| Soils, Crops and Fertilizer Use |
|Chapter 9: Using chemical fertilizers|
You can see now that chemical fertilizers require much more skill to use properly than organic fertilizers in terms of selection, rates, dosage calculations, and application. Now that we've covered most of this, it's easy to understand why the misapplication of chemical fertilizer is a very common problem, whether in the Third World or elsewhere.
Hopefully, this chapter has given you a solid grounding for using chemical fertilizers appropriately. To help tie together all the principles and practices covered, let's practice troubleshooting some common faulty fertilizer recommendations.
What to Look For
When evaluating a fertilizer recommendation, check the following:
• Type of fertilizer
• Amount of fertilizer
• Application method: BC vs. LP, depth, distance
• Timing of applications
• Proportion of total N applied at planting/transplanting
RECOMMENDATION 1: 250 kg/ha of 14-14-14 broadcast and left on the soil surface a day before planting Chinese cabbage, followed by 150 kg/ha of ammonium nitrate (33-0-0) a month later in a band 20 cm out from the row and 2 cm deep.
WHAT'S WRONG?: This recommendation applier a total of 85 kg N, 35 kg P2O5, and 35 kg K2O per hectare. The N rate falls in the medium category, and the P and K rates in the low category, which is OK as long as the soil doesn't have a severe P or K deficiency. However, broadcasting such a low rate of P is a serious mistake and will result in most of it being tied up and unavailable to the crop. It should be banded. Remember that the fertilizer rate table (Table 9-4) is based on localized placement of P; from 3-10 tines or more may be needed if broadcasting is used. It's also a big mistake to leave the 14-14-14 on the soil surface; broadcast P is immobile and won't reach the roots unless thoroughly worked into the top 15-20 cm with a hoe or plow. The proportion of total N (40%) applied at planting is OK. The N sidedressing is applied correctly and at the right time.
RECOMMENDATION 2: 125 kg/ha of urea (45-0-0) applied when grain sorghum is planted, followed by a sidedressing of 200 kg/ha of 16-20-0 at knee high stage.
WHAT'S WRONG?: It's backwards! NP or NPK fertilizer should always be applied at planting (or transplanting), never as a sidedressing. P needs early application, because young plants need a high concentration in their tissues for good early growth and root development. Besides, applying the urea first puts on far more N (63%) than the 1/3-1/2 that should be applied at planting. What about the NPK dosage? It works out to about 88 kg N, 40 kg P2O5 , and 40 kg K2O per hectare which are all in the acceptable range of the rate table a few pages back. However, some extra K might be needed if the soil has a low level.
RECOMMENDATION 3: 300 kg/ha of 12-24-12 when tomatoes are transplanted, applied in a half circle 30 cm out from the stem and 5 cm deep, followed by a sidedressing of 150 kg/ha of ammonium sulfate (21-0-0) every 4 weeks until harvest is over. The sidedressing is applied in a half circle 10 cm out from the stem and 2 cm deep.
WHAT'S WRONG?: To start with, the half-circle for the initial NPK application is placed too far away (30 cm) from the plants. A palm's-width (10 cm) will allow earlier utilization of the fertilizer. The 5 cm depth of the half-circle is correct, as long as there will be sufficient moisture to move the P downward toward the roots.
Secondly, the N sidedressing is placed too close (10 cm) to the stems and may injure the plants; it should be placed about 20-25 cm from the stems.
The N, P, and K dosages seem reasonable, although more K might be needed on a low-K soil. The monthly sidedressings are appropriate, including the rate of 30 kg/ha of actual N per application. In the case of well managed, vine-type (indeterminate) tomatoes, the production period could continue for up to 6 months or more, requiring 6 or more such sidedressings totalling 180 kg/ha or more of actual N. This night seem excessive, but not when you consider the unusually long production period and the potential yield.
RECOMMENDATION 4: 300 kg/ha of 16-20-0 applied when peanuts are planted, followed by a sidedressing of 100 kg/ha of urea (45-0-0) 30 days later.
