Thursday, 20 June 2019




All plants need certain mineral nutrients to survive. These minerals occur naturally in the soil and are taken up from the soil by the roots of the plants. Most soils usually have enough of these minerals to keep plants healthy. However, some nutrients are gradually used up by the plants, or are washed out of the soil, and need to be replaced to maintain optimal growth and appearance. The most common mineral nutrients that need replacing are Nitrogen (N), Phosphorus (P) and Potassium (K).

 Fertilizers are manufactured mixtures of chemical products that contain N, P, K, and other necessary nutrients. They are spread over the soil to re-supply the soil with the proper amount of these nutrients. The three numbers on the front of the fertilizer bag represent the percentage by weight of N, P and K in that particular mixture. These numbers are used to calculate how much of a particular fertilizer to apply at one time.


A typical lawn fertilizer may have the numbers 25-5-10. This means that a 100-pound bag of this fertilizer contains 25% (100 x 25%) total nitrogen, or 25 pounds of N, 5% (5 pounds) phosphorus compounds and 10% (10 pounds) potassium compounds. The three numbers are the “analysis” and are used to figure out how much fertilizer to apply. 

Nitrogen is the most important mineral nutrient for healthy plant growth and the one that plants use the most. The total nitrogen in fertilizers can be supplied as nitrate compounds, ammonium compounds, or urea. Usually, a combination of ammonium and urea are used. Nitrogen keeps the plants green and promotes lush leaf and shoot growth. It is important to keep enough nitrogen in the soil, but do not add too much. Thatch is the layer of brown stems and runners that build up between the soil surface and the green leafy part of the grass. As thatch gets thicker it will give a spongy feel to the lawn and as it continues to become thicker it will eventually cause the mower to sink into the grass and cut lower into the brown stemmy underlayer. This is called “scalping” and causes unsightly brown spots on the grass. Scalping is a problem in highly maintained turf grasses where it is much more visible and unsightly, and where it causes temporary injury to the lawn surface. 

What kind of fertilizer will I use?

HDOT will eventually replace all slow-release fertilizers with the better and advanced technology ultra slow-release formulations; however, until then, you will be using mostly the traditional slow-release N formulations. 

The ultra slow-release fertilizers are specially formulated to slowly release nutrients over a period of one year. Unlike the current slow-release fertilizers, these ultra slow-release formulations can be spread once a year in much higher amounts without danger of burning the plants.

 They also do not require that the plants be watered immediately after application, so irrigation systems may not be needed in areas that are fertilized. The Engineer will supply you with this new ultra slow-release fertilizers formulated for HDOT for use on turfgrasses, trees, shrubs and palms.

Where do I apply the fertilizer?


  • Fertilizing areas you are contracted to maintain will depend on the expected quality of maintenance for that area. The Engineer will determine this and include it in the specifications of the bidding process.
  •  Most rural areas will not need fertilization. The only rural areas needing fertilization are interchanges with turfgrass
  • . Bare soil or sparsely covered slopes subject to erosion should not be fertilized.
  •   Areas most likely to need regular fertilization are high visibility locations with good turfgrass cover and trees and shrubs requiring extra care. Most are combinations of mixed grasses, some broadleaf ground cover, such as wedelia, and various trees and shrubs. Fertilizer application may be required once a year.
  •   A few locations in or near main city highways have a higher level of maintenance expectations. Most of these areas are medians with St. Augustine grass, naupaka hedges and monkeypod trees or shower trees.
  •   A few locations may have other plantings, such as ‘El Toro’ zoysia lawn grass or other species of trees and shrubs that require more specialized care. These high visibility locations must be fertilized once a year, using the special ultra slow-release fertilizers. It is desirable, but not mandatory, that areas getting fertilization have a working irrigation system. 

How do I get the fertilizer for the job?

  • The State will supply you with the fertilizer for the job. You must get approval in writing from the HDOT before using fertilizers. 
  • This approval should be a memo including the time, location, amount of fertilizer issued and the equipment to be used to apply the fertilizer. This memo should be submitted at least two weeks before applying the fertilizer. The Engineer will tell you where to pick up the fertilizer and the spreader.
  •  The type of fertilizer you get will depend on the kind of plants and the location of your maintenance area. In lawn areas, you will be given the ultra slow-release turf type fertilizer higher in nitrogen; for trees and shrubs a more balanced ultra slow-release formulation containing equal amounts of N, P and K, and a special ultra-slow formulation for palms, which is higher in micronutrients and magnesium.
  •  Within two weeks after the fertilizing, you have to submit a record of the area you fertilized, the date applied and the amount used. This is covered in more detail in  “Reporting.” 

