California Crop Fertilization Guidelines

The PPNT is a valuable tool to adjust pre-plant N applications. Samples should be taken to a depth of at least 2 feet ^[N20].

The PPNT measures the N carried over from the previous crop, plus N released from soil organic matter, crop residues, and manure applications ^{[N17, N25]}. Heavy irrigation rates between soil sampling and the time of high N uptake by corn plants may leach nitrate below the root zone, making it unavailable for the plants. In this case, the PPNT overestimates the available soil N ^[N25]. Therefore, the PPNT and high pre-plant N applications are only an efficient fertilization strategy when leaching during the early stages of corn growth is minimal. Furthermore, the PPNT does not include N mineralized between soil sampling and the time of high uptake. Therefore, when most N is sidedressed, the pre-sidedress nitrate test is the better option.

The PSNT provides a measure of the soil N available to the crop prior to sidedressing and can be used to determine the need for sidedress N ^{[N1, N25]}. As the sample is taken just prior to the period of high N uptake rates, the PSNT more accurately determines available residual N than the PPNT.

A soil sample is taken from the top foot of the profile when the corn plants are 6-12 inches tall, corresponding to the 4- to 6-leaf stage ^[N48].

The nutrient concentration in plants changes over time and varies between plant parts. In order to be able to interpret the results correctly, care must be taken to sample the correct plant parts at a specific growth stage (see Table) ^[N36]. For instructions on how to take a representative sample see Plant Tissue Sampling. Plant tissue analysis guidelines for corn ^[N19].

When N concentrations are below the sufficiency range, yield may be decreased even in the absence of deficiency symptoms. Concentrations above the sufficiency range may indicate excess N availability.

The N status of corn plants is reflected in the leaf color, with a light green color indicating low N availability while a dark green color is typical for N sufficient plants. The leaf greenness of corn plants can be determined using a hand-held device, such as the SPAD Chlorophyll Meter, or a canopy reflectance meter.

Leaf chlorophyll readings should be averaged from 20 to 30 plants. Before tassels appear, readings are taken from the most recent leaf with a fully visible collar. The values are more consistent when the reading is taken at approximately the same position on the leaf blade ^[N25].

Leaf greenness readings can be used for real-time N management decisions ^[N64]. Currently, no functions relating absolute leaf greenness readings at a specific growth stage with N fertilizer requirements are available for California. However, relative values can be used to assess the N status of a field. The relative leaf greenness (often referred to as N sufficiency index), is the average leaf greenness of the field relative to the average leaf greenness of a well fertilized strip within the same field with identical management and variety. Studies found that N fertilization is required to prevent yield loss when the relative leaf greenness drops below 90 to 96% of the well fertilized strip ^[N64].

To review and evaluate the N fertilization program and to make adjustments for the following years, a stalk nitrate test at corn maturity can be performed ^{[N10, N11]}.

Samples can be taken just before silage harvest when the milk line is one-fourth of the way down the kernel and through about 3 weeks after black layer formation. During this period, the nitrate concentration in the lower part of the stalk remains relatively stable ^[N27]. A 8-inch long section of the stalk is cut, with the lower cut being about 6 inches above the soil surface. Leaves are removed from the stalk ^[N11]. Ten to 20 samples are taken from a field. The samples are immediately submitted to a laboratory for nitrate-N analysis ^{[N11, N67]}.

Binford and coworkers ^[N11] reported an optimal range of 700 to 2000 ppm nitrate-N. This range has been confirmed in a number of large field studies and is used by the extension service in several states ^{[N6, N33, N39, N41, N53, N60, N67]}. Optimal stalk nitrate-N concentrations in fields where manure was fall applied are generally higher, possibly because a larger proportion of the manure N becomes available later in the season. This N often cannot offset the potential yield loss from early or midseason N deficiency ^[N39]. Oregon State University Extension Service recommends an optimal range of 3,500 to 5,500 ppm nitrate-N ^[N34].

Little recent data about optimal N application rates for corn production in California exist. Brittan ^[N14] recommended total application rates of 200 to 275 lbs N/acre on mineral soils, depending on plant population and previous crop. This is in line with a trial carried out at Davis, where an average grain yield of 225 bu/acre (6.3 tons/acre) was produced with 200 lbs N/acre ^[N3]. Doubling the N rate had no effect on yield or grain protein content.

