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Published in Agron J 98:394-401 (2006)
DOI: 10.2134/agronj2005.0093
© 2006 American Society of Agronomy
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Phosphorus Management

Starter Phosphorus and Broadcast Nutrients on Corn with Contrasting Colonization by Mycorrhizae

Shabtai Bittman*, C. Grant Kowalenko, Derek E. Hunt, Thomas A. Forge and Xiao Wu

Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, Box 1000, Agassiz, BC, V0M 1A0

* Corresponding author (bittmans{at}agr.gc.ca)

Received for publication March 29, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Early P nutrition in corn (Zea mays L.) is important for early growth and arbuscular mycorrhizal fungi (AMF) may help juvenile corn plants access soil P. The role of AMF colonization in manured soils and the impact of AMF on the need for starter P is not well known. This study was designed to assess the response of corn with contrasting levels of AMF colonization to low rates of starter P under inputs of broadcast liquid dairy manure or N and P fertilizer. The study was conducted on a high-P-testing soil in south-coastal British Columbia. Contrasting AMF colonization was achieved by growing corn either after corn or after fallow. Both AMF colonization and starter P improve early growth and, to a lesser extent, final harvest even on a relatively high P soil receiving a heavy application of P as broadcast liquid dairy manure or fertilizer. The effects of AMF colonization and starter P were additive, so both contribute to maximum yield. Our study suggests that practices favoring AMF colonization will help juvenile corn plants take up P. While strategically placed low rates of starter P were beneficial for juvenile plants, these treatments were less effective than conventional sidebanding at 30 kg P ha–1 because the effect did not carry forward to final harvest, even in corn that was well colonized by AMF. Therefore, better nutrient strategies and AMF colonization are needed to ensure that corn crops will gain sufficient P with minimum starter mineral fertilizer on manured soils.

Abbreviations: AMF, arbuscular mycorrhizal fungi


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
THE IMPORTANCE of adequate tissue P concentration for early growth of crops, including corn, is well known (Miller, 2000; Grant et al., 2001, 2005). Deficiencies in early P nutrition in corn cannot be corrected by later applications of P (Barry and Miller, 1989) and seem to affect harvest index (Lauzon and Miller, 1997; Gavito and Miller, 1998b) more than whole crop yield at the end of the season (Plénet et al., 2000). Colonization of corn roots by AMF can enhance P uptake in crops by increasing the effective zone of exploration around the roots and accessing P unavailable to nonmycorrhizal roots (Hayman, 1983). Colonization by AMF is particularly important for juvenile corn plants when root access to soil P lags behind overall plant demand (Miller, 2000); however, colonization of corn roots is diminished by high P status in soil (Fries et al., 1998) and plants (Lu et al., 1994) so the role of AMF is often discounted under high-P conditions. General P recommendations to producers are rarely adjusted for low levels of AMF colonization even when this can be anticipated, such as following nonmycorrhizal crops (Gavito and Miller, 1998a) or when intensive tillage is used (Vivekanandan and Fixen, 1991; McGonigle and Miller, 1993).

Application of small amounts of P fertilizer near the seed (starter P) improves production of corn, particularly on low-P soils (Lauzon and Miller, 1997; Barber, 1958). Juvenile corn plants on high-P soils may also benefit from application of starter P, especially if soils are relatively cold, because their roots are too small to obtain enough soil P to sustain optimum growth (Schröder et al., 1996). Starter fertilizer (P and K), however, is more likely to increase early corn growth than final yield (Bates, 1971).