 How do I determine how much fertilizer to use?

The amount of fertilizer needed for grass, trees, shrubs and palms is based on the amount of nitrogen (N) needed to maintain normal, healthy growth without burning the plants. This amount of fertilizer, given in pounds of nitrogen per 1,000 sq. ft., is called the recommended rate which depends upon the percentage of total nitrogen and also the type of nitrogen formulation. To determine the amount of fertilizer you need for a job you, will need to know the recommended rate. This is usually listed on the fertilizer label.


      Typical recommended rates:

  • Soluble quick-release fertilizers, such as ammonium sulfate, are one pound N per 1,000 sq. ft.
  • Slow-release N products, such as sulfur-coated urea, are 1½ pounds N per 1,000 sq. ft.
  • The new ultra slow-release fertilizers supplied by HDOT can be as high as 4 to 5 pounds of N per 1,000 sq. ft. without danger of burning the grass.

Trees and shrubs

      Typical recommended rates:

  • Quick-release fertilizers are no more than 2 pounds N per 1,000 sq. ft. of the root zone.
  • Slow-release formulations are up to 3 to 4 pounds N per 1,000 sq. ft. of root zone area.
  • Ultra slow-release fertilizers can go as high as 7 to 8 pounds of N per 1,000 sq. ft. of the root zone. The ultra slow fertilizers will allow for one application a year. This will slowly release nutrients over the entire 12 months for all plants.

Soil fertility and its improvement


Fertile and productive soils are vital components of stable societies because they ensure the growth of plants needed for food, fiber, animal feed and forage, industrial products, energy and for an aesthetically pleasing environment. 

Soil fertility integrates the basic principles of soil biology, soil chemistry, and soil physics to develop the practices needed to manage nutrients in a profitable and environmentally sound manner. Soils differ widely in their ability to meet the nutrient requirements of plants; most have only moderate natural soil fertility. To achieve production objectives, more nutrients are usually required than can be supplied by the soil. 

High crop yields mean greater depletion of soil nutrient supplies, which eventually must be balanced by increased nutrient input to maintain the fertile soils needed by our societies. Thus a hallmark of high-intensity agriculture is its dependence on mineral fertilizers to restore soil fertility, and in the broader context of soil productivity, soil fertility regulates the supply of nutrients inherently available in soils or applied as manures and fertilizers to plants. 

Soils with high natural fertility can produce substantial crop yields even without added fertilizer but can produce even higher yields with an additional supply of the critical nutrients. Good soil fertility provides the basis for successful farming and should not be neglected. 

There are a number of ways of making use of soil fertility in farming: 

• nutrient mining–farming without any added fertilizer (e.g., in shifting cultivation); 

• utilization of as many components of soil fertility as possible without compensation and yet without negative yield effects (e.g., by applying only moderate amounts of fertilizer N and P); 

• maintenance and improvement of soil fertility to assure consistent high yields (e.g., by compensating for losses due to removal and by soil amendments to improve fertility). The large differences in fertility between different soil types and sub-types must be taken into account. Some soil characteristics important to nutrient management may be grouped geographically and general recommendations may be summarized as follows:

 Soils of the humid tropics

• partly very acid (liming is required, generally to pH 5.5 or above);

 • often low in available P or liable to P-fixation (use of fertilizer P is therefore often essential, combined if necessary with liming);

 • in very humid areas, often low in available K, Mg and S (therefore there are high fertilizer requirements for these nutrients); 

• often low sorption or storage capacity for nutrients (so fertilizer application should be split between several dressings); 

• often low in available N, although the decomposable organic matter is rapidly mineralized.

 Soils of the sub-tropics 

• water shortage (without irrigation, fertilizer use must be suitably adapted to efficient water use);

 • N is often the main critical nutrient, due to the low humus content; 

• widespread P deficiency, especially in sandy soils;

 • neutral soil reaction (therefore often a shortage of available Fe and Zn); 

• a generally good supply of S, Mn, and B;

 • risk of salinity due to lack of leaching of salts from the root zone. 

Soils of humid temperate zones 

• widespread soil acidity which requires liming; 

• partly obstacles to root growth (e.g., hard layers in subsoil); 

 • often insufficient aeration (poor natural drainage of heavy soils);

 • generally a shortage of available N and often of P, K, Mg

• low nutrient reserves in sandy soils, also only a little storage and therefore considerable leaching with water surplus;
• partial fixation of P and Mo (due to natural soil acidity) and Cu (in organic soils); • climatic cold stress retarding nutrient uptake

No comments:

Post a Comment


Introduction  Although several Extension bulletins are available on greenhouse vegetable production, few of these concentrate on organ...