Pre-plant N fertilization rates should be adjusted by the nitrate-N present in the soil profile in early spring (see Soil Test). One ppm of nitrate-N in the top 2 feet of the profile corresponds to 7-8 lbs/acre (3.5-4 lbs/acre per foot of soil). During the growing season more N will be made available by N mineralizing soil microorganisms, with the amount of N mineralized depending among other factors on soil organic matter content and recent applications of organic material. No University of California recommendations exist for application rates based on pre-plant soil nitrate levels.

Colorado State University Extension recommends reducing the suggested N rate by 8 lbs/ac for each ppm nitrate-N in the soil for a 2-foot sampling depth ^[N20]. University of Nebraska Extension recommends adjusting the pre-plant N application rate based on residual nitrate (PPNT, 0-4 ft), yield goal, and soil organic matter content (see Table). University of Nebraska Extension N fertilizer recommendations (lbs/ac) based on expected yield with adjustments for pre-plant soil nitrate-N and soil organic matter ^[N68]. These values have not been tested in California.

How much of the pre-plant nitrate is crop available depends to a large part on the irrigation management between the time of sampling and the period of high N uptake. High irrigation water applications may leach the nitrate below the root zone making it unavailable to the crop. Therefore, pre-plant N applications are only efficient when leaching during the early stages of corn growth is minimal.

Anhydrous ammonium and aqua ammonia must be injected 6 to 8 inches deep in loamy soils and 8-10 inches deep in sandy soils to avoid losses of gaseous ammonia ^{[N13, N25, N70, N72, N73]}.

Urea and UAN solution should be incorporated to reduce ammonia volatilization losses, especially in no-till corn ^{[N5, N25, N30, N63]}.

Injecting UAN solution in a band below the surface has been found to be more efficient than broadcasting and incorporating it ^[N47]. However, the band should not be placed directly under the corn row, because the release of ammonia can injure roots and substantially lower yield ^[N25].

Common N fertilizers are anhydrous ammonia, aqua ammonia, urea, UAN (sometimes referred to as UN32) and ammonium sulfate ^{[N73, N76]}.

If properly applied, the various mineral N fertilizers are equally effective. Choice therefore can be based on the cost per unit of N and the fertilization plan ^{[N5, N57, N69]}.

Under leaching conditions, losses from fertilizers containing nitrate and fertilizers that are nitrified quickly may be high, reducing the amount of N available for corn uptake ^[N4]. Anhydrous ammonia is the slowest of all N fertilizer forms to be converted to nitrate, reducing the risk of leaching directly after the application ^[N76].

Pre-plant N should be applied close to planting. The longer the time between application and crop uptake, the higher the risk of N losses through leaching or denitrification. Leaching may be caused by rainfall or high irrigation water application rates. For these reasons, fall applications are generally less effective than spring applications ^{[N24, N76]}.

Corn fertilization guidelines in a number of states recommend applying between 10 and 60 lbs N/acre with the starter ^{[N1, N14, N20, N21, N29, N34, N44, N49, N76]}.

In a study carried out with silage corn in the Modesto area, the total N in the aboveground biomass was below 40 lbs/acre one month after planting ^[N31]. Therefore, with careful irrigation management, a starter application rate of 40 lbs/acre together with the residual soil nitrate supplies enough N to the corn plants during the first month of growth.

The maximum amount of starter N that can be applied depends on fertilizer placement and the composition of the material (see Mode of Application).

Starter fertilizer is traditionally applied in a band 2 inches to the side and 2 inches below the seed to reduce the risk of salt damage ^{[N52, N58, N59]}. Even when applied at this distance to the seed, the N plus K₂O applied should not exceed 70-80 lbs/acre ^{[N1, N5, N7]}.

The application of fertilizer in direct contact with the seeds (popup placement) may damage seedlings, especially in sandy soil and when the fertilizer has a high ammonium or K concentration ^[N61]. The maximum rate of N and K₂O applied should not exceed 10-15 lbs/acre in medium and fine textured soils. Cooperative extension in some states even recommends limiting the amount of N and K₂O applied to 8 lbs/acre ^[N76]. Seed placement is not recommended in sandy or dry soils ^[N58].