Silage corn is usually grown for feeding cattle, often near intensive livestock operations, and these crops typically receive large amounts of manure, which build up levels of soil P. In Vermont, dairy farmers often apply about 25 kg P ha–1 of starter to corn even on soils that are P enriched due to previous manure applications (Jokela, 1992). Starter fertilizer was found to increase the yield of corn on soils with medium (but not high) P availability that also received manure during the trial years. In the Netherlands, starter P fertilizer increased corn yield by 8% on manured soils, and this was attributed to the need for P close to the roots 5 to 7 wk after planting (Schröder et al., 1997). In south-coastal British Columbia, at least 20 to 30 kg P ha–1 of starter P fertilizer is routinely applied to silage corn crops after applications of ~50 kg P ha–1 as dairy slurry because starter P seems to provide better early growth and more consistent end-of-season results (Bittman et al., 2004). At these rates, long-term applications of dairy slurry may have little to slightly positive effect on AMF colonization of roots of forage grasses (Christie and Kilpatrick, 1992) but there is little information on the effect of manure on colonization of corn roots and response to starter P. Dairy manure is typically high in orthophosphate and, to a lesser extent, phytic acid (Toor et al., 2005). Because P can accumulate in soils around livestock operations and may transfer to waterways, there is a need to decrease P inputs without sacrificing crop production. The objective of this study was to assess the response of corn with contrasting levels of early AMF colonization to low rates of starter P with high inputs of manure or fertilizer.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
The trials were conducted at the Pacific Agri-Food Research Centre at Agassiz in south-coastal British Columbia (49°10' N lat; 125°15' W long). The soil at the experimental site is a silty to sandy loam with about 6% organic matter, belonging to the Monroe series, and described as an Eutrochrept of moderately good drainage derived from medium textured stone-free Fraser River deposits. Weather measurements were made within 100 m of the trials. Extractable P, determined by the Kelowna extraction method (Van Lierop, 1988), at the experimental site was relatively high at 64 kg P ha–1.

The experiments, conducted in 2000 to 2002, were designed as split plots randomized within four complete blocks. Each four-row plot measured 3 by 10 m. The main plots consisted of a factorial: two forms of broadcast nutrient (manure vs. chemical fertilizer) x two previous crop scenarios (corn vs. fallow). The previous corn and fallow treatments were intended to produce differences in AMF colonization of corn roots during the test years. In the setup years, 1999 to 2001, corn plots were treated the same way as in the trial years, while fallow plots were not fertilized. In the trial years, all plots were planted with corn after one set of previous fallow plots and one set of previous corn plots had received broadcast fertilizer while the other had received liquid dairy manure. The liquid dairy manure (approximately 6% dry matter by weight) was applied with an Aerway SSD applicator (Holland Equipment Ltd, Norwich, ON), which places manure in 19-cm spaced bands over vertical aeration slots (11 slots m–2). Manure application rates were set to provide 60 kg P ha–1. Total N in the manure was determined by Kjeldahl and NH4–N by steam distillation in the presence of MgO. Total P was determined by inductively coupled plasma spectrophotometry after HNO3 digestion. Rates and dates for manure application are shown in Table 1.


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  		Table 1. Rates and dates of application for liquid dairy manure, manure nutrient concentration, and nutrient application rates in the trial and pretrial years of three experiments at Agassiz, BC.

 
Fertilizer plots received NH4NO3 at 175 kg N ha–1 in 2000 and 270 kg N ha–1 in 2001 and 2002. Ammonium nitrate fertilizer was applied at 100 kg N ha–1 to manure plots in 2001 and 2002 to approximately match N application on the fertilizer plots. Average soil P (0–15 cm) in the experimental area in early spring (February–March) rated high in all 3 yr, ranging from 58 to 64 mg kg–1 (Kelowna extract). In 2000, P was not applied to the fertilizer plots because of the high soil test, but 60 kg P ha–1 (as 0:45:0) was applied in 2001 and 2002 to match the manure treatment. Soil P (Kelowna extract) measured between corn rows (away from fertilizer bands) in mid-August ranged from 75 to 80 mg kg–1 across main plot treatments in 2000 and 72–74 mg kg–1 across main plot treatments in 2002 (not measured in 2001). Other nutrients (K, Mg, and S) were applied according to soil test results to ensure that they were not limiting to corn growth.

The subplots consisted of seven starter P (0:45:0) treatments: 1 and 2 kg P ha–1 hand placed in the seed furrow next to each seed; 1, 2, and 7 kg P ha–1 banded in the seed furrow; a positive control representing current farming practice of sidebanding 30 kg P ha–1 5 cm below and 5 cm beside the seed furrow, and a negative control with no starter P.

