| Soils, Crops and Fertilizer Use |
|Chapter 9: Using chemical fertilizers|
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.)