Correct application of fertilizers to vegetables

The demand for information on the application of lime and fertilizers to vegetables grown under plastic greenhouses is growing. The yields of vegetables under protected cultivation, such as peppers, tomatoes and cucumbers, are two to four times higher than the yields obtained in the open field. This has stimulated the carrying out of new scientific research aimed at defining the quantities and ways of applying fertilizers in this modern production system.

This work presents information and recommendations on the management of correctives and fertilizers for the vegetable production system under protected cultivation.

Interpretation of soil analysis

Research carried out at the Campinas Agronomic Institute regarding vegetable fertilization is based on the concept of relative production, that is, it takes into account the response of crops to nutrients applied through fertilization.

Table 1 presents the interpretation of soil analysis for liming and fertilization of vegetables in general (field and protected cultivation).

Table 1. Interpretation of P, K, Ca, Mg, S and V% in soils.

Content / K+exchangeable (mmolc/dm3) / P(resin) (mg/dm3) / Ca++exchangeable (mmolc/dm3) / Mg++exchangeable (mmolc/dm3) / S–SO4-- (mg/dm3 ) / V (%)

very low / 0,0 – 0,7 / 0 – 10 / 0 – 4 / 0 – 2 / 0 – 2 / 0 – 25

Low / 0,8 – 1,5 / 11 – 25 / 5 – 10 / 3 – 5 / 3 – 5 / 26 – 50

medium / 1,6 – 3,0 / 26 – 60 / 11 – 20 / 6 – 10 / 6 – 10 / 51 – 70

high / 3,1 – 6,0 / 61 – 120 / 21 – 40 / 11 – 15 / 11 – 15 / 71 – 90

very high / > 6,0 / > 120 / > 40 / > 15 / > 15 / > 90

Source: Adapted from Raij et al. (1997) - São Paulo and Ribeiro et al. (1999) - Minas Gerais

The interpretation of calcium levels must be adopted with caution taking into account the texture of the soil. Therefore, 15 mmolc Ca++/dm3 of soil can be considered as medium to high content in sandy soil and medium to low content in clayey soil.

Regarding the micronutrients present in the soil, the interpretation aimed at fertilizing vegetables and other crops is presented in table 2.

Table 2. Interpretation of micronutrient content in soils (1).

Content / B (mg/dm3) / Cu (mg/dm3) / Fe (mg/dm3) / Mn (mg/dm3) / Zn (mg/dm3)

Low / 0 – 0,30 / 0 – 0,2 / 0 – 4 / 0 – 1,2 / 0 – 0,5 /

Medium / 0,31 – 0,60 / 0,3 – 0,8 / 5 – 12 / 1,3 – 5,0 / 0,6 – 1,2

High / > 0,60 / > 0,8 / > 12 / > 5,0 / > 1,2

(1) Boron extracted by hot water; Cu, Fe, Mn and Zn extracted by DTPA.

Source: Raij et al. (1997)

N recommendations for vegetables are based on the extraction of this nutrient by plants and export by crops.

An indication of the nitrogen content present in the soil is the amount of organic matter in it. About 5% of soil organic matter is made up of total nitrogen. However, this is not always available to plants. The forms of N in the soil, available to plants, such as nitric (NO3-) and ammonia (NH4+) or even those not available, are unstable, that is, subject to rapid changes, due to the actions of microorganisms in the mineralization of organic matter , leaching caused by rainwater or irrigation, etc. This therefore makes it difficult to interpret the analysis of this element when provided by soil analysis. The organic matter content of the soil also indirectly indicates the texture (granulometry) of the soil. Sandy soil is considered to be soil that contains organic matter up to 15 g/dm3; medium texture soil with organic matter between 16 and 30 g/dm3 and clayey soil with organic matter between 31 and 60 g/dm3. Whenever possible, it is interesting to carry out granulometric analysis (texture) of the soil to know the real quantities of sand, silt and clay in it.

The need for liming is determined by the percentage of base saturation of the soil and the tolerance of the vegetable species to the lowest or highest degree of soil acidity. The equation for calculating liming is given by: NC = CTC (V2 – V1)

10 PRNT

NC = Liming need, in t/ha;

CTC (or T) = Cation exchange capacity expressed in mmolc/dm3 of soil.

V1 = Base saturation given by soil analysis.

V2 = Base saturation to be achieved (generally between 70 and 80%).

The incorporation of limestone must be done up to 20 to 30 cm deep, as many vegetables have a root system as deep as extensive crops. Among the vegetables with a deep root system grown in an agricultural greenhouse, we can mention the tomato. With a moderately deep root system, peppers, cucumbers, eggplants, melons, parsley and chives stand out. Among those with a shallow root system are lettuce, chicory and endive.

The application of limestone must be done at least 30 to 40 days before planting, preferably using finely ground limestone ("filler") with PRNT of 80 to 90% or partially calcined (PRNT of 90 to 100%) . If only common lime is found (PRNT of 60 to 70%), it must be incorporated at least 60 days before planting the vegetables. Limestones that contain a good amount of magnesium in their composition, such as dolomites (above 12% MgO), should be preferred.

Organic fertilizer for vegetables has the following advantages:

a) Improves the physical conditions of the soil, reducing, for example, soil compaction problems.

b) Reduces the incidence of nematodes since organic fertilizers in general enable the development of useful microorganisms in the soil that have an antagonistic action against nematodes.

c) Partially supplies nutrients to plants gradually and continuously.

