a mean of 37.842 ha were sown , decreasing to 22.733 ha in the 2006-2009 period , which indicates a decrease of approximately 40% of the area in 25 years.
As for the nutritional functions and effects of the nutrients in the plant, N must be constantly supplied to the crop to achieve an adequate harvest, especially during panicle formation and development. The final yield of the crop is defined by the number of panicles per m2 and number of tillers, which is defined within the first 10 d after maximum tillering. These components are influenced by N availability during these development stages [47,48]. Meanwhile, the number of spikelets per panicle is closely related to the N content of the leaf sheath during the weeks prior to flowering [47,49]. Nitrogen will later influence an efficient assimilation of carbohydrates on the part of the grain and in a correct filling thereof. The form of absorbed N during the first growth stages the rice plant prefers the ammonium forms (NH4+) [50,53], while in the stages near maturity it prefers N in nitrate form (NO3-). Nitrogen as ammonium is favorable up to the panicle initiation stage , and later nitrate absorption is promoted, especially during the panicle embryonic formation stage, stimulating the increase in the number of panicle flowers and grain weight [52,53]. Ammonium N increases the number of tillers and the number of panicles per plant . Although in flooded soil conditions it is preferred to apply ammonium forms of N or those derived from ammonia (urea), depending on tilling depth, in the first 5-10 cm of soil an aerobic condition appears which would allow transforming ammonium to nitrate, a process that is accelerated by temperature accumulation. Notwithstanding the above, there is no scientific evidence in Chile about the modern varieties that indicates the advantages of applying N in the nitrate form after sowing. Nitrogen excesses, especially associated to abnormal climatic phenomena during the reproductive stage, particularly low temperatures , produce growth and internal distribution disorders of this nutrient that promotes vegetative growth and can delay harvest and reduce yield.
Internal distribution of N in the rice plant is established in studies conducted with N15 and which indicate that between 3 and 5% is found in the roots, between 30 and 44% is found in the aerial vegetative structures, between 37 and 40% is found in the panicle, and between 7 and 13% is found in the senescent structures .
Phosphorus is an element that promotes root growth; therefore, it improves the absorption of water and other nutrients. It also increases the resistance to periods of reduced water availability. An adequate supply of P improves grain flowering and fertilization, increases crop precocity, and increases 1000 grain weight . The highest P absorption occurs during the vegetative development period and decreases after the panicle embryonic formation . In turn, this absorption is promoted by temperature increases with an optimum temperature of 30 ºC. Availability of P in flooded soils is promoted by the redox processes and liberating part of the P retained as iron phosphate and aluminum .
Potassium plays an important role at the beginning of vegetative growth with an emphasis during tiller formation and has a direct influence on the determination of the final number of panicles. During the ripening stage it promotes synthesis and translocation of low molecular weight carbohydrates, is involved in activating phosphorylation processes (energy transport) to activate the transport of soluble N compounds to grains in formation, avoids their accumulation in other tissues, and also promotes 1000 grain weight [53,56]. Furthermore, K increases plant resistance to various diseases, such as stem rot detected in Chile , and adverse climatic conditions (high levels of solar radiation and temperature, or low temperature during tillering and flowering), playing an important role in the economy of use and loss of water by transpiration from the plant .
Moreover, K stimulates cell division, is involved in photon transport in photosynthesis, directs the synthesis of starch, inulin, amino acids, and proteins, modifies cell permeability, interferes with plasmolysis and turgidity mechanisms, and together with P and Mg is actively involved in carbohydrate metabolism .
Sulfur is an important element during the whole growth cycle of the rice plant; it strongly affects grain quality since it is part of some essential amino acids and part of N metabolism in synthesizing proteins and carbohydrates. It reduces nitrate, catalyzed chlorophyll formation along with Cu and Fe, acts as a hydrogen carrier, helps to regulate the tricarboxylic acid cycle, and is part of the sulfur radicals (SH) . The quantities of S absorbed by the crop are relatively low as compared to other nutrients (N and K) since they are covered by the soil’s natural contribution (organic matter mineralization), therefore, it is uncommon to apply S fertilizers in the rice crop. The availability of S is reduced in flooded systems as a result of redox reactions .
Calcium contributes to plant rigidity and resistance to lodging. Magnesium is located in the pyrrole rings making up chlorophyll and is a catalyst in the enzyme activity of nitrate reductases or self-induced enzymes that require molybdenum (Mo) . The availability of Ca and Mg in flooded systems is improved as a result of redox processes .
Another important element for the rice crop is silicon (Si). Silicon extraction by the rice plant is higher than for any other mineral element with a concentration that can fluctuate between 2 and 9% of the plant dry matter . This element is mainly deposited in the leaf epidermal cells and forms a siliceous double layer which is responsible for disease resistance. Studies conducted in Japan point out an extraction mean of 433 kg Si ha-1. In quantitative terms, for each ton of paddy grain yield, the rice plant extracts 100 kg Si, of which a large part is concentrated in the rice husk .