The soil was prepared according to local farming practice by plowing, disking, and harrowing. Corn seed (Pioneer 3845) was planted with a John Deere MaxEmerge planter at 85 000 seeds ha–1 in 75-cm-wide rows in mid-May to early June (Table 2). The seed furrows of all treatments were left open to allow fertilizer placement, then packed down manually. Weeds were controlled with a pre-emergent application of atrazine (6-chloro-N-ethyl-N'-isopropyl-1,3,5-triazine-2,4-diamine;1.1 L ha–1 a.i.) plus metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide; 1.4 L ha–1 a.i.] and post-emergent (4–8 leaves) application of bentazon (3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide; 0.54 L ha–1 a.i.) with Assist oil concentrate (1.1 L ha–1). Sampling was conducted at the three-, six- and nine-leaf stages (based on visible collar), and anthesis by randomly cutting three plants from each of the two middle rows at ground level. Sampling dates are shown in Table 2. These samples were dried and weighed, then ground (1-mm mesh) for determination of P concentration. For the final harvest, 3-m sections of the two middle rows were harvested and weighed in the field, then chopped and sampled for dry matter and P determination. Ears were removed from 10 random plants and these were dried (60°C), shelled, and weighed to determine grain percentage. Ground samples were analyzed for P colorimetrically after a H2SO4–Se–H2O2 digestion (Novozamsky et al., 1983). Phosphorus uptake was determined as P concentration x biomass.


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  		Table 2. Planting and sampling dates for three trials in 2000–2002.

 
For assessment of mycorrhizal colonization, root samples were obtained from the six plants dug out at the three-leaf stage from four subplot treatments across all main plots (control, sideband, seed placed at 2 kg P ha–1, and in furrow at 7 kg P ha–1). The three-leaf stage was chosen because P deficiency symptoms often start to appear after three leaves and diminish by nine leaves. After loose soil was carefully removed, the roots were washed in cold water, then stored in FAA solution (65 mL formalin, 25 mL glacial acetic acid, and 1000 mL 50% ethanol formalin; Sass, 1958). The roots were cleared by autoclaving for 15 min in 10% KOH, washed with water, then stained in 0.01% (w/v) Trypan blue (1:1:1 solution of 85% lactic acid, glycerol, and water) at 90°C for 1 h (Brundrett et al., 1984). Sixteen randomly selected 1-mm-diameter root pieces from each sampled plot were mounted on microscope slides and viewed at 10 x magnification. Ten randomly chosen fields-of-view from each root piece were scored on a scale of 0 to 10, representing the percentage of root area colonized in the field of view. The average percentage of colonization was determined from 160 fields-of-view for each plot.

Statistical analysis was performed with SAS Proc Mixed (SAS Institute, 1999) assuming years and treatments are fixed effects and blocks are random effects. Significance level for all comparisons was set a priori at P < 0.05%.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
The weather conditions in each sampling period varied during the 3 yr of the trials (Table 3). The period from planting to the three-leaf stage accumulated more corn heat units in 2002 than in the other years and this was reflected in higher shoot weights at the three-leaf stage (Tables 4 and 5). For the three- to six- leaf period, most accumulated heat units occurred in 2000 but growth may have been suppressed by low rainfall. Shoot weights at the six-leaf stage were least in 2000, which had the lowest accumulated heat units from planting time. The high whole-crop dry matter content and grain percentage at harvest in 2001 (Tables 6 and 7) reflects the high season-long accumulated heat units (Table 3). All data are presented for individual years du e to the significant interactions of treatments and years for many variables, probably resulting from contrasting weather conditions. The three- and four-way interactions with years were not significant (Tables 4GoGo–7).


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  		Table 3. Mean daily temperature, accumulated precipitation, and corn heat units between samplings in 2000, 2001, and 2002 at Agassiz, BC.

 

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  		Table 4. Effect of year, previous crop, and broadcast nutrient source on mycorrhizal root colonization (at the three-leaf stage), and yield, P concentration, and P uptake of corn shoots at the three- and six-leaf stages in 2000–2002 (values are means of four replicates).

 

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  		Table 5. Effect of starter-P treatments and interactions of starter P with previous crop and broadcast nutrient source on mycorrhizal root colonization (at the three-leaf stage) and yield, P concentration, and P-uptake of corn shoots at the 3- and 6-leaf stages in 2000–2002 (values are means of four replicates).

 

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  		Table 6. Effect of year, previous crop, and broadcast nutrient source on yield at the nine-leaf stage and whole crop yield, dry matter content (DM), grain yield, grain content, P concentration, and P uptake at final harvest in 2000–2002 (values are means of four replicates).

