How to Apply Topdressing to Corn: Best Timing, Fertilizer Types, and Application Rates
Corn takes a long time to absorb fertilizer and has a relatively high nutrient demand. Applying only base fertilizer and seed fertilizer often cannot fully meet its needs, so topdressing is also necessary. So what is the best time for corn topdressing? What fertilizer should be used? One of the key issues in topdressing is to grasp the critical period and the period of highest efficiency for crop nutrient uptake, and to carry out scientific topdressing during the peak nutritional period as much as possible. Only in this way can fertilizer use efficiency be improved and its yield-increasing effect be fully brought into play.
Topdressing Period
The best period for corn topdressing is from the jointing stage to the ear differentiation stage (June 20-27). At this time, corn has unfolded 7-12 leaves, which is exactly the stage when the female ear grows and elongates to the female floret differentiation stage, and it is the key period that determines corn ear length and ear thickness. Topdressing at this time can meet the nutrient needs of jointing and booting, promote ear differentiation, and make corn ears large with many kernels.
Topdressing Methods
There are 3 commonly used topdressing methods in corn production, namely broadcasting on the ridge platform and then immediately plowing soil to cover it; manually digging holes between plants on the ridge platform for deep application, covering with soil, and then plowing the field to cover it; and topdressing in the furrow. Among these 3 topdressing methods, the best effect is manual hole digging, furrow treading, and soil covering. Because of deep fertilization and strict covering, fertilizer use efficiency is high, achieving very good application results, and it will increase yield by 2%-11% compared with broadcasting fertilizer on the ridge.
The next is furrow topdressing. The advantage of this method is that it saves labor and trouble, but the shortcoming is that the fertilizer is slightly farther from the dense root zone, which is not conducive to full absorption of fertilizer by corn roots. Therefore, when using it, be sure to plow a shallow furrow before applying fertilizer and covering with soil, and cover the fertilizer tightly to avoid volatilization loss and waste. Broadcasting on the ridge platform may sometimes leave fertilizer exposed outside due to incomplete ridge closing, causing serious loss and waste. The depth of corn topdressing and the distance from the plant should both be 7-10 cm. This is beneficial for absorption and utilization by corn roots. If it exceeds 10 cm, the fertilizer application effect will be affected; if it is less than 5 cm, it will cause seedling burn. After fertilization, cover with soil in time or cover with soil in combination with hoeing and treading.
Topdressing Frequency
Most corn is topdressed once at the 9-11 leaf stage, around June 25. If the amount used for topdressing is relatively large, it is best to topdress twice (jointing-booting fertilizer and ear-kernel fertilizer). The distribution principle for two topdressings is that the first should account for 2/3 of the total topdressing amount, and the second should account for 1/3. For light-textured soils such as sandy soil, which have poor fertilizer and water retention, topdress in small amounts multiple times to prevent nitrogen leaching loss.
Topdressing Amount
Corn topdressing should be applied in appropriate amounts according to soil fertility, the fertilizer demand characteristics of the variety, and the growth period of the variety, so as to meet the needs of corn growth and development. Plots with lower soil fertility should increase the topdressing amount, while the amount can be appropriately reduced otherwise. For high-fertility plots, the urea application rate is 15-17 kg/mu; for medium-fertility plots, the urea application rate is 18-20 kg/mu; for low-fertility plots, the urea application rate is 21-23 kg/mu. In addition, due to differences in density and cultivation methods, the topdressing amount is also different. With high planting density, the topdressing amount should be increased; for corn with wide ridges and double rows or double plants close together, the fertilization amount should be increased even more, with 35-40 kg of urea topdressed per mu. If potassium fertilizer was not applied as base fertilizer or starter fertilizer, 3-4 kg of potassium chloride or 4-5 kg of potassium sulfate per mu can be applied together with the first nitrogen topdressing.
What Fertilizer to Use for Topdressing
Topdressing mostly uses urea or compound fertilizer, and the amount differs depending on spring corn and summer corn. The amount for spring corn is generally about 33.0% of the total nitrogen fertilizer amount. For soils that are somewhat sandy and have poor fertilizer retention, the topdressing amount can account for about 50% of the total nitrogen fertilizer amount. Because summer corn is replanted in a rush after the previous crop, base fertilizer is generally not applied, and the amount of seed fertilizer is also small, so topdressing becomes even more important. Generally, most or all of the nitrogen fertilizer is used as topdressing. Its topdressing must ensure two applications. The first is after seedling fixing, promptly topdressing to promote seedlings, and this topdressing should mainly use quick-acting nitrogen and phosphorus fertilizers, which can achieve both seedling development and stable growth. The second topdressing should be after jointing at the large trumpet stage, mainly with heavy nitrogen topdressing, at twice the amount of the first nitrogen application, to ensure the large demand for nitrogen during the ear differentiation stage. Topdressing should all adopt the method of hole application and soil covering, and broadcasting and surface application must be eliminated to prevent waste caused by volatilization.
Other Matters
Under dry soil conditions, determine the topdressing time according to the weather forecast, and it is best to topdress before rain. Also topdress in appropriate amounts according to corn growth and seedling conditions, and plots lacking fertilizer should be supplemented in time with an increased topdressing amount. For plots with excessive vegetative growth and late maturity, increase the application of phosphorus and potassium fertilizers to promote earlier ripening of corn.