Silicon is involved in the whole growth cycle of the rice plant, mainly affecting the stage between panicle formation and grain ripening. This essential element for rice cultivation promotes length development and oxidative activity of the root system in addition to protecting plants from Fe and Mn toxicity produced in anaerobic conditions of flooded soil. Finally, a good soil Si level improves P availability for the rice crop. With regard to the micronutrients, Fe participates in chlorophyll formation, prevents chlorosis, and is part of enzyme activity. The excess of Fe can inhibit K absorption . Boron contributes to N uptake, participates in Ca metabolism, and stimulates meristematic activity and pollen formation . Zinc stimulates initial plant development and its deficiency can affect potential crop yield. In flooded systems and as a result of redox processes, availability of Fe, Mn, and Mo increases and availability of Zn and Cu decreases .
Figure 5 shows the nutritional requirements of the rice crop. It can be observed that there is a gradual nutrient extraction in cv. Diamante-INIA (main cultivar used in Chile) between the initial and maximum tillering stages followed by slight increases in N and K accumulation unlike other nutrients that have an increased accumulation until later crop stages. In turn, the nutrient with the highest extraction is K. In productive terms, the nutritional requirements are 12 kg N; 8.4 kg P2O5; 22 kg K2O; 6.2 kg CaO; and 5.6 kg MgO for each ton of yield. As additional information and as a guide, micronutrient requirements for this cultivar are 6.4 kg Fe; 1.88 kg Mn; 56 g Zn; 12 B; and 15 g Cu for each ton of yield.
Although the nutritional requirements of the rice crop are high for some nutrients such as K, the harvested grain from the field is usually extracted and the residue left in place to be incorporated or burned in the following soil preparation. Incorporating residues allows replacing much of the K, Ca, S, and microelements: however, when residues are burned prior to the following crop, there is a loss of N and S content in the residues through the volatilization process to the atmosphere.
Figure 6 shows the nutrient distribution in the rice plant and considers the grain and the residue; it estimates the nutritional contributions produced by incorporating residues and establishes the importance of carrying out this task. This figure shows that incorporating rice residues to the soil returns a large part of K, Ca, and Mg extracted from the previous rice crop and greatly reduces the nutritional requirements of the following crop.
Nutrient management of the rice crop should pay greater attention of the N rate and its form of partialization. The rate must be determined as a function of crop productivity that generates differences in N requirement and the ability of natural soil contribution (with differences between different soil taxonomic orders associated with the evolution of secondary materials and their relationship with active pools of organic matter) since N absorption is mainly from reserves (organic matter mineralization, microbial biomass dynamic cycles, and N-NH4 + fixed in clays) [59-62], N fertilization , as well as a small fraction derived from irrigation water and other environmental and biotic sources. The ability of is determined through mineralization in conditions that are similar to field conditions [64-68]. To determine this soil natural N supply ability, the samples must be incubated under optimal mineralization conditions; flooded soils with soil:water ratio of 1:2.5, a temperature from 20ºC to 40ºC for a period of 14 to 21 d without agitating the samples . Using N rates that are higher than the crop requirements can generate phenological development disorders, increasing the vegetative period and producing the start of the reproductive period at a later time, which can increase flower sterility due to low temperatures that can occur in this period since in the final stage of the crop environmental temperatures start decreasing, negatively affecting grain ripening and ultimately commercial yield. It is common to find situations in which production has not achieved adequate grain maturity associated with the incorrect N rate (rates higher than necessary for the particular edaphoclimatic situation). Furthermore, when crop development time is reduced, it also decreases yield potential and thus N requirements.
About N partialization,  indicated that the highest grain yield is produced when applying N in 2 or 3 phenological stages; (1) 50% at sowing and 50% at panicle initiation, or (2) 33% at sowing, 33% at tillering initiation, and 34% at panicle initiation. This can be observed in Figures 7 and 8. The partialization strategy that the producer must use will depend on the opportunity to carry out the tasks (sowing date and its relationship with the adequate date) and the area sown.
Experiments conducted in Chile with other nutrients have reported a response when applying K, Zn, B, and liming . Rates of P, K, Mg, S, B, and Zn in the rice crop must be adjusted to nutrient requirement (as a function of yield) and the level of availability of each element in the soil. Meanwhile, Ca contributions will be carried out periodically to the extent that liming is performed.
Regarding alternative fertilizers, using organic amendments such as poultry litter has produced crop yield increases as compared to conventional fertilizers with equal N rates , which responds to the accumulative loss of soil organic matter with continuous crop cycles. Adding organic matter to the soil, including as crop residues, modifies the redox potential of the soil system and produces higher availability of some nutrients .