Early studies have shown that P uptake and root growth can be increased by using starter fertilizers that contain P and N ^{[N22, N51]}. Monoammonium phosphate (MAP, 11-52-0) and ammonium polyphosphate (10-34-0) are popular starter fertilizers ^{[N5, N40]}.

Urea, UAN solutions and diammonium phosphate (DAP, 18-46-0) produce free ammonia, which can severely damage seedling roots ^{[N2, N52]}. These fertilizers should only be used at low rates and when the material is placed at least two inches away from the seed ^[N5]. However, in some cases free ammonia from DAP bands has been found to diffuse 3 inches or more away from the band ^[N2].

When losses are minimized, corn needs about 1-1.25 lbs N/bushel of grain, which corresponds to approximately 6-7.2 lbs N/ ton of silage corn ^{[N1, N5, N23, N25, N62, N68]}.

Therefore, for a grain yield of 180 bu/acre (5 tons/acre) or a silage corn yield of 30 tons/acre the plants require between 180 and 216 lbs N/acre. This includes fertilizer N and soil derived N. No official University of California sidedress N application rates based on the pre-sidedress nitrate test (PSNT) exist. Cooperative Extension guidelines in a number of states recommend reducing the fertilizer N rate when the PSNT value is higher than 10 ppm. When the PSNT exceeds 25-30 ppm, it is generally recommended applying no sidedress N ^{[N12, N15, N38]}. The adjustments made for PSNT values between these two thresholds differ. The table gives the range of common recommendations found used in different states. However, these recommendations have not been tested in California. Range of recommended total N application rates (sidedress N plus starter N) based on expected yield and PSNT values from different states. These values have not been tested in California.

Based on recommendations from DE ^[N75], IN, KS ^[N15], PA ^[N5], TE ^[N65], OR ^[N49], WA and ID ^[N16].

Further adjustments may be necessary for nitrate-N in the irrigation water, manure applications in the past years as well as for legumes in the crop rotation ^{[N20, N29, N43, N75]}.

Banding is generally the most efficient method of applying sidedress N ^[N5] because corn roots generally do not reach the middle of the rows until the corn plant has eight fully emerged leaves. Therefore, during approximately the first six weeks after planting, nutrients applied in a band close to the corn row are more likely to be available for plant uptake than nutrients broadcast over the entire soil surface ^[N1]. Furthermore, sidedress broadcasting of granular fertilizer may cause leaf burn ^[N17]. Leaf injury and yield reduction have been found to be more pronounced with UAN and ammonium nitrate than with urea ^[N54]. However, up to 90 lbs N/acre as UAN or urea may be applied at the 4- to 5-leaf stage or up to 60 lbs/acre at the 8-leaf stage without reducing yields ^[N17].

Urea containing fertilizers should be incorporated into the soil to prevent ammonia losses ^{[N5, N17, N54]}. Ammonia volatilization is promoted by crop residue cover on the soil surface, absence of rainfall following the application, high temperatures, and high soil pH ^[N17]. Nitrogen broadcast on fields with a residue cover may be subject to immobilization by the soil microorganisms that decompose the residue ^[N1].

Sub-surface bands should be placed 8-15 inches away from the plants to reduce the amount of root pruning and injury from ammonia ^{[N5, N25]}.

In furrow irrigation systems, N should only be applied with the irrigation water when a tailwater recovery and reuse system is in place ^[N20].

The safest source of N for sidedressing is UAN (UN32) solution ^[N14]. Under the cool and humid climatic conditions prevalent in Quebec, Canada, UAN was found to be more efficient N fertilizer than aqueous ammonia and calcium ammonium nitrate when banded into the soil at sidedress ^[N28]. However, under California conditions, nitrification is generally fast, so that the difference between the materials is small.

However, when the plants are already deficient, nitrate and urea are preferred over ammonium, because the former two are more mobile in the soil and move readily into the root zone with the first irrigation following the fertilizer application ^[N21].

