Any liquid that contains one or more available plant nutrients is classified a liquid fertilizer; whether the plant nutrients are in complete solution or part in solution and part in suspension.
There are many advantages in the use of liquid fertilizers. They provide a greater ease of handling and application, and their liquid consistency allows for a more uniform application. In general, the product is more uniform in analysis and can work in conjunction with a variety of other pesticides; certain compatible pesticides make "weed and feed" programs possible.
However, the use of liquid pesticides may not be ideal in all situations. Liquid fertilizers require special storage tanks, pumps and hoppers, which can greatly add to costs. In addition, the possibilities of salting out in cold weather is increased. The addition of micro-nutrients and magnesium often creates a problem of sedimentation, although micronutrient chelates and polyphosphate materials have overcome some of the problems. Complete N_P_K fertilizers, especially those containing micronutrients, contain relatively low plant-nutrient content.
Nearly all the active ingredients in liquid or dry fertilizers commonly sold are water or citrate-soluble. Although the fertilizer salt may be 100 per cent water soluble, there may be a rapid change to compounds of low water solubility after application to the soil.
Liquid fertilizers seldom show any residues in the soil when band-placed, as do many dry fertilizers. These residues are usually conditioners, inert materials, and by-products such as gypsum and impurities. However, some of these residues may be calcium, magnesium or sulfur which are essential plant nutrients for which no claim has been made.
Unless diluted with water, most liquids of comparable formulation are as toxic as dry fertilizers. Liquid fertilizers made specifically for application to the foliage of plants, on the seed, or for household plants, should have a low salt index value.
Following is a salt index for some common fertilizers:
Salt: Salt Index: ----- -----------
potassium chloride 116 ammonium nitrate 105 sodium nitrate 100 urea 75 potassium nitrate 74 ammonium sulfate 69 calcium nitrate 65 potassium sulfate 46 superphosphate (0-46-0) 10 mono-potassium phosphate 8 gypsum 8 limestone 5
A common inquiry in applying liquid fertilizer is in the calculation of amount required- should the fertilizer be applied by the gallon or by weight? Most liquid fertilizer concentrates weigh 10 to 11 pounds per gallon; hence it takes about 9 gallons of liquid to compare to 100 lbs. of dry fertilizer. These calculations can also help determine appropriate price: if the dry fertilizer costs 9 dollars per hundred, you should be on guard if the liquid fertilizer of the same formulation costs more than one dollar per gallon. Fertilizers of the same grade, formulation, placement and rates give nearly identical response whether liquid or dry.
Either dry or liquid fertilizers can be applied with the seed at planting time. Row spacing, soil moisture conditions and type of seed greatly modify the amount that can be applied, a problem that is accentuated by dry soil. There are, however, some general crop-specific guidelines to follow. For wheat and other small grains planted in moist soil and in 7-inch rows, apply up to about 100 lbs per acre. For field corn planted in 38-inch rows, the upper limit is about 15 lbs. Beans, soybeans, cucumbers and melons are very sensitive to salt injury; fertilizer should not be applied on the seed for these crops. Some vegetable crops such as spinach, red beets, tomatoes and onions can benefit from a small amount of a high phosphate fertilizer placed in the same band as the seed, especially when the soil is cold. Because sandy soils contain much less water, a lower rate of fertilizer should be used. Fertilizers on or near the seed are more likely to benefit small seeds than large seeds such as corn and beans. The salt index value of the various solutions need to be considered when applying fertilizer on the seed.
When applying liquid nitrogen fertilizers, and hydrous ammonia and certain liquid ammonia solutions need to be injected into the soil to prevent ammonia losses. Water mixtures containing ammonium nitrate and urea are popular products, competitively priced, and can be rapidly applied broadcast. However, it is necessary to follow the recommended practices to prevent urea volitization. Losses occur when urea is applied to dry surfaces, the soil temperature is high, and the soil is above pH 7.0. To reduce losses, water the urea into the soil or apply into the moist soil.
Leaf feeding is a method in which plant nutrients are absorbed through the leaves; however the amount of fertilizer that can be applied without burn is limited! The minimum nutrient requirements for plants are rather specific regardless of the method of application; do not expect any benefit from leaf feeding or any other program if the plant already has an ample amount. Leaf feeding can be advantageous where spray programs are necessary for pest control and the fertilizer material is compatible. Leaf feeding should be considered a supplement to, not a substitute for, the regular soil application of fertilizers, useful in the application of micronutrients. Micronutrients can be easily applied by leaf feeding because of the small amounts required for normal plant growth.
Liquid fertilizers are often used to make starter solutions for transplants. The fertilizers in the solution are immediately absorbed, which promotes new growth and better plant survival. The maximum amount of fertilizer in a starter solution will depend upon the carriers, and it is advisable to follow the manufacturer's recommendations during application.
Nitrogation or fertigation are terms sometimes used to describe the application of nitrogen or other fertilizer nutrients through the irrigation system. The most common fertilizer applied is 28% nitrogen solution because it is easily obtained and causes little or no problems when added to the water. The amount of nitrogen that can be applied through the irrigation system is limited only to the capacity of the injector pump. Even with a high rate of injection, the nitrogen will be so dilute that is will seldom cause injury to crops, and most situations call for rates of 20 to 50 pounds of nitrogen per acre per application.
A gallon of liquid fertilizer weighs, at most, about 11 pounds. In midseason, rapidly growing crops such as corn will take up daily almost 4 to 7 pounds of plant nutrients per acre. A good fertilizer program takes into account the plant nutrient uptake of the crop, the soil test and the expected return.
MSU Ag Facts No. 68 "Liquid Fertilizers"; Extension Bulletin E-933, January 1976 By: R. E. Lucas, Crop and Soil Sciences Department Cooperative Extension Service, Michigan State University