 

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  		Table 7. Effect of starter P and interactions of starter P with previous crop and broadcast nutrient source on yield at the nine-leaf stage and whole-crop yield, dry matter content (DM), grain yield, grain content, P concentration, and P-uptake at final harvest in 2000–2002 (values are means of 4 replicates).

 
Colonization of corn roots by AMF at the three-leaf stage was much higher for plants grown after corn (17.2–46.9%) than for those grown after fallow (8.0–12.8%), with differences being particularly great in 2001 (significant year x previous crop interaction; Table 4). The previous crop had no effect on yield or P uptake in corn shoots at the three-leaf stage, but corn grown after corn had higher P concentrations in 2002, when the plants were relatively large at this stage. Previous corn significantly increased yield at the six-leaf stage in all 3 yr, but the effect was greatest in 2002. Similarly, previous corn increased P concentrations and P uptake significantly in all 3 yr, with the greatest effect for P concentration in 2001 and for P uptake in 2002. The effect of the previous crop was evident also at the nine-leaf stage (Table 6) and at anthesis (not shown), but the effect on final dry matter yield was significant only in 2001. At final harvest, previous corn increased dry matter percentage and grain yield in all years, and grain percentage in 2000 and 2002 compared with previous fallow. The effect of the previous crop on final P uptake was not significant but there was a small significant reduction in final P concentration due to previous corn.

Broadcast application of manure had a small but significantly positive effect on AMF colonization (13.1–33.3%) compared with mineral fertilizer (11.9–26.4%) during the 3 yr (Table 4). The manure plots had somewhat higher yields than fertilizer plots after three-leaf stage in 2000 and 2002 (Tables 4 and 6). The nutrient source did not affect P uptake at the three-leaf stage but manure did slightly increase P concentration compared with fertilizer. At the six-leaf stage, manure significantly increased P concentration in all years but increased P uptake only in 2000 and 2002. Manure increased final P uptake and P concentration in all years (Table 6). Manure increased dry matter content only in 2000, and increased grain yield and grain percentage in 2000 and 2002.

There was no significant effect of the conventional sidebanding of P at 30 kg ha–1 on AMF colonization, while the P placed near the seed at 2 kg ha–1 suppressed colonization only in 2002 (Table 5). In contrast, starter P fertilizer banded in the seed furrow at 7 kg ha–1 significantly suppressed AMF colonization in 2001 and 2002, with a similar trend in 2000. The sidebanded P and furrow P (7 kg ha–1) treatments significantly increased yield at the three-leaf stage only in 2002, when the plants were relatively large at this stage. Most of the starter P treatments increased P concentrations at the three-leaf stage, although the effect of 1 kg P ha–1 (seed positioned and furrow banded) was inconsistent. The sideband and furrow treatments at 2 kg ha–1 or more also increased P uptake at the three-leaf stage in 2001 and 2002 but not in the 2000 trial when overall uptake was lowest.

At the six-leaf stage, both the sideband and furrow (7 kg ha–1) treatments produced the highest yields in 2000, while this furrow treatment alone yielded most in 2002 (Table 5). The 2 kg ha–1 rate produced a significant yield increase over the control when placed near the seed in 2000 and near the seed or in the furrow in 2002. The 1 kg ha–1 treatments generally did not significantly affect corn yield at the six-leaf stage. While there were some significant effects of the low rate of starter P on P concentration, the effects were small and inconsistent; however, starter P applied either in furrow (7 kg ha–1) or in a sideband (30 kg ha–1) significantly increased P uptake at the six-leaf stage in all 3 yr. The lower rates of starter P did not produce consistent effects on P uptake.

The relatively high rates of starter P applied as sideband or in furrow increased yield at the nine-leaf (Table 7) and anthesis (not shown) stages for all years while the lower rates of starter P generally produced only small increases over the controls, and these were not always significant. Yield at final harvest was generally improved by starter P in 2000 and 2001 but only the sideband treatment improved final yield in 2002. Averaged across the 3 yr, all starter-P treatments, including the 1 kg ha–1 treatments, significantly increased yield over the control treatments. While only the sidebanded P treatment increased yield above controls consistently during the three trials, on average, furrow (7 kg ha–1) and seed (2 kg ha–1) P applications also significantly increased final dry matter content over the control. Sidebanded P increased grain yield and grain percentage in 2000 and 2002 but the effect in 2001 was somewhat inconsistent. Averaged across years, the sidebanded treatment had significantly higher grain percentages than all other treatments, and all starter-P treatments outyielded the control, with the sidebanded P yielding the most grain.