Corn plants take up little N until they reach the 6-leave stage. Between the 6-leave stage (V6) and the milk stage (R3), however, N uptake rates are high ^[N37]. In a study carried out with silage corn in the Modesto area, maximum N uptake rates reached 4.5 lbs/acre per day ^[N31]. Daily N uptake rates may even reach 8 lbs/acre ^[N37]. When corn reaches the tasseling stage, it has generally taken up more that 60% of the total N ^{[N18, N31, N37]}.

Sidedress N should therefore be applied between the 3- to 4-leaf stage and tasseling ^{[N20, N21, N25, N54]}. The optimal timing of the first sidedress N application depends on the available soil N and the amount of starter N applied. Nitrogen fertilization after silking is generally only beneficial, when the plants are clearly deficient ^{[N9, N34, N68]}. Nitrogen fertilizer applications at milk stage (R3) may increase yield in extremely deficient plants, but may actually decrease yield when corn is only slightly deficient ^[N9].

Soil samples are generally taken in fall or spring (see Soil Test Sampling for sampling instructions). Samples are taken from the top foot of the profile ^[P31], which is the main rooting zone. Studies found 65-85% of the roots of irrigated corn in the top foot of the soil profile and 80-94% in the upper two feet ^{[P14, P20]}. However, corn roots may reach to a depth of 6 feet ^{[P20, P26]}.

In California, the available soil P is generally determined by extracting the soil samples with a sodium bicarbonate solution (Olsen-P test).

Based on earlier studies summarized by Reisenauer and coworkers ^[P31], corn grown in soils with Olsen-P values greater than 12 ppm is unlikely to respond to P applications, while a response is likely with Olsen-P values below 6 ppm (see Table). The intermediate level between 6 and 12 ppm is comparable to optimal Olsen-P values from other states, where the lower limit of this range is generally between 5 and 8 ppm, and the upper limit between 11 and 15 ppm ^{[P4, P7, P30, P32]}.

The optimal soil P test value may vary depending on site- and production-specific factors ^{[P19, P36]}. Combining soil tests with plant analyses and P budgets helps identifying sites where a yield response to P fertilization is likely. Interpretation of P and K soil test levels in the top foot of the soil profile ^[P31].

¹⁾ ppm Olsen-P
²⁾ ppm ammonium acetate extractable K

When Olsen-P values are between 6 and 12 ppm, P fertilizer is applied to prevent yield losses at responsive sites and to maintain soil P availability in the optimal range. This can be achieved by applying the amount of P removed with the harvested grains. Literature values for the amount of P₂O₅ removed with harvested grains ranges from 0.344 to 0.44 lb/bu (6.2 to 7.9 g/kg) ^[P12]. With a yield of 180 bu/acre (5 tons/acre), between 60 and 80 lbs P₂O₅/acre are removed with the grains. The amount of P removed with silage corn may range from 60 to 100 lbs P₂O₅/acre ^[P36]. However, the economically optimal P rate may be lower, depending on site specific condition. Contact your local farm advisor for more information.

For Olsen-P values below 6 ppm, the application rate may be increased, while no or only a small starter P application is needed when the Olsen-P values exceed 12 ppm. Regular soil and plant analyses indicate whether the P fertilization program is adequate. On the long term, the Olsen-P values should reach and remain in the optimal range of 6 to 12 ppm.

In acidic and alkaline soils, P availability is greatly reduced. Under these conditions, soil P availability is better increased by adjusting the soil pH, rather than by applying large amounts of P fertilizer.

Application rates should be based on soil test results (see Soil Test). With intermediate soil P test values of 6-12 ppm Olsen-P, replacing the amount of P removed with the harvested crops ensures that P availability remains in the optimal range over the years. Between 60 and 80 lbs P₂O₅/acre are removed from the field when the grains are harvested and 60-100 lbs P₂O₅/acre with silage corn ^{[P12, P36]}. However, the economically optimal P rate may be lower, depending on site-specific condition. Contact your local farm advisor for more information. When soil P test values are below 6 ppm, higher application rates may be required, while P application rates may be reduced or skipped when soil P test values are above 12 ppm.

Pre-plant applied P can be broadcast and incorporated or applied in a band. However, broadcast applied P has been found to be less effective than band-applied P, especially in fields with a low P availability ^[P27].