Finally, incorporating residues in the rice crop recycles a large part of the absorbed nutrients in the previous crop cycle  as can be observed in Figure 6 and reducing at the same time the rates of elements such as P, K, Ca, and Mg.
2. Nutritional management of main cereals cultivated as a function of yield and soil chemical properties
As abovementioned, the nutritional requirement of a crop is determined by its productivity, whereas the nutrient rate to be applied to the crop must be contrasted with the soil natural nutrient supply ability.
In a conceptual model for the same yield unit (Mg or other), the nutritional requirement can be expressed as a function of this yield unit and then be associated with the yield level as expressed in equation (1):
The nutritional requirement per yield unit is inversely proportional to the natural nutrient supply ability determined by the soil as shown in the conceptual model in Figure 9. Thus, to the extent that the nutrient concentration at issue exhibits a higher level in the soil, the fertilization rate will be lower per yield unit to produce. In turn, there is a minimum and maximum rate for each nutrient, which means that when the nutrient concentration in the soil is less than the minimum value, the maximum rate is applied, whereas when the concentration of this nutrient in the soil is greater than the maximum value shown in the range (minimum rate per yield unit), the minimum rate will be maintained, which for some nutrients could be zero (not applied).
The nutrient rate in corn, wheat and rice crops is calculated in function of production level, expressed in Megagrams per hectare. In corn crop are utilized the figures 10, 11, 12, 13, 14, and 15 for N, P2O5, K2O, CaO, MgO, and S, respectively. For wheat are utilized the figures 16, 17, 18, 19, 20, and 21. And for rice crop are utilized figures 22, 23, 24, 25, 26, and 27, respectively.
Another variable to consider in the fertilization program is the correction soil acidity, for which liming is used by applying a rate determined by specific analyses in each soil and where it is determined if calcium carbonate (CaCO3) is required to increase soil pH (acidity or alkalinity) in a given quantity. When liming is applied, the fixed Ca application is suspended by analyzing the soil. Likewise, if the amendment has to use Mg (CaCO3*MgCO3), the application of fixed Ca and Mg is suspended by analyzing the soil. In general, it is considered that when soil pH is greater than 6.0 liming is not necessary since there are no acidity problems (negative effect of aluminum (Al) on plant development under acid pH conditions, unless a nutritional imbalance in the soil that affects Ca and/or Mg needs to be corrected.
For the elements K, Ca, and Mg, which participate in cation exchange capacity of the soil (CEC) and given the phenomenon of competition or antagonism, the rate should be adjusted as a function of de percentage participation of each one of them in the CEC of the soil, considering that the element that must exhibit the highest participation in CEC is Ca, followed by Mg, and then K. Thus, the reference ranges of the saturation percentage of each one of these elements are shown in Table 4.
|————————— % over CEC ————————|
|K||< 2||2 – 3||"/>3|
|Ca||< 55||55 - 65||"/>65|
|Mg||< 10||10 – 15||"/>15|
2.1. Nutrient rates in corn crop
2.2. Nutrient rates in wheat crop
2.3. Nutrient rates in rice crop
As an example of applying this methodology, sowing corn with an expected yield of 14 Mg ha-1 and its chemical properties are shown in Table 5. The rate of each nutrient to be applied as a function of expected yield and chemical soil properties are also found in the table. To determine the rate to use per yield unit (kg Mg-1), Figures 10, 11, 12, 13, 14, and 15 have been used for N, P2O5, K2O, CaO, MgO, and S, respectively.
|Mineralized N, mg kg-1||30.8||N||322|
|Available P, mg kg-1||18.4||P2O5||104|
|Exchangeable K, cmol+ kg-1||0.35||K2O||133|
|Exchangeable Ca, cmol+ kg-1||8.4||CaO||22|
|Exchangeable Mg, cmol+ kg-1||0.6||MgO||55|
|Soil exchange capacity (CEC), cmol+ kg-1||14.5||-||-|
|CEC used by K, %||2.4||-||-|
|CEC used by Ca, %||57.9||-||-|
|CEC used by Mg, %||4.1||-||-|
|Available S, mg kg-1||6.2||S||38|
It can be observed in Table 5 for this example that it is liming is not necessary, and the total nutrient rates to be applied have been adjusted to both yield and soil chemical properties. In the case where residues are incorporated, their contributions must be considered as was mentioned in this chapter.
Finally, a fertilization strategy must be determined as a function of the dynamics of each nutrient in the soil-plant system, irrigation system (some nutrients can be applied via fertirrigation when there are pressurized irrigation systems), and the response of a partialized application of nutrients such as N.
Finally, the information provided in this chapter allows calculating fertilization rates in cereals using objective measurement tools, such as the productivity level and soil chemical properties, being interrelated contribute to achieving economically adequate yields with an environmental component that allows reducing the negative effects associated with incorrect nutrient use.