The effects of previous crop and broadcast nutrient type were additive (no interaction) for all measured parameters at all stages except yield and P uptake at the six-leaf stage (Tables 4 and 6). For these variables, the effect of the previous crop was greater for the manure than for the fertilizer treatment. The effect of starter-P treatments was additive with previous crop and with nutrient type and there were no significant interactions among starter P, previous crop, and nutrient type (Tables 5 and 7).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
Corn grown on soils that had been fallowed the previous year had less AMF root colonization than corn grown after corn, as previously reported (Vivekanandan and Fixen, 1991). Apparently the network of AMF hyphae from the previous corn crops, which can rapidly attach to newly developing corn roots (Miller, 2000), diminishes during the fallow year so colonization of roots is delayed. The conventional practice of sidebanding P at 30 kg ha–1 had little effect on colonization relative to the control, whereas the lower 7 kg P ha–1 rate applied in the seed furrow suppressed colonization compared with controls. Although application of P decreases overall AMF colonization in corn roots (Fries et al., 1998), colonization is reduced predominantly in roots that actually contact applied P (Lu et al., 1994). In our study, more contact of roots with P in the seed furrow than in the sideband treatment may have caused the difference in colonization. That greater root colonization in the sideband than in the furrow treatment did not result in larger three-leaf shoots may be due to greater uptake of furrow-applied than sidebanded P or to a lag between colonization and tissue-P response. Manure application had a small positive effect (P < 0.05) on AMF colonization, which was not hampered by slightly higher P concentration in the shoot. Moderate rates of dairy slurry did not decrease AMF colonization in forage grasses (Christie and Kilpatrick, 1992). Overall, we found little relationship at the three-leaf stage between colonization and P concentrations in the corn shoots across all treatments. A relationship between AMF colonization and shoot-P concentration may be difficult to discern because plant P tends to have a negative effect on colonization (Lu et al., 1994) while colonization has a positive effect on plant P.

At the six-leaf stage, low shoot weight and P uptake in corn after fallow, compared with corn after corn, appears to be related to the large differences in AMF colonization, as previously reported (Vivekanandan and Fixen, 1991), although it is not possible to exclude other factors related to cropping histories. Nevertheless, analysis of in situ P adsorption onto anion exchange resins in similar plots in 2003 indicated that availability of P was greater after fallow than after corn, suggesting that cropping history effects on P availability cannot explain the improved P uptake for corn after corn. Also, P availability determined by resin adsorption in manured plots was greater than in fertilized plots so the greater colonization of the manured corn cannot be explained by lower available soil P. All poorly colonized corn at the six-leaf stage showed a greater degree of purpling than well-colonized corn, although the greater quantities of starter P reduced the symptoms. The ratio between the corn-after-corn and corn-after-fallow plants at the six-leaf stage, across all treatments and years, was quite consistent for shoot weight (1.4 ± 0.28 SD [standard deviation]), P uptake (1.8 ± 0.33 SD), and P concentration (1.3 ± 0.17 SD). The initial growth advantage for colonized plants helps them to explore more soil and develop more leaf area, but gradually greater uptake of P and other nutrients by the colonized corn plants would suppress AMF symbiosis relative to plants containing less P (McGonigle and Miller, 1993). This may help to explain the diminishing difference in total plant dry matter and P accumulation between the corn-after-corn and corn-after-fallow plants during the growing season; the difference in harvest yield averaged just 650 kg ha–1. However, corn after corn had 15 g kg–1 more dry matter and consistently higher (500 kg ha–1) grain yield than corn after fallow. Colonization by AMF (Miller, 2000) and early P nutrition (Grant et al., 2001) contribute to early maturity, resulting in high grain and dry matter content.

The additive effects of starter P and the previous crop indicates that even the better colonized corn in this trial benefited from starter P fertilizer. In fact, AMF may help corn roots access starter P. Even with the high soil-test P and applications of both manure (50–60 kg ha–1 manure P) and starter P (30 kg ha–1 starter P), well-colonized corn produced 58% more shoot biomass and had taken up 63% more P than poorly colonized corn by the six-leaf stage. By final harvest, the well-colonized, manured corn had 6% greater grain yield and dry matter content and 2% greater whole-plant yield than poorly colonized, manured corn.