Broadcast applied P should be incorporated because P is immobile in the soil and does not leach into the major root zone with rain or irrigation water. Incorporating P into the soil also reduces the risk of losses through runoff ^[P33].

A number of granular and liquid P fertilizers are available. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute.

In general, polyphosphate and orthophosphate fertilizers are equally effective ^{[P8, P21]}, even though polyphosphate fertilizers have been found to promote root growth more than orthophosphate in some soils ^[P34].

For fall or early spring applications, superphosphate should be preferred over blends containing ammonium or other forms of N, because once the ammonium is nitrified, the nitrate is prone to leaching by winter rains or irrigation water (see Pre-plant N).

When smaller application rates are required, all the P may be applied as a starter. When larger quantities are needed, it may be more practical to apply a small amount as a starter and broadcast and incorporate the rest pre-plant. A combination of broadcasting and band-applied starter has been found to be most efficient for corn grown in soil with low soil P test values ^[P27].

Preplant P applications can be made in the fall or spring ^[P17]. Phosphorus is immobile in the soil and does not leach. However, P may be removed with surface runoff, especially when not incorporated into the soil ^[P33].

When only low rates are required, P fertilizer can be applied in alternate years ^[P30].

For high-yielding corn, 40-60 lbs P₂O₅/acre may be applied in a band at planting ^[P30]. When the recommended annual application rate exceeds this amount, the difference between the total and starter can be applied pre-plant ^[P21].

When blends containing ammonium and K are used, the maximum application rate is determined by these two nutrients, because high concentrations of N and K₂O in the root zone may damage the roots of the seedlings (see Starter N).

Starter fertilizer is traditionally applied in a band 2 inches to the side and 2 inches below the seed to prevent salt damage ^{[P23, P27, P28]}. The proper distance to the seed is especially important for starter fertilizers that contain N and K, because salt damage is generally caused by these two nutrients and not by P ^[P23].

Applying starter fertilizer in a band has generally been found to be the most efficient way to supply P to corn. Compared to a broadcast application, P fertilizer applied in a band is restricted to a smaller volume of soil, which reduces interaction with soil particles so that more P remains available for a longer period of time ^[P27]. The increase in P use efficiency is generally highest in soils with low soil P test values, when only a small quantity of P is applied, or when soil temperatures are low ^[P27].

Early studies have shown that P uptake and root growth can be increased by starter fertilizers that contain P and N ^{[P9, P22]}.

Monoammonium phosphate (MAP, 11-52-0) and ammonium polyphosphate (10-34-0) are popular starter fertilizers ^{[P2, P16]}. In contrast, diammonium phosphate (DAP, 18-46-0) should be used with caution because it produces free ammonia, which can severely damage seedling roots ^{[P1, P23]}.

Soil samples are generally taken in fall or spring (see Soil Test Sampling for sampling instructions). Samples are taken from the top foot of the profile ^[K38], which is the main rooting zone. Studies found 65-85% of the roots of irrigated corn in the top foot of the soil profile and 80-94% in the upper two feet ^{[K17, K22]}. However, corn roots may reach to a depth of 6 feet ^{[K22, K26]}.

Plant available K is determined by extracting the soil samples with an ammonium acetate solution ^[K38].

Based on earlier studies summarized by Reisenauer and coworkers ^[K38], corn grown in soils with soil test K values greater than 80 ppm is unlikely to respond to K applications, while a response is likely with soil test K values below 50 ppm (see Table). Interpretation of P and K soil test levels in the top foot of the soil profile ^[P31].

¹⁾ ppm Olsen-P
²⁾ ppm ammonium acetate extractable K

The optimum or medium soil test K levels used in other states tend to be higher, with the lower limit ranging from 60 to 100 ppm and the upper limit from 100 to 150 ppm ^{[K3, K5, K7, K10, K19, K37, K41, K42]}.

However, in a study carried out in Iowa on 28 fields of which 18 had acetate extractable K levels between 80 and 150 ppm in the top 6 inches of the soil, Mallarino and Higashi ^[K20] only found a significant yield response to K fertilizer at one site. Combining soil tests with plant analyses and K budgets helps identifying sites where a yield response to K fertilization may be likely.