WHAT'S WRONG?: Peanuts are a very efficient N-fixing legume which can normally satisfy their entire N needs if the proper strain of rhizobia bacteria is present. Even if the soil lacked the right type of rhizobia, it would be much more economical to innoculate the seed with the bacteria than to buy N fertilizer (see the section on peanuts in Chapter 10). Also, K may be needed
RECOMMENDATION 5: Broadcasting 400 kg/ha of 10-10-20 over a nursery seedbed for raising cabbage transplants and working it into the top 5 cm of soil with a rake. (Assume that manure or compost aren't available).
WHAT'S WRONG?: First, a rake will not move broadcast P deep enough into the soil for good availability to the roots. A good hoeing is needed to work the fertilizer into the top 10 cm of soil.
Second, broadcasting is the only feasible method for applying NPK fertilizer to a nursery seedbed (especially if the seed was broadcast), but the rate of P is far too low this. It's not a simple matter of increasing the amount of 10-10-20 either, because you'll end up applying way too much and especially K in order to put on enough P. The real problem is the fertilizer's ratio (1:1:2). In order to apply the high amount of P needed (remember, up to 10 times more is required when the BC method is used) without overapplying N and K, you want a fertilizer with a ratio of around 1:3:1 or 1:4:1. If this isn't available, you can make your own by mixing the the 10-10-20 with single superphosphate (0-20-0) or triple superphosphate (048-0). This is covered in the fertilizer math section at the end of this chapter.
RECOMMENDATION 6: 100 kg/ha of 16-48-0 applied when maize is planted, followed by a sidedressing of 400 kg/ha of ammonium sulfate (20-0-0) when the tassels and silks emerge.
WHAT'S WRONG?: First, a total of 96 kg/ha of N is being applied in the 2 applications (16 + 80) which is acceptable, as long as moisture is adequate and the crop is well managed. But, remember that 1/3-1/2 of the total N should be applied at planting. In this case, only about 16% of the N was put on at planting, which is too little. As a general rule, at least 30 kg/ha of actual N should be applied at planting, mainly to help avoid temporary tie-up of soil N by the bacteria that are decomposing crop residues. (See Chapter 6 under N.) The problem with 16-48-0 is that its 1:3:0 ratio results in too little N being applied, given the rate needed to supply the 48 kg/ha of P2O5 (a satisfactory amount). One solution would be to add some 20-0-0 to the 16-48-0 to supply the extra N needed at planting.
Second, the N sidedressing is applied too late. In warm climates, maize starts tasseling and silking about 50-70 days after planting. Applying the N this late will give much less of a yield boost, because the plants' need for N begins best time for sidedressing. This is usually when maize has reached the knee-high stage.
Third, the P rate is acceptable, but K may be needed.
RECOMMENDATION 7: Making up a starter fertilizer solution by dissolving 4 cc of urea (45-0-0) in a liter of water and pouring 250 cc around the base of each newly set tomato transplant. No other fertilizer, chemical or organic is applied for the rest of growth.
WHAT'S WRONG?: Urea will have little benefit as a starter solution. What's needed is an NP or NPK fertilizer with a good ratio of P which will help stimulate new root growth. A straight P fertilizer could be used, but N helps promote the uptake of P by the roots. Some examples of fertilizers suitable for making up starter solutions are 12-24-12, 10-3010, 18-46-0, and 16-20-0.
In addition, the starter solution isn't meant to replace the normal NP or NPK fertilizer application nor the sidedressing either. It provides only enough nutrients for the first week or so of growth. (For more information on starter solutions, see the section on vegetables in Chapter 10.)
RECOMMENDATION 8: Applying 10-20-10 to potatoes at planting time by banding it directly under the seed pieces, separated by 3-4 cm of soil. Then applying a sidedressing of urea one month after emergence by sprinkling it on the soil surface along the row.
WHAT'S WRONG?: First, there's danger of burning the seed pieces due to upward movement of fertilizer salts as the soil dries out between rains or waterings. The band should either be applied about 3 fingers-width (7.5 cm) off to the aide or separated vertically from the seed pieces by at least 12 cm of soil.
Second, urea releases free ammonia gas which will cause significant N loss unless the urea is worked into the soil a bit. Rainfall or overhead watering could also wash the fertilizer off the ridges or "hills" on which potatoes are usually grown. Applying it right before the plants are hilled up with soil or weeded is a good way to accomplish this without extra work.