Balanced nutrition in potatoes

Horticultural crops most suited to potassium nitrate applications are those that are sensitive to chloride, those producing high amounts of carbohydrates, those with regular topdressing frequency, crops grown in sandier conditions, lower pH conditions, fertigated and hydroponically grown crops and crops grown in cold climates.

PREVIOUS ARTICLES IN Vegetables & Fruit have alluded to correct fertilizer applications in tomatoes; “The right choice for tomato production” (Sep / Oct, 2020) and in horticultural crops; “Excellent response documented with the use of potassium nitrate (Sep / Oct 2019). Potatoes are no exception.

Ultrasol® K Plus on potatoes* – Ultrasol K Plus **/ Qrop® K*** – KNO3 (potassium nitrate)

The potato is a high value horticultural crop producing high volumes of dry matter. Potatoes are heavy feeders of both nitrogen (N) and potassium (K). The amount of potassium removed by tubers alone is shown in Figure 1. Potassium removed by the whole plant is two thirds to double more than that removed by the tuber alone. This means that 8 – 8,8 kg of potassium is removed per ton of potato crop. The plant must obtain this K either from the soil or from applied potassium. Similarly, potatoes require a total of 4,5 kg of N per ton potato crop. Potassium nitrate supplies both these macro elements.

Potatoes can benefit for the following reasons: • Optimum pH maintained around the roots (rhizosphere) Potatoes are grown preferably on the low buffer (cleaner marketable tubers) and lower pH soils (less diseases) and require large amounts of fertilizer. At starting pH of around pH 5, if ammonium-nitrogen is applied, then the pH of the rhizosphere can decrease to pH 3.5. If nitrate-nitrogen is applied, the pH of the rhizosphere can increase to pH 6.5. The pH of the soil solution in direct contact with the roots can therefore be 1 000x more acid when ammonium- instead of nitrate-nitrogen is used. • Competing cations (uptake antagonism) Plants like to maintain an electro-chemical balance. Ammonium-nitrogen is a positively charged cation. This directly competes with the uptake of other important positive cations such as potassium, calcium and magnesium. Nitrate-nitrogen is a negatively charged anion which synergistically enhances the uptake of the positive cations of potassium, calcium and magnesium. • Nitrification On sandy soils (<5% clay – predominantly in the Sandveld region of the Western Cape) the nitrification process is non-existent and applied ammonium fertilizer remains in the ammonium form. Cold temperatures (winter plantings) as well as regular ammonium nitrogen applications will also influence the ammonium to nitrate ratio in the root zone. Any ammonium to nitrate ratio greater than 25:75 respectively is detrimental to plant growth and yield due to all the factors described in this article. • Energy conservation Nitrate is converted into organic compounds largely in the leaves of plants. The energy for this process is driven by electrons excited by light energy in the chloroplasts of the leaves. On the other hand, ammonium is converted to nitrogen compounds exclusively in roots. The energy used to drive this reaction is derived only from the burning of carbohydrates transported down from the leaves. An excess of ammoniumnitrogen causes a depletion of carbohydrates, which results in reduced dry matter accumulation reducing both yield and quality. • Chloride and Potato Quality Although sometimes conflicting, chloride-free fertilizer has been shown to increase dry matter content, starch content and specific gravity of tubers. Chloride-free fertilizer is the preferred source for processing and chipping potatoes. Growers using potassium sulphate exclusively as a potassium source to avoid chloride application, are applying excess sulphates creating nutrient imbalances in the soil solution. Sulfur, although important in plant nutrition, is required in much lower quantities and applied excesses can be loosely termed as “useless salts” creating unwanted salinity in the root zone. • Balanced nutrition Using potassium nitrate, growers can maximise the application of nitrates and minimise the application of ammonium, a situation that cannot be obtained by using either potassium chloride (KCL) or potassium sulphate (K2SO4.). Therefore, with Ultrasol K Plus, the potato is being supplied with two of its main nutritional elements with the nitrogen in the correct form facilitating the uptake of the important cations.

Figure 1. The amount of potassium removed by tubers.

*All claims in this article can be substantiated with references which can be made available on request. **Reg No K5020 Act 36/1947. Registration holder Sociedad Quimica y Minera (Africa) (Pty)Ltd ***Reg No K5021 Act 36/1947. Registration holder Sociedad Quimica y Minera (Africa) (Pty)Ltd Disclaimer The information herein contained is given to the best of SQM’s knowledge and is believed to be accurate. The conditions of use and application of the sugges- ted recommendations, are beyond SQM’s control. No warranty is made as to the accuracy of any data or statements contained herein. SQM specifically disclaims any responsibility or liability relating to the use of the recommendations and shall under no circumstances whatsoever, be liable for any special, incidental or consequential damages arising from such use.