How to Tell If Crops Are Deficient in Fertilizer? Fertilization Mistakes and Ways to Improve Fertilizer Efficiency

The growth of crops cannot do without fertilizer, and fertilizer use is convenient, so it is deeply loved by farmers. However, there are many mistakes in fertilizer use that prevent fertilizer efficiency from being properly improved. Improper fertilizer application can easily cause waste and is prone to causing fertilizer damage. Below we will look at how to judge whether crops are deficient in fertilizer, and how to avoid some mistakes in fertilizer use and improve fertilizer efficiency.

1. How to judge the symptoms of crop fertilizer deficiency

Crop growth and development require a variety of nutrient elements. Once a certain nutrient element is lacking, deficiency symptoms will first appear on the leaves. The symptoms shown when different nutrient elements are lacking are different, and any kind of deficiency symptom will affect the normal growth and development of crops. Therefore, it is necessary to correctly diagnose deficiency symptoms according to their external manifestations, so as to timely supplement the lacking nutrient elements and reduce losses. Some common deficiency symptoms are listed below.

(1) Nitrogen deficiency

With nitrogen deficiency, the leaves turn pale green or yellowish white, the branches are thin and weak, the new leaves at the top gradually become smaller, and leaves fall easily at the same time.

The main symptoms are thin and short new branches, small upright leaves, grayish-green leaf color, and reddening of petioles, veins and cortex; few flowers and small fruits, fruits ripening early and falling off easily; many fine roots and few large roots, new roots turning yellow, and weakened resistance to pests, diseases and drought.

Taking soybean as an example, if nitrogen fertilizer is deficient during the growth period, starting from the lower leaves, the leaf color becomes lighter and pale green, and then gradually turns yellow and dries up, sometimes with bronze-colored markings appearing on the leaf surface. In severe cases, the plant stops growing and the leaves gradually fall off.

(2) Phosphorus deficiency

Insufficient phosphate fertilizer will hinder the formation of crop flower buds, make crop flowers small and few, and easily lead to poor fruit development. In the early stage of crop growth, the effect of phosphate fertilizer is not very obvious, but in the middle and late stages, phosphorus deficiency will affect the effective accumulation of nitrogen and the synthesis of protein. Different crops show different symptoms when deficient in phosphorus.

The manifestation of phosphorus deficiency in wheat is slow seedling growth, poor root development, reduced tillering, purple base of the stem, dark green leaves with a slight purplish red tinge, small ears and few grains, and a significant decrease in 1000-grain weight.

The manifestation of phosphorus deficiency in corn is slow growth at the seedling stage, with symptoms relatively obvious at the 5-leaf stage, purplish red leaves, short and small cobs with a curved shape, premature bald tips, and mostly unfilled grains.

The manifestation of phosphorus deficiency in soybean is darker leaf color, appearing deep green or black-green, small, pointed and narrow leaves that stand upright, and a thin and small plant with slow growth. In severe cases, the stems may appear red. If soybeans are deficient in phosphorus after flowering, brown spots appear on the leaves.

The manifestation of phosphorus deficiency in fruit trees is poor fruit quality, slow growth, sparse leaves, yellowish-brown leaf color, and a tendency to fall off early.

(3) Potassium deficiency

When potassium nutrition is lacking, the stems are slender, and in severe cases the leaf tips and leaf margins scorch, the leaves wrinkle and curl, and the margins of old leaves curl, turning yellow and scorched and falling off easily. The main manifestation is that the tips of the lower leaves of general crops turn yellow, gradually withering and yellowing along the leaf margins, while both sides of the veins and the midrib remain green.

In severe cases, most leaves wither and yellow, and the leaf margins appear scorched; cereal crops show weakened tillering ability, soft stems, and easy lodging; dicotyledonous plants have curled leaves that gradually shrink, sometimes with incomplete leaves, but the middle part of the leaf still remains green; root tuber crops decrease in quality and have poor quality; the fruit ripening period is inconsistent, the fruit flesh becomes more lignified, and the taste is sour and bitter and unpleasant to eat.

(4) Magnesium deficiency

Magnesium deficiency first causes yellowing between the veins of old leaves, gradually spreading to the upper new leaves, with the mesophyll turning yellow while the veins remain green, and various colored spots appearing between the veins.

(5) Iron deficiency

The symptoms of iron deficiency are similar to those of magnesium deficiency, except that iron deficiency first causes yellowing between the veins of new leaves, while the veins remain green, and then develops into the entire leaf turning yellow or whitish.

(6) Manganese deficiency

The symptoms of manganese deficiency are basically similar to those of iron deficiency, with chlorotic spots appearing between the veins and gradually forming stripes, but the veins remain green.

(7) Boron deficiency

With boron deficiency, young leaves lose green, leaves become thick and shrunken, leaf margins curl upward, the root system is underdeveloped, the growing points of the terminal bud and young roots die, and flowers and fruits fall off.