On the other hand, organic fertilizer has some limitations:

a) The incorporation of organic fertilizers into the soil must be carried out at least 30 to 40 days before planting, the time necessary for the curing or decomposition process to occur, otherwise there may be "burning" of the seeds or vegetable seedlings.

b) Some poorly decomposed organic fertilizers can introduce weed seeds into the site.

c) Animal manure, especially from birds, can carry salt residues and other products present in feed, causing problems such as soil salinization.

Among the best organic fertilizers used on vegetables, organic compost, earthworm humus and pre-fermented castor bean cake stand out.

The quantities of organic fertilizers to be applied also depend on their local availability and the cost of transportation and application. Table 3 shows organic fertilizer recommendations for different groups of vegetables, valid both for protected cultivation and in the field, in the open.

Table 3. Organic fertilizer recommendations for vegetables

Group of vegetables / Well-tanned cattle manure or compost (kg/m2 of bed) / Chicken/chicken manure, pig/sheep manure and earthworm humus / Castor bean cake (pre-fermented)

Hardwoods (lettuce, arugula, etc.) / 2 - 4 / 0,5 - 1 / 0,1 - 0,2

Fruits ((tomatoes, peppers, etc.) / 2 - 4 / 0,5 - 1 / 0,1 - 0,2

Bulbs and Roots (onion, carrot, etc.) / 1 - 2 / 0,25 - 0,50 / 0,02 - 0,05

Note: Higher doses of organic fertilizers for low fertility soils. Apply about 30 days before planting. Incorporate 20 to 30 cm deep, throughout the bed.

The recommendations for fertilizing vegetables based on the nutrient content in the soil and also on the extraction of nutrients by the vegetables are described below. The nutrient doses were determined based on experiments carried out in the soil and climate conditions of the State of São Paulo, and should be adopted with caution for other regions.

Pre-planting soil fertilization must be carried out across the entire area of ​​the bed or in the planting furrow. It is recommended to apply lime and fertilizers from the surface to a depth of 20 to 30 cm to provide better growth and distribution of the plants' root system.

The distribution of fertilizers to be applied as top dressing must take into account the nutrient absorption rate of the crop. For vegetables, it is recommended to apply 10% of nutrients in the first quarter of the crop cycle (beginning of growth); 20% of nutrients in the second phase of development; 40% of the nutrients in the third phase of the cycle (period of greatest formation of fresh mass of leaves and fruits) and 30% in the fourth phase of the crop cycle. Depending on the species and group of vegetables, nutrients such as potassium are concentrated in the stage of maximum fruit production.

Tables 4; 5; 6 and 7 show the amounts of nutrients needed for various vegetables that can be produced under protected cultivation.

Mix fertilizers into the soil at least 10 days before sowing or transplanting seedlings. Add 1 kg of B/ha and 3 kg of Zn/ha to the planting mineral fertilizer for all the vegetables mentioned above. New applications of micronutrients will only be carried out after chemical analysis of the soil.

Lettuce – 60 to 120 kg/ha of N; 20 to 40 kg/ha of P2O5 and 30 to 60 kg/ha of K2O, splitting the applications through fertigation; American-type lettuce should receive 20 to 40% higher doses of potassium compared to smooth and curly types. Almeirão and Chicória – 60 to 120 kg/ha of N, dividing the doses through fertigation; Kale - 60 to 120 kg/ha of N and 20 to 40 kg/ha of K2O, dividing the doses through fertigation. Every 30 days spray the plants with 0,5 g of ammonium molybdate and 1 g of boric acid per liter of water; Chives: 60 to 90 kg/ha of N and 20 to 40 kg/ha of K20, distributed through fertigation; Arugula: 90 to 150 kg/ha of N and 20 to 40 kg/ha of K2O, splitting the doses through fertigation; Parsley: 30 to 90 kg/ha of N and 20 to 40 kg/ha of K2O, distributed through fertigation.

Mineral top dressing: Apply 200 to 300 kg/ha of N; 60 to 120 kg/ha of P2O5 and 120 to 240 kg/ha of K2O, dividing the doses through fertigation.

Fertigation systems in protected cultivation

The main fertigation system is one that uses hoses in the form of ribbons or casings, on the surface or sub-surface of the soil. These hoses contain micro-holes, in the form of pores. Irrigation hoses may or may not be covered with plastic placed along the rows planted with vegetables.

Another fertigation system is carried out using drip tubes that are arranged along the irrigation lines and drip water with fertilizers onto plastic pots containing substrates of different compositions (figures 5 and 6). Substrates must be previously sterilized against pathogens, and fertilized according to chemical analysis that identifies their nutrient content.

In greenhouses known as high tunnels (in the form of arches and chapels), when leafy vegetables are planted, a sprinkler system with bars containing sprinklers at a height of 50 to 60 cm is also used. Clean water must be applied to vegetables after applying fertilizers via irrigation water.

In the case of producing vegetable seedlings, fertigation can be carried out using the nebulization system, where it is important to irrigate with clean water after applying highly soluble fertilizers, so that the leaves do not "burn" due to possible fertilizer residues.

Another system, less used, is the application of fertilizers to flood water, where vegetable seedlings, inside perforated cups, are placed in "pools", where water and nutrients are absorbed by the plants.

Horticulture Center / IAC / APTA

THANKS

The author would like to thank Mr. André Luis Trani (Institute of Chemistry/USP, São Carlos-SP), for reviewing and editing the text.