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close this book Soils, Crops and Fertilizer Use
close this folder Chapter 9: Using chemical fertilizers
View the document What are chemical fertilizers?
View the document Are chemical fertilizers appropriate for limited-resource farmers?
View the document An introduction to chemical fertilizers
View the document Common chemical fertilizers and their characteristics
View the document The effect of fertilizers on soil pH
View the document Fertilizer salt index and "burn" potential
View the document Basic application principles for N, P, and K
View the document Fertilizer application methods explained and compared
View the document Troubleshooting faulty fertilizer practices
View the document Getting the most out of fertilizer use: crop management as an integrated system
View the document Understanding fertilizer math
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Basic application principles for N, P, and K

Before covering the specific application methods for chemical fertilizers, let's go over some important principles that affects how N, P, and K can be best applied.

NITROGEN APPLICATION PRINCIPLES

Remember that nearly all chemical fertilizer N is mobile and leachable in the soil, because ammonium N is rapidly converted to mobile nitrate in warm soils. The sandier the soil and the higher the rainfall, the greater the potential leaching losses.

How to Combat Leaching Losses of N

If all the N is applied at planting or transplanting, much may be lost by leaching, especially since young plants have relatively small N needs. For annual crops, such as maize, tomatoes, and cabbage, it's far better to "spoonfeed" the N by applying only 1/3-1/2 of the total (but no less than 30 kg/ha actual N) at planting or transplanting, usually as part of an NP or NPK fertilizer. The remaining 1/2-2/3 is applied in one to several sidedressings along the crop row, starting about 4 weeks after the initial NPK application. Sidedressings usually consist of a straight N fertilizer like urea or ammonium sulfate.

Guidelines for Sidedressing N

The number of sidedressings over which the remaining N is divided depends on 2 factors:

• The potential for leaching losses as influenced by texture and rainfall.

• The length of growing period for the crop.

Here are some examples:

Maize: Usually needs one sidedressing around knee-high stage (about 4 weeks after planting in warm areas). Under high rainfall, especially on sandy soils, 2 sidedressings are recommended: one at knee high, one at tasseling.

Vegetables: A very short season crop like radishes doesn't need a sidedressing. Leafy vegetables such as lettuce, pak choy, and amaranth may get one to several sidedressings (at 3-4 week intervals), depending on whether the whole plant is harvested at once or picked a few leaves at a time over a longer period. Short-term cucurbits like summer squash and cucumber can use 1-2 sidedressings, while longer-tare ones like melons and winter squash might need 2-3. Tomatoes will need from 2 to as many as 6 or more, depending on leaching conditions and length of production. A good interval between sidedressings is 3-4 weeks.

Where to Place Sidedressed N: We'll cover this under application methods in a few pages.

How Deep to Place N: Since N is so mobile, it doesn't have to be placed deep in order to reach the roots, but just enough (2-5 cm deep) to avoid being washed away by rain or losing N as ammonia gas (refer to the section on N fertilizers).

PHOSPHORUS APPLICATION PRINCIPLES

The yield response obtained from applying fertilizer P to P-deficient soils depends a lot on how and when it's applied. Learn these important guidelines:

• Apply P early: Young seedlings need a high concentration of P in their tissues for early growth and root development. One study showed that young maize seedlings take up 22 times more P per unit of length than plants 11 weeks old. P should be applied at planting or transplanting time.

• Remember that applying P in combination with N (if needed) helps stimulate P uptake.

• Application method has a big influence on the soil's ability to tie up applied P. Broadcasting (spreading) fertilizer P usually results in far more tie-up than using a localized placement method (band, hole, or half circle) since it maximizes the contact of each fertilizer granule with soil particles than can cause tie-up. These methods will be explained in the upcoming section on fertilizer application

• Place broadcast P deep: It should be thoroughly mixed into the topsoil with a plow or hoe, except when spread around tree crops (this will be explained farther along under application methods).

• Don't "spoonfeed" P: Depending on application method, the mobility of P varies from nothing to very moderate. Leaching is never a problem, so all of the P can be applied in one application. There's no advantage to making sidedressings as growth proceeds unless P hunger signs develop.

POTASSIUM APPLICATION PRINCIPLES

K ranks midway between N and P in terms of mobility and leaching. As with P, all the K can usually be applied at planting or transplanting as part of an NPK fertilizer or as a straight K fertilizer. Where leaching losses are likely to be high, split applications of K may be needed. Split applications are also recommended for pastures to avoid "luxury consumption" of K. (Refer the section on potassium in Chapter 6.)