(8) Calcium deficiency

With calcium deficiency, the terminal bud is damaged and causes necrosis of the root tip, young leaves lose green, leaf margins curl upward and scorch, and leaf tips often become hook-shaped.

(9) Sulfur deficiency

With sulfur deficiency, the leaf color turns pale green or even white, spreading to new leaves, the leaves are slender, the plant is short, flowering is delayed, and the roots are noticeably elongated.

(10) Zinc deficiency

With zinc deficiency, the internodes of the plant noticeably shrink and become rigid, leaves turn yellow or become smaller, yellow spots appear between the veins and spread to new leaves, and young leaves are hard and small and become yellow-white.

(11) Molybdenum deficiency

With molybdenum deficiency, young leaves are yellow-green, leaves lose green and wither, and necrosis is easily caused.

(12) Copper deficiency

With copper deficiency, leaf tips turn white, young leaves shrink, and white leaf spots appear.

2. Common mistakes in current fertilizer application

(1) Drying organic fertilizer in the sun

Human manure and chicken manure have become the main base fertilizers for vegetable production in greenhouses, but vegetable growers often spread human manure and chicken manure in the field to dry and lose water for convenience of application. This practice causes fly maggots to breed, nitrogen to volatilize, and the nitrogen nutrient of the fertilizer to be lost.

(2) Applying calcium magnesium phosphate as a base fertilizer on alkaline soil

Calcium magnesium phosphate is a weakly acidic fertilizer that is insoluble in water. Only under weakly acidic conditions can it gradually convert into water-soluble phosphate and be absorbed by crop roots. When applied on alkaline soil, it cannot solve the urgent need of crop seedlings for phosphorus, causing physiological phosphorus deficiency.

(3) Broadcasting superphosphate on the soil surface as a topdressing

Phosphorus has little mobility in the soil, generally moving only within 1 to 3 cm from the application point at 0.5 cm. Therefore, surface application can hardly reach the crop rhizosphere and thus cannot play the role of supplementing phosphorus in the crop.

(4) Watering immediately after broadcasting urea

Urea is an amide nitrogen fertilizer that is easily soluble in water. After being applied to the soil, it must be decomposed before it can be transformed into ammonium bicarbonate and absorbed and utilized by crops. Watering immediately after surface application easily causes urea to be lost with the water and reduces fertilizer efficiency.

(5) Broadcasting ammonium bicarbonate with irrigation water

This method often causes more fertilizer at the water inlet, uneven crop growth, and difficulty in management, while in the afternoon when the greenhouse temperature rises, ammonia gas escapes from the soil, scorching the lower leaves of the crops and causing fertilizer damage.

3. Main methods for improving fertilizer efficiency

(1) Composting and decomposing organic fertilizer as a base fertilizer

In late July, in a sunny place, take 4000 kg of human manure, add 100 kg of calcium magnesium phosphate and 400 kg of chopped wheat straw (bran), mix them and pile them into a steamed bun shape, cover tightly with worn plastic film or seal with mud, and compost for 30 days to form a high-quality organic fertilizer for use as a base fertilizer in greenhouses.

(2) Concentrated application of superphosphate as a base fertilizer

Open an 8 cm deep furrow in the transplanting row, broadcast in phosphate fertilizer and then cover with 4—5 cm of soil, then transplant crops in the shallow furrow, shortening the distance between phosphate fertilizer and crop roots to make up for the weakness of low phosphorus mobility.

(3) Early application, deep application and foliar application of urea

According to the crop development stage and the demand for fertilizer and water, apply topdressing in advance and deeply, which increases the utilization rate by 28% compared with shallow application. When the greenhouse temperature is 15—20°C, apply 7 days in advance; when the greenhouse temperature is 20—25°C, apply 5 days in advance. When topdressing, open an 8—10 cm furrow, broadcast the fertilizer and then cover the soil tightly. For crops with large plant and row spacing, deep hole application can be used.

Water every 5—7 days according to the greenhouse temperature, so that it has enough time in the soil to fully ammonify, facilitating crop absorption and utilization. During the crop growth period, a 0.3% urea solution can be sprayed on the leaf surface, once every 7 days, using 100 kg of solution per mu, for 2—3 consecutive times.

(4) Deep application of ammonium bicarbonate

Ammonium bicarbonate is an ideal quick-acting fertilizer for topdressing in winter warm greenhouses for vegetable production. Because it rarely volatilizes at a soil temperature of 20°C, after being applied to the soil it can dissociate into ammonium ions and be adsorbed by the soil, then slowly release for absorption by crop roots. Even at a soil temperature of 5°C, it can also be transformed and decomposed for absorption and utilization by crops.

When topdressing, open a 10 cm deep furrow 8—10 cm away from the crop roots and stems, scatter the fertilizer and then cover it tightly with soil, which can increase the utilization rate by 10—30%, increase yield by 10% compared with shallow application, and increase yield by 7.8% compared with broadcasting with irrigation water.