Tubers with dry matter above 18-20% tend to be more susceptible to bruising and tubers disintegrate more readily when cooked. However, for processing high dry matter content is required to achieve a good fry colour and often 20-25% is specified. Nitrogen, potassium and magnesium can all have influences on tuber dry matter content.
Nitrogen is an essential nutrient to fuel canopy growth, however in temperate climates, an excess supply of nitrogen at later stages of growth will keep the crop growing, delaying maturity and may reduce starch and dry matter content, reducing processing potato crop quality.
This trial in Norway demonstrate that too much late nitrogen can reduce tuber dry matter.
Too much late growth in cool climate crops can also result in a crop with many 'oversize' potatoes and cause internal cracking.
The form of potassium has an effect on dry matter. Sulphate of potash - SOP (potassium sulphate) can achieve higher dry matters than muriate of potash - MOP (potassium chloride) and therefore is frequently the preferred form for processing potatoes. This is due to the chloride in the muriate of potash having a negative effect on tuber dry matter content.
This study from Denmark demonstrates the higher dry matter content achieved with SOP (potassium sulphate) rather than MOP (potassium chloride).
Low levels of magnesium will reduce the starch content of tubers and trials have confirmed the positive benefits of magnesium on tuber dry matter and specific gravity.