Nutrient Needs and Fertilization Program for Kiwi Plants

Kiwifruit is a unique and nutritious fruit popular among consumers for its high vitamin C content and numerous health benefits. For successful kiwifruit production, the farmers need to understand and monitor the nutrient needs of their kiwi plants and create a suitable fertilization program.

Before diving into specific numbers and information, mentioning some key best practices a farmer should adopt is essential. Regular soil tests to determine nutrient levels and pH are important to help adjust the fertilization programs. Additionally, leaf analysis can be performed during the growing season to assess nutrient status and availability, allowing the grower to take immediate measures and correct any deficiencies with soil or foliar applications. Overfertilization (usually nitrogen) is suggested to be avoided because it can cause toxicities, imbalance to other nutrients, and environmental pollution. Irrigation or mechanical incorporation into the soil is generally needed, especially for granular and crystal fertilizers. Finally, it is important to avoid overirrigation, which can lead to nutrient leaching and runoff.

Nutrient Needs of Kiwi Plants

Nutrient removal during kiwifruit harvest is an important factor to consider when developing an effective fertilization program. Understanding how much of each nutrient is removed from the soil with the harvested fruit can help you replenish the soil, maintain its fertility, and support the plants next season. More specifically, per metric ton of fruit harvested, the following amounts are estimated to be removed from the field/soil:

2.5–3.5 kg (5.5-7.7 lb) of Nitrogen (N)
0.3–0.5 kg (0.66-1.1 lb) of Phosphorus (P)
3–4.5 kg (6.6-10 lb) of Potassium (K)
0.2–0.4 kg (0.44-0.88 lb) of calcium (Ca) and magnesium (Mg)
from the micronutrients boron, iron, zinc, copper, and magnesium, the amounts vary

Each of these nutrients plays an essential role, and luck may affect the absorption of others, causing significant problems in plant growth and productivity.

Nutrient Needs During Different Developmental Stages

Kiwifruit plants have specific nutrient requirements that vary depending on the growth stage. Proper nutrient management ensures healthy growth, maximum yield, and high-quality fruit.

Young Plants (Establishment Phase) vs Mature-Fruit-Bearing Plants

Our primary focus for the young kiwi plants is on helping them establish a strong root system and vigorous vegetative growth. As a result, in the first 1-3 years after planting, we should supply the plants with sufficient amounts of Nitrogen (N) that promotes leafy growth and shoot elongation, Phosphorus (P) that supports root establishment, and Potassium (K) that enhances disease resistance and overall vigor. It is important to use a small amount of fertilizers during the first year after planting to prevent excessive growth that frost could damage.

On the other hand, mature kiwi plants require a nutrient management strategy that supports fruit production and maintains plant health. Emphasis shifts to potassium and calcium for fruit development and quality. More specifically, potassium is vital for fruit sizing and sugar content, while calcium ensures fruit firmness and reduces physiological disorders. Of course, we should not forget the micronutrients (iron, zinc and boron that support enzymatic functions and fruit set). These micronutrient are usually applied as chelated iron, zinc sulfate, and borax.

Fertilization Program for Kiwi Plants

Developing a fertilization program tailored to the specific needs of kiwi plants involves understanding nutrient requirements, application methods, and timing. Below, we will see some common practices many kiwi growers follow during the different developmental stages in a growing season for mature, fruit-bearing kiwifruit plants. You should not follow any of them without first doing your own research and consulting a local licensed agronomist. Keep in mind that every field is different, and no one can advise you on your crop fertilization program without knowing your field's crop history and soil analysis.

Important note: Kiwi plants are very susceptible to iron deficiency, which is one of the most common problems of kiwi farmers (especially in fields with alkalic pH soil). Iron deficiencies should be addressed early in the season (at the start of the new vegetation), and at any moment (with foliar applications) the farmer observes relevant symptoms (after confirming with a leaf analysis). Yellowish kiwi fruits have lower commercial value and are unwanted.

