Polish trial shows higher yield with Qrop® in potato

Through the trials in Poland, SQM clearly demonstrated that the investment in applying high quality fertilizers providing 2.6 times more potassium, in addition to Ca and P, without adding Cl, at the right time to support early plant development and increased tuber caliber, pays off. An income of more than 12% total yield and with improved tuber quality was obtained.

Nitrate stimulates lateral plant growth (cytokinin formation), leading to improved canopy formation, as observed in this trial. Early closure of planting rows is important to provide shade to the root zone. This reduces soil water evaporation at the ridges, which protects the developing tubers from drought and also helps control weeds.

It is important to highlight the benefit of potassium nitrate to provide potassium together with nitrate nitrogen in the root zone, which generates a synergistic effect on cation uptake, faster root and canopy development, and consequently, better tuber filling. Showing these benefits in local agricultural practice is very rewarding. Especially for northern Europe, an early supply of nitrate nitrogen is important due to the colder spring conditions, when nitrification of ammonia or urea by soil bacteria is slow.

The application of nitrate nitrogen as the most efficient source of nitrogen uptake by plants, compared to ammoniacal or urea nitrogen sources, allows less nitrogen to be applied per hectare. Because of lower levels of residual nitrate and higher yields per unit of nitrogen applied, this not only gives better economic results, but also reduces environmental impact.

Poland ranks third in the list of EU potato producing countries, after Germany and France. Statistics show a shift of production from the domestic market of fresh products to the production of potatoes for starch and processed foods (e.g. French fries and frozen French fries). This is a consequence of the dynamic development of the food industry in Poland and changing consumer preferences, which reduces the amount of fresh potatoes consumed locally per capita.

Farmers aim to increase the average tuber production per hectare, which in the country is less than 30 MT/ha on average - including fresh potatoes - is lower compared to higher producing EU countries such as the Netherlands, Belgium or Germany (on average more than 46 MT/ha). In addition to phytosanitary concerns, the main challenge is drought, as most Polish farmers do not irrigate the crop.

A switch to the cultivation of local potato varieties, to varieties specially developed for processing into frozen potato chip products, requires the development of fertilization programs to achieve the expected high yield, tuber size class and corresponding nutrient demand of these crops.

Despite growing awareness of the negative impact of excessive chloride application on potato production, KCl remains the dominant K source in Western, Central and Eastern Europe. The cost of KCl is relatively low and the prevailing belief is that the negative impact of chloride on yield and quality is alleviated by applying KCl prior to winter so that chloride can be removed during the wet winter months. SQM's global experience with the use of potassium nitrate in Qrop® K PluS and Qrop® mix to increase the _NO3:NH4 ratio, decrease the total N:_K2Oratio and reduce the amount of chloride applied to the crop was translated into Polish farming practices.

A trial was organized in southeastern Poland in collaboration with Polish input experts from the Polboto company and an international ware potato buyer (Figure 1). The SQM program was evaluated in comparison with the usual local practice. The objective was to evaluate an improvement in productivity due to the reduction in chloride applied as KCl after winter, and increasing the amount of _K2Oapplied in spring and summer as _KNO3.

Figure 1. Chief agronomist Witold Choroś and SQM representative in Poland, Mariusz Hoszowski, evaluated yield and quality results.

Study design

A local agricultural practice treatment was compared with a treatment based on two Qrop® products: Qrop® K PluS 12-0-46 and Qrop® mix Development 20-10-16 +3CaO+ 2MgO. Both the local farming practice and the SQM treatments were applied in fields with similar soil characteristics and homogeneous growing conditions. The area of both fields was 6 ha each. Crop development during the season was monitored with pre-harvest evaluations on the number of tubers in 1 linear meter on two dates. Total yield was determined on a 0.9 ha area of each field, and tuber size grading was evaluated on 2.25 linear meter samples duplicated twice near harvest according to an industry standard protocol.

The nutrient programs are described in Table 1. The farmer does not apply potassium chloride after February, for fear of chloride toxicity. The main improvement sought with the application of Qrop® K PluS and Qrop® mix were split potassium applications without adding chloride, just before planting and during tuber filling. The additional potassium applications improved the total N: K2O ratio in favor of potassium compared to local farming practice. Qrop® mix provided more readily available potassium and nitrate nitrogen at the vegetative stage, with the added benefit of phosphorus and calcium. Qrop® K PluS allowed an application of potassium and nitrate nitrogen to meet crop demand at the tuber filling stage.

Outstanding Results

Early canopy development was observed in the crop after transplanting in late May. This allowed the plants to produce more tubers. Faster tuber formation was observed in June in the Qrop® treatment, resulting in 17% more tubers per linear meter in the August pre-harvest evaluation. (Table 2, Figures 2 and 3).

Table 3 shows a higher percentage of tubers in the size classes >40 mm for the Qrop® treatment, and together with the higher number of tubers per plant, this resulted in a total yield increase of 6 MT/ha, 12% improvement over the farmers' practice.

A selection of marketable tubers was packed in 3 x 10 kg net bags. These were weighed and stored for 2 months after harvest in a well-ventilated storage room without climate control, and weight loss after storage was determined. Weight loss during storage was twice as great in the agricultural practice compared to Qrop® treated potatoes. When the difference in weight loss is taken into account, the total gain in marketable tuber yield was 7 MT/ha (+17%).

At a modest market price of 125 euros / MT, the yield required to cover the costs of the investment in high-yield fertilizer is 2 MT / ha, or 4% over the yield currently achieved in farming practice. In this trial, the yield benefit was three times the break-even point, and net income doubled with Qrop® applications, compared to local farming practice (Table 3).

For this trial, an exceptionally dry period during early plant development (8 mm of rainfall during April, compared to a historical average of 38 mm) may have emphasized the importance of facilitated uptake of water, K, and Ca, in synergy with nitrate in early crop development (Figure 4). In addition, application of KCl in February with farmer practice may have affected carbohydrate production during tuber development in the control treatment. This potentially damaging effect was avoided by applying Qrop® mix, which does not contain KCl. The lower tuber weight loss during storage, related to better tuber and skin quality, can also be attributed to improved tuber K and Ca levels.

Fertilizer and nutrient application before planting and during crop development. Urea-AN = solution of Urea and ammonium nitrate 32% N + nitrification inhibitor.

Figure 2. Qrop® application increased the number of developing tubers in a 1 linear meter evaluation on June 15, 2020.

Figure 3. Higher tuber quantity and better size grading with Qrop® K Plus visible in a pre-harvest evaluation of 1 linear meter on August 13, 2020.

Early tuber formation in June (75 days after planting) and August (134 days after planting).

Table 3. Harvest evaluation in September (156 days after planting): total yield, tuber grades and percentage weight loss after 2 months of storage and financial analysis. A total cost of 4900 €/ha excluding fertilizer cost was estimated to be equal between treatments.

An overview of the test field in May, showing the dry conditions prevalent during early crop development.

What's next?

On a worldwide scale, SQM has demonstrated that the application of prilled potassium nitrate in Qrop® facilitates the improvement of potato yields, both in quantity and quality. The use of Qrop® mix formulas in Europe should be suggested because of their better balance between nutrients and highly efficient nutrient sources. The formula is based on potassium and nitrogen directly available to plants, without chlorides, but with additional nutrients to meet crop demand. This results in better yield and quality in a crop and, in addition, helps to improve soil conditions in the root zone and crop resistance to drought.