When soil test K values are between 50 and 80 ppm, applying the amount of K removed at harvest ensures that the K availability remains in the optimal range over the years. Literature values for the amount of K₂O removed with harvested grains ranges from 0.2 to 0.3 lb/bu (3.6 to 5.2 g/kg) ^[K14]. With a yield of 180 bu/acre (5 tons/acre), between 37 and 52 lbs K₂O/acre are removed with the grains. Potassium removal with silage ranges from 280 to 365 lbs K₂O/acre ^[K44]. However, the economically optimal K rate may be lower, depending on site-specific condition. Contact your local farm advisor for more information.

For soil test K values below 50 ppm, the application rate may be increased, relative to crop removal, especially for grain corn because the total K uptake of the plant far exceeds the K removed with the grains. In contrast, when soil test values are higher than 80 ppm, K application rates may be reduced or skipped. Regular soil and plant analyses indicate whether the K fertilization program is adequate. On the long term, the soil test K values should reach and remain in the optimal range of 50 to 80 ppm.

Grain corn trials carried out in Nebraska showed that soil test K levels can be maintained with annual applications of 30 lbs K/acre (37 lbs K₂O/acre). The average grain yield in these studies ranged from 150 to 155 bu/acre (4.2-4.3 tons/acre) ^{[K34, K35, K36]}. This application rate roughly corresponds to the amount of K removed with the harvested grains. With a yield of 180 bu/acre (5 tons/acre), between 37 and 52 lbs K₂O/acre are removed with the grains.

Corn stalk and leaves contain a lot of K. When the whole plant is harvested for silage, between 280 to 365 lbs K₂O/acre may be removed ^[K44]. However, the economically optimal K rate may be lower, depending on site-specific condition. Contact your local farm advisor for more information.

Band application has been found to be more effective than broadcast application, especially in soils with a low K availability ^[K28]. If K is broadcast, the fertilizer is best incorporated to increase its availability to plant roots.

Potassium chloride (KCl), potassium sulfate (K₂SO₄), and potassium magnesium sulfate (K₂SO₄, 2Mg SO₄) are common K fertilizers. They all contain readily available K. The choice may therefore be made based on price and whether the application of chloride, sulfate or magnesium is beneficial.

Unlike some other crops, corn is not very susceptible to high concentrations of chloride ^{[K11, K27]}. In contrast, chloride in KCl may suppress corn stalk rot ^[K12]. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute.

In most soils, K is not leached and can be applied in fall or spring. However, leaching of K may occur in sandy soils with a low soil organic matter content, due to the low cation exchange capacity. In these soils, spring K applications, close to the time it is needed by the plants, are more efficient ^[K18].

For K deficient soils, the recommended rates should be applied annually ^[K42]. When only low rates are required, K fertilizer can be applied in alternate years ^[K37].

Applying potassium with the starter fertilizer generally has no effect on corn development and yield ^[K21]. However, Brittan ^[K2] recommended using a starter blend containing K in the Clarksburg area. Trials have shown that 20-40 lbs K₂O/acre banded is sufficient in most cases ^[K2].

Heckman and Kamprath ^[K13] compared a band application 2 inches to the side and 2 inches below the seed with an incorporated broadcast application in a sandy soil with low K test values. Banding 56 lbs K₂O/acre reduced early K uptake, presumably due to the high salt concentration, but had no effect on grain yield.

Starter fertilizer is traditionally applied in a band 2 inches to the side and 2 inches below the seed to prevent salt damage ^{[K23, K30, K31]}. Even when applied at the proper distance, the total amount of N plus K₂O should not exceed 70-80 lbs/acre ^{[K16, K42]}.

The application of fluid fertilizer in direct contact with the seed (popup placement) may damage seedlings, especially in sandy soil and when the fertilizer has a high ammonium or K concentration ^[K33]. The maximum rate of N and K₂O applied should therefore not exceed 10-15 lbs/acre in medium and fine textured soils. Cooperative extension in some states even recommends limiting the amount of N and K₂O applied to 8 lbs/acre ^[K42]. Seed placement is not recommended in sandy or dry soils ^[K30].