1. Early Growth Stage (Spring)

In the early growth stage in spring, kiwi plants require significant amounts of nitrogen (N) to support rapid vegetative growth. During this period, the plants develop shoots, leaves, and new roots, crucial for establishing a strong framework for supporting the fruits that will be formed later. Farmers usually apply their basic fertilization at the end of winter and early spring. Some common options for slow-release synthetic fertilizers are 12-8-16, 15-9-15, or 15-15-15, with S, Mg, and micronutrients.

For a field with an average production of around 40-45 tons per hectare, the farmers may apply the following:

120-180 kg of N per hectare (or 107- 160 lb/acre) (as urea, ammonium nitrate, and ammonium sulfate) - It is advised to be applied into 2-3 doses – time moments within the growing season.
40-130 kg of P per hectare (35.7-116 lb/acre) (as superphosphate and triple superphosphate).
150-180 kg of K per hectare (134-160 lb/acre) (as potassium sulfate and potassium chloride).

2. Flowering and Fruit Set (Late Spring to Early Summer)

During flowering and fruit set, the plant's nutrient needs shift. Adequate nutrient supply during this period is critical for successful pollination and fruit development. For full-bearing kiwi plants, farmers apply 1 pound of nitrogen per plant, equivalent to 150 pounds per acre (1.12 kg of N per plant and 168 kg per hectare). At least 50-60% of nitrogen should be applied in March-April (north hemisphere), with the remainder applied in May, June, and July. Liquid feeding via an irrigation system is often used, applying about 100 ppm or 10 pounds of nitrogen per acre per week.

During this period, potassium and calcium play a crucial role in fruit formation. Some commonly used amounts are 60-80 kg of potassium and 20-30 kg of calcium (as calcium nitrate or calcium carbonate) per hectare (53-71 lb P/acre, 18-27 lb Ca/acre), which are applied at the start of the fruit set.

3. Fruit Development and Maturation (Summer to Early Fall)

During the fruit development and maturation phase, the nutrient requirements of kiwi plants focus on enhancing fruit size, quality, and flavor. For this reason, farmers invest in potassium applications (essential for fruit enlargement and sugar accumulation). Up to 100 kg/ha (89 lb/acre) of potassium might be applied as the fruit matures, while most farmers choose to perform foliar sprays. Additionally, calcium (foliar applications) can improve fruit firmness and reduce physiological disorders.

4. Post-harvest (Late Fall to Winter)

After harvesting, kiwi plants start preparing for dormancy, during which nutrient uptake decreases. However, adequate nutrition at this stage is essential for replenishing nutrient reserves and preparing for the next growing season.

Fertigation Program for Kiwi Plants

Fertigation, applying fertilizers through irrigation systems, is a highly efficient method for delivering nutrients to kiwi plants. It allows for precise nutrient management and reduces the risk of nutrient leaching. To apply fertigation as a complementary fertilization option (together with soil and foliar applications), you need to have a drip irrigation system and suitable (soluble) fertilizers. You must consult a local licensed agronomist to select the most appropriate products to avoid problems.

In a common fertigation schedule, a farmer may apply 100 ppm nitrogen solution (e.g., urea or ammonium nitrate) once a week in early spring to promote rapid vegetative growth. Later, during the flowering and fruit set, a balanced solution of N-P-K, e.g., 20-20-30 (with more potassium), is usually applied twice a week. Finally, during fruit development and until maturation, the amount of potassium added can increase by adding 15-5-40 fertilizers once or twice a week, depending on soil and plant conditions.

Organic Fertilization Options

Organic growers can use a more integrated approach to enhance soil fertility by applying compost and manure (from organically grown animals) to meet the nutrient needs of kiwi plants. Some may use bone meal or rock phosphate as a phosphorus source. If other products are used, they need to be suitable and certified for organic cultivation.

By understanding the nutrient needs of kiwi plants, implementing a well-planned fertilization program, and following best practices, growers can ensure healthy plant growth, high fruit yields, and excellent fruit quality.