Accumulation of soil P is a serious environmental problem on many dairy farms, so it is desirable to reduce the amount of fertilizer applied and it would be much easier and cheaper for farmers to reduce fertilizer than manure applications. At present, it is common for farmers to apply starter P to corn grown on soils with a history of manure applications exceeding crop P requirements, as is often found on dairy operations (Jokela, 1992). In coastal British Columbia, corn fields typically receive 20–30 kg ha–1 of P as sidebanded starter P, usually in combination with about 10 to 20 kg ha–1 of N. In our trial,sidebanded P increased biomass and P uptake of well-manured and well-colonized corn at the six-leaf stage by 18 and 20%, respectively, and increased final yield, grain yield, and dry matter by 5 to 8%. These results show that adding starter P on manured soils may be justified, regardless of AMF colonization, when the intention is to obtain maximum early growth to compete with weeds and produce the best possible crop. These trials were planted &!sim;2 wk later than average planting dates for local farms so the responses to P observed were on relatively warm soils. Greater responses may be expected for corn planted earlier in colder soils. In addition to the effect of weather on the availability of P in the soil, soil characteristics such as the ability to bind P and the degree of P saturation may influence the ability of the crop to use soil P (Schroeder et al., 2004). That manure enhanced colonization indicates that AMF are important in highly manured soils. The results suggest that banded manure might be used to replace mineral fertilizer as a source of starter P, as suggested by Schröder et al. (1997).

The uptake of P in aboveground biomass at final harvest did not exceed 25 kg ha–1 for any of the treatments. Therefore, P from soil and nutrients applied for maximum yield substantially exceeded removal levels, so that continued application at these rates would cause a buildup of P in the soil. Moreover, at the six-leaf stage when greatest P deficiency symptoms usually appear, P uptake by the corn shoots was <1 kg ha–1, and probably not more than 1 kg ha–1 was taken up in plants and roots. Since uptake of P in the shoots at the six-leaf stage of even the poorest treatments exceeded 0.3 kg P ha–1, the amount of P required to avoid early P deficiency is <1 kg ha–1. Despite P applications of 50 to 60 kg ha–1 as broadcast manure or fertilizer P and a P soil test of ~60 kg ha–1 (Kelowna extract or ~55 kg ha–1 Bray 1 extract; Van Lierop, 1988), the juvenile corn plants responded to P fertilizer placed in the furrow or beside the seed at rates as low as 2 kg ha–1, while the 7 kg ha–1 furrow application was particularly effective. While the low application rates were very effective at the six-leaf stage, they were less effective at the nine- leaf stage and were significantly less effective than the standard sidebanding treatment at final harvest, even in the well-colonized corn.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
This study showed that both AMF colonization and starter P improve early growth and, to a lesser extent, final harvest of corn even on a soil of relatively high soil-test P receiving a large amount of P as broadcast liquid dairy manure or fertilizer. The effects of AMF colonization and starter P were additive, so both are required for maximum yield. Our results support the farmer's decision to sideband 30 kg ha–1 to maximize yield; however, accumulating P in soils and new standards for purity of surface waters requires that P inputs be closely matched to P removal by the crop. On many livestock farms, use of mineral P fertilizer and even manure must be greatly reduced to meet this standard. Our study shows that AMF colonization contributes to P nutrition so management practices such as reduced tillage (McGonigle and Miller, 1993) and the use of previous mycorrhizal crops (Vivekanandan and Fixen, 1991) can be expected to help corn take up P. While strategic placements of low rates of P were beneficial for juvenile plants, the effect did not carry forward to final harvest, even in corn that was well colonized by AMF. Therefore, better nutrient strategies, in addition to effective AMF colonization, are needed to ensure that corn crops will have sufficient available P with minimum mineral fertilizer on manured soils.


   ACKNOWLEDGMENTS
 
We thank F. Bounaix, A. Friesen, M. Schaber, M. Hansen, and K. Kropp and for their technical assistance. We gratefully acknowledge the support of the Potash and Phosphate Institute, Westco Fertilizers, and Agriculture and Agri-Food Canada Matching Investment Initiative.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 





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